Ultraviolet light induced reactions in polymers: An international symposium sponsored by the Division of Organic Coatings and Plastics Chemistry at the ... April 8-11, 1975 (ACS symposium series ; 25)

Ultraviolet Light Induced Reactions in Polymers Santokh S. Labana, EDITOR Publication Date: June 1, 1976 | doi: 10.102...

Author: Santokh S. Labana

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Ultraviolet Light Induced Reactions in Polymers Santokh S. Labana,

EDITOR

Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.fw001

Ford Motor Co.

An international symposium sponsored by the Division of Organic Coatings and Plastics Chemistry at the 169th Meeting of the American Chemical Society, Philadelphia, Penn., April 8-11,

1975

25

ACS SYMPOSIUM SERIES

AMERICAN CHEMICAL SOCIETY WASHINGTON, D. C.

1976


Library of Congress

Data

Ultraviolet light induced reactions in polymers. (ACS symposium series; 25 ISSN 0097-6156) Includes bibliographical references and index. 1. Polymers and polymerization, Effect of radiation on —Congresses. 2. Ultra-violet rays—Congresses. I. Labana, Santokh S., 1936II. American Chemical Society. Division of Organic Coatings and Plastics Chemistry. III. Series: American Chemical Society. ACS symposium series; 25. QD381.8.U45 ISBN 0-8412-0313-X

547'.84

76-3421 ACSMC8 25 1-495

Copyright © 1976 American Chemical Society A l l Rights Reserved. N o part of this book may be reproduced or transmitted in any form or by any means—graphic, electronic, including photo copying, recording, taping, or information storage and retrieval systems—without written permission from the American Chemical Society. PRINTED IN T H E UNITED STATES O F AMERICA

ACS Symposium Series


Robert F. G o u l d , Editor

Advisory Board Kenneth B. Bischoff Jeremiah P. Freeman E. Desmond Goddard Jesse C. H. Hwa Philip C. Kearney Nina I. McClelland John B. Pfeiffer Joseph V . Rodricks Aaron Wold


FOREWORD The ACS SYMPOSIUM SERIES was founded in 1974 to provide

a medium for publishing symposia quickly in book form. The format of the SERIES parallels that of the continuing ADVANCES IN CHEMISTRY SERIES except that in order to save time the papers are not typeset but are reproduced as they are submitted by the authors in camera-ready form. As a further means of saving time, the papers are not edited or reviewed except by the symposium chairman, who becomes editor of the book. Papers published in the ACS SYMPOSIUM SERIES are original contributions not published elsewhere in whole or major part and include reports of research as well as reviews since symposia may embrace both types of presentation.

iv

PREFACE O t u d y of the effects of ultraviolet light on polymers has attracted con^

siderable interest for a long time.

Some of the early research was

Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.pr001

directed towards understanding the photodegradation processes in poly mers, especially for polymers used in paints and in articles exposed to sunlight. In general this research aimed to make polymers more durable to light. A number of very effective photostabilizers were developed as a result.

More recently, because of ecological concerns, notable efforts

are being made to develop polymers with controlled rate of photodegra dation or to develop polymers which are stable when kept away from sunlight but which degrade rapidly outdoors.

Significant progress has

been made in this direction in recent years. Ultraviolet light can be used to induce reactions for making new polymers, modification of existing polymers, crosslinking of polymers, and degradation of polymers. This book deals with the recent develop ments on both the mechanisms of these reactions and discusses their use. A l l of these have provided bases for a number of important industrial materials and processes.

Various papers describe the use of ultraviolet

light for curing of paints, printing inks, microelectronics, photostabilization, photodegradation, and theoretical treatment of the excited species in the photochemical reactions. The need to reduce solvent emissions and to conserve energy in coatings and printing inks has stimulated investigations of the applica tions of photopolymerization reactions.

T h e ultraviolet light curable

coatings and printing inks generally produce much less organic emissions and cure with less energy consumption as compared with thermally cured compositions. The growth of uv-curable coatings so far has been limited by lack of rapid photopolymerization in highly pigmented or thick (over 25 μ) coatings. Approximately half of the papers in the book deal with photopolym erization reactions including the role of photoinitiators and photosensi tizes. The other half of the papers deal with photodegradation reactions including the use of photostabilizers and photodegradable polymers. I take pleasure in acknowledging F o r d Motor C o . and S. Gratch for their interest in, and support of, this project. vii

I also wish to express my

appreciation to Carol Gestwicki, Joan Gorski, and Anne Oslanci for their secretarial assistance. Finally, I thank the Division of Organic Coatings and Plastics Chemistry, American Chemical Society for sponsoring the Symposium. Ford Motor Co.

SANTOKH S. L A B A N A

Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.pr001

Dearborn, Mich. December 16, 1975

viii

1 Participation of Excited Species in Propagation Step in Photopolymerization NORMAN

G. GAYLORD

Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch001

Gaylord Research Institute, Inc., New Providence, N.J. 07974

The use of ultraviolet light in polymerization processes takes advantage of the fact that exposure of organic molecules to irradiation results in the generation of reactive species, e.g. excited species, at ambient or lower temperatures. The role of UV light has generally been considered to be restricted to the initiation of polymerization. However, propagation may also involve, or even be limited to, species generated by photo -excitation. UV Light in Polymerization Initiation When a polymerizable monomer is exposed to UV light in the presence of a photosensitizer, polymerization may "be initiated by radicals generated by dissociation of the excited sensitizer, e.g. benzoin methyl ether. Alternatively, the excited photosen sitizer may transfer its excitation energy to the monomer which, in turn, undergoes excitation and radical initiated polymeriza tion. [p-p]* --> P·

P-P

P-M.

M-M.

P-P + M* --> M.

[P-P]*

(1) (2)

The initiation of polymerization in the presence of benzo phenone requires the presence of a hydrogen donor such as a sol vent or polymer. The excited sensitizer abstracts a hydrogen from the donor resulting in the formation of a radical which initiates monomer polymerization. Ρ

*

Ρ

S-M.

PH + S.

(3)

Polymerization of monomers occurs in the absence of a photosensitizer when the functional groups in the monomer undergo 1


2

UV LIGHT INDUCED REACTIONS IN POLYMERS

excitation upon exposure to l i g h t of the appropriate wavelength, i.e. energy l e v e l . Although the result of photoactivation of a monomer is gen e r a l l y the generation of a r a d i c a l species, ionic polymerization has been reported where a c a t i o n i c a l l y polymerizable monomer, i.e. isobutylene, underwent photoionization in the vapor state and the ionized fragments were separated from t h e i r electrons by an electric field (1). Irradiation of a donor monomer-electron acceptor charge transfer complex, e.g. N-vinylcarbazole-sodium chloroaurate (2), -nitrobenzene (2) or -p-chloroanil (3) and β-propiolactone-uranyl nitrate (4), results i n the initiation of cationic polymerization. Irradiation of an acceptor monomer-electron donor charge transfer complex i n i t i a t e s anionic polymerization i n the case of nitroethylene-tetrahydrofuran (5) and r a d i c a l polymerization i n the case of methyl methacrylate-triphenylphosphine (6). Free r a d i c a l copolymerization is i n i t i a t e d upon UV i r r a d i a t i o n of mixtures of isobutyl vinyl ether and a c r y l o n i t r i l e (7) presumably as a result of photoexcitation of the comonomer charge transfer complexes. The excited complexes dissociate into ionradicals which initiate r a d i c a l propagated copolymerization. 5

JVE-ÂN]—[VETTAN]

VEÎ

+

aAN

+

bVE

*VET

+

TAN

00

(5)

* VE- (AN -co-VE^)

Mixtures of styrene and a c r y l o n i t r i l e also y i e l d free r a d i c a l copolymers under UV i r r a d i a t i o n (Table I) (8). Table I. Photoinitiated Copolymerization of Styrene and Acrylonitrile Light Dark 2537 A. 3500 A. 3500 A. Dark

Time, hr. 2k.O 0.5 0.5 6.0 18.0 1.0 2k.O

Conversion,$ 0.0 0.5 0.9 12.2 31.6 1.8 2.3

S/AN mole r a t i o Found Theory

58/U2

59Al

S/AN mole r a t i o = 1; 30°C Although i n i t i a t i o n by monomer ion-radicals may also be operative i n t h i s case, alternatively, homopolymerization of excited comon omer complexes may occur to a limited extent, due to the low con centration of such complexes, followed by ion pair coupling or dissociation to i n i t i a t e r a d i c a l copolymerization.

1.

GAYLORD

Excited Species in Propagation Step

[S-*AN]

> (SÎTAN)

3

v (S-AN) S 7AN X

(6)

+

^y (S-AN) S-AN( S - A N ) s TAN

^ ^ ^ S ( S - A N ) S·

X

+

(8)

AN

> (S-AN) S-AN- ( S - C O - A ^ )

(S-AN) S-AN- + aS + bAN x

x

* (S-AN) S-(S

(S-AN) S- + aS + bAN

X

x


(7)

X

-co-AI^)

(9) (10)

Thus, i n these cases, UV r a d i a t i o n provides a method f o r t h e i n i t i a t i o n o f r a d i c a l or i o n i c polymerization. A f t e r the i n i t i a t i o n s t e p , c h a i n propagation proceeds as though i n i t i a t i o n r e s u l t e d from the use o f c o n v e n t i o n a l c a t a l y s t s f o r r a d i c a l and ionic polymerization. UV L i g h t i n P o l y m e r i z a t i o n

Propagation

P h o t o a c t i v a t e d Copolymerization. Although p o l y m e r i z a t i o n and c o p o l y m e r i z a t i o n g e n e r a l l y i n v o l v e t h e a d d i t i o n o f a monomer t o a r e a c t i v e c h a i n end, t h e ground s t a t e charge t r a n s f e r complex generated by t h e i n t e r a c t i o n o f an e l e c t r o n donor monomer and a s t r o n g e l e c t r o n acceptor monomer, a c t s as a s i n g l e u n i t and, upon e x c i t a t i o n o f t h e complex, b o t h monomers enter t h e c h a i n . D

+ A

i ^ A ]

|i>Î:Âj

~(DA) D TA

Jllïâ^

X

+

(11) -(DA) N

(12)

The N - v i n y l p y r r o l i d o n e - m a l e i c anhydride charge t r a n s f e r com p l e x undergoes homopolymerization upon p h o t o e x c i t a t i o n i n a i r t o y i e l d the a l t e r n a t i n g copolymer (9). Complexation o f a r e l a t i v e l y poor e l e c t r o n a c c e p t i n g monomer w i t h a Lewis a c i d o r organoaluminum compound converts the accept or monomer t o a stronger e l e c t r o n acceptor, thus promoting the f o r m a t i o n o f ground s t a t e comonomer complexes. The l a t t e r under go p h o t o e x c i t a t i o n and homopolymerization. A

+ MX

D

+ A...MX - — * |p-*A...Mx]

JDÎ7A...MXJ

ν A...MX

(13)

> - ( D A ) - + nMX n

|pÎTA...Mx]

(lh)

(15)

Complexation o f methyl methacrylate and a c r y l o n i t r i l e w i t h t r i e t h y l a l u m i n u m converts these poor e l e c t r o n a c c e p t i n g monomers i n t o stronger e l e c t r o n a c c e p t o r s . Ground s t a t e charge t r a n s f e r complexes a r e generated when styrene i s added t o these monomers i n the presence o f t r i e t h y l a l u m i n u m . P h o t o e x c i t a t i o n o f t h e

4

UV

L I G H T INDUCED REACTIONS IN

POLYMERS

complexes under UV i r r a d i a t i o n r e s u l t s i n the formation of exc i p l e x e s which homopolymerize t o y i e l d equimolar copolymers over a wide range of comonomer r a t i o s ( l O ) . Thus, UV l i g h t c o n t r o l s the propagation step by promoting the formation of polymerizable s p e c i e s , i . e . comonomer e x c i p l e x e s . As shown i n Table I I , the p h o t o a c t i v a t e d c o p o l y m e r i z a t i o n of styrene and methyl methacrylate i n the presence of A l E t ^ y i e l d s equimolar, a l t e r n a t i n g copolymers when the i n i t i a l monomer charge contains excess styrene. However, when the comonomers are pre sent i n equimolar r a t i o , the copolymer i s r i c h i n methyl meth acrylate.


Table I I . P h o t o a c t i v a t e d P o l y m e r i z a t i o n of S - M M A . . . A l E t ^ Charge S/MMA/A1

mole r a t i o

a

Copolymer mole r a t i o Found Theory S/MMA

Conversion, °j 0

50/50/10

I.7U

37/63

50/50

70/30/5

2.79

51Λ9

60/UO

80/20A 85/15/3 90/10/2

2.83 2.95 3.19

51Λ9 51Λ9 50/50

70/30 75/25 8U/16

* 500W tungsten lamp; 5°C; Based on l / l S/MMA

[ÂlEt Ί = 10 mmoles; 2 hr L 41

Since MMA...AlEt^ i s a stronger e l e c t r o n acceptor than uncomplexed MMA, i t forms charge t r a n s f e r complexes w i t h the l a t t e r as w e l l as w i t h styrene. MMA S

+

MMA..

.AlEt

+

MMA..

.AlEtg

MMA. -.MMA.. .AlEt^j +

3

stTMMA..

.AlEt^J

(16) (17)

The c o p o l y m e r i z a t i o n of the two complexes y i e l d s an MMA-rich co polymer. However, when the i n i t i a l monomer charge contains ex cess s t y r e n e , there i s l i t t l e or none of the complex from Eq. (l6) and the equimolar copolymer r e s u l t s from the homopolymeriza t i o n of the comonomer complex i n Eq. (17)· The c o p o l y m e r i z a t i o n of styrene and a c r y l o n i t r i l e i n the presence of A l E t ^ under UV i r r a d i a t i o n y i e l d s equimolar, a l t e r n a t i n g copolymers when the i n i t i a l comonomer charge i s equimolar or contains excess a c r y l o n i t r i l e and products w i t h compositions intermediate between t h a t of the equimolar copolymer and t h a t of the r a d i c a l copolymer when the i n i t i a l charge i s r i c h i n styrene (Table I I I ) ( l O ) . The intermediate compositions may represent mixtures of equimolar and r a d i c a l copolymers, b l o c k copolymers generated as shown i n Eq. (6)-(l0) or random copolymers r e s u l t i n g from c o p o l y m e r i z a t i o n of complexes and monomers.

1.

GAYLORD

5


Table I I I . P h o t o a c t i v a t e d P o l y m e r i z a t i o n o f S-AN...AlEt^ Copolymer S/AN mole r a t i o Found Theory

Charge S/AN/AI

Conversion,

mole r a t i o 30/70/6 50/50/6 70/30/6 80/20/6 90/10/6

8.81 5.67 7.12 lh.33 27.10

5k/U6 59Al 67/33 7U/26 8k/l6

51Λ9 51Λ9 6V36 63/37 72/28


t 3500 A. Hg lamp; 30°C; l A l E t J ] == 12 mmoles; 2 h r L 4! Based on l / l S/AN Ethylaluminum s e s q u i c h l o r i d e (EASC) and d i c h l o r i d e are even more e f f e c t i v e than A l E t i n promoting the formation o f styrenemethyl methacrylate and s t y r e n e - a c r y l o n i t r i l e charge t r a n s f e r complexes. The c o n c e n t r a t i o n o f ground s t a t e complexes i s s u f f i c i e n t l y high so t h a t a u t o e x c i t a t i o n occurs, p o s s i b l y through c o l l i s i o n , and p o l y m e r i z a t i o n proceeds even i n the dark. However, upon exposure t o UV l i g h t , t h e c o n c e n t r a t i o n o f e x c i p l e x e s and the r a t e o f homopolymerization are g r e a t l y increased t o y i e l d a l t e r n a t i n g copolymers over a wide range o f comonomer compositions (Table IV) (10). a

Table IV. P h o t o a c t i v a t e d P o l y m e r i z a t i o n o f S-AN...EASC Copolymer S/AN mole r a t i o Found Theory

Charge S/AN/AI

Conveys

mole r a t i o 12.5/87.5/5 25/75/5 37.5/62.5/5 50/50/5 62.5/37.5/5 75/25/5

U8/52 50/50 52/U8 52/1(8 5lA9 55A5°

10.7 15 Λ 16.5 21.5 17.1 12.3

c

c

^5/55 50/50 5h/U6 59Λ1 62/38 69/31

10 mmoles; 1 h r Ambient l i g h t ; 30°C; JEASCJ Based on l / l S/AN A l t e r n a t i n g s t r u c t u r e confirmed by N M R The i n f l u e n c e o f UV l i g h t on the course o f c o p o l y m e r i z a t i o n , i . e . on copolymer composition, i s d r a m a t i c a l l y shown i n the t e r p o l y m e r i z a t i o n o f butadiene, methyl methacrylate and a c r y l o n i t r i l e i n the presence o f ethylaluminum d i c h l o r i d e . The r e a c t i o n a c t u a l l y i n v o l v e s t h e c o p o l y m e r i z a t i o n o f two complexes, i . e . B D - M M A . . . .EtAlClp and BD-AN.. . E t A l C l . When the r e a c t i o n i s con ducted i n the dark w i t h an i n i t i a l B D / M M A / A N = 6θ/2θ/20 molar p

6

UV L I G H T INDUCED REACTIONS IN P O L Y M E R S


charge, the terpolymer composition v a r i e s w i t h temperature, rang i n g from a B D / M M A . ^ A N composition o f 50/U1/9 at 0°C t o a U7/17/36 composition a t 50 C. I n c o n t r a s t , when the r e a c t i o n i s conducted under U V l i g h t , the composition i s e s s e n t i a l l y independent o f temperature and remains a t about 50/33/17 over the 0-50 C range (Table V ) ( l l ) . A p p a r e n t l y thermal a c t i v i a t i o n y i e l d s d i f f e r i n g r e l a t i v e amounts o f the copolymerizable e x c i t e d complexes as the temperature i s v a r i e d w h i l e U V l i g h t e x c i t e s a l l o f the ground s t a t e complexes whose r e l a t i v e concentrations are independent o f temperature. The y i e l d o f terpolymer i s a t a maximum between 330 and 350 myUL, c o n s i s t e n t w i t h the U V s p e c t r a which i n d i c a t e the formation o f a charge t r a n s f e r complex w i t h a b s o r p t i o n i n t h i s r e g i o n (12). Table V . T e r p o l y m e r i z a t i o n o f BD-MMA.-AN i n Presence of E t A l C l B D / M M A / A N / A 1 = 60/20/20/2 mole r a t i o Dark Temp., Terpolymer,mole

°c

BD

0 19 30 1+0 50

50 52 56 1+7 1+7

MMA.

1+1 26 20 18 17

AW

9 23 21+ 35 36

ratio MMA/AN

h.55 1.13 Ο.83

0.51 0.1+7

UV Light Temp., Terpolymer,mole °C

BD

MMA.

2 21.5 30 1+0 50

50 50 1+9 50 52

31+ 33 33 33 32

AN

16 17 18 17 16

p

ratio MMA./AN

2.12 I.9I+

1.83 1.9l* 2.02

In the t e r p o l y m e r i z a t i o n o f s t y r e n e , methyl methacrylate and a c r y l o n i t r i l e ( S / M M A . / A N = 50/25/25 mole r a t i o ) i n the presence o f EASC, the terpolymer composition i s approximately 50/36/lU, i n dependent o f the temperature w i t h i n the range o f 10-90 C, whether the r e a c t i o n i s conducted i n the dark o r under U V r a d i a t i o n (10). However, the t e r p o l y m e r i z a t i o n r a t e i s i n c r e a s e d 2-5 times under UV light. The r e a c t i o n o f a conjugated diene such as butadiene o r i s o prene and an e l e c t r o n acceptor monomer such as m a l e i c anhydride proceeds through a ground s t a t e complex which undergoes c y c l i z a t i o n t o y i e l d the D i e l s - A l d e r adduct. However, under U V l i g h t , i n a i r o r i n the presence o f s e n s i t i z e r s , the adduct i s accompa n i e d by the equimolar, a l t e r n a t i n g copolymer which r e s u l t s from e x c i t a t i o n o f the ground s t a t e complex, f o l l o w e d b y homopolymer i z a t i o n o f the e x c i t e d complex (13), as shown i n Eq. ( l 8 ) . When the e l e c t r o n acceptor monomer i s a r e l a t i v e l y weak acceptor, e.g. a c r y l o n i t r i l e , the r e a c t i o n w i t h a conjugated diene such as butadiene, t o y i e l d the D i e l s - A l d e r adduct, pro ceeds s l o w l y . However, i n the presence o f aluminum c h l o r i d e , t h e a c r y l o n i t r i l e i s converted t o a s t r o n g e r e l e c t r o n acceptor and the formation o f the ground s t a t e complex and the adduct therefrom proceeds more r a p i d l y . When the r e a c t i o n i s c a r r i e d out under U V i r r a d i a t i o n , the y i e l d o f adduct i s g r e a t l y reduced and the e q u i -


1.

GAYLORD

7


molar, a l t e r n a t i n g copolymer becomes t h e predominant product (ik) (Table V l ) , i n accordance w i t h t h e r e a c t i o n sequence shown i n Eq.

(19). Table V I . P h o t o a c t i v a t e d Copolymerization o f Butadiene and A c r y l o n i t r i l e i n Presence o f A l C l ^ and E t A l C l 2

AlCl Dark Adduct

BDJVN

Copolymer ( B D - A N ) A

23.5 0

J

n

UV

12.8 lk7.3

EtAlClp Dark UV

20.0

18.3

15.8

126.8

B D / A N / A 1 = 266/266/2.66 mmoles; 20°C; kO min Y i e l d i n 10~3 mmoles/min

8

UV

L I G H T INDUCED REACTIONS IN P O L Y M E R S


As shown i n Table V I , when the r e a c t i o n between butadiene and a c r y l o n i t r i l e i s c a r r i e d out i n t h e presence o f E t A l C l p , a u t o e x c i t a t i o n occurs even i n t h e dark t o y i e l d b o t h adduct and equimolar, a l t e r n a t i n g copolymer. N e v e r t h e l e s s , UV l i g h t pro motes t h e f o r m a t i o n o f the copolymer. Thus, i n these r e a c t i o n s e x c i t a t i o n under UV l i g h t r e s u l t s i n a change i n t h e course o f the r e a c t i o n from adduct f o r m a t i o n t o c o p o l y m e r i z a t i o n . P h o t o a c t i v a t e d Homopolymerization. The p a r t i c i p a t i o n o f e x c i t e d monomer has been proposed i n t h e p o l y m e r i z a t i o n o f e t h y l ene under UV ( l 5 , l 6 ) as w e l l as gamma i r r a d i a t i o n (17) i n t h e presence o f t r a c e amounts o f oxygen. I t has been suggested t h a t under i r r a d i a t i o n , ethylene undergoes e x c i t a t i o n per se o r as a r e s u l t o f t h e p e r t u r b i n g e f f e c t o f oxygen, t o generate t r i p l e t e x c i t e d e t h y l e n e . The l a t t e r r e a c t s w i t h ground s t a t e ethylene t o generate a d i r a d i c a l dimer (l6) o r an e x c i t e d dimer (17). Propagation i n v o l v e s a d d i t i o n o f the dimer t o t h e growing c h a i n end which i s presumed t o be a r a d i c a l . I n one p r o p o s a l (17)> a d d i t i o n o f t h e e x c i t e d dimer r e s u l t s i n t h e i n c o r p o r a t i o n o f one ethylene u n i t i n t o t h e c h a i n and t h e r e g e n e r a t i o n o f e x c i t e d ethylene monomer.

I f t h e e x c i t e d ethylene dimer e x i s t s as an i o n r a d i c a l p a i r , propagation may i n c o r p o r a t e b o t h monomeric u n i t s o f the dimer, analogous t o t h e behavior o f comonomer charge t r a n s f e r complexes such as butadiene-maleic anhydride, o r o n l y one u n i t , as noted i n the homopolymerization o f N - v i n y l c a r b a z o l e i n t h e presence o f e l e c t r o n a c c e p t i n g monomers such as a c r y l o n i t r i l e and maleic anhydride. Although l o n g considered i n c a p a b l e o f homopolymerization, m a l e i c anhydride i s r e a d i l y polymerized under UV i r r a d i a t i o n i n the presence o f a p h o t o s e n s i t i z e r (l8,19)> The p o l y m e r i z a t i o n presumably i n v o l v e s propagation o f e x c i t e d m a l e i c anhydride (20). I t i s noteworthy t h a t t h e s t r u c t u r e o f p o l y ( m a l e i c anhydride) contains fused cyclopentanone and s u c c i n i c anhydride u n i t s , d e r i v e d from two i n t e r a c t i n g m a l e i c anhydride u n i t s . This sug gests t h a t propagation i n v o l v e s e x c i t e d dimer r a t h e r than monomer.

X = H or COOH

(23)

1.

GAYLORD


9


C h e m i c a l l y Induced Photopolymerization i n the Absence o f L i g h t The i n t e r a c t i o n o f UV l i g h t with organic molecules i s not the o n l y method o f g e n e r a t i n g e l e c t r o n i c a l l y e x c i t e d s p e c i e s . The same e x c i t e d s p e c i e s t h a t a r e formed under i r r a d i a t i o n a r e produced by c e r t a i n chemical r e a c t i o n s i n the absence o f l i g h t . Such c h e m i c a l l y generated e x c i t e d s p e c i e s undergo t h e same chem i c a l r e a c t i o n s as t h e e x c i t e d s p e c i e s formed by t h e a b s o r p t i o n o f l i g h t , i n c l u d i n g t h e t r a n s f e r o f e x c i t a t i o n energy (21). The a l t e r e d WSt s p e c t r a c h a r a c t e r i s t i c o f c h e m i c a l l y induced dynamic p o l a r i z a t i o n (CIDNP) (22) are observed d u r i n g the photol y t i c decomposition o f v a r i o u s peroxides i n the presence o f p h o t o s e n s i t i z e r s a t ambient temperatures, as w e l l as the thermal decomposition o f these peroxides i n the absence o f l i g h t a t temp eratures where they have a short h a l f - l i f e . The r e a c t i o n s which are r e s p o n s i b l e f o r CLDNP are apparently s i m i l a r i n both cases. The p r e c u r s o r s o f such r e a c t i o n s are photοlytically and chemi c a l l y generated s p e c i e s , r e s p e c t i v e l y , capable o f t r a n s f e r r i n g energy t o s u i t a b l e a c c e p t o r s . The e x c i t a t i o n o f comonomer donor-acceptor complexes occurs i n the presence o f p e r o x i d e s , under c o n d i t i o n s where the l a t t e r are undergoing r a p i d decomposition. The peroxide-induced e x c i t e d complexes then undergo homopolymerization t o equimolar, a l t e r n a t ing copolymers. Thus, styrene-methyl methacrylate, s t y r e n e a c r y l o n i t r i l e , a - o l e f i n - a c r y l o n i t r i l e and b u t a d i e n e - a c r y l o n i t r i l e y i e l d equimolar copolymers i n t h e presence o f A1C1- and/or organoaluminum h a l i d e s under these c o n d i t i o n s . E x c i t e d complexes are a l s o the polymerizable s p e c i e s i n the peroxide-induced copolymer i z a t i o n o f conjugated dienes and maleic anhydride as w e l l as i n the formation o f copolymers d u r i n g the r e t r o g r a d e rearrangement of eyelopentadiene-maleic anhydride D i e l s - A l d e r adducts. Ethylene and maleic anhydride undergo peroxide-induced homo p o l y m e r i z a t i o n under s i m i l a r c o n d i t i o n s , i . e . i n the presence o f peroxides undergoing r a p i d decomposition, presumably through the propagation o f e x c i t e d monomers or dimers. Summary I r r a d i a t i o n o f a s u i t a b l e monomer under UV l i g h t i n t h e presence o f a p h o t o s e n s i t i z e r or complexing agent r e s u l t s i n the i n i t i a t i o n o f c o n v e n t i o n a l r a d i c a l or i o n i c p o l y m e r i z a t i o n , wherein t h e monomer adds t o the propagating c h a i n end. I r r a d i a t i o n o f ground s t a t e comonomer charge t r a n s f e r complexes r e s u l t s i n e x c i t a t i o n f o l l o w e d by homopolymerization, wherein the comono mer e x c i p l e x adds t o the propagating c h a i n end t o y i e l d equimolar, a l t e r n a t i n g copolymers, e.g. S-MMA. and S-AN i n t h e presence o f RJU. or RA1X. The t e r p o l y m e r i z a t i o n o f BD-MMA.-AN i n the presence or RAlClg y i e l d s terpolymers whose composition v a r i e s w i t h temp e r a t u r e i n the dark and i s independent o f temperature under UV light. The ground s t a t e comonomer complexes from BD-maleic

10

UV LIGHT INDUCED REACTIONS IN

POLYMERS

anhydride and BD-AN-A1C1.- and - E t A l C l ^ undergo c y c l i z a t i o n t o y i e l d adducts i n the dark and e x c i t a t i o n followed by homopolymer i z a t i o n under UV l i g h t . E x c i t e d ethylene monomer or dimer par t i c i p a t e s i n the propagation step i n the p o l y m e r i z a t i o n o f e t h y l ene under UV l i g h t i n the presence o f oxygen. Homopolymerization of maleic anhydride under UV l i g h t i n the presence o f a photosen s i t i z e r proceeds through propagation o f the e x c i t e d monomer or dimer.


Literature Cited 1. Sparapany, J. J . , J. Amer. Chem. Soc. (1966) 8 8 , 1357. 2 . Tazuke, S., A s a i , M., Ikeda, S., Okamura, S., J . Polym. S c i . , Part Β (1967) 5 , 4 5 3 . 3 . Natsuume, T., Akana, Y., Tanabe, K., Fujimatsu, Μ., Shimizu, M., Shirota, Y., Hirata, H., Kusabayashi, S., Mikawa, Η., Chem. Commun. (1969) 189. 4. Sakamoto, M., Hayashi, Κ., Okamura, S., J . Polym. S c i . , Part Β (1965) 3 , 2 0 5 . 5 . I r i e , M., Tomimoto, S., Hayashi, K., J . Polym. S c i . , Part Β (1972) 1 0 , 699, 6 . Mao, T. J . , Eldred, R. J . , J . Polym. S c i . , Part A-1 (1967) 5 , 1741. 7. Tazuke, S., Okamura, S., J . Polym. S c i . , Part A-1 (1969) 7, 715. 8 . Gaylord, N. G., D i x i t , S. S., J . Polym. S c i . , Part Β (1971) 9, 823. 9 . Tamura, Η., Tanaka, Μ., Murata, Ν., B u l l . Chem. Soc. Japan (1969) 42, 3042. 10. Gaylord, N. G., D i x i t , S. S., M a i t i , S., Patnaik, Β. K., J . Macromol. S c i . (Chem.) (1972) A6, 1495. 11. Furukawa, J . , Kobayashi, E., Iseda, Y., A r a i , Y., J . Polym. S c i . , Part Β (1971) 9, 179. 1 2 . Furukawa, J . , Kobayashi, Ε., A r a i , Y., J . Polym. S c i . , Part Β (1971) 9 , 8 0 5 . 1 3 . Gaylord, N. G., M a i t i , S., D i x i t , S. S., J . Macromol. S c i . (Chem.) (1972) A 6 , 1521. 14. Furukawa, J . , Kobayashi, Ε., Haga, Κ., Iseda, Y., Polym. J . (1971) 2 , 4 7 5 . 15. Machi, S., Hagiwara, M., Kagiya, T., J . Polym. S c i . , Part Β (1966) 4, 1019. 16. Hagiwara, Μ., Okamoto, Η., Kagiya, T., Kagiya, T., J . Polym. S c i . , Part A-1 (1970) 8 , 3295. 17. Hagiwara, Μ., Okamoto, H., Kagiya, T., J . Polym. S c i . , Part A-1 (1970) 8 , 3303. 1 8 . Bryce-Smith, D., G i l b e r t , Α., Vickery, B., Chem. Ind. (Lon don) (1962 ) 2 0 6 0 . 1 9 . Lang, J . L., Pavelich, W. Α., Clarey, H. D., J . Polym. S c i . , Part A (1963) 1, 1123.

1. 20.

GAYLORD


11


Gaylord, N. G., M a i t i , S., J . Polym. S c i . , Polym. L e t t . Ed. (1973) 11, 2 5 3 . 2 1 . White, Ε. H., Miano, J . D., Watkins, C. J . , Breaux, E. J . , Angew. Chem. Internat. Ed. (1974) 1 3 , 229. 2 2 . Lepley, A. R., Closs, G. L., eds., "Chemically Induced Mag n e t i c P o l a r i z a t i o n " , Wiley, New York, 1973.

2 Benzoin Ether Photoinitiated Polymerization of Acrylates (1) S. PETER PAPPAS, ASHOK K. CHATTOPADHYAY (2), and L E L A N D H . CARLBLOM


Department of Polymers and Coatings, North Dakota State University, Fargo, N. D. 58102

Our interest in the photochemistry of benzoin ethers was prompted by two major reasons: (1) a discrepancy existed between recent mechanistic studies on the photochemistry of benzoin ethers and earlier reports on the benzoin ether photo initiated polymerization of reactive monomers, and (2) the extensive, commercial utilization of benzoin ethers as photo initiators in uv curable coatings and printing inks warranted further investigation of this discrepancy. The photochemistry of benzoin ethers (α-alkoxy-α-phenyl -acetophenones) has been examined in considerable detail, recently, by quenching (3, 4), sensitization (4), CIDNP (5), and radical scavenging (6) studies. These investigations indicate that benzoin ethers undergo a facile, photocleavage (Norrish type I) to yield benzoyl and benzyl ether radicals, as shown in eqn 1. This α-scission is not retarded by conventional triplet

+

(1)

quenchers which led to the suggestion (3) that reaction occurs via the excited singlet state. However, the sensitization (4) studies are best interpreted in terms of triplet reactivity. The (3,4) constitutes an facility of α-cleavage (k 10 sec ) important reason for the effectiveness of benzoin ethers as photoinitiators for uv curing in air (7) since scission is not quenched by oxygen or reactive monomers, such as styrene (8). Unfortunately, the facility of cleavage does not prevent air -inhibition of uv curing by reaction of the initiator or growing polymers radicals with oxygen (7). Based on the photochemistry of eqn 1, one would reasonably predict that, in the presence of reactive monomer, a conven-10

-1

12

2.

PAPPAS E T A L .

13

Benzoin Ether Photoinitiated Polymerization

t i o n a l r a d i c a l c h a i n p o l y m e r i z a t i o n w o u l d o c c u r , i n i t i a t e d by the b e n z o y l and b e n z y l e t h e r r a d i c a l s , as r e c e n t l y p r o p o s e d (9). O n e w o u l d f u r t h e r p r e d i c t that a m a x i m u m of two i n i t i a t o r f r a g m e n t s w o u l d be i n c o r p o r a t e d p e r p o l y m e r m o l e c u l e d e p e n d i n g u p o n the r e l a t i v e extent of t e r m i n a t i o n by d i s p r o p o r t i o n a t i o n a n d c o u p l i n g . H o w e v e r , these p r e d i c t i o n s a r e not i n a c c o r d w i t h e a r l i e r s t u d i e s . U t i l i z i n g C - l a b e l l e d b e n z o i n m e t h y l e t h e r as photo i n i t i a t o r i n the p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e ( M M A ) , M o c h e l and c o w o r k e r s r e p o r t e d that 12-14 r a d i o a c t i v e f r a g m e n t s o r m o l e c u l e s of i n i t i a t o r w e r e i n c o r p o r a t e d p e r p o l y m e r (10). N o r a d i o a c t i v i t y w a s i n c o r p o r a t e d when the p o l y m e r i z a t i o n w a s i n i t i a t e d t h e r m a l l y w i t h (X , ( X ' - a z o b i s i s o b u t y r o n i t r i l e ( A I B N ) i n the p r e s e n c e of r a d i o l a b e l e d b e n z o i n m e t h y l e t h e r ( B E ^ . F u r t h e r m o r e , b a s e d on q u a n t u m y i e l d data f o r b e n z o i n , it w a s e s t i m a t e d that l e s s than 2 quanta a r e u t i l i z e d / p o l y m e r m o l e c u l e f o r m e d . S u b s e q u e n t l y , these r e s u l t s t o g e t h e r w i t h u n p u b l i s h e d d a t a w e r e c i t e d as e v i d e n c e f o r c o p o l y m e r i z a t i o n of p h o t o e x c i t e d b e n z o i n e t h e r s and b e n z o i n w i t h r e a c t i v e m o n o m e r s , s u c h as M M A and s t y r e n e (11). This pro p o s a l i s c o n s i s t e n t w i t h the f i n d i n g that no r a d i o a c t i v i t y was i n c o r p o r a t e d w h e n the p o l y m e r i z a t i o n w a s i n i t i a t e d t h e r m a l l y w i t h A I B N i n the p r e s e n c e of B E (10). H o w e v e r , the i n c o r p o r a t i o n of 12-14 p h o t o e x c i t e d b e n z o i n e t h e r m o l e c u l e s by the a b s o r p t i o n of l e s s than 2 quanta r e q u i r e s that about 7 e x c i t e d m o l e c u l e s be p r o d u c e d / q u a n t u m a b s o r b e d . T h i s w o u l d constitute a c h a i n p r o c e s s f o r l i g h t a b s o r p t i o n f o r w h i c h we a r e u n a w a r e of any precedent. We f e l t that the u n i q u e n e s s of these r e s u l t s w a r ranted a reinvestigation.


1 4

X

T h e C - l a b e l l e d b e n z o i n m e t h y l e t h e r ( Β Ε χ ) , u t i l i z e d by M o c h e l ' s g r o u p (10), w a s p r e p a r e d by m e t h y l a t i n g b e n z o i n w i t h C - m e t h y l i o d i d e . In o r d e r to o b t a i n a d d i t i o n a l i n f o r m a t i o n , we a l s o p r e p a r e d doubly and t r i p l y l a b e l l e d a n a l o g s , B E and B E , as o u t l i n e d on S c h e m e 1. 1 4

1 4

2

3

I r r a d i a t i o n s w e r e c o n d u c t e d at 366 n m ( C o r n i n g 7-83 f i l t e r c o m b i n a t i o n ) i n a m e r r y - g o - r o u n d a p p a r a t u s in w h i c h the s a m p l e s r o t a t e d about a s t a t i o n a r y 4 5 0 - W a t t H a n o v i a m e d i u m p r e s s u r e l a m p f o r constant l i g h t e x p o s u r e . T h e s a m p l e s c o n s i s t e d of d e g a s s e d 5 m l s o l u t i o n s of the photo i n i t i a t o r s i n neat M M A c o n t a i n e d in P y r e x t u b e s . T w o i n i t i a t o r c o n c e n t r a t i o n s w e r e u t i l i z e d : 1.05 χ 10" and 4 . 1 1 χ Ι Ο " Μ , w h i c h c o r r e s ponded to 16 and > 99% l i g h t a b s o r p t i o n , r e s p e c t i v e l y . S a m p l e s w e r e i r r a d i a t e d to about 7% m o n o m e r c o n v e r s i o n s w h i c h r e q u i r e d 15 m i n f o r the o p t i c a l l y d e n s e s o l u t i o n s and 30 m i n f o r the tubes w i t h low i n i t i a t o r c o n c e n t r a t i o n . The resulting poly3

2

14


SCHEME 1 Ο OH PhC-CHPh

14

+

2

CH I

Ο 0 CH PhC-CHPh 1 4

Ag 0

2>

3

3

B Eχ


Ph

1 4

Ο

NaCN

CHO

OH

14 M

Ph

BE

1 4

14 1

C—

CHPh

BE

2

3

m e r s w e r e p r e c i p i t a t e d i n m e t h a n o l , and the n u m b e r of m o n o m e r s r e a c t e d / q u a n t u m a b s o r b e d (0 ) was d e t e r m i n e d by s i m u l t a n e o u s i r r a d i a t i o n of a b e n z o p h e n o n e / b e n z h y d r o l a c t i n o m e t e r f o r w h i c h 0 w a s t a k e n as 0 . 8 5 (12). T h e i n t e n s i t y of l i g h t a b s o r b e d w a s a p p r o x i m a t e l y 4 χ 10 q u a n t a / m i n at h i g h initiator concentration. m

17

A f t e r d e t e r m i n a t i o n of m o n o m e r c o n v e r s i o n s , the p o l y m e r s w e r e d i s s o l v e d i n benzene and r e p r e c i p i t a t e d two a d d i t i o n a l t i m e s p r i o r to d e t e r m i n a t i o n of i n t r i n s i c v i s c o s i t i e s ( [ η ] ) and specific a c t i v i t i e s . Intrinsic v i s c o s i t i e s were obtained in b e n z e n e at 3 0 ° C w i t h a n o . 1 U b b e l o h d e v i s c o m e t e r (solvent flow t i m e s w e r e 6 9 . 1 - 0.1 s e c ) . N u m b e r average m o l e c u l a r w e i g h t s ( M ) w e r e c a l c u l a t e d f r o m the e x p r e s s i o n : [ η ] = 8 . 6 9 χ 10" M (13). S p e c i f i c a c t i v i t i e s w e r e d e t e r m i n e d by _ liquid scintillation counting. P o l y m e r activities (A), 0 and M v a l u e s a r e p r e s e n t e d in T a b l e I. n

5

0

n

,

7

6

m

n

I n i t i a t o r a c t i v i t i e s (a) t o g e t h e r w i t h a v e r a g e v a l u e s f o r p o l y m e r a c t i v i t i e s (A) a r e p r e s e n t e d i n T a b l e II. In m a r k e d c o n t r a s t to the e a r l i e r r e p o r t s (10, 11), it i s seen that p o l y m e r a c t i v i t i e s a r e l e s s than that of the c o r r e s p o n d i n g i n i t i a t o r i n e a c h c a s e . U n f o r t u n a t e l y , we c a n o f f e r no r a t i o n a l e f o r t h i s d i s c r e p a n c y . H o w e v e r , the p r e s e n t data a r e r e a d i l y i n t e r p r e t e d in t e r m s of b e n z o i n e t h e r p h o t o c l e a v a g e into b e n z o y l and b e n z y l e t h e r r a d i c a l s (eqn 1), w h i c h initiate M M A p o l y m e r i z a t i o n . T h e i n c o r p o r a t i o n data m a y be a n a l y z e d by eqns 2 - 4 , w h e r e i n a i , a and a r e f e r to the s p e c i f i c a c t i v i t i e s of i n i t i a t o r s B E i , B E and B E , r e s p e c t i v e l y , A A and A r e p r e s e n t the c o r r e s p o n d i n g p o l y m e r a c t i v i t i e s , w h i l e B a n d Ε a r e the n u m b e r 2

2

3

3

it

2

3

2.

PAPPAS E T AL.

T A B L E I.

P h o t o i n i t i a t e d P o l y m e r i z a t i o n of M M A

BE ( Μ χ 10 )

M χ ΙΟ"

3

B E j (1 .05)



BE

2

(1 .05)

BE

3

(1 .05)

B E χ (41.1)

BE

2

(41.1)

BE

3

(41.1)

a

A

n

Cm 1020 1100 a 953 1030 1000 1000 1070 1070 248 307 310 258 271 280 341 240 235 338 258 272 262 332 318

8.20 7,86 8,54 7.97 7.86 8.54 8.08 8.31 8.96 6.32 4.97 5.17 5.89 5.07 5.07 6.21 5.62 5.48 5.69 5.69 5.60 5.17 4.97 5.17

4

dpm/mol χ 1.73 1.71 1.63 3.10 3.18 3.39 4.82 4.86 5.00 2.59 2.46 2.52 2.87 2.53 2.47 2.94 4.62 4.17 3.58 6.78 6.49 6.30 7.00 7.15

T h e weight of the p r e c i p i t a t e d p o l y m e r w a s i n a d v e r t e n t l y not

r e c o r d e d i n this e x p e r i m e n t .

15

16

UV L I G H T INDUCED REACTIONS I N P O L Y M E R S

T A B L E II.

Specific Activities A

BE ( Μ χ 10 )

a

BEj (1.05) (41.1)

3.13

BE (1.05) (41.1)

6.24

BE (1.05) (41.1)

9.31

d p m / m o l χ 10

3

1.69 2.62

2

3.22 4.12


3

4.89 6.74

of b e n z o y l and b e n z y l e t h e r r a d i c a l s , r e s p e c t i v e l y , i n c o r p o r ated/polymer. In t h i s c o n t e x t , E _ m a y be d e r i v e d i n d e p e n d e n t l y Ai A A

3

2

=

=

£a

Ia

2

= 2

a E

(2)

x

( Ε + B)

( E + B)

(3)

+ (a - a ) Ε 3

(4)

2

f r o m eqn 2 and s i m u l t a n e o u s s o l u t i o n o f eqns 3 and 4 . T h e r e s u l t i n g Ε and Β v a l u e s at e a c h i n i t i a t o r c o n c e n t r a t i o n a r e p r o v i d e d i n T a b l e III. A v e r a g e v a l u e s f o r 0 , M and 0p, the c o r r e s p o n d i n g n u m b e r of p o l y m e r m o l e c u l e s p r o d u c e d / q u a n t u m a b s o r b e d , a r e a l s o p r e s e n t e d i n T a b l e III. T h e s e v a l u e s w e r e found to be independent of r a d i o l a b e l w i t h i n e x p e r i m e n t a l e r r o r , e s t i m a t e d a s ί 1 0 % . H o w e v e r , a s s h o w n , they a r e h i g h l y dependent o n i n i t i a t o r c o n c e n t r a t i o n . m

T A B L E III.

1.05 41.1

S u m m a r y of Data f o r M M A

0

[BE] Μ χ 10

n

3

1030 280

0

Ε

Β

8.25

1.26

0.54

0.49

5.49

0.52

0.85

0.48

n χ 10" M

4

T h e t o t a l i n c o r p o r a t i o n at low i n i t i a t o r c o n c e n t r a t i o n of a p p r o x i m a t e l y one i n i t i a t o r f r a g m e n t / p o l y m e r i s i n r e a s o n a b l e a g r e e m e n t w i t h v a l u e s o b t a i n e d o n the p o l y m e r i z a t i o n of M M A

2.

PAPPAS E T A L .


14

u t i l i z i n g C-labeled AIBN (14). The data are indicative of a conventional polymerization, initiated equally effectively by ben zoyl and benzyl ether radicals, and terminated predominately by disproportionation of the growing polymer ends. Since the maxi mum number of polymers/quantum absorbed i s 2 for termination by disproportionation, the value of 1.26 for 0p at low i n i t i a t o r concentration corresponds to a minimum quantum efficiency of 63%. This signifies that at least 63% of the absorbed light results i n polymer formation. At the high i n i t i a t o r concentration, i n i t i a l conversion of monomer to polymer increased by a factor of 1.6 and ^ decreased, as expected. However, as shown i n Table III, these changes were accompanied by a drop i n quantum efficiency to 25% (0 = 0.52), together with an increase i n incorporation of radioactivity (1.33 as compared to 1.03 i n i t i a t o r fragments/polymer). The reduced quantum efficiency may be attributed to self-reactions of the i n i t i a t o r radicals, as well as to chain termination by i n i t i a t o r radicals, i . e . , primary radical termination. The increase i n incorporation of radioactivity constitutes additional support for primary radical termination. The discrepancy i n the incorporation of benzoyl (B) and benzyl ether radicals (E) at high i n i t i a t o r concentration appears to be significant. Further studies on this interesting finding are i n progress. Similar studies are also being carried out with methyl acrylate (MA). In this case, the samples, which were irradiated i n t r i p l i c a t e , consisted of degassed 8 ml solutions of the photoi n i t i a t o r s and MA (25% by volume) i n benzene. Intrinsic v i s c o s i ties were obtained i n benzene at 35°C with a Cannon-Ubbelohde size 50 viscometer, and number average molecular weights were calcu lated from the expression: [η] = 1.28 χ ΙΟ" * M^- * (15). The pertinent results are summarized i n Table IV. As shown i n Table IV, the total incorporation values are greater than 2 at both i n i t i a t o r concentrations. Since MA termi nates predominately by combination (16), the presence of more than 2 i n i t i a t o r fragments/polymer i s suggestive of an additional mode of incorporation. A possible mechanism i s H-abstraction from polymer by i n i t i a t o r radicals which provides radical sites for chain branching. However, further discussion of these find ings must await confirmation of the molecular weight data by more direct means. p


17

1

711

18


a

TABLE IV. Summary of Data for MA [BE] (Μ χ 10 )

Κm

1.04

2560

3

41.0

Μ χ ΙΟ" η

5

"Ρ

Ε

Β

b

4.48

0.51

1.4

1.3

C

2.60

0.35

1.8

1.9

1040

estimated error ±10%. 15% monomer conversions.


10% monomer conversions.

ACKNOWLEDGMENT. We wish to express our appreciation to the Alcoa Foundation for financial assistance and to Dr. Zeno Wicks, J r . for helpful comments. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

16.

Taken, i n part, from the Ph.D. Thesis of A.K.C., North Dakota State University, 1974. Present address: DeSoto, Inc., 1700 So. Mt. Prospect Road, Des Plaines, I l l i n o i s 60018. Heine, H.-G., Tetrahedron Lett. (1972) 4755. Pappas, S. P. and Chattopadhyay, A. K., J . Amer. Chem. Soc. (1973) 95, 6484. Dominh, T., Ind. Chim. Beige. (1971) 36, 1080; Chem. Abstr. (1972) 76, 126080p. Ledwith, Α., Russell, P. J . and S u t c l i f f e , L. H., J . Chem. Soc., Perkin Trans. 2 (1972) 1925. Pappas, S. P., Progr. Org. Coatings (1974) 2 (4), 333. Heine, H.-G., Rosenkranz, H.-J., and Rudolph, H., Angew. Chem., Int. Ed. Engl. (1972) 11, 974. Hutchison, J . and Ledwith, Α., Polymer (1973) 14, 405. Mochel, W. E., Crandall, J . L. and Peterson, J . H., J . Amer. Chem. Soc. (1955) 77, 494. Bevington, J . C., "Radical Polymerization," Academic Press, New York, 1961, p. 77. Pappas, S. P., Alexander, J . E. and Zehr, R. D., Jr., J . Amer. Chem. Soc. (1970) 92, 6927. Fox, T. G., Kissinger, J . B., Mason, H. E. and Shuele, Ε. Μ., Polymer (1962) 3, 71. Bevington, J . C., Melville, H. W., and Taylor, R. P., J . Polym. Sci. (1954) 14, 463. Sen, J . N., Chatterjee, R. and P a l i t , S. R., J . S c i . Ind. Research (India) (1952) 11b, 90; Chem. Abstr. (1952) 46, 7847a. Bamford, C. H., Dyson, R. W. and Eastmond, G. C., Polymer (1969) 10, 885.

3 Photocrosslinkable Polymers G E O R G E B. B U T L E R and W I L L I A M I. F E R R E E , JR.


Center for Macromolecular Science and Department of Chemistry, University of Florida, Gainesville, Fla. 32611

Summary This paper describes some polymeric systems which have recently been shown to undergo photocrosslinking. Photocrosslink ing of poly-(m-vinyloxyethoxy)-styrene can be carried out with several different acceptors, although several others are ineffec t i v e for reasons unknown. The reaction is not quenched apprecia bly by H O , oxygen, or γ-collidine, but is quenched by t r i e t h y l amine, and cannot be i n i t i a t e d by azoisobutyronitrile, thus sup porting the supposition that the mechanism i s cationic but not protonic. Quenching by hydroquinone may also be consistent with t h i s conclusion. The reaction is favored by low temperature and by tetrabutylammonium perchlorate, also supporting a cationic mechanism. Small concentrations of H O , added before and after i r r a d i a t i o n , induce a postcrosslinking process that proceeds to comple tion within a short time and appears to be proton-initiated. The photocrosslinking is completely quenched by 0.005 M thio cyanate ion, and subsequent addition of 0.5 M H O causes the formation of postcrosslinked polymer containing an unexplained IR absorption band in the carbonyl region. Effects of varying light intensity, wavelength, acceptor concentration, and of rotating the sample tube suggest that the tetrachlorophthalic anhydride-catalyzed reaction is biphotonic. The chloranil-catalyzed reaction shows no intensity dependence but unexplainably w i l l not proceed with 334 nm l i g h t , whereas 366 nm light causes very rapid reaction. 2

2

2

Introduction The increasing u t i l i t y of photocrosslinkable polymers in microelectronics, p r i n t i n g , and UV-curable lacquers and inks is providing an incentive for development of new v a r i e t i e s of photopolymers. After a brief overview of some of the commonly used materials, some of the interesting recent developments in t h i s 19

20

U V L I G H T INDUCED REACTIONS IN P O L Y M E R S


f i e l d w i l l be d i s c u s s e d . Some r e c e n t e x p e r i m e n t a l r e s u l t s o b t a i n e d i n o u r l a b o r a t o r y on photoionîc c r o s s ! i n k i n g o f c e r t a i n p o l y m e r s w i l l a l s o be p r e s e n t e d . P h o t o p o l y m e r s i n Common U s e . I n c r e a s i n g i n d u s t r i a l u s e i s b e i n g made o f U V - c u r a b l e u n s a t u r a t e d p o l y e s t e r f o r m u l a t i o n s . The p o l y e s t e r component i s t y p i c a l l y p r e p a r e d f r o m m a l e i c a n h y d r i d e , an a r o m a t i c a n h y d r i d e , and a d i o l . T h i s i s combined w i t h s t y r e n e , a s e n s i t i z e r , and m i s c e l l a n e o u s a d d i t i v e s . The u n s a t u r a t e d s i t e s associated with t h e maleate e s t e r linkages i n t h e p o l y e s t e r chain undergo a r a d i c a l c o p o l y m e r i z a t i o n w i t h s t y r e n e t o e f f e c t c r o s s I i n k i n g . The s e n s i t i z e r s e r v e s b o t h t o a b s o r b l i g h t and t o generate r a d i c a l s . P a r a f i n , f a t t y e s t e r s , o r s i m i l a r agents a r e o f t e n i n c l u d e d t o r e d u c e o x y g e n i n h i b i t i o n and t h u s improve surface hardening. The p r i n c i p l e u s e f o r t h e s e f o r m u l a t i o n s i s as U V - c u r a b l e l a c q u e r s i n t h e f u r n i t u r e i n d u s t r y . In a t y p i c a l c o m p o s i t i o n , an u n s a t u r a t e d p o l y e s t e r i s p r e p a r e d f r o m a 1:2:3 m i x t u r e o f p h t h a l l i c a n h y d r i d e , m a l e i c a n h y d r i d e , and 1,2-propaned i o l , and a 66% m i x t u r e o f t h e p o l y e s t e r i n s t y r e n e c o n t a i n i n g 0.)% p a r a f f i n , \% [ ( C H 3 ) C 0 C S 2 ] 2 ^ and 0.5% s e n s i t i z e r c o n s t i t u t e s t h e U V - c u r a b l e p o l y e s t e r . T y p i c a l s e n s i t i z e r s a r e b e n z o i n methyl e t h e r , b e n z i l , x a n t h o n e , and a c e t o n a p h t h o n e A s i m i l a r mix t u r e has been used t o i m p r e g n a t e f i b e r g l a s s f a b r i c f o r U V - h a r d e n ed o r t h o p e d i c c a s t s (2). The e f f i c i e n t l i g h t - i n i t i a t e d d e c o m p o s i t i o n o f a z i d e s has been t h e b a s i s f o r c o m m e r c i a l l y i m p o r t a n t p h o t o r e s i s t f o r m u l a t i o n s f o r t h e s e m i c o n d u c t o r i n d u s t r y . A common a p p r o a c h i s t o mix a d i a z i d e , s u c h a s d i a z a d i b e n z y l i d e n e c y c I o h e x a n o n e ( I ) , with an u n s a t u r a t e d h y d r o c a r b o n p o l y m e r . E x c i t a t i o n o f t h e d i f u n c t i o n al s e n s i t i z e r produces h i g h l y r e a c t i v e n i t r e n e s which c r o s s l i n k t h e p o l y m e r by a v a r i e t y o f p a t h s i n c l u d i n g i n s e r t i o n i n t o b o t h c a r b o n - c a r b o n d o u b l e bonds and c a r b o n - h y d r o g e n b o n d s , and by generation of radicals. The p o l y m e r component i n t h e most w i d e l y used r e s i s t s i s p o l y i s o p r e n e w h i c h has been p a r t i a l l y c y c l i z e d by r e a c t i o n w i t h p - t o l u e n e s u I f o n i c a c i d (5). O t h e r p o l y m e r s used i n c l u d e p o l y c y c l o p e n t a d i e n e and t h e c o p o l y m e r o f c y c l o p e n t a d i e n e and α-methylstyrene (A). A s i m p l e , p h o t o s e n s i t i v e p o l y m e r t h a t has f o u n d w i d e u s e a s a p h o t o r e s i s t and i n p r i n t i n g p l a t e s i s p o l y ( v i n y l a l c o h o l ) ( I I ) (5). B e c a u s e r a d i c a l p o l y m e r i z a t i o n o f t h e monomer p r e f e r s t h e c y c l o p o l y m e r i z a t i o n p a t h , II i s p r e p a r e d by r e a c t i n g p o l y ( v i n y l a l c o h o l ) w i t h cinnamoyl c h l o r i d e . The mechanism o f p h o t o c r o s s I i n k i n g o f II has been a m a t t e r o f c o n t r o v e r s e y , w h e t h e r p r o c e e d i n g by a r a d i c a l p r o c e s s , by photodîmerization o f c i n n a m a t e g r o u p s t o c y c l o b u t a n e s , o r b o t h i n c o m p e t i t i o n (6). The i n i t i a t i n g species i s d e f i n i t e l y t h e t r i p l e t , consequently t h e c r o s s l i n k i n g e f f i c i e n c y and a b s o r p t i o n b a n d w i d t h a r e g r e a t l y i n c r e a s e d i n t h e p r e s e n c e o f t r i p l e t s e n s i t i z e r s (6). 2

D e v e l o p m e n t s E m p l o y i n g P o t e n t i a l l y Photodimerîzable G r o u p s .


3.

BUTLER AND FERRÉE

Photocrosslinkable Polymers

21

Most r e s e a r c h i n t o new p h o t o p o l y m e r s has d e v e l o p e d a l o n g t h e l i n e s of a t t a c h i n g r e a c t i v e groups t o a polymer backbone. Several polymers c o n t a i n i n g pendant cinnamate groups a r e found t o improve on t h e p h o t o s e n s i t i v i t y o f I I . R e a c t i o n o f poly(β-hydroxyethyI aery I ate) w i t h cinnamoyl c h l o r i d e y i e l d s a polymer ( I I I ) t h a t when s e n s i t i z e d by 5 - n i t r o a c e n a p h t h e n e i s more t h a n t w i c e a s r e a c t i v e a s II (7). P o l y ( g l y c i d y I c i n n a m a t e ) ( I V ) , p r e p a r e d by r e a c t i n g p o l y ( e p i c h l o r o h y d r i n ) with potassium cinnamate, i s a l i q u i d a t room t e m p e r a t u r e and p h o t o c r o s s I i n k s w i t h a n e f f i c i e n c y e i g h t t i m e s t h a t o f II ( 8 ) . A s e r i e s o f s i m i l a r p o l y m e r s (V) has been p r e p a r e d d i r e c t l y f r o m monomers by c a t i o n i c p o l y m e r i z a t i o n o f t h e v i n y l e t h e r g r o u p o f t h e monomer, t h e c i n n a m o y l g r o u p b e i n g u n r e a c t i v e u n d e r t h e s e c o n d i t i o n s (9_). P o l y C v i n y l c h a l c o n e ) ( V I ) r e s e n b l e s II i n s t r u c t u r e , and t h e mechanism o f i t s c r o s s l i n k i n g has s i m i l a r l y been a r g u e d i n f a v o r of r a d i c a l o r d i m e r i z a t i o n pathways ( 1 0 , 1 1 ) . One o f t h e more i n t e r e s t i n g and s u c c e s s f u l o f r e c e n t p h o t o polymers i s p o l y ( v i n y l 2 - f u r y l a c r y l a t e ) ( V I I I ) . Its photosensi t i v i t y i s r e p o r t e d t o be more t h a n a n o r d e r o f m a g n i t u d e g r e a t e r than t h a t o f I I , whether s e n s i t i z e d o r u n s e n s i t i z e d samples a r e compared. The mechanism o f c r o s s l i n k i n g i s p r o p o s e d t o i n v o l v e photodîmerization o f p e n d a n t g r o u p s t o c y c l o b u t a n e s ( 1 2 ) . Recently, r e s u l t s o f i n c o r p o r a t i n g i n polymers e f f i c i e n t l y photodîmerîzable g r o u p s f a m i l i a r t o modern p h o t o c h e m i s t s have been r e p o r t e d . Reaction o f poly(vinyl alcohol) with 1,2-diphenylc y c l o p r o l e n o y I c h l o r i d e y i e l d s a very p h o t o s e n s i t i v e polymer ( V I I I ) t h a t c r o s s l i n k s by a m i x t u r e o f c y c l o b u t a n e r i n g f o r m a t i o n and s i n g l e bond f o r m a t i o n between two d i p h e n y l eye I o p r o p e n e g r o u p s a f t e r h y d r o g e n - a t o m t r a n s f e r ( 1 3 ) , mechanisms w e l l c h a r a c t e r i z e d i n t h e p h o t o c h e m i s t r y o f t h e s e m o i e t i e s (_T4). Hyde, K r i c k a , a n d L e d w i t h r e p o r t t h a t N-acryloyldîbenz(b,f)azepîne ( I X ) c a n be c o p o l y m e r i z e d w i t h s t y r e n e , methyl methacryI a t e , N - v i n y I c a r b a z o l e , o r m a l e i c a n h y d r i d e under r a d i c a l c o n d i t i o n s t o p r o v i d e polymers t h a t p h o t o c r o s s I i n k by c y c l o b u t a n e r i n g f o r m a t i o n ( V 5 ) . Two more p o l y m e r s d e s i g n e d t o c r o s s l i n k by t h e p h o t o d i m e r i z a t i o n mechanism a r e X and XI (]6). The methoxy s u b s t i t u e n t s a r e r e s p o n s i b l e f o r t h e e f f i c i e n c y o f c r o s s l i n k i n g , p r e s u m a b l y , i n p a r t , by i n c r e a s i n g the l i f e t i m e of t h e r e a c t i v e s i n g l e t e x c i t e d s t a t e . Recent Azido Photopolymers. A c l a s s o f u s e f u l photopolymers t h a t c r o s s l i n k s by non-dîmerizable g r o u p s i s t h a t c o n t a i n i n g a z i d o g r o u p s . P o l y ( v i n y l - p - a z t d o B e n z o a t e ) CXI I ) , p r e p a r e d f r o m p o l y ( v i n y l a l c o h o l ) , i s one e x a m p l e ( 1 7 ) . S i n c e no u n s a t u r a t i o n i s p r e s e n t , c r o s s l i n k i n g e f f i c i e n c y depends on t h e p h o t o - g e n e r a t e d n i t r e n e s r e a c t i v i t y t o w a r d i n s e r t i o n i n t o c a r b o n - h y d r o g e n bonds and g e n e r a t i o n o f r a d i c a l s . Various t r i p l e t s e n s i t i z e r s increase t h e e f f i c i e n c y by one t o two o r d e r s o f m a g n i t u d e . R e a c t i o n o f p o l y ( v i n y l c h l o r i d e ) w i t h s o d i u m a z i d e i n dîmethyl formamide i n c o r p o r a t e s a z i d e g r o u p s t o y i e l d a p h o t o c r o s s l i n k a b l e f

22

UV



polymer ( 1 8 ) . PhotocrossIinking of poly(vinyI-p-azidocinnamate) (XIII) i s reported t o i n v o l v e c o m p e t i t i v e p h o t o d i m e r i z a t i o n o f cinnamoyl groups t o c y c l o b u t e n e s a l o n g w i t h t h e dominant n i t r e n e r e a c t i o n s o f i n s e r t i o n and r a d i c a l a b s t r a c t i o n . The r e a c t i o n c a n be triplet-sensitized (19). R e c e n t P h o t o p o l y m e r s C r o s s l i n k e d by R a d i c a l P r o c e s s e s . P o l y m e r s ( X I V ) c o n t a i n i n g t h e p h o t o r e a c t i v e a e r y IoyI and m e t h a c r y o y l g r o u p s have been p r e p a r e d by a c a t i o n i c s y n t h e s i s s i m i l a r t o t h a t m e n t i o n e d e a r l i e r (2(D). P h o t o d i m e r i z a t i o n i s n o t e x p e c t e d to play a r o l e i n c r o s s l i n k i n g i n these cases. A p o l y m e r c o n t a i n i n g p e n d a n t benzophenone g r o u p s (XV) i s f o u n d t o p h o t o c r o s s I i n k . The mechanism i s i n t e r m o l e c u l a r c o u p l i n g o f r a d i c a l s formed v i a p h o t o r e d u c t i o n i n t h e t r i p l e t s t a t e . In s o l u t i o n , c r o s s l i n k i n g i s c o n c e n t r a t i o n dependent as i n t r a m o l e c u l a r eye Iîzation competes ( 2 1 ) . D i a l l y I p h t h a l a t e , w h i c h has been c o n v e r t e d under r a d i c a l c o n d i t i o n s t o a low p o l y m e r , p h o t o c r o s s I i n k s i n t h e p r e s e n c e o f N - p h e n y I m a l e i m i d e and M i c h l e r ' s k e t o n e s e n s i t i z e r (22_). P o s s i b l y the c r o s s l i n k i n g i n v o l v e s c o p o l y m e r i z a t i o n of unreacted a l l y l groups with t h e maleimide. A l a r g e number o f e x a m p l e s o f p o l y m e r s c o n t a i n i n g t e r t i a r y amines a r e r e p o r t e d t o p h o t o c r o s s I i n k i n t h e presence o f a l k y l halides. One c o m b i n a t i o n w o u l d be p o l y ( p - d i m e t h y I a m i n o s t y r e n e ) and e t h y l i o d i d e ( 2 3 ) . The mechanism i s n o t r e p o r t e d , b u t may i n v o l v e c h a r g e - t r a n s f e r e x c i t a t i o n t o form r a d i c a l s . The o x i d a t i v e c o u p l i n g o f monomers s u c h a s t h e d i p r o p a r g y l e t h e r o f bîs-phenol A ( X V I ) r e s u l t s i n p h o t o c r o s s l i n k a b l e m a t e r i a l ( 2 4 ) . The mechanism i s unknown, b u t t h e r a t e i s g r e a t e s t i n examples i n which carbonyl groups a r e a t t a c h e d t o t h e a r o m a t i c rings. A r e c e n t s t u d y (23) r e p o r t e d on t h e r a t e s o f p h o t o c r o s s I i n k i n g and t h e p h o t o s e n s i t i v i t i e s o f p o l y ( v i n y l α-cyanocinnamate) and p o l y ( v i n y l α-cyanocinnamoxyacetate). The a u t h o r s concluded t h a t photocrossIînking o f t h e s e p o l y m e r s p r o c e e d e d m a i n l y t h r o u g h r a d i c a l a d d i t i o n , and t h a t t h e s e p o l y m e r s showed h i g h e r p h o t o s e n s i t i v i t i e s t h a n p o l y ( v i n y l c i n n a m a t e ) and p o l y ( v i n y l c i n n a m o x y a c e t a t e ) i n s p i t e o f t h e lower r a t e s o f p h o t o c r o s s I i n k i n g o f t h e α-cyano-substituted p o l y m e r s . The p h o t o s e n s i t i v i t i e s and t h e r a t e s o f photocrossIînking o f t h e s e α-cyano-substituted p o l y m e r s were a l s o compared w i t h t h a t o f p o l y ( v i n y l β-styryI a e r y l o x y acetate). A r e c e n t r e v i e w (2(5) c o v e r e d f o u r - c e n t e r t y p e photopolymeriz a t i o n i n t h e c r y s t a l l i n e s t a t e . Although these p h o t o i n i t i a t e d p o l y m e r i z a t i o n s by t h e f o u r - c e n t e r mechanism may n o t be c o n s i d e r e d to involve a photocrosslinkable step, the extensive discussion of mechanism by t h e s e a u t h o r s may be o f v a l u e t o t h o s e i n t e r e s t e d i n p h o t o c r o s s I i n k i n g a s t h e r e a r e some marked s i m i l a r i t i e s between t h e s e two c o n c e p t s .


3.

BUTLER AND FERRÉE


23

P h o t o i o n i c C r o s s l i n k i n g o f C e r t a i n P o l y m e r s . C e r t a i n mono mers a r e r e a d i l y p r e p a r e d h a v i n g two p o l y m e r i z a b l e f u n c t i o n a l g r o u p s w h i c h p r o p a g a t e e x c l u s i v e l y by d i f f e r e n t mechanism, s e l e c t i n g among r a d i c a l , a n i o n i c , and c a t i o n i c p r o p a g a t i o n . These monomers may be p o l y m e r i z e d t h r o u g h one f u n c t i o n a l g r o u p and c r o s s l i n k e d i n a s e p a r a t e s t e p by a d i f f e r e n t c l a s s o f i n i t i a t o r . The f a c t t h a t p h o t o e x c i t a t i o n o f c h a r g e - t r a n s f e r c o m p l e x e s c a n i n i t i a t e i o n i c p o l y m e r i z a t i o n s o f c e r t a i n monomers s u g g e s t e d t h e f e a s i b i l i t y o f using photo-generated ions r a t h e r than photogenerated r a d i c a l s . The two p o s s i b i l i t i e s a v a i l a b l e f o r t h i s p u r p o s e a r e t h e p h o t o c a t i o n i c c r o s s l i n k i n g o f a polymer c o n t a i n i n g donor o l e f i n i c g r o u p s i n t h e p r e s e n c e o f a n a c c e p t o r compound, and t h e p h o t o a n i o n i c c r o s s l i n k i n g o f a polymer c o n t a i n i n g acceptor o l e f i n i c g r o u p s i n t h e p r e s e n c e o f a d o n o r compound. The p o l y m e r c o n t a i n ing donor o l e f i n i c groups, p o l y - ( m - v i n y l o x y e t h o x y ) s t y r e n e ( X V I I I ) was p r e p a r e d i n o u r l a b o r a t o r i e s by Mr. Shaw Chu ( 2 7 ) , and has a m o l e c u l a r w e i g h t o f 110,000. The p o l y m e r c o n t a i n i n g acceptor o l e f i n i c groups, p o l y - [ v i n y I ( 2 - n i t r o s t y r y l o x y e t h y I ) e t h e r ] , (XIX) was s y n t h e s i z e d a c c o r d i n g t o t h e p r o c e d u r e o f J.W. Schwîetert ( 3 0 ) and has a low m o l e c u l a r w e i g h t ( p r o b a b l y 5000 o r l e s s ) . R e s u l t s and D i s c u s s i o n I n i t i a l experiments involved t h e i r r a d i a t i o n o f XVIII i n t h e p r e s e n c e o f c h l o r a n i l i n m e t h y l e n e c h l o r i d e and t h e i r r a d i a t i o n o f X I X i n t h e p r e s e n c e o f Ν,Ν-dîmethylaniIine i n m e t h y l e n e chloride. Samples were p r e p a r e d on t h e vacuum l i n e w i t h p u r e d r y m a t e r i a l s and w i t h c a r e f u l removal and e x c l u s i o n o f m o i s t u r e and o x y g e n . However, no t r a c e o f photocrossIînking was o b s e r v a b l e in e i t h e r system. In a c c o r d a n c e w i t h t h e e x p e c t a t i o n t h a t i n i t i a t i o n by a c h a r g e - t r a n s f e r e x c i t e d s t a t e must be p r e c e d e d by s e p a r a t i o n o f t h e r a d i c a l c a t i o n - r a d i c a l a n i o n p a i r by s o l v e n t , i n o r d e r t o compete w i t h r e c o n v e r s i o n t o t h e g r o u n d s t a t e w i t h i n t h e s o l v e n t c a g e , more p o l a r s o l v e n t s were employed t o f a c i l i t a t e t h i s s t e p . D + A

(D,A)

h V ( t

7

> (Ο+,Α ) }

^

>

D* + A

T

Thus, i r r a d i a t i o n o f XVIII i n t h e presence o f c h l o r a n i l o r t e t r a c h l o r o p h t h a l I i c a n h y d r i d e (TCPA) i n a c e t o n i t r i l e u n d e r d r y , oxygen-free conditions yielded rapid p r e c i p i t a t i o n o f c r o s s l i n k e d p o l y m e r . The i r r a d i a t i o n o f X I X i n t h e p r e s e n c e o f N,N-dimethyI a n i l i n e i n d i m e t h y I f o r m a m i d e u n d e r e x c l u s i o n o f a i r and m o i s t u r e produced a very s l i g h t i n d i c a t i o n o f c r o s s l i n k i n g o c c u r r i n g . U n t i l the present time, the f a c t that the photo-cat ionic c r o s s l i n k i n g o f XVIII i s s u c c e s s f u l while t h e photo-anionic crossl i n k i n g o f X I X i s much l e s s s o has prompted much f u r t h e r work o n

UV

24

LIGHT INDUCED REACTIONS

IN

POLYMERS


t h e f i r s t and none on t h e s e c o n d . Other reasons a l s o e x i s t f o r t h e l a c k o f f u r t h e r work on X I X . The p h o t o - a n i o n i c c r o s s l i n k i n g o f X I X i s p r o b a b l y quenched by t r a c e s o f w a t e r , and i t may be e x t r e m e l y d i f f i c u l t t o p r e p a r e s o l u t i o n s o f t h e p o l y m e r i n an absolutely dry state. A l s o , t h e polymer p a r t i a l l y c r o s s l i n k s s p o n t a n e o u s l y by a i r o x i d a t i o n d u r i n g s t o r a g e , i s p r e p a r e d i n low y i e l d , h a s a low m o l e c u l a r w e i g h t , a n d i s v e r y d i f f i c u l t t o d i s s o l v e , making i t l e s s d e s i r a b l e f o r t h e purpose i n t e n d e d . Never t h e l e s s , f u r t h e r a t t e m p t s t o c r o s s l i n k X I X by means o f o t h e r d o n o r s , s u c h a s a n t h r a c e n e , w o u l d be o f i n t e r e s t . The p r o c e s s a p p a r e n t l y b e i n g o b s e r v e d i n t h e p h o t o - c a t i o n i c c r o s s l i n k i n g o f X V I I I i s d e p i c t e d b e l o w , where R0CH=CH2 r e p r e s e n t s XVI I I :

ROCH=CH

2

+ A

hv CH CN

+ R O C H - C H . + A*

3

CH:

I

ROCH=CH. ->

+

2

ROCH-CH -CHOR 2

CH; ROCH -f CH -CH -KX 2 , η OR 0

The t e r m i n a t i o n s t e p o f a p r o p a g a t i n g c h a i n c o u l d i n v o l v e p r o t o n t r a n s f e r t o monomer, c o m b i n a t i o n w i t h t h e a c c e p t o r r a d i c a l a n i o n , o r e l e c t r o n t r a n s f e r from t h e a c c e p t o r r a d i c a l a n i o n t o t h e carbonium i o n t o y i e l d a t e r m i n a l r a d i c a l , a s below. CH;

Ruin

pu · R0CH=CH j 2 ^ R O C H -f C H - C H ->- CH=CH + ROCHCH, 2 η j -> OR O

-f CH -CH * CH -CH 2 j η 2 ι OR 0

0

0

CH; I ROCH -f C H - C H 2 2

0

CH;

η

CH -CH-A ζ ι OR 9

ROCH « - C H - C H + C H - C H 2 η 2 ι OR OR 0

ι

0

P r e s u m a b l y t h e r a d i c a l s i t e s g e n e r a t e d do n o t p r o p a g a t e b u t a r e d e s t r o y e d i n some c h a i n - t r a n s f e r r e a c t i o n o r r a d i c a l - r a d i c a l combination. The c h a r g e - t r a n s f e r c o m p l e x h a s n o t been i n c l u d e d i n t h e i n i t i a t i o n mechanism f o r r e a s o n s t o be d i s c u s s e d . The e f f e c t s o f d i f f e r e n t a c c e p t o r s , o x y g e n , w a t e r , quenchers,

3.

BUTLER A N D FERRÉE


25


r a d i c a l i n i t i a t o r s , and o t h e r a d d i t i v e s , t e m p e r a t u r e , g e o m e t r y o f i r r a d i a t i o n v e s s e l , a c c e p t o r c o n c e n t r a t i o n l i g h t i n t e n s i t y , wave l e n g t h o f l i g h t , and s p o n t a n e o u s c r o s s l i n k i n g w i l l be d i s c u s s e d . Effect of Different Acceptors. Although both o f t h e e a r l i e s t a c c e p t o r s t e s t e d w o r k e d , most o f t h e o t h e r s l a t e r examined d i d n o t e f f e c t p h o t o c r o s s I i n k i n g f o r unknown r e a s o n s . Table I l i s t s the a c c e p t o r s t e s t e d , a l o n g w i t h two m e a s u r e s o f a c c e p t o r s t r e n g t h and approximate e f f i c i e n c i e s o f p r e c i p i t a t i o n o f c r o s s l i n k e d polymer. The c r o s s l i n k e d p o l y m e r formed w i t h c h l o r a n i l i s p r e d o m i n a n t l y s u s p e n d e d i n t h e l i q u i d w h i l e t h a t formed on t h e t u b e w a l l where t h e l i g h t beam i m p i n g e s . The p r o d u c t o f c r o s s l i n k i n g X V I I I u s i n g c h l o r a n i l o r TCPA c o n t a i n s no d e t e c t a b l e IR bands due t o t h e a c c e p t o r s . The IR s p e c t r u m o f X V I I I c r o s s l i n k e d by i r r a d i a t i o n i n t h e p r e s e n c e o f TCPA i s i d e n t i c a l t o t h a t o f X V I I I c r o s s l i n k e d by B F by Shaw Chu. Spontaneous C r o s s l i n k i n g . In t h e a b s e n c e o f TCPA, e v a p o r a t e d s o l u t i o n s o f X V I I I c a n be c o m p l e t e l y r e d i s s o l v e d i n f r e s h s o l v e n t i n a few m i n u t e s . And s o l u t i o n s o f 0.05 M X V I I I c o n t a i n i n g 0.007 M TCPA i n a c e t o n i t r i l e a r e s t a b l e i n d e f i n i t e l y ( a t l e a s t t h r e e weeks) a t room t e m p e r a t u r e . However, on e v a p o r a t i o n o f t h e s o l v e n t under vacuum, t h e m i x t u r e f o r m s a v e r y t o u g h f i l m t h a t i s i n s o l u b l e i n a c e t o n i t r i l e o r h o t DMF, does n o t m e l t up t o 300°, and i s r e s i s t a n t t o a t t a c k by c h r o m i c a c i d . T h e IR s p e c t r u m i s i d e n t i c a l t o t h a t o f X V I I I c r o s s l i n k e d B F 3 o r by i r r a d i a t i o n i n t h e p r e s e n c e o f TCPA. A p p a r e n t l y a s t h e s o l u t i o n becomes h i g h l y c o n c e n t r a t e d , c r o s s l i n k i n g o c c u r s by t h e r m a l e x c i t a t i o n o f c h a r g e t r a n s f e r c o m p l e x e s o f TCPA and X V I I I . The IR e v i d e n c e and t h e i n s o l u b i l i t y b e h a v i o r a p p e a r t o d i s c o u n t an a l t e r n a t i v e p o s s i b i l i t y o f s i m p l e a s s o c i a t i o n o f p o l y m e r c h a i n s f a c i l i t a t e d by t h e p r e s e n c e o f t h e a c c e p t o r compound. The s p o n t a n e o u s c r o s s l i n k i n g i s a l s o o b s e r v e d w i t h t h e a c c e p t o r s c h l o r a n i l and 1 , 3 , 5 - t r i n i t r o b e n z e n e , and p r e s u m a b l y w o u l d o c c u r w i t h o t h e r s . W h i l e t h e s p o n t a n e o u s c r o s s l i n k i n g i s i n t e r e s t i n g and c e r t a i n l y worthy o f f u r t h e r i n v e s t i g a t i o n , i t p l a c e s l i m i t a t i o n s on t h e methods o f s a m p l e p r e p a r a t i o n f o r t h e p h o t o c r o s s I i n k i n g when d r y c o n d i t i o n s a r e r e q u i r e d . T h a t i s , one c a n n o t d r y t h e a c c e p t o r and X V I I I i n t h e same s t e p by e v a p o r a t i o n o f a s t o c k s o l u t i o n c o n t a i n i n g t h e two t o d r y n e s s on t h e vacuum l i n e . The method t h a t was used t o a v o i d t h e s p o n t a n e o u s c r o s s l i n k i n g was t o prepare a stock s o l u t i o n o f XVIII i n dry a c e t o n i t r i l e , evaporate t h e s o l v e n t on t h e vacuum l i n e , h e a t t h e p o l y m e r f i l m t o 80° b r i e f l y , and t h e n a l l o w i t t o d r y u n d e r vacuum f o r one h o u r , t h e t u b e removed f r o m t h e l i n e and a c c e p t o r w e i g h e d i n , t h e n d r y p o l y m e r and a c c e p t o r d r i e d u n d e r vacuum f o r one h o u r . E x c e s s d r y a c e t o n i t r i l e was c o n d e n s e d i n t o t h e l i q u i d - n i t r o g e n - c o o l e d t u b e d i r e c t l y f r o m P2®5> m e l t e d t h e e x c e s s e v a p o r a t e d by h i g h vacuum, and t h e t u b e s e a l e d w i t h a f l a m e . I f t h e d r y i n g u n d e r vacuum o f t h e combined d r y m i x t u r e o f X V I I I and TCPA i s c a r r i e d o u t f o r a long p e r i o d o f t i m e , p a r t i a l spontaneous c r o s s l i n k i n g i s observed 3

-1.31

PhthalIic

0.56

0.56

(est.

0.70

0.77

1.37

E.A.,

0.4)

eV

300

275,

334

300, 254,

236

275 236

334 ( 3 3 4 , 313,

334 ( 3 3 4 , 313,

334

λ(+6.4),

366 )

. 366)

nm

D

d

e

0.75

o.o '

0.75

0.0

0.0

0.0

2.3

d

Eff iciency

9

e

c

C o n c n . XVI1 I was 0.050 M ( i n monomer u n i t s o f M.W. 190) i n a c e t o n i t r i l e , c o n c n . a c c e p t o r was 0.007 M. A c c e p t o r s t r e n g t h d a t a f r o m book by F o r s t e r . ^Samples c o n t a i n i n g 0.50 M H2O; a l l o t h e r s p r e p a r e d u n d e r r i g o r o u s l y d r y c o n d i t i o n s . A l l s a m p l e s s e a l e d u n d e r vacuum g r e a t e r t h a n 10-4 t o r r . E x p r e s s e d i n t e r m s o f m o l e s o f monomer u n i t s o f p o l y m e r p r e c i p i t a t e d p e r e i n s t e i n of protons absorbed. ^ I r r a d i a t e d i n q u a r t z t u b e s a t room temp, o f 25°; a l l o t h e r s were P y r e x t u b e s i r r a d i a t e d w h i l e immersed i n an i c e b a t h a t 3±1°. A f i n e p r e c i p i t a t e formed t h a t q u i c k l y r e d i s s o l v e d .

Anhydride

-0.86

TCPA

a

-0.86

TCPA

( e s t . -0.8).

-0.60

1,3,5-Trinitrobenzene

1,3,5-Tricyanobenzene

-0.51

/ v

Benzoqu i none

1/2 +0.01

L

Chlorani1

Acceptor

in the Photo-cat i o n i c C r o s s l i n k i n g of X V I I I

Acceptor Strength

Tests of V a r i o u s Acceptors

Table I


3.

BUTLER AND FERRÉE


27


by i n s o l u b i l i t y i n f r e s h s o l v e n t , p a r t i c u l a r l y where t h e TCPA p a r t i c l e s d i r e c t l y c o n t a c t e d t h e polymer, but t h e r e a l s o appears t o be some c r o s s l i n k i n g c a u s e d by TCPA v a p o r . T h i s p r o c e s s h a s a l s o been o b s e r v e d w i t h c h l o r a n i l , w h i c h i s more v o l a t i l e t h a n TCPA. E f f e c t o f Oxygen. A l t h o u g h a c a t i o n i c p r o p a g a t i o n i s g e n e r a l l y n o t a f f e c t e d by o x y g e n , t h e e x c i t e d s t a t e g i v i n g r i s e t o t h e p h o t o - c a t i o n i c c r o s s l i n k i n g m i g h t be e x p e c t e d t o be s u s c e p t i b l e t o oxygen quenching, p a r t i c u l a r l y i f t h e t r i p l e t e x c i t e d s t a t e i s i n v o l v e d . To t e s t t h i s , d r y r e a c t i o n s a m p l e s were p r e p a r e d on t h e vacuum l i n e a s a b o v e , t h e n a i r a d m i t t e d t h r o u g h a d r y i n g t u b e and t h e sample t u b e s s e a l e d o f f and i r r a d i a t e d . W i t h TCPA a s t h e a c c e p t o r , t h e photocrossIînking o c c u r r e d w i t h c o m p a r a b l e e f f i c i e n c y t o t h a t o f t u b e s k e p t s e a l e d u n d e r vacuum ( T a b l e I I ) . Thus b o t h a t r i p l e t e x c i t e d s t a t e i n i t i a t o r and a r a d i c a l p r o p a g a t i o n mechanism a r e d i s f a v o r e d by t h i s e v i d e n c e when TCPA i s u s e d . However, w i t h c h l o r a n i l a s t h e a c c e p t o r , p h o t o c r o s s I i n k i n g i s c o m p l e t e l y quenched by a i r . S i n c e s e v e r a l r e a s o n s t o be d i s c u s s e d below d i s f a v o r a r a d i c a l p r o p a g a t i o n mechanism f o r t h e p h o t o c r o s s I i n k i n g , i t i s b e l i e v e d t h a t t h e oxygen quenching b e h a v i o r i n d i c a t e s t h a t t h e c r o s s l i n k i n g i s i n i t i a t e d by t r i p l e t e x c i t e d chloranil. Q u e n c h i n g o f a s i n g l e t e x c i t e d i n i t i a t o r would have been o n l y p a r t i a l . I t would be o f i n t e r e s t t o t e s t t h e h y p o t h e s i s o f t r i p l e t i n i t i a t o r by e x a m i n i n g t h e e f f e c t o f s m a l l c o n c e n t r a t i o n s (0.02 M) o f c y c l o o c t a t e t r a e n e , a q u e n c h e r o f low t r i p l e t e n e r g y (39 K c a l . ) , i n t h e c a s e o f c h l o r a n i l . E f f e c t o f W a t e r . A s a m p l e was p r e p a r e d a s d e s c r i b e d a b o v e e x c e p t t h a t t h e s o l v e n t condensed i n t o t h e tube c o n t a i n i n g dry X V I I I and TCPA was a measured a l i q u o t o f a s o l u t i o n o f 0.010 M H 0 in dry a c e t o n i t r i l e . I r r a d i a t i o n produced p h o t o c r o s s I i n k i n g i n a p p a r e n t l y c o m p a r a b l e amounts t o t h a t f o r a d r y s a m p l e . However, a f t e r o p e n i n g t h e t u b e and s t o p p e r i n g , t h e n a l l o w i n g t o s t a n d o v e r n i g h t a t room t e m p e r a t u r e , a d d i t i o n a l c r o s s l i n k i n g o c c u r r e d , a s a p r e c i p i t a t e t h a t s e t t l e d t o t h e bottom o f t h e t u b e , i n a d d i t i o n t o t h e p a t c h e s a d h e r i n g t o t h e a r e a s where t h e l i g h t beam had impinged. T h e t o t a l c o n v e r s i o n i s o l a t e d was 93% o f t h e 18.1 mg X V I I I s t a r t e d w i t h . On r e p e a t i n g t h e e x p e r i m e n t w i t h 0.50 Μ H 0 , t h e p h o t o c r o s s I i n k i n g o c c u r r e d w i t h t y p i c a l e f f i c i e n c y , but a f t e r i r r a d i a t i o n , a s t h e t u b e was b e i n g a l l o w e d t o warm t o room t e m p e r a t u r e , t h e c r o s s l i n k i n g resumed s p o n t a n e o u s l y t o f i l l t h e s o l u t i o n w i t h heavy p r e c i p i t a t e w i t h i n a few m i n u t e s . T o t a l c o n v e r s i o n was 95% t o t h e 18.1 mg X V I I I s t a r t e d w i t h . The s e a l e d s a m p l e had p r e v i o u s t o i r r a d i a t i o n s t o o d a t room t e m p e r a t u r e o v e r n i g h t w i t h no c h a n g e . The i n d i c a t i o n o f two s e p a r a t e p r o c e s s e s , a p h o t o c r o s s l i n k i n g and a w a t e r - i n d u c e d p o s t c r o s s Iînking, h e r e a f t e r s h o r t e n e d t o p o s t l i n k i n g , was v e r i f i e d d r a m a t i c a l l y by t h e f o l l o w ing experiment. A c o m p l e t e l y d r y s a m p l e was p r e p a r e d and i r r a d i a t e d , t h e n t h e t u b e opened and enough H 0 (180 mg/2.0 ml a c e t o 2

2

2

UV

28

Table


POLYMERS

I I

E f f e c t s o f V a r y i n g C o n d i t i o n s and o f A d d i t i v e s on t h e P h o t o c r o s s I i n k i n g o f X V I I I i n t h e P r e s e n c e o f TCPA o r C h l o r a n i l . Acceptor TCPA

Additive or Variation


TCPA

mg pcpt. 7.3

E f f ί ο ciency 0.82

334

Time Irrad. 60 m i n .

334

60

6.0

0.68 0.96

TCPA

Air

334

60

8.5

TCPA

0.0010 M TCPA

334

30

0.0

0.00

TCPA

Narrow S1 i t s

334

120

1.3

0.073

TCPA

n-Bu.NCIO. 0.10 Ή

334

20

4.1

1.4

TCPA

0.0050 M HQ

334

20

0.0

0.00

TCPA

0.010 M

334

20

0.0

0.00

TCPA

Air, H 0 0.0005 M E t N

334

10

0.0

0.00

0.010 M γ-CoI I i d i ne

334

10

>0.0

0.50 M γ-Col1îdine

334

10

0.0

0.00

TCPA

0.005 M KSCN

334

10

0.0

0.00

TCPA

0.10 M AIBN

366

10

0.0

0.00

366

4

8.3

2.6

4

Et N 3

?

3

TCPA TCPA

Chlorani1 Chlorani1

Narrow s I i t s

366

12

6.2

2.0

Chlorani1

25°

366

8

5.0

0.80

Chlorani1

Air

366

8

0.0

0.00

334

10

0.0

0.00

C h l o r a n î1 a

C o n c . X V I I I was 0.050 M i n A c e t o n i t r i l e ; c o n e , a c c e p t o r was 0.0070 M e x c e p t i n t h e e x c e p t i o n n o t e d ; i r r a d i a t i o n s p e r f o r m e d on t u b e s immersed i n an i c e b a t h e x c e p t where n o t e d . ^ D i s p e r s i o n o f monochromator was 6.4 nm, e x c e p t where narrow s l i t s were e m p l o y e d , r e d u c i n g d i s p e r s i o n t o 3.2 nm. Intensities f o r t h e two s l i t c o n d i t i o n s w e r e , f o r 334, 4.7 and 1.64 χ 1 0 ^ p h o t o n s / m i n . , and f o r 366 nm, 2.5 and 0.83 χ 1 0 photons/min. E x p r e s s e d i n t e r m s o f m o l e s o f monomer u n i t s o f p o l y m e r p r e c i p i t a t e d p e r e i n s t e i n o f photons absorbed. 1 8

c

3.

BUTLER AND FERRÉE

29


nîtrile) t o make t h e s o l u t i o n 0.5 M i n H 0 was added and t h e s o l u t i o n shaken g e n t l y t o m i x . No c h a n g e o c c u r r e d i m m e d i a t e l y , b u t on r e e x a m i n a t i o n a f t e r one h o u r i n t h e d a r k t h e w h o l e sample had crosslinked. F i l t r a t i o n p r o v i d e d 99% c o n v e r s i o n . Dry s a m p l e s t h a t a r e i r r a d i a t e d and k e p t s e a l e d have n e v e r been o b s e r v e d t o p o s t l i n k even a f t e r p e r i o d s o f a few d a y s . Addi t i o n o f 0.5 M H 0 t o an u n i r r a d i a t e d s a m p l e c a u s e d no p r e c i p i t a t i o n f o r s e v e r a l days i n t h e dark. A f t e r s i x days a m i l k y c a s t a p p e a r e d , a n d f o r m a t i o n o f a t h i n p r e c i p i t a t e a p p e a r e d t o have reached c o m p l e t i o n i n about e l e v e n days. 2


2

E f f e c t o f Quenchers. The p h o t o c r o s s I i n k i n g o f X V I I I i n t h e p r e s e n c e o f 0.007 M TCPA i s quenched c o m p l e t e l y by 0.010 M t r i e t h y l a m i n e o r even by 0.005 M t r i e t h y l a m i n e ( a i r and a d v e n t i t i o u s H 0 were p r e s e n t i n t h e l a t t e r r u n ) . T h i s i s b e l i e v e d t o be good evidence t h a t t h e propagating s p e c i e s i s c a t i o n i c r a t h e r than radical. T h e s e s a m p l e s a l s o do n o t p o s t l i n k when 0.5 M H 0 i s added. The h i n d e r e d amine γ-colIidîne (2,4,6-trîmethyIpyridine) was t h e n e x a m i n e d a s a q u e n c h e r s i n c e i t w o u l d s e l e c t i v e l y bond t o p r o t o n s b u t n o t t o l a r g e r c a t i o n s . I t was o b s e r v e d t h e 0.010 M γ-collidine does n o t quench t h e p h o t o c r o s s I i n k i n g i n t h e p r e s e n c e o f TCPA c o m p l e t e l y , b u t p o s t l i n k i n g a f t e r a d d i t i o n o f 0.5 M H 0 was c o m p l e t e l y i n h i b i t e d u n t i l two weeks l a t e r . T h i s experiment s h o u l d be r e p e a t e d on a l a r g e r s c a l e t o o b t a i n q u a n t i t a t i v e measurements o f t h e i n h i b i t i o n . The e x p e r i m e n t a p p e a r s t o p r o v e t h a t t h e photocrossIînking i s c a t i o n i c b u t t h e i n i t i a t i n g s p e c i e s a r e n o t p r o t o n s , w h i l e t h e p o s t l i n k i n g i s i n i t i a t e d by p r o t o n s . A t a c o n c e n t r a t i o n o f γ-collîdîne o f 0.50 M t h e p h o t o c r o s s I i n k i n g i s c o m p l e t e l y i n h i b i t e d , w h i c h c o u l d be due t o c o m p e t i t i v e c h a r g e t r a n s f e r c o m p l e x a t i o n o f g r o u n d s t a t e o f e x c i t e d s t a t e TCPA r a t h e r than i n t e r f e r i n g with c a t i o n i c propagation. ( F o r s t e r ' s book on c h a r g e - t r a n s f e r c o m p l e x e s shows t h a t γ-collîdîne c a n f o r m compI e x e s . ) H y d r o q u i n o n e a t a c o n c e n t r a t i o n o f 0.0050 M q u e n c h e s p h o t o c r o s s l i n k i n g i n t h e p r e s e n c e o f TCPA c o m p l e t e l y . Possibly the b a s i c i t y o f hydroquinone i s r e s p o n s i b l e f o r t h e i n h i b i t i o n , s i n c e t h e r a d i c a l mechanism i s d i s f a v o r e d compared t o t h e c a t i o n i c mechanism by s e v e r a l o t h e r p i e c e s o f e v i d e n c e . The i d e a l q u e n c h e r w h i c h was on o r d e r , i s d i p h e n y I p i c r y I h y d r a z y I , w h i c h a f f e c t s r a d i cal but not c a t i o n i c processes. 2

2

2

E f f e c t o f A z o i s o b u t y r o n i t r i l e (AIBN). AI t h o u g h XVI I I i s formed f r o m t h e monomer u n d e r r a d i c a l c o n d i t i o n s w i t h o u t r a d i c a l propagation o c c u r r i n g along t h e v i n y l ether groups, t h e p o s s i b i l i t y t h a t i n t h e p r e s e n c e o f an a c c e p t o r s u c h a s TCPA o r c h l o r a n i l t h e c o m p l e x e d v i n y l e t h e r g r o u p s w o u l d be s u s c e p t i b l e t o r a d i c a l p r o p a g a t i o n seemed p l a u s i b l e enough t o t e s t t h i s p o s s i b i l i t y . Thus a s a m p l e o f 0.050 M X V I I I and 0.007 M TCPA c o n t a i n i n g 0.10 M AIBN i n d r y a c e t o n i t r i l e was i r r a d i a t e d a t 366 ± 6.4 nm, where

UV

30


POLYMERS

AIBN a b s o r b s and decomposes t o r a d i c a l s w i t h a quantum y i e l d o f 0.43 (0.86 r a d i c a l s p e r p r o t o n ) ( 2 8 K No p h o t o c r o s s I i n k i n g occurred. T h i s seemed t o show t h a t t h e c r o s s l i n k i n g i n t h i s s y s tem i s n o t r a d i c a l - i n i t i a t e d . ( I t m i g h t be m e n t i o n e d t h a t t h e o n l y p o s s i b i l i t y l e f t open by t h i s e x p e r i m e n t f o r a r a d i c a l p r o c e s s i s t h a t d i r e c t p h o t o - i n i t i a t i o n p r o d u c e s a r a d i c a l o f much g r e a t e r r e a c t i v i t y than (CH3) C-CN which i s a b l e t o i n i t i a t e c r o s s l i n k i n g in the presence of the acceptor.)


2

E f f e c t o f M i s c e l l a n e o u s A d d i t i v e s . The s o u r c e o f t h e p r o t o n s r e s p o n s i b l e f o r p o s t l i n k i n g i s not y e t c l e a r . The r e s u l t s o b t a i n ed r e q u i r e t h a t i r r a d i a t i o n p r o d u c e s e i t h e r i n t h e c r o s s l i n k e d p o l y m e r o r t h e s o l u t i o n an a c i d i c s p e c i e s t h a t does n o t c o n t i n u e t o p r o p a g a t e c a t i o n i c c r o s s l i n k i n g when i r r a d i a t i o n i s s t o p p e d , b u t w h i c h can r e a c t w i t h H 0 t o r e l e a s e p r o t o n s . One p o s s i b i l i t y t h a t has been u n d e r c o n s i d e r a t i o n i s t h a t d u r i n g p h o t o c r o s s I i n k i n g some o f t h e p r o p a g a t i n g c a t i o n i c c e n t e r s become t r a p p e d i n c r o s s l i n k e d polymer, such t h a t molecules of H 0 c o u l d d i f f u s e in and bond t o t h e c a r b o n i u m i o n and t h e n r e l e a s e a p r o t o n . Thus H 0 would a c t as a c h a i n t r a n s f e r a g e n t . If t h i s t h e o r y i s c o r r e c t , other small molecules could s i m i l a r l y i n t e r c e p t the trapped c a r bonium i o n s . One a p p r o a c h t h a t was t o be a t t e m p t e d i s t o add v i a s e a l e d ampoule t e c h n i q u e s β-chloroethyI v i n y l e t h e r t o an i r r a d i a t e d m i x t u r e o f X V I I I and a c c e p t o r i n a c e t o n i t r i l e and a n a l y z e f o r i n c o r p o r a t i o n of c h l o r i n e in t h e c r o s s l i n k e d polymer, assuming the β-chloroethyI v i n y l e t h e r i s a b l e t o d i f f u s e t o t h e c a r b o n i u m c e n t e r s and f o r m g r a f t e d c h a i n s by c a t i o n i c p r o p a g a t i o n . This exper iment i s p r e f e r a b l y t o be c a r r i e d o u t w i t h an a c c e p t o r t h a t ab s o r b s a b o v e t h e P y r e x c u t o f f o f 310 nm and does n o t c o n t a i n chlorine. The r e c e n t l y r e c e i v e d p y r o m e l l i t i c d i a n h y d r i d e was t o be i n v e s t i g a t e d f o r t h i s p u r p o s e . A n o t h e r a g e n t i n v e s t i g a t e d a s an i n t e r c e p t o r o f t h e t r a p p e d c a r b o n i u m i o n s i s p o t a s s i u m t h i o c y a n a t e , KSCN. T h i s i s t h e o n l y common s a l t t e s t e d t h a t i s s i g n i f i c a n t l y s o l u b l e i n a c e t o n i t r i l e . The t h i o c y a n a t e a n i o n has a m o l e c u l a r s i z e c o m p a r a b l e t o t h a t o f H 0 and i t s i n c o r p o r a t i o n i n t o t h e p o l y m e r s h o u l d be e a s i l y de t e c t e d by t h e s t r o n g , s h a r p i n f r a r e d a b s o r p t i o n n e a r 2100 cm-1. A c c o r d i n g l y , a s a m p l e o f X V I I I and TCPA i n d r y a c e t o n i t r i l e was i r r a d i a t e d f o r 20 min. t o p r o d u c e c r o s s l i n k i n g , t h e n t h e t u b e opened and an e q u a l volume o f 0.10 M KSCN i n d r y a c e t o n i t r i l e a d d ed. A f t e r s t a n d i n g i n i c e - w a t e r f o r 2 h o u r s ( i t was assumed t h a t c o m b i n a t i o n o f t h i o c y a n a t e and c a r b o n i u m c e n t e r s w o u l d be r a p i d ) , t h e s o l u t i o n was made 0.5 M i n H 0 t o o b s e r v e w h e t h e r t h e p o s t l i n k i n g would o c c u r . A f t e r 3 h o u r s a t room t e m p e r a t u r e , no p o s t l i n k i n g had o c c u r r e d , w h e r e a s p o s t l i n k i n g was c o m p l e t e i n an i d e n t i c a l i r r a d i a t e d sample not t r e a t e d w i t h t h i o c y a n a t e i o n . But a f t e r t h r e e d a y s n e a r l y c o m p l e t e p o s t l i n k i n g (93%) had o c c u r r e d . The i n f r a r e d s p e c t r u m o f t h e f i l t e r e d p o l y m e r showed no d e t e c t a b l e band i n t h e 2100 cm"! r e g i o n n o r any o t h e r d i f f e r e n c e f r o m s i m i l a r s a m p l e s n o t t r e a t e d w i t h t h i o c y a n a t e i o n . However, d u r i n g t h e 2

2

2

2

2


3.

BUTLER AND FERRÉE

PhotocwssUnkable Polymers

31

same s e t o f r u n s a s a m p l e o f X V I I I a n d TCPA i n d r y acetonitrîle c o n t a i n i n g 0.005 M KSCN was p r e p a r e d a n d i r r a d i a t e d , b u t no p h o t o crosslinking occurred, ( A l t h o u g h TCPA and KSCN f o r m a y e l l o w c h a r g e - t r a n s f e r c o m p l e x i n acetonitrîle, a b s o r p t i o n o f t h e c o m p l e x i s i n s i g n i f i c a n t a t t h e c o n c e n t r a t i o n s used h e r e . I t might a l s o be n o t e d t h a t t h e s t r o n g e r a c c e p t o r c h l o r a n i l f o r m s a r e d c o m p l e x w i t h KSCN t h a t s p o n t a n e o u s l y r e a c t s t o f o r m a brown p r e c i p i t a t e t h a t p r o b a b l y r e s u l t s from d i s p l a c e m e n t o f c h l o r i n e s from t h e 2 and 5 p o s i t i o n s by t h i o c y a n a t e i o n . ) A d d i t i o n o f a n 0.5 M c o n c e n t r a t i o n o f H2O p r o d u c e d no p o s t l i n k i n g i n t h r e e h o u r s b u t c o m p l e t e p r e c i p i t a t i o n (measured 107% based o n s t a r t i n g p o l y m e r a l o n e ) i n t h r e e days. H e r e we seem t o have an e x a m p l e o f a more r a p i d p o s t l i n k i n g i n an i r r a d i a t e d s a m p l e t h a n i n an u n i r r a d i a t e d s a m p l e e v e n t h o u g h no v i s i b l e p r e c i p i t a t i o n due t o p h o t o c r o s s I i n k i n g occurred. F u r t h e r , a l t h o u g h t h e p r o d u c t d o e s n o t c o n t a i n a band n e a r 2 1 0 0 c n r H , t h e r e i s a m o d e r a t e l y s t r o n g and s l i g h t l y b r o a d band a t 1737 c r r H , w i t h no o t h e r o b s e r v a b l e d i f f e r e n c e i n t h e spectrum from t h a t t y p i c a l o f c r o s s l i n k e d X V I I I . Evidently the s p e c i e s p r o d u c i n g t h e 1737 c r r H band must have i n c o r p o r a t e d i n t h e polymer d u r i n g t h e i r r a d i a t i o n p e r i o d , o t h e r w i s e i t would a l s o be p r e s e n t i n t h e p r o d u c t o f t h e e x p e r i m e n t d e s c r i b e d j u s t p r e viously. One p o s s i b i l i t y i s t h a t t h e t h i o c y a n a t e i o n quenched p h o t o c r o s s I i n k i n g by c o m b i n a t i o n w i t h t h e i n i t i a t i n g r a d i c a l c a t i o n o r t h e p r o p a g a t i n g c a t i o n a n d t h e n was h y d r o l y z e d t o a t h i o l c a r b a m a t e o n a d d i t i o n o f H 2 O . However, t h e band maximum d o e s n o t a g r e e w i t h t h o s e r e p o r t e d f o r N - p h e n y l - and N - p r o p y l t h i o I carbamates ( 2 9 ) , R-S(C0)NH2, which g i v e sharp d o u b l e t s i n t h e 1600-1650 cm-1 r e g i o n a n d a n o t h e r band n e a r 1300 α τ Η . The p o s i t i o n o f t h e band i s more t y p i c a l o f an e s t e r o r k e t o n e , b u t i t s e x a c t o r i g i n i s a s y e t unknown. The e f f e c t o f t e t r a b u t y I ammonium p e r c h I o r a t e was e x a m i n e d , s i n c e i t c o u l d i n c r e a s e the e f f i c i e n c y o f t h e r e a c t i o n i n two ways. T h e p e r c h I o r a t e i o n w o u l d c o m p e t e e f f e c t i v e l y w i t h t h e r a d i c a l a n i o n o f TCPA a s a c o u n t e r i o n t o t h e i n i t i a t i n g r a d i c a l c a t i o n and t o t h e p r o p a g a t i n g c a t i o n , t h u s p o s s i b l y d e c r e a s i n g both r a d i c a l a n i o n - r a d i c a l c a t i o n r e c o m b i n a t i o n and whatever t e r m i n a t i o n s t e p s t h e r a d i c a l a n i o n o f TCPA i s i n v o l v e d i n . I t was f o u n d t h a t a s a m p l e c o n t a i n i n g 0.10 M t e t r a b u t y I ammonium p e r c h l o r a t e i n c r e a s e d t h e p h o t o c r o s s I i n k i n g y i e l d t o about double t h a t o b s e r v e d i n t h e a b s e n c e o f t h e e l e c t r o l y t e ( T a b l e I I ) . The a p p e a r a n c e o f t h e p r o d u c t was a l s o b e t t e r i n t h a t i t was w h i t e r and more c o a g u l a t e d . A p o s s i b l e e x p e r i m e n t o f i n t e r e s t w o u l d be t o i r r a d i a t e a s i m i l a r sample i n t h e s o l v e n t methylene c h l o r i d e , where t h e p r e s e n c e o f t h e e l e c t r o l y t e m i g h t a l l o w t h e p h o t o c r o s s I i n k i n g t o p r o c e e d and t a k e a d v a n t a g e o f t h e improved p r o p a g a t i o n of t h e c a t i o n i c process i n methylene c h l o r i d e than i n a c e t o nitrîle. E f f e c t o f Temperature. In t h e p r e s e n c e o f c h l o r a n i l , t h e e f f i c i e n c y o f p h o t o c r o s s I i n k i n g a t room t e m p e r a t u r e i s d e c r e a s e d

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t o a b o u t o n e - t h i r d o f t h a t a t an i c e - b a t h t e m p e r a t u r e ( T a b l e I I ) . T h i s i n d i c a t i o n of a negative temperature c o e f f i c i e n t i s s t r o n g e v i d e n c e t h a t a c a t i o n i c mechanism i s i n v o l v e d i n t h e p r o p a g a t i o n of the c r o s s l i n k i n g . The e f f e c t o f t e m p e r a t u r e s as low as -40°C w o u l d be o f i n t e r e s t . In t h e p r e s e n c e o f TCPA, t h e e f f i c i e n c y o f p h o t o c r o s s I i n k i n g a l s o a p p e a r e d t o be r e d u c e d a t room t e m p e r a t u r e but because of f i l t r a t i o n d i f f i c u l t i e s a r e l i a b l e q u a n t i t a t i v e measurement was n o t o b t a i n e d i n t h e e x p e r i m e n t r u n . E f f e c t o f R o t a t i n g Sample Tube D u r i n g I r r a d i a t i o n . A l l of t h e e a r l y e x p e r i m e n t s were p e r f o r m e d on 2.0 ml s a m p l e s c o n t a i n i n g 0.05 M X V I I I (19 mg) and u s u a l l y 0.007 M a c c e p t o r . The s a m p l e s were i r r a d i a t e d w h i l e s t a t i o n a r y , u s u a l l y i n an i c e b a t h , and t h e t u b e was t u r n e d t o a f r e s h s u r f a c e a t c o n s t a n t f r e q u e n t i n t e r v a l s . In t h e f i r s t a t t e m p t a t o b t a i n i n g q u a n t i t a t i v e quantum y i e l d m e a s u r e m e n t s , a s e r i e s o f 10 ml s a m p l e s c o n t a i n i n g 0.05 M X V I I I (95 mg e a c h ) and TCPA c o n c e n t r a t i o n s o f 0.001, 0.003, and 0.007 M were p r e p a r e d . T h e s e t u b e s were i r r a d i a t e d w h i l e t u r n e d by an e l e c t r i c m o t o r a t a b o u t 1 r . p . s . and w h i l e immersed i n an i c e b a t h , i n t e n d i n g t o form a u n i f o r m c o a t i n g of c r o s s l i n k e d polymer on t h e t u b e w a l l r a t h e r t h a n t h e s e r i e s o f p a t c h e s o b t a i n e d i n t h e previous runs. However, u n d e r t h e s e c o n d i t i o n s no s i g n i f i c a n t s o l i d formed on t h e t u b e w a l l s b u t i n s t e a d o n l y s l i g h t l y m i l k y s o l u t i o n s were p r o d u c e d , d e c r e a s i n g i n c l o u d i n e s s w i t h d e c r e a s i n g TCPA c o n c e n t r a t i o n . Even t h e p r e c i p i t a t e formed i n t h e 0.007 M TCPA s a m p l e was n o t f i l t e r a b l e . However, t h e s e i r r a d i a t e d sam p l e s , a f t e r u n s e a l i n g , s t o p p e r i n g , and b e i n g l e f t s t a n d i n g f o r a week, formed g e l s o c c u p y i n g t h e w h o l e l i q u i d volume o f t h e s a m p l e . The g e l s were no more opaque t h a n t h e r e s p e c t i v e o r i g i n a l s o l u t i o n s a f t e r i r r a d i a t i o n , t h e 0.001 M s a m p l e h a v i n g l i t t l e o p a c i t y . A p p a r e n t l y a d v e n t i t i o u s H2O c a u s e d a s l o w p r o t o n - i n i t i a t e d c r o s s Ii nking. E f f e c t of Acceptor C o n c e n t r a t i o n . With the f a i l u r e of the r o t a t i n g - s a m p l e - t u b e e x p e r i m e n t s , a l l t h e quantum e f f i c i e n c y measurements were p e r f o r m e d w i t h 10 ml s a m p l e s i n s t a t i o n a r y t u b e s turned at constant, frequent i n t e r v a l s t o a f r e s h surface. The c o n c e n t r a t i o n o f TCPA used i n most o f t h e r u n s was 0.007 M, w h i c h i s near the s o l u b i l i t y l i m i t in a c e t o n i t r i l e or methylene chloride. The a b s o r b a n c e a t 334 nm a t t h i s c o n c e n t r a t i o n f o r t h e 0.9 cm p a t h l e n g t h o f t h e t u b e s i s 20, a b o u t t e n t i m e s t h a t n e c e s sary t o absorb a l l the l i g h t . When a s a m p l e c o n t a i n i n g o n l y 0.0008 M TCPA ( a b s o r b a n c e 2.3 f o r t h e 0.9 cm p a t h l e n g t h , t h u s s t i l l a b s o r b i n g o v e r 99% o f t h e l i g h t ) was p r e p a r e d and i r r a d i a t e d , no p h o t o c r o s s I i n k i n g was o b s e r v a b l e w i t h i n 10 min. A second s a m p l e c o n t a i n i n g 0.0010 M TCPA p r o d u c e d no p h o t o c r o s s I i n k i n g a f t e r 30 min. o f i r r a d i a t i o n ( T a b l e I I ) . T h i s o b s e r v a t i o n c o r r e l a t e s with the q u a l i t a t i v e observations in the rotating-tube run. The o b s e r v a t i o n o f a c o n c e n t r a t i o n dependence o f c r o s s l i n k i n g e f f i c i e n c y i s v e r y s u r p r i s i n g , and a f u r t h e r s t u d y o f

3. BUTLER AND FERRÉE


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c o n c e n t r a t i o n e f f e c t s w o u l d be o f i n t e r e s t . E f f e c t o f L i g h t I n t e n s i t y . As n o t e d , t h e f a i l u r e o f t h e r o t a t e d TCPA s a m p l e s t o r e s e m b l e t h e b e h a v i o r o f s t a t i o n a r y sam p l e s d u r i n g i r r a d i a t i o n i s v e r y u n e x p e c t e d . However, t h e d e c r e a s e o f c r o s s l i n k i n g e f f i c i e n c y when e i t h e r t h e s a m p l e t u b e i s r o t a t e d o r t h e TCPA c o n c e n t r a t i o n i s r e d u c e d p o t e n t i a l l y h a s a common e x p l a n a t i o n , w h i c h i s t h a t t h e r e a c t i o n e f f i c i e n c y depends s t r o n g l y on t h e c o n c e n t r a t i o n o f e x c i t e d s p e c i e s . In a s t a t i o n a r y t u b e c o n t a i n i n g 0.007 M TCPA, t h e r e i s a h i g h c o n c e n t r a t i o n o f l i g h t e x c i t e d s p e c i e s j u s t i n s i d e t h e t u b e w a l l where t h e l i g h t beam impinges. Rotating t h e tube o r decreasing the concentration o f TCPA d e c r e a s e s l o c a l c o n c e n t r a t i o n s o f e x c i t e d s t a t e s w i t h o u t changing t h e i r t o t a l c o n c e n t r a t i o n . (Whether r o t a t i o n o f t h e tube could s i g n i f i c a n t l y a f f e c t local concentrations o f e x c i t e d s t a t e s i s by no means c e r t a i n b u t i s a p o s s i b i l i t y . ) These con s i d e r a t i o n s made a t e s t o f t h e e f f e c t o f c h a n g i n g l i g h t i n t e n s i t y on t h e r e a c t i o n i m p e r a t i v e , e v e n t h o u g h p h o t o r e a c t i o n s w h i c h a r e more e f f i c i e n t a t h i g h e r l i g h t i n t e n s i t i e s ( b i p h o t o n i c r e a c t i o n s ) a r e g e n e r a l l y v e r y r a r e , a n d t h e assumed mechanism does n o t r e q u i r e a b i p h o t o n i c e x c i t a t i o n scheme. Thus a s a m p l e c o n t a i n i n g 0.0070 M TCPA a c c e p t o r was i r r a d i a t e d w i t h n a r r o w e r s l i t s i n t h e monochromator t o reduce l i g h t i n t e n s i t y by a b o u t t w o - t h i r d s , and t h e i r r a d i a t i o n t i m e i n c r e a s e d proportionately. The o b s e r v e d quantum y i e l d o f p h o t o c r o s s I i n k i n g was i n f a c t r e d u c e d by a f a c t o r o f t e n ( T a b l e I I ) , s u p p o r t i n g t h e supposition t h a t a biphotonic process i s involved i n t h e r e a c t i o n c a t a l y z e d by TCPA. On t h e o t h e r hand, a s i m i l a r e x p e r i m e n t w i t h c h l o r a n i l a s t h e a c c e p t o r f a i l e d t o show a s i g n i f i c a n t e f f e c t o f d e c r e a s i n g l i g h t i n t e n s i t y ( T a b l e I I ) . Thus a b i p h o t o n i c mechanism d o e s n o t a p p e a r t o be i n v o l v e d i n t h e c h l o r a n i l s y s t e m . As a s i d e comment, i t had been o b s e r v e d i n t h e p h o t o p o l y m e r i z a t i o n o f f u m a r o n i t r i l e and d i v i n y l e t h e r i n methanol t h a t t h e r e a c t i o n w o u l d n o t p r o c e e d a s u s u a l when t h e s e a l e d s a m p l e was magnetically s t i r r e d . T h i s may be s i m i l a r i n c a u s e t o t h e e f f e c t of r o t a t i n g t h e tube above, t h a t i s t h a t t h i s r e a c t i o n a l s o depends on t h e l o c a l c o n c e n t r a t i o n o f e x c i t e d s t a t e s . E f f e c t o f Wavelength. In e a r l y t e s t e x p e r i m e n t s w i t h 0.007 M TCPA a c c e p t o r , i t was o b s e r v e d t h a t a t 366 nm, where a b s o r b a n c e o f t h e s o l u t i o n i s l e s s t h a n 0.10, b u t i n w h i c h a n y c h a r g e - t r a n s f e r a b s o r p t i o n s u p e r i m p o s e d on t h e t a i l o f t h e a c c e p t o r a b s o r p t i o n band w o u l d be a t t h e optimum p e r c e n t a g e o f t h e l i g h t a b s o r b ed by t h e s o l u t i o n , no p h o t o c r o s s I i n k i n g was o b s e r v e d . Also, in q u a r t z t u b e s a t 275 nm a n d 236 nm ( i c e - b a t h c o o l i n g c o u l d n o t be used i n t h e s e c a s e s s i n c e a s u i t a b l e q u a r t z v e s s e l was n o t on h a n d ) , no p h o t o c r o s s I i n k i n g was o b s e r v e d . In t h e l i g h t o f s u b s e quent f i n d i n g s t h a t t h e r e a c t i o n e f f i c i e n c y decreases markedly w i t h decreased l i g h t i n t e n s i t y , these r e s u l t s a r e adequately


34

UV


a c c o u n t e d f o r , s i n c e t h e 275 nm and 236 nm i n t e n s i t i e s a r e much weaker t h a n t h e 334 nm l i n e , w h i l e t h e 366 nm l i n e i s o n l y a b o u t 5 t o 6 t i m e s a s i n t e n s e a s t h e 334 nm l i n e and o n l y a s m a l l f r a c t i o n i s a b s o r b e d . However, some u n e x p l a i n e d w a v e l e n g t h e f f e c t s a r e d e s c r i b e d below. When t h e measurement o f quantum y i e l d s i n t h e c h l o r a n i l s y s tem was t o be begun, i t was d e c i d e d t o u s e t h e 334 nm l i n e r a t h e r t h a n 366 nm s i n c e w i t h t h e h i g h e f f i c i e n c y o b s e r v e d i n t h e t e s t r u n w i t h c h l o r a n i l a t 366 nm, i t a p p e a r e d d e s i r a b l e t o u s e a l e s s i n t e n s e e x c i t a t i o n band i n o r d e r t o have more c o n t r o l l a b l e r e a c t i o n t i m e s , a s w e l l a s t o u s e t h e same w a v e l e n g t h used i n t h e TCPA s y s t e m f o r c o n v e n i e n c e and d i r e c t c o m p a r i s o n . S u r p r i s i n g l y , no p h o t o c r o s s I i n k i n g w a t s o e v e r c o u l d be i n i t i a t e d a t 334 nm i n t h r e e d i f f e r e n t c h l o r a n i l s a m p l e s p r e p a r e d . However, when t h e e x c i t a t i o n w a v e l e n g t h was changed t o 366 nm, t h e same s a m p l e s p r o d u c e d photocross Iinking very r a p i d l y . This observation i s very puzzling and d e s e r v e s f u r t h e r i n v e s t i g a t i o n . Experimental A c c e p t o r compounds (TCPA, c h l o r a n i l , 1,3,5-triηitrobenzene, b e n z o q u i n o n e , p h t h a l l i c a n h y d r i d e , and 1 , 3 , 5 - t r i c y a n o b e n z e n e ) were r e c r y s t a I I i z e d and s u b l i m e d . Acetonîtrîle (MaI I i n c k r o d t r e a g e n t ) was d i s t i l l e d f r o m P2O5 and r e d i s t i l l e d , r e t a i n i n g m i d d l e c u t s . D i c h l o r o m e t h a n e (MaI I i n c k r o d t r e a g e n t ) was d r i e d o v e r c a l c i u m h y d r i d e . X V I I I was p r e p a r e d by Shaw Chu (27) and has a m o l e c u l a r w e i g h t o f 110,000 by membrane osmometry. X I X was p r e p a r e d a c c o r d i n g t o t h e p r o c e d u r e o f J.W. S c h w i e t e r t ( 3 0 ) . The i r r a d i a t i o n s o u r c e i s a 2500 Watt M e r c u r y - X e n o n lamp ( H a n o v i a t y p e 929B-9U) c o n t a i n e d i n t h e S c h o e f f e l LH 152N/2 Lamp Housing ( s u p p l i e d w i t h 2 i " diameter v a r i a b l e focus double q u a r t z c o n d e n s o r , p a r a b o l i c r e f l e c t o r , c o o l i n g f a n , and f i n n e d h e a t s i n k s f o r t h e a r c l a m p ) . The o u t p u t beam i s d e f l e c t e d t h r o u g h a S c h o e f f e l LHA 165/2 S t r a y L i g h t R e d u c i n g & I l l u m i n a t i o n P r e d i s p e r s i o n P r i s m A s s e m b l y i n t o a S c h o e f f e l GM 250 H i g h I n t e n s i t y Monoc h r o m a t o r ( f o c a l l e n g t h 0.25 M., l i n e a r d i s p e r s i o n 3.2 nm/mm., g r a t i n g b l a z e d a t 300 nm w i t h 1180 grooves/mm., a p e r t u r e r a t i o f / 3 . 9 ) . The power s u p p l y f o r t h e lamp i s a S c h o e f f e l LPS 400 e q u i p p e d w i t h t h e S c h o e f f e l LPS 400S s t a r t e r , w h i c h o p e r a t e t h e lamp a t 50 V and 50 A. A l e n s s u p p l i e d by S c h o e f f e l f o c u s e s t h e e x i t beam o f t h e monochromator i n t o a v e r t i c a l n a r r o w l i n e s u c h t h a t o n l y t h e c e n t e r p o r t i o n o f a sample tube i s i r r a d i a t e d . Sample v e s s e l s were e i t h e r P y r e x t u b e s o f 11MM. o . d . (9mm. i . d . ) o r 199 mm. o . d . (16 mm. I.d.) o r q u a r t z t u b e s o f 12 mm. o . d . (10 mm. i . d . ) s e a l e d t o g r a d e d s e a l s . The p r e p a r a t i o n o f s a m p l e s was a s f o l l o w s . An a l i q u o t o f a s o l u t i o n o f t h e p o l y m e r i n d r y acetonîtrîle was e v a p o r a t e d t o a f i l m i n t h e sample t u b e on a vacuum l i n e , h e a t e d t o a b o u t 80° b r i e f l y , and d r i e d f o r o n e h o u r . The t u b e was removed f r o m t h e vacuum l i n e , weighed a c c e p t o r ( a n d any n o n v o l a t i l e a d d i t i v e ) were a d d e d , t h e t u b e reevâcuated f o r o n e


3.

BUTLER AND FERRÉE

Photocwsslinkable Polymers

35

h o u r , and e x c e s s d r y acetonîtrîle t r a n s f e r r e d f r o m P 2 O 5 by c o o l i n g t h e t u b e i n l i q u i d n i t r o g e n on t h e vacuum l i n e a t 10~4 f o r r o r less. The f r o z e n s o l i d i s m e l t e d and e x c e s s s o l v e n t e v a p o r a t e d a t 25° u n t i l t h e marked v o l u m e l e v e l i s r e a c h e d , a n d t h e t u b e sealed o f f w i t h a flame. F o r experiments i n which v o l a t i l e a d d i t i v e s s u c h a s t r i e t h y l a m i n e were u s e d , a measured a l i q u o t o f t h e a d d i t i v e i n d r y acetonîtrîle was d r i e d o v e r c a l c i u m h y d r i d e , d e g a s s e d by f r e e z e - t h a w c y c l e s , and t r a n s f e r r e d i n t o t h e sample t u b e o n t h e vacuum l i n e . S i m i l a r l y , f o rtesting the e f f e c t of H2O, measured a l i q u o t s o f m i x t u r e s o f H2O i n d r y acetonîtrîle were d e g a s s e d and c o n d e n s e d i n t o t h e s a m p l e t u b e s on t h e vacuum l i n e . F o r t e s t i n g t h e e f f e c t o f a i r , t h e s a m p l e was p r e p a r e d a s a b o v e , a i r a d m i t t e d t h r o u g h a d r y i n g t u b e (CaS04) and t h e sample s e a l e d off. The sample t u b e s were u s u a l l y i r r a d i a t e d w h i l e immersed i n an i c e b a t h . The l i g h t beam f o r m s an image 24 mm. by 4 mm. on t h e c e n t e r o f t h e sample s o l u t i o n . F o r experiments i n which polymer formed on t h e i r r a d i a t e d w a l l , t h e sample t u b e was t u r n e d t o a f r e s h s u r f a c e a t c o n s t a n t , f r e q u e n t i n t e r v a l s , u s u a l l y 2.0 m i n . , w h i l e u s e o f i n t e r v a l s o f t w i c e t h i s l o n g were f o u n d t o y i e l d n e a r l y equal c r o s s l i n k i n g e f f i c i e n c i e s . P r e c i p i t a t e was measured by w e i g h t a f t e r f i l t r a t i o n , w a s h i n g w i t h a l a r g e q u a n t i t y o f a c e t o n e , and d r y i n g t w o h o u r s u n d e r vacuum a t 5 5 ° . L i g h t i n t e n s i t i e s were measured by p o t a s s i u m f e r r i o x a l a t e actinometry (31). ( N o t e on f u t u r e e x p e r i m e n t a l : F i l t r a t i o n may be d i f f i c u l t , but i s b e s t performed w i t h c o u r s e s i n t e r e d g l a s s f u n n e l s , which s h o u l d be c l e a n e d by d r a w i n g s o l v e n t b a c k w a r d s t h r o u g h them, n o t using a c i d . Sample t u b e s were f u s e d t o g e t h e r b e f o r e f i n a l c l e a n i n g , s o a k e d o v e r n i g h t w i t h d i c h r o m a t e c l e a n i n g s o l u t i o n , washed w i t h w a t e r , l e t s t a n d s e v e r a l h o u r s c o n t a i n i n g d i l u t e (1-2#) KOH s o l u t i o n , r i n s e d w i t h d i s t i l l e d H 2 O , and d r i e d i n an o v e n . ) Literature

Cited

1. Hartmann, H.; Krauch, CH.; and Marx, M.; Ger. Of fen. (Jul.9, 1970), 1,813,011. Chem. Abst. 74, 78650 (1970). 2. Beightol, L.E.; B r i t . 1,245,937, Sep. 15, 1971. Chem. Abst. 76, 26114 (1972). 3. Agnihotri, R.K.; Falcon, D.L.; Hood, F.P.; Lesoine, L.G.; Needham, C.D.; and Offenbach, J.Α.; Tech. Pap., Reg. Tech. Conf., Soc. Plast. Eng., Mid-Hudson Sect. (1970, Oct. 15-16), 33 4. Kolbe, G.; Schmidt, Α.; Schmitz-Josten, R; and Wolff, E.; Ger. Offen. 2,019,598, Nov. 11, 1971. Chem. Abst. 76, 73294 (1972). 5. Minsk, L.M.; Van Deusen, W.P.; and Robertson, E.M.; U.S. Pat. 2,610,120 (1952). 6. Delzenne, G.A.; J . Mac. S c i . , Rev. Polym. Technol. (1971), 185.

36

7. 8. 9. 10. 11.


12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31.

UV L I G H T INDUCED REACTIONS IN

POLYMERS

Nishikubo, T.; Watauchi, T.; Maki, Κ.; and Takaoka, T.; Nippon Kagaku Kaishi; (1972); 9, 1626. Taditomi, Ν.; Ichijyo, T.; and Takaoka, T.; Nippon Kagaku Kaishi; (1973), 10, 35. Kato, M.; Ichijo, T.; I s h i i , K.; and Hasegawa, M.; J . Polym. Sci.(1971); 9; (A-1); 2109. Lyalikov, K.S.; Gaeva, G.L.; and Evlasheva, N.A.; Tr. Leningrad Inst. Kinoinzh. (1970). No. 16, 42. Oster, G.; Encyclopedia of Polymer S c i . & Tech.; (1969); 10; 145. Tsuda, M.; J . Polym. S c i . ; (1969); 7; 259. Deboer, C.D.; J . Polym. S c i . (1973); 11; (B); 25. Deboer, C.D.; Wadsworth, D.H.; and Perkins, W.C.; J . Amer. Chem. Soc.; (1973); 95, 861. Hyde, P.; Kricka, L.J.; and Ledwith, Α.; J . Polym. S c i . ; (1973); 11 (B); 415. Stuber, F.A.; U l r i c h , H.; Rao, D.V.; and Sayigh, A.A.R; J . Appl. Polym. S c i . (1969(; 13; 2247. Tsunoda, T.; Yameoka, T.; Nagamatsu, G.; and Hirohashi, M.; Tech. Pap., Reg. Tech. Conf., Soc. Ρlast. Eng., Mid-Hudson Sect.; (1970); 25. Takeishi, M. and Okawara, M., J . Polym. S c i . , (1969), 7, (b), 201. Yabe, Α.; Tsuda, M.; Honda, K.; Tanaka, H.; J . Polym. S c i . ; (1972); 10; (A-1); 2379. Ichijo, T.; Nishikubo, T.; and Maki, Κ.; Japan Kokai; (Oct. 7, 1972); 72; 22,490. Chem. Abst. 78, 30778 (1973). David, C.; Demarteau, W.; and Geuskins, G.; Polymer (1969); 10; 21. Kleeburg, W.; Rubner, R.; and Kuehn, E.; Ger. Offen. 130,904; Dec. 28, 1972. Chem. Abst. 78, 91045 (1973). Hrabak, F.; Bezdek, M.; Hynkova, V.; and Bouchal, Κ.; Ger. Offen. 2,150,076; Apr. 13, 1972. Chem. Abst. 77., 49485 (1972). Hay, A.S.; Bolon, D.A.; Leimer, K.R.; and Clark, R.F.; J . Polym. S c i . ; (1970); 8; (B); 97. Nishikubo, T.; Ichiiyo, T.; and Takaoka, T.; J . Applied Polymer S c i . ; (1974); 18; 2009. Hasegawa, M.; Susuki, Y.; Nakanishi, H.; and Nakanishi, F.; "Progress in Polymer Science, Kodansha-Tokyo," Vol. 5; pp. 143-210; John Wiley & Sons; New York; 1973. Chu, Shaw, Ph.D. Dissertation, University of Florida, 1974. "Photochemistry,: Calvert & P i t t s . Sadtler Standard Infrared Spectra No. 4291. Schwietert, J.W.; Ph.D. Dissertation; University of Florida; 1971. Hatchard, Parker, Proc. Roy. Soc. (London) A235, 518 (1956).

4 Synthesis of N e w Photocrosslinkable Polymers Derived from Cinnamic A c i d C . P. PINAZZI and A . F E R N A N D E Z


Laboratoire de Chimie et Physico-Chimie Organique et Macromoléculaire, Equipe de Recherche Associée au C N R S N ° 311, Route de Laval—Le Mans, France

Dimerization of trans-cinnamic acid under u l t r a v i o l e t radiations i s a well known phenomenon since i t has been studied a long time ago by Bertram and Kürsten (1). Presence of cinnamic structures in macromolecules leads, after i r r a d i a t i o n , to crosslinking polymers which have found applications i n modern lithographic techniques and electronic microminiaturization (1). Photodimerization of cinnamic acid only occurs in the s o l i d state and i r r a d i a t i o n of i t s melt or solutions does not give dimer. One control factor in such reaction i s the geometry of the crystal structure of the species (2). Photocrosslinking of polymers carrying cinnamic structures cannot be explained by the only cyclobutanic structures formations. Nakamura and Kikuchi have proposed according to EPR data the intervention of two different kindsof radical intermediates, one of them being produced by hydrogen abstraction from the main chain of the polymer (1). The photocrosslinking of cinnamic polymers is accelerated by introduction of organic compounds. The absence of fluorescence of these sensitizers and their high intersystem crossing efficiencies (S *—T *) show that triplet state is the precursor of the reaction. Introduction of these photocrosslinkable structures in macromolecular chains can be performed by e s t e r i f i c a t i o n of hydroxylated polymers with cinnamoyl chloride. Cellulose {3),condensation products (4,5J and mainly p o l y v i n y l alcohol) have been treated(6) by this method. Other chemical modifications have been studied as ester interchange of poly (vinyl acetate) (_7) and Knoevenagel reaction on polyesters (8). Very few results on the synthesis of such photocrosslinkable polymers by polymerization have been reported* Therefore free radical polymerization of cinnamic acid vinyl derivatives did not lead to the expected polymers, but to insolubil i z a t i o n reactions. Howewer cationic procedure can be a good way in some cases since Kato et a l . could polymerize by this way with high yields p-vinyl phenyl cinnamate (9) and 3-vinyloxyethyl cinnamate (10). In this paper, synthesis of new photosensitive polymers 1

1

37

38


d é r i v a t e d from cinnamic a c i d are d e s c r i b e d and t h e i r i n s o l u t i l i z a t i o n p r o p e r t i e s pointed o u t . In connection w i t h t h i s work p r o t e c t i v e a b i l i t i e s o f new polymeric absorbers d e r i v e d from 2-hydroxy benzophenone and phenylbenzoate are examined.


Synthesis And P o l y m e r i z a t i o n Of P h o t o s e n s i t i v e Polymers Monomers have been prepared i n benzene s o l v e n t by Knoevanagel r e a c t i o n between 4 - v i n y l benzaldehyde and malonic d e r i v a t i v e s a t room temperature, by use o f two c a t a l y s t s chosen according to the nature o f compounds (Table I ) . For c a r b o x y l i c a c i d s as malonic o r c y a n a c e t i c a c i d s p i p e r i d i n e can be used ; f o r e s t e r s as e t h y l m a l o nate o r e t h y l cyanacetate pi p e r i d i n i urn benzoate i s p r e f e r e d . Monomers 1,2 and 4 have been p u r i f i e d by r e c r i s t a l l i z a t i o n . Compound 3 , an o i l , i s heated a t 100°C under reduced pressure to d r i v e o f f the aldehyde and used w i t h o u t f u r t h e r p u r i f i c a t i o n . N o d e c a r b o x y l a t i o n o f the c a r b o x y l i c a c i d d e r i v a t i v e s has been o b s e r ved i n t h i s s t e p . P o l y m e r i z a t i o n s have been c a r r i e d out under n i t r o g e n i n THF o r benzene w i t h a z o b i s i s o b u t y r o n i t r i l e (3% weight) as i n i t i a t o r (Table I I ) . The r e s u l t s are d i f f e r e n t according to the nature o f the cinnamic double bond s u b s t i t u e n t s . Monomers 1 and 2 and c o r responding polymers can indeed l o s e one a c i d f u n c t i o n during p o l y m e r i z a t i o n s c a r r i e d out a t r e f l u x temperature. This decarboxylat i o n i s i n r e l a t i o n w i t h the y i e l d o f gel product and molecular weight. Therefore monomers 3 and 4 f o r which no d e c a r b o x y l a t i o n c o u l d o c c u r , have been polymerized without gel formation w i t h high y i e l d s . Likewise d e c a r b o x y l a t i o n r e a c t i o n seems to induce transfert reaction. I r r a d i a t i o n Of The P h o t o s e n s i t i v e Polymers D i f f e r e n t methods have been used to f o l l o w c r o s s l i n k i n g r e a c t i o n s . When polymers were i r r a d i a t e d by U .V. l i g h t i n d i l u t e THF s o l u t i o n above 200 nm (Method A) or 300 nm (Method B ) , the ^ s o l u b i l i z a t i o n r e a c t i o n was f o l l o w e d by measuring the s o l u t i o n u l t r a v i o l e t absorbance versus time. Likewise the course o f disappearance o f cinnamic s t r u c t u r e was measured on polymers f i l m s placed on quartz a t 9cm o f the lamp and i r r a d i a t e d above 200 nm (Method C ) . Lamp used f o r methods A and C was a PCQ 9 G - l which emits i n the U.V. region a t 253.7 (2.5 W), 3 1 2 . 5 , 365 nm , the o t h e r rays being i n v i s i b l e . A Pyrex f i l t e r was placed beetween 450 W Hanovia lamp and s o l u t i o n when only higher wave lenghts were expected. Lamps powers were c o n t r o l e d before and a f t e r i r r a d i a t i o n w i t h a Black-Ray U l t r a - V i o l e t I n t e n s i t y Meter. N i t r o - 2 f l u o r e n e (Ετ=59 kcal/mole) and M i c h l e r ' s ketone (ET=61 kcal/mole) are e f f i c i e n t s e n s i t i z e r s f o r p o l y ( v i n y l c i n namate) and poly ( v i n y l f u r y l a c r y l a t e ) ( 1 1 ) . For t h i s reason, we have t e s t e d t h e i r e f f i c i e n c y i n the case o f s y n t h e s i z e d polymers. Ravve e t a l . found t h a t xanthene-9 one (E =74 kcal/mole) i n c r e a f

2

C0 Et

4

. C a r a c t e r i c t i c s o f polymersobtained from monomers 1 , 2 , 3 , 4 .

TABLE II

1 C Q

cm"

1720

1725

1680

1680-1710

v

24,300

311

288.5

312

290

λ max (nm)

-

-1 cm'

220

-

2230

C N

18,500

21,400

18,400 (1

-

ε max

v

Y i e l d s were c a l c u l a t e d from i n i t i a l monomer q u a n t i t y . P o l y m e r i z a t i o n c o n d i t i o n s : 100g/l w i t h 3% weight AIBN at s o l v e n t r e f l u x d u r i n g 5 hours, except (a) : 12 hours. Number average molecular weightswere obtained from GPC i n THF w i t h polystyrene r e f e r e n c e s . λ max was found on a DK2A Beckmann Spectrophotometer i n THFFor ε max, c o n c e n t r a t i o n s were taken i n unit-mole/1 ( u n i t / 1 ) . (b) : ε max was c a l c u l a t e d from pure d i a c i d polymer c o n c e n t r a t i o n .

0

2,500

-

70

0

2,000

S=0 -*~^ÎS-0~ and i n c r e a s e s t h e f o r c e c o n s t a n t o f t h e bond. T h i s was r e v e a l e d by t h e s p e c t r a l s h i f t o f t h e SO band i n t h e i n f r a r e d , i n a c c o r d a n c e w i t h the e l e c t r o n - a t t r a c t i n g a b i l i t y of the S-attached s u b s t i t u e n t s ( i n t r a n s i t i o n from n - a l k y l - S O - n - a l k y l t o Ph-SOPh t h e s p e c t r a l s h i f t o f t h e band 1008.0cm"" up t o 1042.0cm"" , was d i s c o v e r e d ) . I t seems t h a t i n t h e c a s e o f a r o m a t i c o r mixed s u l p h o x i d e s , t h e S-0 bond c h a r a c t e r w i l l be t h e most i m p o r t a n t f a c t o r d e t e r m i n i n g t h e s e n s i t i z e r e f f e c t i v e n e s s ; t h e mole f r a c t i o n o f t h e SO grouping i s here of secondary importance. Among t h e i n v e s t i g a t e d compounds, d i p h e n y l s u l p h o x i d e , t h e s t r u c t u r a l analogue o f c l a s s i c a l s e n s i t i z e r - benzophenone - was f o u n d t o be t h e most e f f e c t i v e . According to s e v e r a l authors [ l ] , e f f e c t i v e sen s i t i z e r s seem t o be s u b s t a n c e s t h a t decompose p h o t o c h e m i c a l l y t o f r e e r a d i c a l s . I t has a l r e a d y been found [8] t h a t , d u r i n g t h e p h o t o l y s i s o f benzophenone, t h e r a d i cals C H and ÇgHsÔO a r e p r o d u c e d , t h e l a t t e r decom p o s i n g p a r t l y t o C H and CO. T h i s f i n d i n g means i n f a c t t h a t i n t h e i r r a d i a t e d system, t h e c o n c e n t r a t i o n o f t h e s e n s i t i z e r d e c r e a s e s owing t o i t s g r a d u a l decom p o s i t i o n . Speaking o f benzophenone [9, 35] t h e t r i p l e t s t a t e f o r m a t i o n i s p o s t u l a t e d i n which t h e hydrogen atom i s a b s t r a c t e d from t h e polymer m o l e c u l e g i v i n g a m a c r o r a d i c a l [36], whereas benzophenone i t s e l f gives 1

1

6

5

6

5


5.

HIPPE

Photosensitized Crosslinking of Polymers

55

b e n z o p i n a c o l [37, 38] o r b e n z o h y d r o l [ 1 ] . In c o n t r a s t , o t h e r workers e.g. Schenck [39] expected t h a t the s e n s i t i z i n g agent would t r a n s f e r t h e absorbed energy on t h e polymer m o l e c u l e s i n i t i a t i n g t h e r e a c t i o n o f c r o s s l i n k i n g , without e n t e r i n g i n t o the r e a c t i o n i t s e l f , i . e. i t s m o l e c u l e s would be r e g e n e r a t e d a f t e r t e r m i n a t i o n o f t h e k i n e t i c c h a i n . The i n v e s t i g a t i o n s performed p r o ved, t h a t i n c o n d i t i o n s u s e d , t h e s u l p h o x i d e s a c t as thrue s e n s i t i z e r s f o r c r o s s l i n k i n g : the e x c i t a t i o n energy was t r a n s f e r r e d t o polymer m o l e c u l e s but t h e s e n s i t i z e r s d i d not change p h o t o c h e m i c a l l y t h e m s e l v e s , i . e . t h e y r e g e n e r a t e t h e i r i n i t i a l m o l e c u l e s when t h e k i n e t i c c h a i n was completed. The above p o s t u l a t e d v i e w may be w e l l assumed i n t h e c a s e o f s e n s i t a t i o n o f s o l i d s t a t e r e a c t i o n s s i n c e the p o s s i b i l i t y of recombination o f t h e r a d i c a l s formed o f a s e n s i t i z e r m o l e c u l e has been advanced t o a c c o u n t f o r t h e c a g e - e f f e c t o f F r a n c k - R a b i n o v i t s c h [40] . The p r o p e r t i e s o f t r i d i m e n s i o n a l network formed i n t h e p o l y ( v i n y l b u t y r a l ) on i r r a d i a t i o n were e s t i m a t e d by means o f u l t r a s o n i c and t h e r m a l methods. I t was found, f o r i n s t a n c e , t h a t t h e e l a s t i c modulus changed o v e r from 3 . 3 3 x l 0 dynes cm." to 4.03xl0 dynes cm" f g e i c o n t e n t i n c r e a s e from 0.082 up t o 0.923. Simultaneously, the thermal s t a b i l i t y of the c r o s s l i n ked polymer r i s e d from 167°C up t o 417°C. [42] In c o m p l e t i o n o f above mentioned s t u d i e s , t h e dependence between i n c i d e n t l i g h t i n t e n s i t y , I , and t h e c r o s s l i n k i n g y i e l d , was e s t i m a t e d . I t has been found, t h a t the dependence e x p l o r e d may be r e p r e s e n t e d by t h e e q u a t i o n : 1 0

2

1 0

2

o

r

a

(l)

g e l content

(% by wt.)

=(l.5I

a

+ 45.l) +

I - ^

2

Assuming t h a t t h e y i e l d f o r g e l f r a c t i o n i s p r o p o r t i o n a l t o t h e r a t e o f c r o s s l i n k i n g , i t seemed t h a t t h e o b s e r v a t i o n on p o s s i b i l i t y o f r e s o l v i n g t h e e x p e r i m e n t a l r e s u l t s i n t o terms p r o p o r t i o n a l t o I and l a / , p o i n t s o u t t h e d u a l mechanism o f p h o t o i n d u c e d c r o s s l i n k i n g o f p o l y ( v i n y l b u t y r a l ) . The o v e r a l l c h a r a c t e r o f t h e e q u a t i o n ( l ) s u g g e s t s t h a t t h e main r o l e i n the d i s c u s s e d r e a c t i o n p l a y s the r e l a t i o n : g e l con t e n t <JR Q

L

59

case.

Energy ab sorption a t Gel content i n t e r the i n c i d e n t v a l surf. quanta 20 g 0.00032X>-0.2400

quanta/g, f i l m t h i c k n e s s

- i n the Table I I .

R ^ R* c a s e . L

Energy ab sorption a t Gel content i n t e r the incident v a l surface, R quanta 20 Q

x l Q

280.8 561.6 842.4 1123.2 2246.4 4492.8

0.00l20X>-0.2400 quanta/g, f i l m t h i c

The r e s u l t s p r e s e n t e d proved o u r assumption t h a t t h e use o f S h u l t z ' s e q u a t i o n s , s h o u l d be i n some c i r c u m s t a n c e s d i s s u a d e d . Even S h u l t z h i m s e l f suggested i n f47] t h a t i n some c i r c u m s t a n c e s ( a s i n t h e c r o s s l i n k i n g o f m e t h y l m e t h a c r y l a t e - e t h y l e n e d i m e t h a c r y l a t e system) n e g a t i v e , p h y s i c a l l y u n r e a l X v a l u e s were o b t a i n e d . I n t h i s c a s e , a s p e c i a l f a c t o r ( e . g . - l ) has t o be used

60

UV LIGHT

INDUCED

REACTIONS IN

POLYMERS


f o r making a change o f t h e s i g n . The o r i g i n o f t h e f a c t o r i s not q u i t e c l e a r . Equations ( 4 ) , ( 5 ) and ( δ ) were d e r i v e d under the assumption t h a t t h e a b s o r p t i o n c o e f f i c i e n t k remains c o n s t a n t d u r i n g t h e e x p o s u r e . I t may t h u s be supposed t h a t the d e r i v a t i o n o f r e l a t i o n between X and g e l f r a c t i o n c o n t e n t o f t h e polymer, w i t h t h e assumption that t h e a b s o r p t i o n c o e f f i c i e n t changes d u r i n g i r r a d i a t i o n may y i e l d u s e f u l r e s u l t s . I f t h e l i g h t absorption by polymer f i l m i s p r o p e r l y measured, t h e f o l l o w i n g formu l a may be d e r i v e d : ^

where g i s g e l f r a c t i o n c o n t e n t o f t h e polymer a f t e r energy R^^ has been a b s o r b e d ; R.^ - i n t e g r a l energy a b s o r p t i o n by t h e f i l m , and R. - i n t e g r a l energy a b s o r p t i o n , a t g e l a t i o n p o i n t . R e l a t i o n (7) was t e s t e d f o r p o l y v i n y l butyra]) , and r e c e n t l y , f o r o t h e r p o l y m e r s . R

Polyester Glycol i n

of M a l e i c / P h t a l i c Anhydrides Styrène V

and

Ethylene

E x p e r i m e n t a l . The g e n e r a l c o n d i t i o n s o f experime n t s , and methods o f i n v e s t i g a t i o n s were s i m i l a r to those d e s c r i b e d previously.However, extremely p r e c i s e and r e p r o d u c i b l e r e s u l t s o f c r o s s l i n k i n g were a c h i e v e d owing o r i g i n a l l y d e v e l o p e d d e v i c e f o r exposure ( F i g l) i n which t h e specimens may be r o t a t e d w i t h chosen r a t e . A d d i t i o n a l l y , t h e s e l e c t i o n o f U V - s e n s i t i z e r s was ext e n t e d , and b e s i d e s d i p h e n y l s u l p h o x i d e , a l s o d i n a p h t y l sulphoxide, methyl - benzyl sulphoxide, b i s - p - c h l o r o phenyl sulphoxide, b i s - 2,4-dichlorophenyl sulpho x i d e , were u s e d . A l s o , some o t h e r compounds were e x a mined, namely: d i p h e n y l s u l p h i d e and i t s p - n i t r o - p amino- as w e l l ο,ο-dicarboxy-derivatives, methiomethyl e n e - d i e t h y l phosphate and u r a n y l a c e t a t e .

T h i s p a r t o f work was done w i t h c o l l a b o r a t i o n Mrs. B. Guzowska and Mr. J . L i p i n s k i

of


5.

HIPPE


61

Figure 1. Device for controlled irradiation of polymerfilms—rotatingplate with specimens (a), source (b), ventilator (c), and electronics for controlling of light intensity, specimens temperature, and rate of rotations, (d)

R e s u l t s and d i s c u s s i o n . The most e f f e c t i v e sub s t a n c e was found t o be (among t h e S-compounds) d i p h e n y l s u l p h i d e . On t h e o t h e r s i d e , p-nitro-p-amino-diphenyl s u l p h i d e .showed n e a r l y complete l a c k o f a c t i v i t y . I n the group o f s u b s t i t u t e d s u l p h o x i d e s and s u l p h i d e s , may be n o t i c e d t h e dependence o f s e n s i t i z i n g e f f i c i e n c y on the e l e c t r o m e r i c e f f e c t s i n the molecule. A t present s t a t u s o f t h e work i t i s i m p o s i b l e t o p r e d i c t whether t h i s r e s u l t has as i t s own b a s i s an I - o r M - e f f e c t . However, d e c r e a s e o f s e n s i t i z i n g a b i l i t y o f d i s c u s s e d compounds may a l s o be c o n n e c t e d w i t h d i s t u r b a n c e of coplanarity of photosensitizers structures. High a c t i v i t y o f u r a n y l a c e t a t e as c r o s s l i n k i n g s e n s i t i z e r s h o u l d be n o t e d . P o s s i b l y , t h i s may be ex p l a i n e d from d i f f e r e n t p o i n t s o f v i e w : u r a n y l a c e t a t e b e l o n g s t o t h e compounds t h a t y i e l d e a s i l y t o p h o t o c e m i c a l e x c i t a t i o n , and a l s o u r a n y l a c e t a t e as r a d i o a c t i v e compound may produce i t s e l f f r e e r a d i c a l s i n neighboring polyester molecules. The d e t a i l e d i n v e s t i g a t i o n s a l o n g t h e s e l i n e s a r e now i n t h e c o u r s e .

62

UV


IN

POLYMERS


L i t e r a t u r e Cited 1. Oster, G., Oster, G.K., Moroson, H., J.Polym.Sci., (1959), 34, 671 2. Oster, G.K., Oster, G., Int.Symposium on Macromolec. Chemistry, Moscow, 1960, section 3, 289-292 3. Oster, G., Polymer L e t t e r s , (1964), 2, 1181 4. Luck, W., Hrubesch, Α., Suter, H., German Pat. 1,046,883 (1958) 5. W i l s k i , H., Angew.Chem., (1959), 71, 612 6. W i l s k i , H., German Pat. 1,062,007 (1959) 7. Soumelis, K., W i l s k i , H., Kunststoffe,(1961), 52,471 8. Pao-Kung Chien, Ping-Cheng Chiang, En-Chien Hou, Κ ο Hsueh Τ ung Pao 1958 , 663 (chem.Abstr.(1959), 53, 120151) 9. Pao-Kung Chien, Ping-Cheng Chiang, Yu-Chen Liao, Ying-Chin Liang, Hsia-Yu Wang, Chni-Chang Fang, Sci. S i n i c a , (1962), 11, 903 (Chem.Abstr. (1965), 61,10539e) 10.- i b i d . , (1962), 11, 1513 (Chem. Abstr. (1963), 58, 10320e) 11.Tsunada, Yamaoka, J.Appl.Polymer S c i . , (1964), 9,1763 12.Ducalf, B., Dunn, A.S., i b i d . , (1964), 8, 1763 13.French Pat. 1,091,323 (1955) 14.French Pat. 1,229,883 (1960) 15.Stuber, F.A., U l r i c h , H., Rao, D.V., Sayigh, A.A.R., J.Appl.Polym.Sci., (1969), 13, 2247 16.US Pat. 2,816,091 (1957) 17.UK Pat. 820,173 (1959) 18.UK Pat. 825,948 (1959) 19.Hutchison, J . , Redwith, Α., Advances i n Polymer S c i . V o l . 14, p. 20.German Pat. 2,102,366 (1971) 21.German Pat. 2,232,365 (1971) 22.German Pat. 2,337,813 (1971) 23.Andruntchenkho, A.A., Bogdan, L.S., Khatchan, A.A., Shrubovitsch, W.A., Vysokho Molekh.Soiedin. (1972), 14, 594 24.US Pat. 2,719, 141 (1954) 25.Marimoto, K., Hayashi, I., Inami, Α., Bull.Chem.Soc. Japan (1963), 36, 1651 26.Tsubahiyama, K., F u j i s a k i , S., J.Polym.Sci., (1972), B, 10, 341 27.Gaylord, N.G., D i x i t , S.S., Patnaik, B.K., i b i d . , (1971), Β 9, 927 28.Ogawa, T., Taninaka, T., i b i d . , (1972), A-1, 10, 2056 29.Kaeriyoma, K., Shimura, Y., i b i d . , (1972), A-1, 10, 2833 30. Dack, M.R., J.Chem.Educ., (1973), 50, 109


5.

HIPPE


63

31.Hatchard, C.G., Parker, C.A., Proc.Roy.Soc. (1956), A235, 518 32.Strecker, W., Spitaler, R., Chem.Rev. (1926),59, 754 33. Vogel, A.J., J.Chem.Soc.(London), (1948), 1820, 1833 34. Gillis et a l l , R.S., J.Am.Chem.Soc. (1958), 80, 2999 35. Charlesby, Α., Grace, C.S., Pilkington, F.B., Proc. Roy.Soc., (1962), A268, 205 36.Yang-Hsu Liang, Hsia-Yu Wang, Chni-Chang Fang, PingCheng Hsiang, Pao-Kung Chien, Sci.Sinica (1962), 11 903 (Chem.Abstr. (1965) , 62, 10589a) 37.Trudelle, I., Neel, I., Compt.Rend.Hebd.Seances Acad.Sci. (1965), 260, 1950 38.Charlesby, Α., Rad.Chemistry Proc.Tihany Symp., Tihany, 1962 39.Schenck, G.O., Ind.Eng.Chem. (1963), 55, 40 40.Franck, I., Rabinovitsch, E., Trans.Faraday Soc., (1934), 30, 120 41.Hippe, Ζ., Bull.Acad.Polon.Sci.,Ser.Sci.Chim.,(1967) 15, 261 42.Hippe, Z., The P.R.I.Research Project No 43.Stevens, B., Chemical Kinetics, Chapman and Hall, London, 1961, p. 78 44.Shultz, A.R., J.Chem.Phys., (1958), 29, 200 45.Hippe, Z., Dębska, Β., Bull.Acad.Polon.Sci.,Ser.Sci. Chim., (1972), 20, 1001 46. Hippe, Z., Dębska, B., ibid., (1972), 20, 1075 47.Shultz, A.R., J.Am.Chem.Soc. (1958), 80, 1834

6 Polymerization and Crosslinking by the BenzeneBismaleimide Photoaddition Reaction MALCOLM P. STEVENS


Chemistry Department, University of Hartford, West Hartford, Conn.

06117

Maleimide and Ν-substituted maleimides undergo photoaddi tion to benzene and alkylbenzenes to form 2:1 adducts (1).1-3 Intermediate in the reaction i s homoannular diene (formed by initial 2 + 2 cycloaddition) which subsequently undergoes 2 + 4 (Diels-Alder) cycloaddition to give diimide (Reaction 1). Maleic anhydride reacts similarly4,5 to give a dianhydride (2) having the analogous pentacyclic structure; however, it has Reaction 1

(R-H, alkyl)

(R'-H, alkyl, aryl)

1

6.

STEVENS

Benzene-Bismaleimide Photoaddition Reaction

65


been shown6,7 quite conclusively that the mechanism in this case involves an excited charge-transfer intermediate rather than homoannular diene.

2 (R=H,

alkyl)

In the case of photoaddition to alkylbenzenes, the sub stituent R group has been shown, on the basis of ultraviolet8 and NMR9 spectroscopy, to be located principally on the double bond of the adduct (as shown), but the actual direction of cycloaddition has also been shown to vary with temperature.10 Thus i t is quite probable that isomer mixtures result in the photopolymerization reactions described below. It has also been reported that N-phenylmaleimide and meta- and para-substi tuted N-phenylmaleimides give very poor yields of photoadduct.6 This has been attributed to the existence of low-lying, excited states that quench the photoexcited maleimide when the benzene and maleimide rings are coplanar. With N-alkylmaleimides and N-phenylmaleimides having ortho substituents (which prevent co planarity), photoaddition proceeds readily.1 More recent work11 has shown that N-phenylmaleimides substituted in the para posi tion with strongly electron-attracting groups (NO , CO CH3, SO C H ) also form photoadducts with benzene in good yield. A logical extension of maleimide photoaddition i s the light -catalyzed cyclopolymerization reaction of bismaleimides with benzene or alkylbenzenes to form polyimides. This reaction, and the application of bismaleimide photoaddition to polystyrene crosslinking, are discussed in the following sections. 2

2

6

2

5

Polyimide Formation Under the influence of light, benzene and alkylbenzenes copolymerize with bismaleimides by successive 2 + 2 and 2+4 cycloaddition reactions12"l^ (Reaction 2). Although only the head-to-tail repeating unit is shown in Reaction 2, i t should be recognized that head-to-head and t a i l - t o - t a i l units can

U V LIGHT INDUCED REACTIONS

66

IN

POLYMERS

also form depending upon the sequence of cycloaddition reactions during propagation. Following i s a review of polyimide syntheses performed in our l a b o r a t o r i e s . ! Similar results have been reported by Zhubanov and Akkulova.lâ 12

13

Reaction 2

h-9 0 Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch006

(R=H, alkyl)

0

f

(R «alkyl,aryl)

N-R».

4» head-to-head and t a i l - t o - t a i l structures

The photopolymerization reactions may be carried out in ordinary borosilicate glassware, using acetone to effect solubility of the bismaleimides and acetophenone as photosensitizer. A water-cooled Hanovia 450-W ultraviolet lamp was used in this work, although ordinary sunlight i s also effective . Polymers precipitated as pale yellow or white powders or were deposited as b r i t t l e films on the walls of the irradiation tubes. They were insoluble in the usual solvents, including N,N-dimethylformamide, Ν,Ν-dimethylacetamide, trifluoroacetic acid, and hexamethylphosphoramide, probably because of crosslinking during polymerization. This could occur via copolymerization of maleimide groups with the double bond arising from cycloaddition. Polymer structure was established from the following observations: 1. Infrared spectra closely resembled those of the corre sponding polyimides prepared from dianhydride, 2, and the corre sponding diamines, although absorptions arising from unreacted amine and anhydride were present in the latter. (A comparison i s shown in Figure 1, along with the spectrum of a model compound.) 2. Elemental analyses of representative polymers were con sistent with structure.


67


6. STEVENS

3

4

5

6

7

8

9 MICRONS

10

11

12

13

14

15

Journal of Polymer Science

Figure 1. Infrared spectra (KBr) of (A) polyimide prepared by photolysis of benzene solutions of Ν,Ν'-hexamethylenebismaleimide; (B) polyimide prepared from benzenemaleic anhydride photoadduct (2) and lfi-hexanediamine; and (C) the N,N'-fris(nhexyl) derivative of diimide 1


68

3. U l t r a v i o l e t spectra of a c e t o n i t r i l e e x t r a c t s of polymers e x h i b i t e d maxima c o n s i s t e n t with both maleimide and homoannular diene end groups* 4. The NMR spectrum of d i l u t e t r i f l u o r o a c e t i c a c i d e x t r a c t s of one polymer was c o n s i s t e n t with s t r u c t u r e . ( S o l u b i l i t y was too low f o r the o t h e r s . ) 5. Polymers underwent thermal d e c y c l i z a t i o n at 420 to 4 8 5 ° C r e g a r d l e s s o f bismaleimide s t r u c t u r e ; and DTA thermograms o f polyimides prepared photochemically c l o s e l y resembled those of the model p o l y i m i d e s . TABLE 1


Polyimides Prepared with Bismaleimides and B e n z e n e u

R

a

1

of Bismaleimide

Approx. Y i e l d , %a

I n i t i a l dec. Temp.°C b

95

420

80

420

70

410

65

485

80

440

15

420

30

420

85

450

A f t e r i r r a d i a t i o n f o r 18 h r .

Âpproximate temperature observed i n m e l t i n g p o i n t

capillaries.


6.

STEVENS


69

Table 1 shows approximate yields and i n i t i a l decomposition temperatures of polyimides prepared from a variety of aliphatic and aromatic bismaleimides. DTA thermograms, which confirmed the i n i t i a l decomposition temperatures, also exhibited strong endothermic transitions in the range 480-510°C corresponding to more extensive decomposition. Polymer yields from aliphatic bismaleimides decreased with chain length, possibly because of competing intramolecular c y c l i z a t i o n . i l Aromatic bismaleimides having ortho methyl groups gave higher yields than the unsubstituted ones with the exception of 4,4'-bismaleimidodiphenylsulfone which contains a strongly electron-attracting group.11 Percentage yields and decomposition temperatures for a series of polyimides prepared from three bismaleimides and four alkylbenzenes are given in Table 2, together with the same data for the corresponding polyimides prepared from benzene. Best yields from alkylbenzenes were obtained from toluene and t-butylbenzene with a l l three bismaleimides. This probably reflects a balance between inductive effects, which would favor both 2 4-2 and 2 + 4 cycloadditions in the order t-butyl> i-propyl> ethyl > methyl, and steric effects which would hinder the reaction in the same order. Additional steric effects arising from the ortho methyl groups of Ν,Ν'-4,4'-(3,3 dimethyl biphenyl)bismaleimide are evident since no polymer was formed from this monomer with ethylbenzene or i-propylbenzene (Zhubanov and Akkulova reported!^ their best yields were obtained with 1,10-decamethylenebismaleimide and i-propylbenzene.) Other workerslè have reported the homopolymerization of certain substituted bismaleimides in solution by successive 2 + 2 cycloaddition reactions, and they have appropriately de fined this type of process as a true photopolymerization; i . e . a polymerization in which every chain-propagating step involves a photochemical reaction. Another such example i s the solidphase 2 + 2 cyclopolymerization of divinyl monomers.1? By contrast, the polymers described above result from a combina tion of photocycloadditions and Diels-Alder cycloadditions. 1

Polystyrene Crosslinking The extension of bismaleimide photoaddition to polystyrene crosslinking represents a practical application of photopoly merization with potential in the area of light curable coatings. I n i t i a l crosslinking experiments have been concerned with the effect of both aliphatic and aromatic bismaleimides on gelation times of polystyrene solutions. A commercial atactic poly styrene with number average molecular weight of 270,000 was used for this work. Both dimethylene- and hexamethylenebismaleimide were found effective in gelling methylene chloride solutions of polystyrene in sunlight at bismaleimide concentrations as low as

UV L I G H T INDUCED REACTIONS

70

IN

POLYMERS

TABLE 2 Polyimides Prepared with Bismaleimides and Alkylbenzenesl3Ethyl- ^-Propyl- t-ButylBenzene Toluene benzene benzene benzene 35

*' -(CH ) ~ Irradiation time, hr. Approx. yield, % Dec. temp. C* 2

6

18 70 410

24 40 380

24 23 390

24 28 390

24 34 425

Irradiation time, hr. Approx. yield, % Dec. temp. ° C

18 65 485

40 45 390

40 0 -

40 0 -

40 44 370

Irradiation time, hr. Approx. yield, % Dec. temp. C

18 85 450

5 89 400

5 50 360

5 60 420

5 85 400


0

R»*

a

a

approximate temperature observed i n melting point c a p i l l a r i e s .

1 mole percent based on phenyl equivalents i n the polymer. Re sults are shown in Table 3. Surprisingly, gelation time did not decrease with increasing bismaleimide concentration. In the case of hexamethylenebismaleimide, several test runs confirmed the shortest gelation time for the unsensitized solution containing 1.0 mole percent bismaleimide. Somewhat different results were obtained using an a r t i f i c i a l ultraviolet source (G.E. 375 watt sunlamp), as shown in Table 4. The data i n this table are for a distance of 60 cm between lamp and sample. Much faster gelation times were observed at shorter distances (2 hr. for 57· bismaleimide at 15 cm.). Aromatic bismaleimides were much less effective (Table 3), partly because of low solubility i n the polystyrene solutions and partly because the rates of photoaddition were much lower than those of the aliphatic bismaleimides. Not unexpectedly (in view of the results shown i n Table 2), the bismaleimide prepared from o,o -dimethylbenzidine (o-tolidine) was least effective. Only in the case of 4,4*-bismaleimidodiphenylsulfone did photosensitizer (benzophenone) noticeably shorten gelation 1

6. STEVENS

71


TABLE 3 Polystyrene Crosslinking

f

R of Bismaleimide


2

6

-ττ 255 nm absorption i s of primary importance. The extinction coefficient of the ΠΓΗΓ band i s too small to be very active and the strong absorption below 210 nm does not overlap a significant portion of our lamp's emission. Baseline Determinations and Relative Extinction Coefficients. For the 255 nm absorption baseline determination, we define polyethylene as transparent


80

UV L I G H T

INDUCED

REACTIONS I N P O L Y M E R S

at 450 nm and subtract the absorption at that wave length from a l l other values. Next, we obtained a measure of the surface and/or interior scattering of the films and possible base polymer absorption. The spectra of several Alathon® 15 polyethylene films of different thicknesses were measured and, for wave lengths greater than 220 nm, the Α (λ)-A(450 nm) values were identical for a l l samples. This indicated that the polyethylene "absorption spectrum" was primarily due to surface scattering and could be used as the baseline for the 255 nm absorption maximum. We examined the spectra of 5 blend samples to investigate the p o s s i b i l i t y of peak overlap effects. After subtracting baselines, the TT-MT* absorptions ranged from 0.3 to 1.0; and i n each case, the ab sorptions were determined at a series of wavelengths between 220 and 3βθ nm. Relative extinction c o e f f i cients were obtained for each sample at the various wavelengths by dividing the individual absorptions by the maximum absorption (257.5 nm). These values were then averaged and plotted versus the reciprocal wave length. The resulting symmetrical peak i s shown i n Figure 2.

7.

IKEDA E T A L .

Photocrosslinking of Polyethylene

81

We conclude that our baseline determination permits a reasonable determination of the sensitizer TT+TT ab sorption characteristics. With linear polyethylenes, the scattering correc tion was thickness dependent. While extensive compu tations were not carried out with these samples, the thickness-adjusted scattering corrections appeared to be as satisfactory i n the baseline determination as was found for Alathon® 15.


Extinction Coefficients. 255 nm absorption. Table II summarizes our measurements of the apparent extinction coefficient for this band. There i s considerable scatter i n the data but error analysis of the individual experiments showed that most of the variations could not be attributed to random measurement errors. The extinc tion coefficient (base e) for the model compound, p-benzoyl phenyl isobutyrate was measured as 5A7 x lCr* l-mole^-cm" - a significantly greater value than most of the numbers i n Table I I . 1

TABLE g

255 nm EXTINCTION COEFFICIENTS BASE 371-31-1A 371-3I-2A 371-31-βΑ -9B 371-31-βΑ 371-31-βΑ -88 371-31-9A -9B 371-31-IGA 371-31-11A 371-31-12A 37M48-2A -2B 371-148-3A -3B 371-14β-4Α -48 371-148-βΑ -5Β 297-21-Α 373-105-5 367-69

ΑΙ5 Α15 Α15 ΑΙ5 Α15 Α7040 Α7040 SUPERC SUPERC AÏS A15 A15 A15 A15 A15 A15 AI5 A15 AIS A15 A15 A15 A7030

Co(Wt%) (IR)

(255 M )

*mt

UlMlt)

0.493 0.493 0.336 0.336 0.420 0.343 0.343 0.269 0.269 0.495 0.214 0.502 0.102 0.106 0.119 0.142 0.172 0.176 0.188 0.220 0.56

1.220 1.143 0.700 1.745 0.734 0.439 1.97 0.417 1.143 0.900 0.516 1.549

1.55 1.β4 1.33 3.20 1.12 0.92 3.55 1.23 3.03 1.37 1.55 1.75

0.5

—-

-

— — —

-

0.269 0.268 0.372 0.313 0.304 0.403

«(•)(l/ftoli-c»)X Κ Γ

4

4.06 3.20 3.96 4.11 3.96 3.53 4.11 3.20 3.48 3.35 3.96 4.47 4.36 4.47 5.74 4.77 5.47 4.54 4.11 4.64 2.89 2.89 2.79

340 nm absorption. The data available for this absorption are summarized i n Table I I I . The corrected absorptions are small and the variations of the computed extinction coefficients i n Table III can be attributed to random errors.


82

TABLE III 340 nm EXTINCTION COEFFICIENTS SAMPLE

«le)U/mole-cm)xiO-

371-31-1A -2A-1 -2A-2 -2B -2C

77 5.1 4.8 5.8 5.6

AV6


2

(5.811.3)

Gel Measurements. An example of the gel fraction data is given i n Figure 3 with gel fraction, G, plotted against the fraction of sensitizer reacted, x. The solid line i s given by: 0,

(I ~ Umax ) ( | -

X c

)

-, ( X - X ^

The terms are defined i n the figure. We can only specify Xc i n the 0.1 to 0.2 wt % ABP range. Gmax i s modest (50$ - 70$ range) with the blends and there i s a definite correlation with the total ABP concentration although the scatter i n the correlation i s excessive relative to the experimental precision, Figure 4.

0.8.

0 8,

Ο 0.4k

I

ι

0 (X)

0.5 FRACTION REACTED

1 1.0

0

0.5 WT % ABP

10

Figure 3. Gel fraction and the fraction Figure 4. Maximum gel fraction and the init. sensitizer concentration reacted


7.

i K E D A ET AL.


83

Using well-known relationships for swollen gels (3)3 we computed "apparent crosslink densities" from the gel swell ratios and the solvent-polymer interaction parameter determined from the temperature dependence of those ratios (4)· With lower ABP con centrations, the gels produced from the blends were fragile, leading to appreciable experimental uncer tainties. With the higher ABP concentrations, more meaningful information was obtained. In comparison experiments, we used two samples, each with 1.2 wt # ABP - one an E/ABP co polymer and the other an E//E/ABP blend. Due to the sample thickness (2 mils) and the high ABP concen tration, nearly 100$ of the incident radiation was absorbed by the samples i n the i n i t i a l part of the reaction. The samples were irradiated for various periods and the gel fractions and crosslink densities determined. These data are tabulated i n Table IV and plotted i n Figure 5. TABIiE IV COPOLYMERS & BLENDS GEL FRACTIONS & CROSSLINK DENSITIES EXPOSURE IMIN) 1 2 5 0

2

COPOLYMER 40 1.62 49 3.58 60 9.26 85 21

CR0SSLINKS/5000

BLEND 21 0.70 31 1.13 35 1.61 65 6

C-AT0MS

84


POLYMERS

100

*50


0

Figure 5. Gel fractions and crosslink densities for copolymers and blends

2 4 6 EXPOSURE IM IN)

EXPOSURE

CO

(MIN)

Two factors were worthy of note. F i r s t , as we ex pected from the total absorption of the incident radiation, i n i t i a l l y there i s a linear increase i n the "apparent crosslink density" with time. Secondly, the "apparent crosslink density" efficiency of the sensitizer moiety i s greater with the copolymer sample and much more of the total polymer (i.e., 90%) can be involved i n the crosslinked portion. Discussion The gel swelling results can be explained by assuming that the E//E/ABP blends are two-phase materials with domains of E/ABP dispersed i n a poly ethylene matrix. In this picture, a significant fraction of the matrix would have no contact with the crosslinking copolymer and would remain soluble after photolysis, leading to low maximum gel fractions. The gel from such blends should have a "tight" core (10% - 20% of the gel) surrounded by a more open


7.

IKEDA E T A L .


85

structure which would have few crosslinks. The over a l l appearance would be that of a fragile, l i g h t l y crosslinked gel. The Gtnax correlation can be sharpened by the twophase argument. With the crosslinking agents con fined to their own phase, the quantity of gel should be determined by the total surface area of that phase and, hence, the dispersion of the E/ABP i n the poly ethylene matrix. If we assume that better blending i s achieved by matching the component melt viscosities, then Grjjax should be dependent on the sensitizer melt index (MI), the relative volumes of the components blend ratio, BR - and the total sensitizer concentra tion (Co). Prom least squares analyses of our re sults, we find: Gfcax « 0.174 + 0.0296 log MI + 0.475 Co + 0.0149 BR The f i t of this expression to the experimental data i s excellent as indicated by the last two columns i n Table V and underscores the usefulness of the twophase proposal. TABLE V MAXIMUM GEL FRACTION CORRELATIONS Να 371-31- 1 -31- 2 -31- 3 -31- 5 -31- 6

log MI

Co (WT %)

0.857 -0.143 2.060 2958 1.710

0.493 0.493 0.883 0.336 0.420

BLEND RATIO 13 13 4.63 6 13.65

GEL (MAX) EXP 0.638 0.593 0.724 0.512 0.626

GEL (MAX) CALC 0.628 0.598 0.724 0.511 0.628

The two-phase model can also explain the low values of our measured 255 nm extinction coefficients and their v a r i a b i l i t y . Consider a clear polyethylene matrix with strongly absorbing dispersed domains of E/ABP. I f the dispersion i s gross (i.e., large particles), then some photons w i l l pass through the film and see no ABP. Other photons w i l l pass through one domain and be mostly absorbed while s t i l l others w i l l pass through more than one domain. For the latter, passage through the second, or third, domain



86

w i l l remove very l i t t l e of the incident radiation since most of the beam was absorbed passing through the f i r s t domain. Because of this strong absorption by the domains, when the total absorption i s averaged over the entire f i l m surface, the effect w i l l be a low apparent absorption. This notion can be quantified i n a model. Assume that the E/ABP phases are cubes with sides, d/Z. Here, d i s the film thickness and Ζ i s some large integer. Divide the f i l m volume into a matrix of such cubes and randomly disperse the sensitizer i n this matrix. The light transmitted by such a sample is:

1

=

2

Ρί")

where P(n) i s the probability of finding a column of Ζ cubes containing η E/ABP cubes and I(n) i s the light transmitted by such a column and i s given by the expression:

le») -' I,(exp)R£ ) û

Here I i s the incident radiation and A i s the true absorbance (base e) for a molecularly dispersed sysbem. The probability, P(n) i s the same as that for flipping a coin Ζ times and coming up with η heads. Thus: 0

t

where V i s the volume fraction of E/ABP. Sub stituting and rearranging the terms, we obtain:

e * (,-v)%^|o-vrJ|[i^v]]e A

s

ζ

7.

i K E D A ET AL.


87

This expression i s useful since i t contains measur able true and apparent absorbances (At and A ) ; measurable volume fractions, V; and only one unknown, the size parameter, Z. Some typical computational re sults are shown i n Figure 6 where Aa/At values are plotted versus the size parameter, Z. Note that A approaches At for very large Z's, i . e . , molecular dispersion. a


a

0.5

/

-

A

t

ο

1.85

3.70 •Γθ.85 L3.70

V 0.0715 0.0715 0.02 0.04

•

100

SIZE PARAMETER ( Z)

Figure 6.

Apparent absorption and the size parameter

For At, we can use the extinction coefficient measured for the model compound, p-benzoyl phenyl isobutyrate (5A7 x 10* l-mole^-cm" ) and then use the experimental A s to compute the E/ABP phase sizes, (d/Z). These are shown i n Table VI. 1

f

a


88

TABLE VI COMPUTED PARTICLE SIZES SAMPLE

, «O/moH-cirtxlOr

BLEND RATIO 13 13 10.7 6 13.65 9 9 22 22 8.58

4.06 3.20 3.70 4.04 3.96 3.53 332 3.35 3.96 4.47

MA -2A -4A -5A(B) -6A


4

-8A(B) -9A(B) -10A -11A -12A

PHASE SIZE pu»)

llog MI*| | 4 I

0.49 0.91 0.80 1.40 0.58 1.26 2.19 0.40 0.60 0.43

0.255 0.745 0.976 2.355 1.109 2.325 2.325 0.559 0.301 0.301

*MELT INDEX OF THE POLYETHYLENE » 4

The calculated phase sizes have reasonable dimensions. As a check on the calculations, we again assume that the blending w i l l be better (i.e., particles smaller) i f the polyethylene and the E/ABP have the same v i s cosity. We have chosen to use the melt index (MI) function: ι

log

MIe/ASP

MIe

as a measure of the similarity, or dissimilarity of the viscosities and have plotted this parameter versus the computed-phase sizes i n Figure 7. There i s a positive correlation which supports this hypothesis.

|LOG^|

Figure 7.

Particle size and the melt in dex mismatch

7.

IKEDA E T A L .


89

Summary


The crosslinking behavior of blends of polyethy lene and E/ABP photosensitizers can be explained on the basis of a two-phase mixture. The particle size of the disperse phase (E/ABP) has a significant bear ing on the degree of crosslinking, approaching theoretical as particle size decreases. Improved dis persion can be attained by matching the melt viscosi ties of the components. The behavior of such similar materials underscores the d i f f i c u l t y i n getting molecular compatibility with polymers. Literature Cited 1.

M. J . Roedel, U.S. Patent 2,484,529, 10/11/49

2.

S. Tocker, U.S. Patents, 3,214,492 (1965), 3,265,772 (1966), and 3,315,013 (1967)

3. R. F. Boyer, J . Chem. Phys., 13 363 (1945) 4.

M. L. Huggins, Annals, N.Y. Acad. Sci., 44 431 (1943)

8 Calorimetric Analysis of Photopolymerizations J. E. MOORE, S. H. SCHROETER, A. R. SHULTZ, and L. D. STANG


General Electric Corporate Research and Development, Schenectady, Ν. Y. 12345

The p r e s e n t h i g h l e v e l o f i n d u s t r i a l a c t i v i t y i n t h e f i e l d o f l i g h t i n i t i a t e d p o l y m e r i z a t i o n s has s t i m u l a t e d r e s e a r c h i n b a s i c p o l y m e r i z a t i o n phenomena i n c l u d i n g p o l y m e r i z a t i o n k i n e t i c s . ( 1 - 4 ) U n f o r t u n a t e l y , p r e v i o u s l y used methods o f d e t e r m i n i n g monomer c o n v e r s i o n s u c h a s d i l a t o m e t r y o r measurements o f u n r e a c t e d monomers are n o t e a s i l y adapted t o t h i n c o a t i n g f i l m s . In a d d i t i o n , t h e p r e s e n c e o f m u l t i f u n c t i o n a l monomers y i e l d i n g n e t w o r k s a t low c o n version i n photopolymerizable formulations a l s o complicates analyses. D i f f e r e n t i a l s c a n n i n g c a l o r i m e t r y (DSC) h a s been used t o i n v e s t i g a t e a d d i t i o n and c o n d e n s a t i o n p o l y m e r i z a t i o n s i n b u l k , s o l u t i o n , and e m u l s i o n s y s t e m s . ( 4 - 8 ) B o t h i s o t h e r m a l and t e m p e r a t u r e s c a n n i n g modes have been e m p l o y e d . In t h e p r e s e n t p r e l i m i n a r y work we d e m o n s t r a t e t h e u t i l i t y o f DSC i n s t u d y i n g t h e k i n e t i c s o f light initiated free radical polymerizations. Differential calor i m e t r y was c h o s e n b e c a u s e i t was a n t i c i p a t e d t h a t d i r e c t , c o n t i n uous m o n i t o r i n g o f t h e r e a c t i o n e x o t h e r m w o u l d g i v e t h e most i n f o r m a t i o n on r a p i d a c r y l a t e p o l y m e r i z a t i o n s . Lauryl a c r y l a t e pro v i d e d a monomer o f low v o l a t i l i t y upon w h i c h r e c e n t c o n v e n t i o n a l k i n e t i c s t u d i e s (9,10) e x i s t e d . 1,6-Hexanediol d i a e r y I a t e , neop e n t y l g l y c o l d i a c r y l a t e and t r i m e t h y l o l p r o p a n e t r i a c r y l a t e were chosen t o determine t h e c a p a b i l i t y o f t h e d i f f e r e n t i a l c a l o r i m e t r i c approach as a p p l i e d t o network-forming photopolymerizations. Experimental Materials. L a u r y l a c r y l a t e was f r e e d o f i n h i b i t o r by t h r e e s e p a r a t o r y f u n n e l e x t r a c t i o n s w i t h 5% s o d i u m c a r b o n a t e s o l u t i o n , t h r e e d e i o n i z e d w a t e r w a s h i n g s , f o l l o w e d by t h r e e c r y s t a l l i z a t i o n s f r o m m e t h a n o l . R e s i d u a l m e t h a n o l and w a t e r were removed by 24 h o u r s p a r g i n g w i t h n i t r o g e n a t room t e m p e r a t u r e . B o t h o r i g i n a l ( i n h i b i t e d ) a n d p u r i f i e d l a u r y l a c r y l a t e (LA) were p o l y m e r i z e d . I , 6 - H e x a n e d i o l d i a c r y l a t e (HDDA), n e o p e n t y l g l y c o l d i a c r y l a t e 90

8.

MOORE ET A L .

Calorimetric Analysis of Photopolymerizations

91


(NPGDA), t r i m e t h y l o l p r o p a n e trîacrylate (TMPTA), T r i g o n a l 14 (Noury Chem. C o r p . ; m i x t u r e o f b e n z o i n b u t y l e t h e r s ) , b e n z o y l p e r o x i d e , and t - b u t y l p e r b e n z o a t e were used a s o b t a i n e d f r o m commer c i a l s u p p l i e r s without f u r t h e r p u r i f i c a t i o n . Apparatus. A P e r k i n - E l m e r DSC-IB d i f f e r e n t i a l s c a n n i n g c a l o r i m e t e r was m o d i f i e d by r e p l a c i n g t h e g l a s s windows o f t h e s a m p l e h o l d e r c o v e r w i t h q u a r t z windows. F i g u r e I d i s p l a y s s c h e m a t i c a l l y t h e m o d i f i e d DSC-IB s a m p l e h o l d e r ( A ) , r e f e r e n c e h o l d e r ( B ) , sam p l e h o l d e r c o v e r ( C ) , a r e a d i l y moved s u p e r s t r u c t u r e c o n t a i n i n g n e u t r a l d e n s i t y f i l t e r s (D), s h u t t e r ( E ) , c i r c u l a t i n g water heat f i l t e r ( F ) , a n d a G e n e r a l E l e c t r i c H3T7 medium p r e s s u r e m e r c u r y lamp (G) mounted i n a r e f l e c t o r e n c l o s u r e . T h i s a p p a r a t u s was em p l o y e d i n t h e i n i t i a l s c r e e n i n g s t u d i e s . The m a j o r p o r t i o n o f t h e d a t a r e p o r t e d h e r e were o b t a i n e d w i t h a m o d i f i e d P e r k i n - E l m e r DSC-2 i n s t r u m e n t . T h i s i s we 1 1 - r e p r e s e n t e d by F i g u r e I e x c e p t t h a t t h e i r r a d i a t i o n s u p e r s t r u c t u r e was mounted i n a p r e s s e d - w o o d c o m p o s i t i o n b o a r d p l a t f o r m c o n t a i n i n g a r e c t a n g u l a r q u a r t z window. The b o t t o m o f t h i s p l a t f o r m r e s t e d on a r u b b e r O - r i n g g a s k e t s e a t e d d i r e c t l y on t h e s u r f a c e o f t h e r e f r i g e r a t e d b l o c k ( t u r r e t removed) c o n t a i n i n g t h e s a m p l e h o l d e r s . A t w o pen, s t r i p c h a r t r e c o r d e r ( H i t a c h i model 5 6 ) was c o n n e c t e d t o t h e DSC-2. Procedure. S o l u t i o n s were p r e p a r e d by w e i g h i n g . A l l concen t r a t i o n s o f i n i t i a t o r s a r e s t a t e d as weight i n i t i a t o r per t o t a l weight o f mixture. S i n c e t h e s o l u t i o n s were p r e p a r e d i n a i r , a l l s o l u t i o n s s h o u l d be assumed e s s e n t i a l l y e q u i l i b r a t e d w i t h a i r . The DSC i n s t r u m e n t s were c a l i b r a t e d by t h e h e a t o f f u s i o n o f i n d i u m (6.80 c a l g n H ) . T o t a l i n c i d e n t l i g h t i n t e n s i t y (UV + v i s i b l e ) was d e t e r m i n e d i n t h e DSC-2 a p p a r a t u s by p a i n t i n g t h e s a m p l e . h o l d e r w i t h a t h i n l a y e r o f c a r b o n ( A q u a d a g ) , a d d i n g 10 mg o f U v i n u l N-539 t o c o v e r t h e c a r b o n w i t h a l i q u i d l a y e r , a n d i l l u m i n a t i n g t h e sample h o l d e r ( r e s t o f sample h o l d e r c a v i t y s h i e l d e d ) w i t h t h e r e f e r e n c e h o l d e r and c a v i t y s h i e l d e d f r o m t h e l i g h t . The d i f f e r e n t i a l h e a t i n g r a t e was 3.52 meal s e c " ' o v e r a 0.503 c m a r e a o r 6.99 meal cm" 2 s e c - I f o r t h e H3T7 lamp o n a t y p e LD-S b a l l a s t a t 400W. An empty a l u m i n u m c u p w i t h c r i m p e d - o n l i d was used a s a s e c o n d a r y , w o r k i n g s t a n d a r d f o r s u b s e q u e n t i n t e n s i t y m e a s u r e m e n t s . T h e t o t a l s a m p l e h o l d e r c a v i t y was i l l u m i n a t e d . A f a c t o r o f 2.28 was r e q u i r e d t o c o n v e r t o b s e r v e d meal s e c ' t o i n t e n s i t y i n meal cm~2 s e c " l u s i n g t h i s s e c o n d a r y s t a n d a r d . Long p a s s f i l t e r s show 7\% o f t h e l i g h t e n e r g y l i e s b e l o w 470 nm wave l e n g t h and 58% o f t h e l i g h t e n e r g y l i e s below 380 nm w a v e l e n g t h . The p r i n c i p a l UV l i g h t e n e r g y o u t p u t o f t h e H3T7 l i e s i n t h e t h r e e r e g i o n s a r o u n d 254 nm, 3 1 3 nm, a n d 365 nm. T h e r m a l l y - a c t i v a t e d p e r o x i d e i n i t i a t e d p o l y m e r i z a t i o n s were p e r f o r m e d i n t h e normal s c a n n i n g t e m p e r a t u r e mode (+20° m i n ~ f , 40 mm min"' c h a r t s p e e d ) w i t h t h e u n m o d i f i e d DSC-2 a p p a r a t u s . The s c a n b a s e l i n e was o b t a i n e d by a s e c o n d s c a n o f t h e r e a c t e d ma terial. T e m p e r a t u r e r a n g e 30°-250°C. The c u r v e a r e a i n t e g r a t i o n s 2

-

UV


92


POLYMERS

were a c h i e v e d by X e r o x i n g t h e c u r v e s , c u t t i n g , and weighing. A t y p i c a l l i g h t - i n i t i a t e d polymerization involved weighing 7-9 mg o f a c r y l a t e w i t h i n i t i a t o r i n t o t h e s t a n d a r d DSC a l u m i n u m cup. The cup was p l a c e d i n t h e DSC s a m p l e h o l d e r and an empty a l u m i n u m cup was p l a c e d i n t h e r e f e r e n c e h o l d e r . The u s u a l n i t r o gen f l o w w i t h i n t h e c e l l e n c l o s u r e was e s t a b l i s h e d w i t h t h e UV lamp, f i l t e r , and s h u t t e r a s s e m b l y p o s i t i o n e d o v e r t h e h o l d e r s . The g u i l l o t i n e s h u t t e r was o p e r a t e d m a n u a l l y . A h o l d e r t e m p e r a t u r e o f 52°C was used w i t h t h e DSC-IB w h i l e i s o t h e r m a l t e m p e r a t u r e s f r o m 0° t o 80° were employed w i t h t h e DSC-2. A r e c o r d i n g c h a r t s p e e d o f 4 cm min"I was used w i t h t h e DSC-2 w i t h most r u n s o b s e r v e d f o r 15 m i n u t e s . The two pens were s e t a t d i f f e r e n t s e n s i t i v i t i e s so t h a t one t r a c e w o u l d r e m a i n on s c a l e t h r o u g h o u t t h e e x o t h e r m w h i l e t h e s e c o n d t r a c e a m p l i f i e d t h e l a t e r , low r a t e c u r v e s e c t i o n . C u r v e a r e a i n t e g r a t i o n s and d i g i t a l r a t e d a t a were o b t a i n e d by t r a c i n g t h e c u r v e s on a CALMA d i g i t i z e r , s t o r i n g t h e d i g i t a l c o o r d i n a t e d a t a on m a g n e t i c t a p e , and p r o c e s s i n g t h e d a t a f i l e s on a GE 600 s e r i e s d i g i t a l c o m p u t e r . R e s u l t s and

Discussion

The r e s u l t s o f p e r o x i d e i n i t i a t e d and T r i g o n a l 14 p h o t o i n i t i ated p o l y m e r i z a t i o n s of l a u r y l a c r y l a t e (LA), I,6-hexanediol d i a c r y l a t e (HDDA), n e o p e n t y l g l y c o l d i a c r y l a t e (NPGDA), and t r i m e t h y l o l p r o p a n e t r i a c r y l a t e (TMPTA) w i l l f i r s t be p r e s e n t e d . These e x p e r i m e n t s were d e s i g n e d t o o b s e r v e t o t a l h e a t s o f p o l y m e r i z a t i o n u n d e r p r e s c r i b e d c o n d i t i o n s . The r e s u l t s o f more e x t e n s i v e r a t e s t u d i e s on T r i g o n a l 14 p h o t o i n i t i a t e d LA p o l y m e r i z a t i o n s w i l l t h e n f o l low. Mono-, P i - , and T r i - A c r y l a t e P o l y m e r i z a t i o n s : Peroxide Initiation. Table I presents the observed heats of polymerization o f LA, HDDA, NPGDA, and TMPTA c o n t a i n i n g \% b e n z o y l p e r o x i d e +\% t - b u t y l p e r b e n z o a t e upon +20°/min t h e r m a l s c a n n i n g f r o m 30° t o 250°C. The c o m b i n e d p e r o x i d e i n i t i a t o r s y s t e m a s s u r e d a h i g h p r o b a b i l i t y o f r e a c t i o n o f n e a r l y a l l " a c c e s s i b l e " d o u b l e bonds i n t h e n e t w o r k - f o r m i n g monomers. The -19.2 k c a l m o l " l (-80.0 c a l gm"h t o t a l h e a t o f p o l y m e r i z a t i o n f o u n d f o r LA i s c l o s e t o t h e -18.8, -18.6, and -18.6 k c a l mol"' r e p o r t e d f o r m e t h y l , e t h y l , and η-butyl a c r y l a t e s , ( 1 1 , 1 2 ) r e s p e c t i v e l y . The -17.6, - 1 6 . 7 , and -15.4 k c a l p e r mole o f C=C o b s e r v e d f o r HDDA, NPGDA, and TMPTA, r e s p e c t i v e l y , a r e b e l i e v e d t o show t h e p r o g r e s s i v e l y d e c r e a s i n g h e a t s o f p o l y m e r i z a t i o n due t o u n r e a c t e d c a r b o n - c a r b o n d o u b l e bonds on t h e p o l y m e r n e t w o r k s . Mono-, P i , and T r i A c r y l a t e P o l y m e r i z a t i o n s : T r i g o n a l 14 Photoinitiation. The h e a t s o f p o l y m e r i z a t i o n o f t h e monomers a c h i e v e d by p h o t o i n i t i a t i o n ( l i g h t i n t e n s i t y a p p r o x i m a t e l y 5.2 meal cm"2 s e c " ' ) u s i n g T r i g o n a l 14 a t a p p r o x i m a t e l y \% c o n c e n t r a t i o n ( T a b l e I) were o b t a i n e d f r o m r a t e v s . t i m e PSC-2 t r a c e s s i m i l a r t o t h e one shown i n F i g u r e 2 ( c f . s e q . ) . Pue t o t h e r a p i d

8.


MOORE E TA L .

93

< ·> J F Ε L

_U

Figure 1. Schematic of modified DSC-IB apparatus: (A) sample holder, (B) reference holder, (C) sample holder cover with quartz windows, readily moved superstruc ture with (D) neutral density filters, (E) guillotine shutter, (F) circulating water heatfilter,and (G) G. Ε. H3T7 medium pressure mercury lamp in a reflector enclosure

,

0


He

Β

Α

TABLE I H e a t s o f P o l y m e r i z a t i o n o f A c r y l i c Monomers Thermally-activated Peroxide I n i t i a t i o n . S c a n 30°-250°C. P h o t o - a c t i v a t e d T r i g o n a l 14 I n i t i a t i o n . * * 40° and oCPC. I n i t i a t o r Cone. l 0 C ( g m gm-1) 2

Temp. °c

ca1

- Δ Hp gm~ '

kcal moHc=C

Monomer

Initator

LA (C-13) (C-5) (C-19)

peroxides TR-14 TR-14 TR-14

1 +1 0.989 1.01 1.13

scan 30 40 40

80.0 73. 71. 74.

19.2 17.5 17.0 17.8

HDDA

peroxides TR-14 TR-14 TR-14 TR-14

1 +1 1.312 1.000 1.312 1.000

scan 40 40 40 60

155.4 99. 106. 106. 147.

17.6 11.2 12.0 12.0 16.6

NPGDA

peroxides TR-14 TR-14 TR-14

1 +1 1.236 1.236 1.236

scan 40 40 60

157.5 76. 78. 91.

16.7 8.1 8.3 9.7

TMPTA

peroxides TR-14 TR-14 TR-14 TR-14

1 +1 1.045 1.045 1.045 1.045

scan 40 40 40 60

156.1 94. 94. 88. 103.5

15.4 9.3 9.3 8.7 10.2

**400W H3T7 lamp; no f i l t e r , a p p r o x i m a t e l y

5.2 meal cm"^ s e c "


94

UV


POLYMERS

i n i t i a l p o l y m e r i z a t i o n r a t e s o f t h e m u l t i f u n c t i o n a l monomers, sam p l e w e i g h t had t o be r e s t r i c t e d t o 2-3 mg. The 40° p h o t o a c t i v a t e d , T r i g o n a l 14 i n i t i a t e d r u n s y i e l d e d p o l y m e r i z a t i o n h e a t s o f -17.4, - 1 1 . 7 , - 8 . 2 , and -9.1 k c a l p e r m o l e o f C=C f o r LA, HDDA, NPGDA, and TMPTA, r e s p e c t i v e l y . A s s u m i n g Δ H =- 19.2 k c a l p e r mole o f C=C f o r c o m p l e t e c o n v e r s i o n , t h e LA, HDDA, NPGDA, and TMPTA p h o t o p o l y m e r i z a t i o n c o n v e r s i o n s a t 40° were 9 1 $ , 6\%, 43%, and 47%. Based on t h e i r p a r t i c u l a r p o l y m e r i z a t i o n h e a t s i n t h e t h e r m a l s c a n s , t h e LA, HDDA, NPGDA, and TMPTA photopoIymerizations a t 40° were 9\%, 66%, 49%, and 59%, r e s p e c t i v e l y , o f t h e i r " u l t i mate" p o s s i b l e c o n v e r s i o n s . The m u l t i f u n c t i o n a l a c r y l a t e s e x h i b i t i n c r e a s e d c o n v e r s i o n d u r i n g p h o t o p o l y m e r i z a t i o n a t 60°. B a s e d o n -19.2 k c a l p e r mole o f C=C f o r c o m p l e t e c o n v e r s i o n , t h e 60° p h o t o p o l y m e r i z a t i o n c o n v e r s i o n s o f HDDA, NPGDA, and TMPTA a r e 86%, 5\%, and 53%, r e s p e c t i v e l y . A g a i n , b a s e d o n t h e i r p a r t i c u l a r p o l y m e r i z a t i o n h e a t s i n t h e t h e r m a l s c a n s , t h e HDDA, NPGDA, a n d TMPTA p h o t o p o I y m e r i z a t i o n s a t 60° were 94%, 5 8 , a n d 66%, r e s p e c t i v e l y , of t h e i r " u l t i m a t e " conversions. The i n c r e a s e d m o b i l i t y o f t h e n e t w o r k s b r o u g h t a b o u t by r a i s i n g t h e t e m p e r a t u r e p r o b a b l y a c c o u n t s f o r t h e i n c r e a s e d c o n v e r s i o n o f C=C bonds i n g o i n g f r o m 40° t o 60°. Two f u r t h e r c o n c l u s i o n s may be drawn. F i r s t , t h e g r e a t e r f l e x i b i l i t y and r e s u l t a n t c o n f i g u r a t i o n p o s s i b i l i t i e s o f HDDA n e t w o r k u n i t s r e l a t i v e t o NPGDA n e t w o r k u n i t s p e r m i t f o r t h e f o r m e r s l i g h t l y h i g h e r C=C bond c o n v e r s i o n s i n t h e t h e r m a l s c a n s and m a r k e d l y h i g h e r c o n v e r s i o n s a t 40° and 60° i n t h e p h o t o p o I y merizations. S e c o n d l y , t h e p h o t o p o l y m e r i z a t i o n o f TMPTA a t 40° and 60° i s s l i g h t l y more e f f i c i e n t t h a n t h a t o f NPGDA i n s p i t e o f a l o w e r " u l t i m a t e " c o n v e r s i o n o f TMPTA s u g g e s t e d by t h e t h e r m a l scan. I t i s i n t e r e s t i n g t o s p e c u l a t e on t h e p o s s i b i l i t y t h a t t h e p o l y m e r n e t w o r k p r o d u c e d by t h e TMPTA p o l y m e r i z a t i o n p r o v i d e s g r e a t e r a c c e s s i b i l i t y f o r r e a c t i o n o f t h e p e n d a n t d o u b l e bonds a t 40° a n d 60° t h a n t h a t w h i c h e x i s t s i n t h e NPGDA n e t w o r k . L a u r y l A c r y l a t e P o l y m e r i z a t i o n by P h o t o - a c t i v a t e d T r i g o n a l 14 Initiation. A more e x t e n s i v e p r e l i m i n a r y s t u d y o f t h e c a p a b i l i t y o f c a l o r i m e t r y t o f o l l o w r a p i d p h o t o p o l y m e r i z a t i o n r e a c t i o n s was made o f t h e k i n e t i c s o f LA p o l y m e r i z a t i o n i n i t i a t e d by p h o t o - a c t i v a t e d TR-14. F i g u r e 2 i s a t r a c i n g o f an i s o t h e r m a l e x o t h e r m c u r v e o f LA c o n t a i n i n g TR-14 d u r i n g c o n t i n u o u s i r r a d i a t i o n by a 400W H3T7 medium p r e s s u r e m e r c u r y lamp. A t h i g h l i g h t i n t e n s i t i e s and h i g h TR-14 c o n c e n t r a t i o n s no i n d u c t i o n p e r i o d i s s e e n . Differential h e a t i n g i s noted a t about 2 s e c a f t e r s h u t t e r opening; t=0 i s t a k e n a t t h i s p o i n t . The d i f f e r e n t i a l h e a t i n g peak ( e x o t h e r m peak) i s seen t o o c c u r a t about t=7 s e c w i t h subsequent d e c l i n e a s monomer ( a n d i n i t i a t o r ) c o n c e n t r a t i o n d i m i n i s h e s . The u p p e r c u r v e i s t h e b a s e l i n e o b t a i n e d by r e p e a t i n g t h e i r r a d i a t i o n c y c l e a f t e r 15 min i r r a d i a t i o n . The " r a t e a t peak" i s used i n t h e f o l l o w i n g data p r e s e n t a t i o n a s a c o n v e n i e n t comparator o f r a t e s . The sub sequent computer treatment o f t h e data r e v e a l s f o r t h i s run t h a t

8.

MOORE

ET

AL.


95


\0% o f t h e t o t a l p o l y m e r i z a t i o n h e a t (69.4 c a l g i r H f o r t h i s sam p l e ) i s o b s e r v e d p r i o r t o t h e peak. It i s possible to extrapolate the rate vs. conversion curve t o zero conversion ( c f . s e c t i o n Monomer C o n c e n t r a t i o n E f f e c t ) , b u t f o r t h e p r e s e n t t h i s i s n o t done. The r e a d e r ' s a t t e n t i o n i s d i r e c t e d t o t h e f a c t t h a t t h e s e are very r a p i d p o l y m e r i z a t i o n s being described here. 50% o f t h e t o t a l h e a t a p p e a r s by t=25 s e c i n t h e run r e p r e s e n t e d by F i g u r e 2. D a t a o b t a i n e d f r o m c u r v e s s u c h a s t h e one shown i n F i g u r e 2 a r e p r e s e n t e d i n T a b l e s II t h r o u g h V. The dependence o f LA p o l y m e r i z a t i o n r a t e on t e m p e r a t u r e , i n i t i a t o r c o n c e n t r a t i o n , l i g h t i n t e n s i t y , and monomer c o n c e n t r a t i o n (conversion) i s discussed in the f o l l o w i n g s e c t i o n s . Temperature E f f e c t . T a b l e II l i s t s d a t a on LA p o l y m e r i z a t i o n r a t e a t t h e e x o t h e r m peak i n t h e t e m p e r a t u r e r e g i o n 0°-80°C a t two l i g h t i n t e n s i t i e s . A \% TR-14 c o n c e n t r a t i o n was u s e d . The d a t a a r e p r e s e n t e d g r a p h i c a l l y i n F i g u r e 3. For the high l i g h t i n t e n s i t y s e r i e s a low a p p a r e n t a c t i v a t i o n e n e r g y o f 0.63 k c a l mol"' i s o b s e r v e d f r o m t h e t e m p e r a t u r e d e p e n d e n c e o f t h e r a t e a t e x o t h e r m peak. ( T h i s a c t i v a t i o n energy i s c a l c u l a t e d from t h e c a l gm"' s e c " I r a t e s r a t h e r t h a n f r o m mol I"I s e c " ' r a t e s . Use o f t h e l a t t e r would d e c r e a s e t h e c a l c u l a t e d a c t i v a t i o n energy s i n c e about o n e - t h i r d o f t h e r a t e i n c r e a s e w o u l d be d i m i n i s h e d by t h e s p e c i f i c volume i n c r e a s e i n t h e 50° t e m p e r a t u r e i n t e r v a l . Of c o u r s e , t h e i n i t i a t o r c o n c e n t r a t i o n on a mol I " * b a s i s w o u l d a l s o d e c r e a s e about 5%.) The l o g ( r a t e a t p e a k ) v s . I/T d a t a ( S e r i e s 11) a t low l i g h t i n t e n s i t y a p p e a r c o m p l e x ( F i g u r e 3 ) . No a t t e m p t w i l l be made a t t h i s time t o e x p l a i n the observed negative apparent a c t i v a t i o n e n e r g y f o r t h e r e a c t i o n i n t h e 0°-30° t e m p e r a t u r e r a n g e . I n i t i a t o r Concentration Effect. Table III presents data f o r f i v e s e r i e s o f p h o t o p o l y m e r i z a t i o n s o f LA u s i n g v a r i o u s c o n c e n t r a t i o n s o f TR-14. The s e r i e s d i f f e r f r o m one a n o t h e r i n i n c i d e n t l i g h t i n t e n s i t y a n d / o r t e m p e r a t u r e . A l s o , S e r i e s I I I was p e r formed w i t h LA monomer s t i l l c o n t a i n i n g f r e e - r a d i c a l i n h i b i t o r . F i g u r e 4 r e p r e s e n t s t h e d a t a o f S e r i e s IV, V, and VI run i n t h e m o d i f i e d DSC-2 a p p a r a t u s p l u s a n o n - t a b u l a t e d s e r i e s on i n h i b i t e d monomer i n t h e DSC-IB a p p a r a t u s . S e r i e s III data (not p l o t t e d ) w o u l d l i e s l i g h t l y b e l o w and p a r a l l e l t o t h e S e r i e s V d a t a . One n o t e s t h a t t h e d a t a (C=.00l t o .01) a t h i g h i n t e n s i t y i l l u m i n a t i o n may be a p p r o x i m a t e d by s t r a i g h t l i n e r e l a t i o n s i n F i g u r e 4. The slope of these l i n e s y i e l d s (rate at peak)* C . The ( r a t e a t peak) α C ' r e p r e s e n t s t h e d a t a f o r S e r i e s I I I and V (400W lamp o p e r a t i o n ) l e s s w e l l and o v e r a more l i m i t e d c o n c e n t r a t i o n r a n g e . The same power r e l a t i o n a p p e a r s t o a p p r o x i m a t e t h e low i n t e n s i t y d a t a ( S e r i e s V I ; 400W lamp o p e r a t i o n w i t h n e u t r a l d e n s i t y f i l t e r ) o n l y i n t h e r e g i o n n e a r C = .01. 0 e

0

3 5

The c o m p l e x i t y o f t h e i n i t i a t o r m i x , f i l m t h i c k n e s s v a r i a t i o n , and o x y g e n p r e s e n c e i n t h e s e p h o t o p o l y m e r i z a t i o n s does n o t

U V L I G H T INDUCED REACTIONS

IN

POLYMERS

TIME (SEC)


0

60

120

180

240

300

Figure 2. Photopolymerization exotherm trace. Rate of sample heat change vs. time at 40°C. Photoactivated Trigonal 14 initiation of lauryl acrylate polymerization.

TABLE II Temperature E f f e c t Lauryl A c r y l a t e Photopolymerization R a t e a t Peak -dH/dt ( c a l gm"' s e c " ' )

Series*

Run

T(°C)

1

T-l T-2 Î-3 T-4 T-5

10 30 40 50 60

2.29 2.47 2.50 2.69 2.68

T-6 T-7 T-8 T-9 T-10 T-l 1 T-12

0 10 20 30 40 60 80

0.615 0.601 0.586 0.563 0.573 0.601 0.729

II

*Series

Series

2

I

8Ô0+ w a t t s ; i n t e n s i t y 16.7 meal c m " T-14 I02c = 0.989 gm gm"'

II

400 w a t t s , 0.1 n e u t r a l d e n s i t y f i l t e r ; i n t e n s i t y 0.88 meal cm-2 s e c " I ; T-14 I0 C = 0.989 gm gm"' 2

1

sec" ;

MOORE

Calonmetric Analysis of Photopolymerizations

ET AL.

TABLE III I n i t i a t o r Concentration E f f e c t Lauryl Acrylate Photopolymerization Half Reaction Total Heat Time, f "ΔΗ 5 ( ca 1 gm 1 ) (sec)

Run C-l C-2 C-3 C-4 C-5 C-6 C-7

0.101 0.205 0.405 0.604 1.01 2.01 4.03

0.80 1.19 1.68 1.88 2.05 2.11 2.42

34. 47. 57. 71 . 71 . 74. 79.

30 28 25 29 25 26 26

C-8 C-9 C-10 C-l I C-l 2 C-l 3

0.0101 0.0375 0.159 0.352 0.634 0.989

0.023 0.59 1.74 2.33 2.79 3.34

17. 48. 59. 68. 73.

25 21 18 17 17

C-14 C-l 5 C-l 6 C-17 C-18 C-19 C-20 C-21

107 140 257 257 569 13 2.06 4.59

1.01 0.92 I .56 I .59 2. 18 2.55 2.65 2.55

39. 39. 60. 54. 69. 74. 77. (83.)

28 32 29 24 25 22 21 25

VI

C-22 C-23 C-24 C-25 C-26 C-27 C-28 C-29 C-30 C-31 C-32 C-33 C-34

0.107 0. 140 0. 159 0.257 0.352 0.569 0.634 0.989 1.13 1.13 2.06 4.59 4.59

0.057 0.046 0.132 0.262 0.313 0.515 0.451 0.546 0.570 0.637 0.693 0.588 0.612

5.5 4.6 14.6 30. 35. 54. 42. 52. 52. 57. 65. 68. 70.

64 68 82 90 91 101 79 86 85 85 95 122 I 16

VII

C-35 C-36 C-37 C-38

0.0375 0.159 0.352 0.634

(0) 0.182 0.352 0.432

(0) 23. 41 . 47.

I 16 120 122

2

Series* III


Rate a t Peak -dH/dt (cal am sec"')

Trigonal 14 I0 C (gm gm"1)

IV

*Series I 11 Series IV Series V Series VI Series VII

-1

11

2

400 watts; i n t e n s i t y 6.6 meal cm" s e c - l ; T=40°C; inhibited monomer 800+ watts; i n t e n s i t y 16.7 meal cm" sec"'; T=30°C 400 watts; i n t e n s i t y 6.3 meal cm" s e c - l ; T=40°C 400 watts; 0.1 neutral density f i l t e r ; i n t e n s i t y 0.89 meal cm~2 sec"!; T=40°C 400 watts; 0.1 neutral density f i l t e r ; intensity 0.89 meal cm- sec"I; 6 0 ° C 2

2

2

U V LIGHT INDUCED REACTIONS IN

POLYMERS

2.0 RATE AT PEAK (CAL GUT'SEC ) -1

1.0 0.8

ο

0.6

2.8


2.6

3.0

3.2

3.4

3.6

3.8

I0 /T 3

Figure 8. Temperature dependence of —dH/dt exotherm rate at peak for lauryl acrylate containing 0.989 gm gm' of Trigonal 14 (cf. Table II). Upper points at 16.7 meal cm~ sec total light intensity; lower points at 0.88 meal cm' sec' total light intensity.

1

2

1

2

10.0 h

1

1

1

—

1

' ι · · "I

oc 0.1

J_jj

0.001

0.010

TRIGONAL 14 CONCENTRATION (GM/GM MIXTURE)

Figure 4. Dependence of —dH/dt exotherm rate at peak on initial Trigonal 14 concentration in photo polymerizations of lauryl acrylate. ·, 800 -f- W, inhib ited monomer, Τ = 52°C, DSC-IB; 0,800 + W, Τ = 80° (Series IV); 3 , 4O0W, Τ = 40°C (Series V); €, 4O0W, 0.1 neutral densityfilter,Τ = 40°C (Series VI).


8.

MOORE

ET AL.

CalofimeMc Analysis of Photopolymerizations

99

encourage d e t a i l e d a n a l y s i s o f t h e i n i t i a t o r c o n c e n t r a t i o n e f f e c t . However, t h e r a p i d d e c r e a s e i n t h e peak r a t e w i t h l o w e r i n g o f C b e l o w 0.001 a p p e a r s a s s o c i a t e d w i t h i n i t i a t o r l o s s t o s i d e r e a c t i o n s , most l i k e l y t o d e p l e t i o n due t o o x y g e n p r e s e n c e . A t v e r y low C a n e x t e n d e d i n h i b i t i o n p e r i o d i s n o t e d i n t h e i s o t h e r m a l DSC p h o t o p o l y m e r i z a t i o n s l e a d i n g i n e x t r e m e i n s t a n c e s , e . g . , r u n C-35, t o no o b s e r v e d p o l y m e r i z a t i o n . T h e a c c e n t u a t e d d r o p - o f f a t low l i g h t i n t e n s i t y ( S e r i e s V I ) i n t h e peak r a t e w i t h i n i t i a t o r c o n c e n t r a t i o n d e c r e a s e i s n o t r e a d i l y e x p l i c a b l e . We p r e s e n t l y a s c r i b e t h e d e c r e a s e i n t h e r a t e a t peak a t h i g h TR-14 c o n c e n t r a tions t o optical thickness effects. T h i s aspect r e q u i r e s study w i t h b e t t e r f i l m t h i c k n e s s geometry, s i n g l e i n i t i a t o r s p e c i e s , and l i m i t e d wavelength ranges. Figure 5 d i s p l a y s t h e v a r i a t i o n o f t o t a l heat o f r e a c t i o n w i t h i n i t i a l i n i t i a t o r c o n c e n t r a t i o n f o r S e r i e s I V , V, V I , a n d VII. T h i s e f f e c t i s most a s s u r e d l y a s s o c i a t e d w i t h i n i t i a t o r d e p l e t i o n throughout t h e continuous i r r a d i a t i o n . Some enhancement of t o t a l c o n v e r s i o n a t given i n i t i a t o r c o n c e n t r a t i o n i s noted i n r a i s i n g t h e p o l y m e r i z a t i o n t e m p e r a t u r e f r o m 40° ( S e r i e s V I ) t o 60° ( S e r i e s V I I ) . A l a r g e decrease i n t o t a l heat o f r e a c t i o n i s noted i n g o i n g f r o m t h e 16.6 meal cm~2 s e c - I a n d 10 meal cm" 2 s e c " ' i l l u m i n a t i o n s t o t h e 0.89 meal cm-2 s e c " ' i l l u m i n a t i o n s . Light Intensity Effect. The e f f e c t o f l i g h t i n t e n s i t y upon fhe p o l y m e r i z a t i o n r a t e a t peak i s d i s p l a y e d by t h r e e s e r i e s o f r u n s ( c f . T a b l e I V ) . The s e r i e s d i f f e r s l i g h t l y i n i n i t i a l TR-14 c o n c e n t r a t i o n and t e m p e r a t u r e . Figure 4 presents these data a s log ( r a t e a t p e a k ) v s . l o g ( i n t e n s i t y ) . The r a t e s p l o t t e d have been c o n v e r t e d t o r a t e s a t C = 0.0100 and T=3I3°K (40°C) by u s i n g ROjÔO = r C χ ( C / . 0 I ) 0 . 3 5 +3l6(T"' -313"'). C o n s i d e r i n g t h e 30f o l d i n t e n s i t y r a n g e 0.48 meal c m s e c " ' t o 14.3 meal c m " s e c " ' ( F i g u r e 6) and f i r s t a v e r a g i n g t h e r a t e a t peak v a l u e s a t g i v e n i n t e n s i t i e s , we f i n d t h a t t h e l e a s t - s q u a r e s s t r a i g h t l i n e t h r o u g h the r e s u l t i n g t e n data p o i n t s y i e l d s a slope corresponding t o ( r a t e a t peak) α ( i n t e n s i t y ) * T h i s i s near t h e expected de pendence o f p o l y m e r i z a t i o n r a t e upon t h e s q u a r e r o o t o f t h e l i g h t i n t e n s i t y f o r s i m p l e i n i t i a t o r p h o t o l y s i s and r a d i c a l + r a d i c a l termination o f k i n e t i c chains i n solution. x

e

- 2

2

0

Monomer C o n c e n t r a t i o n E f f e c t . In b u l k p o l y m e r i z a t i o n s s u c h a s t h o s e c o n d u c t e d i n t h e p r e s e n t s t u d y , t h e dependence o f p o l y m e r i z a t i o n r a t e o n monomer c o n c e n t r a t i o n c a n be d e t e r m i n e d o n l y on t h e b a s i s o f t h e dependence o f r a t e o n t h e e x t e n t o f r e a c t i o n . Re d u c t i o n o f t h e r a t e v s . t i m e DSC t r a c e s t o d i g i t a l d a t a f i l e s p e r m i t s c o m p u t e r c a l c u l a t i o n o f r e a c t i o n r a t e a s a f u n c t i o n o f mono mer c o n v e r s i o n . A computer program which y i e l d s p r i n t - o u t o f t h e r a t e and t i m e a t g i v e n f r a c t i o n s o f t h e t o t a l heat r e l e a s e a l l o w s computation o f t h e order o f r e a c t i o n with respect t o carbon-carbon d o u b l e bond c o n c e n t r a t i o n . A s s u m i n g -80.0 c a l gm"' r e p r e s e n t s t h e

100


POLYMERS

TOTAL HEAT REACTION °l" (CALGM-1, 4

Figure 5. Total heat of reaction, — ΔΗ , plotted against initial Trigonal 14 concentration, C, for lauryl acrylate polymerizations (cf. Table III). ·, Series IV; d , Series V; O, Series VI; €, Series VII.


Ρ

10'C (GM/GM MIXTURE)

TABLE IV

Lauryl

Series*

Run

Light Intensity Effect Acrylate Photopolymerization

Light Intensity (mca1 c m " s e c " ' ) 2

R a t e a t Peak -dH/dt ( c a l gm"' s e c " ' )

l-l 1-2 1-3 1-4 1-5

0.69 1.42 3.52 9.0 14.3

0.70 1.00 1.91 2.26 3.32

IX

1-6 1-7 1-8 1-9 1-10 l-l 1

0.34 0.34 1.53 1.53 14.3 14.3

0.074 0.059 0.727 0.763 2.74 3.02

X

1-12 1-13 1-14 1-15 1-16 1-17 1-18 1-19

VI 11

*Series VIII Series Series

IX X

0.107 0.214 0.48 0.48 0.84 2.80 4.34 4.34

0.007 0.187 0.197 0.504 0.80 1 .59 1.96 1.93 2

800+ w a t t s ; TR-14 I 0 C = 1.00; 52°C; i n h i b i t e d monomer; DSC-IB 800+ w a t t s ; TR-14 I 0 C = 0.989; 30°C 400 w a t t s ; TR-14 I 0 C = 1.13; 40°C 2

2

8.

MOORE


ET AL.

101

t o t a l Δ Η o f LA, .the r e s i d u a l h e a t o f p o l y m e r i z a t i o n a t t i m e t , H ( t ) = δθ.0 - / x ( d H / d t ) - d t , i s p r o p o r t i o n a l t o t h e u n r e a c t e d monomer c o n c e n t r a t i o n i n t h e s y s t e m a t t h a t t i m e . F i g u r e s 7 and 8 are p l o t s o f t h e exotherm r a t e s , - ( d H / d t ) v s . H ( t ) on l o g - l o g c o o r d i n a t e s f o r t h e S e r i e s IV a n d S e r i e s V d a t a . τ a t which t h e p o i n t s a r e shown a r e t o> o.3> "*"(). 4' + *> ' w h i c h 0.2, 0.3, 0.4, e t c . , a r e t h e f r a c t i o n s o f total h e a t f o r e a c h r u n . T h u s , t h e upper f o u r p o i n t s on t h e t o p c u r v e o f F i g u r e 7 r e p r e s e n t f o r r u n C - l 3 t h e r a t e s o f r e a c t i o n a t 0.2 χ 73 = 14.6, 0.3 χ 73=21.9, 0.4 χ 73 = 29.2, a n d 0.5 χ 73 = 36.5 c a l gm" f o r -H c u m u l a t i v e = " £ x ( d H / d t ) d t , χ = .2, .3, .4, .5. I f t h e r e were l i t t l e i n i t i a t o r d e p l e t i o n and o t h e r f a c t o r s remained c o n s t a n t , one would e x p e c t t h e c u r v e s i n F i g u r e s 7 a n d 8 t o be p a r a l l e l s t r a i g h t l i n e s whose s l o p e s w o u l d be t h e o r d e r o f r e a c t i o n w i t h r e s p e c t t o mon omer c o n c e n t r a t i o n . O b v i o u s l y , c u r v a t u r e e x i s t s i n t h e p l o t s and the curve slopes i n t h e e a r l y stages o f conversion appear t o i n c r e a s e a s t h e i n i t i a l i n i t i a t o r c o n c e n t r a t i o n i s lowered. Despite t h e s e d e v i a t i o n s we may e x p e c t t h e s l o p e s o f t h e c u r v e s i n F i g u r e s 7 a n d 8 a t low c o n v e r s i o n and h i g h i n i t i a l i n i t i a t o r c o n c e n t r a t i o n s t o represent nearly the order o f reaction with respect t o monomer c o n c e n t r a t i o n . Table V presents the r e s u l t s o f least-squares f i t t i n g o f s t r a i g h t l i n e s t o t h e 0.2, 0.3, 0.4, a n d 0.5 f r a c t i o n a l h e a t p o i n t s o f t h e l o g - l o g p l o t s ( o f F i g u r e s 7 a n d 8) o f p o l y m e r i z a t i o n rates (-dH/dt) and r e s i d u a l h e a t s o f r e a c t i o n f o r S e r i e s I I I , IV, and V d a t a . T h e s l o p e s , B, p r o g r e s s i v e l y i n c r e a s e a s i n i t i a l i n i t i a t o r concentrations decrease. However, r e s t r i c t i n g o u r c o n s i d e r a t i o n t o t h e t e n r u n s i n w h i c h - A H >68 c a l gm"' t h e a v e r a g e Β i s 1.65. F o r t h e s i x r u n s ( S e r i e s I V a n d S e r i e s V) f o r w h i c h -ΔΗ > 68 c a l gm"' and no f r e e r a d i c a l i n h i b i t o r was i n i t i a l l y p r e s e n t t h e a v e r a g e Β i s 1.57. We t h e r e f o r e c o n e i u d e , w i t h c o n s i d e r a b l e r e s e r v a t i o n , t h a t -dH/dt Ql .6 f l/\ where M i s t h e monomer c o n c e n t r a t i o n . ρ

T

R

x

t x

R

x

χ

T

e

c

n

Q

1


+

+ x

p

o

r

Summary. T h e f o r e g o i n g s e c t i o n s l e a d t o t h e c o n c l u s i o n t h a t f o r t h e p h o t o - a c t i v a t e d T r i g o n a l 14 p o l y m e r i z a t i o n o f l a u r y l a c r y l a t e , t h e r a t e o f p o l y m e r i z a t i o n may be a p p r o x i m a t e l y e x p r e s s e d a s -dM/dt

= Κ, χ (-dH/dt) 0

= K C 2

3 5

|°-

5

3

M

L

6

e*

3

,

6

/

T

(I)

where C i s t h e T r i g o n a l 14 c o n c e n t r a t i o n , I i s t h e i n c i d e n t l i g h t i n t e n s i t y , OQ i s t h e l a u r y l a c r y l a t e monomer c o n c e n t r a t i o n , a n d Τ i s t h e absolute temperature. This expression holds best f o r t h e h i g h e r l i g h t i n t e n s i t i e s , 0.001 £ C < 0.010 r a n g e , a n d M / M Q >_ 0.5 r e g i o n . A l t h o u g h t h e f o r e g o i n g i n f o r m a t i o n was o b t a i n e d o n l a u r y l a c r y l a t e i n i t i a l l y e q u i l i b r a t e d w i t h a i r and i n i t i a t e d by a com p l e x c o m m e r c i a l p h o t o - i n i t i a t o r , i t i s s t i l l p r o f i t a b l e t o compare


102 10.0

-i—ι—ι ι ι ι ι ι

RATE AT PEAK (CAL GM" SEC")


1

1

QJI I I 1 I I I I _l I I I I I I I I #

10

INCIDENT LIGHT INTENSITY (MCAL CM" SEC' ) 2

1

Figure 6. Exotherm rate at peak, —dH/dt (peak), vs. incident light intensity for photoactivated Trigonal 14 initiated lauryl acrylate polymerizations (cf. Table IV). ·, Series VIII; O, Series IX and Series X.

EXOTHERM RATE (CAL GM" SEC"' ) 0.6| 1

Figure 7. Exotherm rates, (—dH/dt) , vs. residual polymerization heats, H (t ), for Series TV lauryl acrylate polymeriza tions. Points correspond to 0.2, 0.3, 0.4, etc., fractional conversions for each run based on its total —ΔΗ . tx

R

Ρ

x

10

20 RESIDUAL

60 80

8.

MOORE

E T AL.

Cahrimetric Analysis of Photopolymerizations


4.0h

20

40

60 80

RESIDUAL HEAT (CAL GM' ) 1

Figure 8. Exotherm rates, (—dU/dt) , vs. residual polymerization heats, H (t ), for Series V lauryl acrylate polymerizations. Points correspond to 0.2, 0.3, 0.4, etc., fractional conversions for each run based on its total — Δ Η . tx

R

x

Ρ

103

104

U V L I G H T INDUCED REACTIONS IN

POLYMERS

TABLE V P o l y m e r i z a t i o n R a t e Dependence on R e s i d u a l P o l y m e r i z a t i o n H e a t Lauryl A c r y l a t e Photopolymerization TR-14 I0 ( 3 (gm gm" h

-Δ H ( c a l grn ')

A

Β

C-13 C-12 C-ll C-10

0.989 0.634 0.352 0.159

73 68 59 48

-4.476 -5.156 -6.182 -8.835

1.34 1.47 1.67 2.20

C-21 C-20 C-19 C-18 C-l7,16 C-15

4.59 2.06 1.13 0.569 0.257 0.140

83 77 74 69 57 39

-5.618 -5.756 -5.463 -7.278 -9.631 -14.12

1.56 1.60 1.52 1.90 2.37 3.29

C-7 C-6 C-5 C-4 C-3 C-2 C-l

4.03 2.01 1.01 0.604 0.405 0.205 0.101

79 74 71 71 57 47 34

-5.789 -6.824 -7.019 -7.146 -9.318 -10.817 -16.729

1.59 1.81 1.83 1.84 2.31 2.57 3.83

2


Run

In R -R

p

= A + Β ΙηΔΗ ( r e s i d u a l ) Ρ = r a t e ( c a l gm"' s e c " 1 )

Ρ A H ( r e s i d u a i ) = 80.0 - (-ΔΗ c u m u l a t i v e ΔH cumulative

1

[ c a l gm" ])

= i n t e g r a t e d heat t o chosen f r a c t i o n o f t o t a l heat

A and Β o b t a i n e d f r o m l e a s t - s q u a r e s l i n e a c c o r d i n g t o a b o v e e q u a t i o n u s i n g 0.2, 0.3, 0.4, and 0.5 f r a c t i o n a l h e a t d a t a p o i n t s . Β = 1.65 a v e r a g e f o r t h e t e n r u n s h a v i n g

- Δ Η >68 c a l gm"'.


8.

MOORE ET A L .


105

t h e c a l o r i m e t r i c s t u d y o f p o l y m e r i z a t i o n r a t e w i t h more c o n v e n t i o n a l d i l a t o m e t r i c s t u d i e s . (9,10) O u r p r e s e n t d a t a a r e i n a g r e e m e n t w i t h t h e o b s e r v a t i o n o f no Trommsdorf e f f e c t i n LA b u l k p o l y m e r i z a t i o n . ( 9 ) O u r d e t e r m i n a t i o n o f a p p r o x i m a t e monomer o r d e r 1.6 f o r t h e v e r y r a p i d p h o t o p o l y m e r i z a t i o n s i s h i g h e r t h a n t h e 1.4 o r d e r f o u n d p r e v i o u s l y f o r b u l k p o l y m e r i z a t i o n and n e a r t h e 1.6 o r d e r f o u n d f o r s o l u t i o n p o l y m e r i z a t i o n . (£) I t s h o u l d be r e c o g n i z e d t h a t t h e r a t e s o f p o l y m e r i z a t i o n i n t h e p r e s e n t work a r e a s much a s 3 0 0 t i m e s t h e r a t e s i n t h e d i l a t o m e t r i c s t u d y . * The e f f e c t o f i n i t i a t o r d e p l e t i o n on t h e computed monomer o r d e r i n t h e present treatment i s t o increase t h e apparent order. T h e 0.35 o r d e r i n T r i g o n a l 14 c o n c e n t r a t i o n c o r r e s p o n d s q u a l i t a t i v e l y w i t h t h e 0.45 o r d e r i n i n i t i a t o r c o n c e n t r a t i o n f o u n d i n t h e d i l a t o m e t r i c work. I t i s i n t e r e s t i n g t h a t a l t h o u g h t h e T r i g o n a l 14 o r d e r i s a p p r o x i m a t e l y 0.35, t h e o r d e r i n l i g h t i n t e n s i t y i s n e a r 0.5. The p r e s e n t p r e l i m i n a r y s t u d y o f r a p i d p h o t o p o l y m e r i z a t i o n r e a c t i o n s by d i f f e r e n t i a l c a l o r i m e t r y shows t h i s method t o have great potential u t i l i t y . Future s t u d i e s i n v o l v i n g simpler r e a c t i o n s y s t e m s and b e t t e r c o n t r o l l e d r e a c t i o n c o n d i t i o n s w i l l d i s p l a y t h e power a n d s o p h i s t i c a t i o n o f t h e d i f f e r e n t i a l c a l o r i m e t r i c t e c h nique i n photochemical research.

Abstract Differential calorimetry has been applied to the study of rapid photopolymerizations. This new technique holds great promise for basic and applied research on photopolymerization and other photochemical reactions. The method requires only a few milligrams of sample, can be used on network-forming systems, and can approx imate actual conditions of thin f i l m and coating technologies. Lauryl acrylate polymerizations i n i t i a t e d by a photo-acti vated mixture of benzoin butyl ethers (Trigonal 14) were performed in Perkin-Elmer model DSC-IB and DSC-2 apparata modified by attach ment of a heat-filtered medium pressure mercury lamp. Within spec i f i e d variable l i m i t s , the rate of polymerization may be approxi mated by the relation R = const. |0.53 0.35[ ]|.6 -316/T. | is light intensity; C i s initiator concentration; [M] i s monomer con centration; Τ i s absolute temperature. Peroxide i n i t i a t e d polymerizations (scan 30°-250°C) of lauryl acrylate (LA), 1,6-hexanediol diacrylate (HDDA), neopentyl glycol diacrylate (NPGDA), and trimethylol propane t r i a c r y l a t e (TMPTA) revealed total polymerization heats per mole of C=C of 19.2, 17.6, 16.7, and 15.4 kcal, respectively. Photoactivated Trigonal 14 i n i t i a t e d polymerizations at 40° yielded total polymerization heats per mole of C=C of 17.4, 11.7, 8.2, and 9.1 kcal for LA, HDDA, NPGDA, and TMPTA, respectively. At 60°C the photopolymerization heats for the latter three monomers increased to 16.6, 9.7, and 10.2 kcal per mole of C=C, respectively. p

C

Μ

e

106

UV


POLYMERS


Literature Cited 1. McGinnis, V.D., Holsworth, R.M. and Dusek, D.M., A.C.S. Coat ings and Plastics Preprints, (1974), Vol. 34, (No. I ) , p. 669 2. Osborn, C.L. and Sander, M.R., i b i d . , p. 660. 3. Schroeter, S.H., Moore, J.E. and Orkin, O.V., i b i d . , p. 751. 4. McGinnis, V.D. and Dusek, D.M., J . Paint Tech., (1974), 46, 23. 5. Faru, R.A., Polymer, (1968) 9, 137. 6. Horie, K., Mita, I. and Kambe, H., J . Poly. S c i . , (1968), Vol. 6, (A-1), 2663. 7. Burrett, K.E.J, and Thomas, H.R., B r i t . Polymer J . , (1970), Vol. 2, p. 45. 8. Willard, P.E., Polym. Eng. and S c i . , (1972), Vol.12, (No. 2), 120. 9. Scott, G.E. and Senogles, E, J . Macromol. Sci.,-Chem., (1970), A4, 1105. 10. Scott, G.E. and Senogles, E., i b i d . , (1974), A8, 753. 11. Joshi, R.M., Makromol. Chem., (1963), 66, 114. 12. Joshi, R.M., J . Polymer S c i . , (1962), 56, 313.

9 Applications of Radiation Sensitive Polymer Systems W . M O R E A U and N . V I S W A N A T H A N


I B M Corp., Systems Products Division, E . Fishkill, N.Y. 12533

Over the l a s t f i v e years, s i g n i f i c a n t advances have been made i n the r a d i a t i o n processing of commercial products. The products range i n size from cured films on automobiles to picosecond microelectronic devices 500A° i n three dimensional s i z e . Four major techno l o g i c a l areas encompass the scope of the applied tech nologies, ( c f . , Table I ) . These include; (1) the r a d i a t i o n curing or a l t e r a t i o n of coatings and f i b e r s , (2) the photoproduction of lithographic p r i n t i n g plates, (3) the processing of f a b r i c a t i n g e l e c t r o n i c devices with r e s i s t s , and (4) the commercial development of photodegradable p l a s t i c s . The treatment of photoconductive polymers for xerography w i l l not be reviewed but i s treated i n several reviews (see bibliography). In this report we w i l l survey the catagories of a p p l i c a tion by examining the chemical classes of polymeric systems, the processing sequences, and the e f f i c i e n c y of the various systems. The exposure of polymeric systems has involved the full electromagnetic spectrum (Table I I ) . The i r r a d i a tion wavelength i s usually i n excess of the bond energy of the carbon bonds or i n excess of the a c t i v a t i o n energy of chemical reaction. High energy x-rays, gamma, ion, electron, u l t r a v i o l e t and i n f r a r e d beams have been used to degrade, polymerize, or c r o s s l i n k polymeric f i l m s . The most widely used wavelengths are the 3000-5000A region of mercury lamps for the expo sure of polymer f i l m s . The thicknesses of exposed films range from Angstroms to thousands of microns (cf., Table I I I ) . 0

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U V L I G H T INDUCED REACTIONS I N P O L Y M E R S

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TABLE I


R a d i a t i o n P r o c e s s i n g o f polymers Degradation

Polymerization

Grafting

Crosslinking

Polymer

Fiber treatment

Negative resists

Positive resists

Ink C u r i n g

Membrane

Ink curing

Photodegrading plastics

Coating curing

"Casing"

Coating curing Dental c o a t i n g s; Heat shrinks

Plastic weathering Paint weathering Polymer analysis

Synthesis

TABLE I I Radiation source

Exposure Sources of Polymer Approximate Energy Wavelength A° Kcal 3

290 x l O 6 28 x l O 5 35 xlO

X-Rays

1-10

Gamma(1MEV)

0.01

Ε-Beam(100KV)

0.008

E-beam(lOeV)

1200

250

UV UV

1000-3000 3000-4000

190 100

UV

4000-5000

60

Laser

3000-6000

70

Laser ION(IOOKV) ION (lOev)

3

10.6 μ 0.13 1000

2

xlO 300

6

Systems Application Resist exposure Polymer synthesis Resist exposure Plasma processing UV c u r i n g Ink C u r i n g Lithography Photoresists Graphic A r t s Holography Resist exposure Plastic Evaporation Resist Etching Plasma Chem. treatment o f films, fibers, membranes·


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The p o l y m e r i c systems are u s u a l l y composed of a polymer which imparts the m a j o r i t y of p h y s i c a l p r o p e r t i e s and a c t i n i c a d d i t i v e s . In simple systems such as c u r i n g f i l m s or e l e c t r o n beam r e s i s t s , the polymer i s a l s o the r a d i a t i o n s e n s i t i v e s p e c i e s . In most c a s e s , the f o r m u l a t i o n s behave s i m i l a r i l y i n t h e i r response to h i g h energy i r r a d i a t i o n . P r a c t i c a l l y any polymer can be made r a d i a t i o n s e n s i t i v e by d i r e c t exposure to i o n i z i n g e n e r g i e s or by f o r m u l a t i o n w i t h a d d i t i v e s such as f r e e r a d i c a l p r e c u r s o r s . Thermally s e n s i t i v e poly mers are a l s o l i k e l y to undergo a s i m i l a r r e a c t i o n when exposed. The p r o c e s s i n g of polymer f i l m s i n v o l v e s wet and dry t e c h n i q u e s . Some systems are s o l v e n t l e s s l a m i n a t e s such as dry r e s i s t s . In most cases one-dimen s i o n a l f i l m s r e q u i r e o n l y c o a t i n g and e x p o s u r e . For m u l t i d i m e n s i o n a l images, s o l v e n t s are used to form images by d i s s o l u t i o n of unwanted a r e a s . A general t r e n d f o r economic and e c o l o g i c a l purposes w i l l i n v o l v e s o l v e n t l e s s c o a t i n g s and dry p r o c e s s i n g . The i n c r e a s e d i n t e r e s t i n energy s e n s i t i v e p o l y mers p r o b a b l y e v o l v e s from the shortcomings of the c o n v e n t i o n a l image r e c o r d i n g m e d i a - s i l v e r h a l i d e emulsion. I t i s both d i f f i c u l t and e x p e n s i v e to a p p l y emulsion f i l m s as p r o t e c t i v e l a y e r s or use as p r i n t i n g p l a t e s or e t c h r e s i s t s . S y n t h e t i c polymers are one to two o r d e r s of magnitude l e s s r a d i a t i o n s e n s i t i v e than h a l i d e e m u l s i o n , but t h e i r unique p r o p e r t i e s of chemi c a l and p h y s i c a l r e s i s t a n c e e a s i l y overcome t h i s d i s advantage, ( c f . , T a b l e I V ) . R a d i a t i o n s e n s i t i v e f i l m s are a v a i l a b l e from a wide source of cheap s y n t h e t i c monomers or polymers. The r e s o l u t i o n of p o l y m e r i c f i l m s g r e a t l y exceeds t h a t of s i l v e r e m u l s i o n . Images as m i c r o s c o p i c as 50A° l i n e s and spaces (200,000 lines/mm.) have been r e c o r d e d i n r e s i s t f i l m s by exposure i n a s c a n n i n g e l e c t r o n m i c r o s c o p e . M o l e c u l a r a d d i t i v e s such as diazonium s a l t s or o r g a n i c h a l i d e s can be added to produce c o l o r e d images of h i g h o p a c i t y . P o l y m e r i c systems can be e n v i s i o n e d as meeting a m a j o r i t y of t e c h n o l o g i c a l needs f o r i n f o r m a t i o n r e c o r d i n g and i n the f a b r i c a t i o n of m i n i a t u r e e l e c t r o n i c or m e c h a n i c a l components. In the a p p l i e d t e c h n o l o g i e s , the c u r i n g of p a i n t s or i n k s by e l e c t r o n beam or u l t r a v i o l e t l i g h t has

UV

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TABLE I I I Exposure P e n e t r a t i o n i n P r o c e s s i n g


Exposing Radiation

Depth o f Penetration

Plasma (UV E m i s s i o n ) Plasma(1-lOeV) Ion (60KV) E-Beam(20KV) Ε-Beam(1MEV) X-Rays (60KV) UV (3650A ) 0

Polymer T h i c k n e s s

1-10 / i 0.005-0.05 2 8 150 10 1-125 p

μ μ μ μ

Polymers

Surface μ

Treatment

S u r f a c e Treatment 0.1 - 1 μ 0.1 - 2 μ 25 - 200 μ 0.5 - 2.0 μ 0.1 - 10

μ

T a b l e IV Comparison of C o n v e n t i o n a l S i l v e r Emulsion and U n c o n v e n t i o n a l Polymer Image R e c o r d i n g Silver

Emulsion F i l m

Higher c o s t Higher p r o p r i e t a r y formulations Precoated films Thickness r e s t r i c t i o n R e s o l u t i o n to Ija Wet C h e m i c a l P r o c e s s High speed (lO'^cm^/erg) Poor c h e m i c a l r e s i s t a n c e Poor p h y s i c a l r e s i s t a n c e One or two d i m e n s i o n a l images Quantum a m p l i f i c a t i o n 200-1000 nm

sensitivity

Polymeric

Films

Low cost Wide p a t e n t base Easily applied Molecular films R e s o l u t i o n to .00]yu Dry and Wet P r o c e s s i n g Slow speed (lO'^cm^/erg Excellent resistance Excellent resistance Multi-dimensional images Low quantum a m p l i fication 200-500 nm s e n s i tivity


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f i n a l l y been a c c o m p l i s h e d on an i n d u s t r i a l s c a l e . The e l e c t r o n beam c u r i n g of c o a t i n g s has f o r some time a t t r a c t e d a g r e a t d e a l of a t t e n t i o n s i n c e i t i s c l a i m e d t h a t c o a t i n g s can be f u l l y cured i n a f r a c t i o n of a JL second. The c o a t i n g s a r e p r i m a r i l y based on the c r o s s l i n k i n g o f a c r y l i c or epoxy r e s i n s i n a p o l y u r e t h a n e or p o l y e s t e r base ( i . e . , m o d i f i e d p a i n t s ) . E l e c t r o n beam c u r i n g has reached f u l l y commercial s t a g e s i n the USA and Europe.— P r o t e c t i v e coatings have been e l e c t r o c u r e d on automotive f i n i s h e s , wood p r o d u c t s , p l a s t i c p r o d u c t s , and s t r u c t u r a l m e t a l s . The p r o d u c t s u s u a l l y have to be f l a t f o r the most e f f i c i e n t processing. The h i g h s e n s i t i v i t y of s o l v e n t l e s s c o a t i n g s c o u p l e d w i t h more e f f i c i e n t s o u r c e s has enabled p r o c e s s speeds to a t t a i n 50-100 f e e t per ο

minute S i n c e the e l e c t r o n beam (150 KV) p e n e t r a t e s and c u r e s f i l m s up to 50 m i l s i n t h i c k n e s s e s , the f i l m s can be h i g h l y pigmented ( p r e f e r a b l y w i t h h i g h atomic number a d d i t i v e s to enhance e l e c t r o n absorption). The u l t r a v i o l e t c u r i n g of i n k s and c o a t i n g s has a l s o r e c e i v e d new a t t e n t i o n . The renewed i n t e r e s t i n c u r i n g i n k s has been i n f l u e n c e d by e c o l o g i c a l and eco nomic c o n s i d e r a t i o n s T h e i n k s can be f o r m u l a t e d as s o l v e n t f r e e systems which a r e s e n s i t i z e d to mercury 3650A i r r a d i a t i o n . The c u r i n g energy i s o n l y a f r a c t i o n of t h a t needed w i t h heat c u r a b l e systems. The i n k f o r m u l a t i o n s u s u a l l y c o n t a i n an u n s a t u r a t e d monomer or prepolymer, a r a d i c a l p r e c u r s o r and/ or an a c t i n i c s e n s i t i z e r , a polymer v e h i c l e base, and pigmentation. Recent f o r m u l a t i o n s i n c l u d e p o l y u n s a t u r a t e d p o l y e n e s , and p o l y a c r y l a t e s w i t h t h i o l or ben z o i n r a d i c a l s o u r c e s . ! The a c i d or h y d r o x y l c o n t e n t of the r e s i n base i s u s u a l l y v a r i e d to a d j u s t the water o r o l e o p h i l l i c w e t t i n g a c t i o n of the c o m p o s i t i o n . A l t h o u g h the p e n e t r a t i o n range of u l t r a v i o l e t l i g h t i s o n l y about 25 microns (1-2 m i l s ) , the c u r i n g time i s i n the second range and throughputs approach the speeds of e l e c t r o n beam cured f i l m s . The e l e c t r o n and u l t r a v i o l e t c u r i n g of r a d i a t i o n can be compared as follows. E l e c t r o n c u r i n g has a wider range of a p p l i c a t i o n and i s s u i t a b l e f o r t h i c k e r , pigmented systems. E l e c t r o n beam s o u r c e s a r e more e x p e n s i v e to o p e r a t e and not r e a d i l y adapted to a s m a l l i n d u s t r i a l s c a l e or f o r 0


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small parts. Both t e c h n i q u e s overcome the s o l v e n t p o l l u t i o n of heat c u r a b l e c o a t i n g s but a r e s t i l l r e s t r i c t e d to m a i n l y f l a t s u r f a c e s . In the p r i n t i n g i n d u s t r y one of the a p p l i c a t i o n s of p h o t o p o l y m e r i z a t i o n i s i n the area of g e n e r a t i n g l i t h o g r a p h i c p r i n t i n g p l a t e s . As i s commonly known i n a l l l i t h o g r a p h i c systems, the p r i n t i n g p l a t e s c o u l d be generated by p o s i t i v e l y or n e g a t i v e l y a c t i n g r e s i s t systems. The p r i n t i n g p l a t e s can be b r o a d l y c l a s s i f i e d as S u r f a c e p l a t e s or Deep e t c h p l a t e s . In the former c l a s s , the p h o t o p o l y m e r i s e d c o a t i n g i s a hard r e s i d u e on the developed p l a t e ; and the p o l y m e r i s e d m a t e r i a l i s u s u a l l y o l e o p h i l l i c and h y d r o phobic. The background ( u s u a l l y a m e t a l s u b s t r a t e ) i n c o n t r a s t i s u s u a l l y h y d r o p h i l l i c and o l e o p h o b i c . In a c t u a l p r a c t i c e , the background m e t a l l a n d s a r e p e r i o d i c a l l y d e s e n s i t i z e d w i t h a s u s p e n s i o n of c o l l o i d a l p a r t i c l e s i n an a c i d i c medium. The Deep e t c h p l a t e s , on the o t h e r hand, a r e u s u a l l y etched m e t a l p l a t e s u s i n g a n e g a t i v e l y working p h o t o r e s i s t and an e t c h i n g l a c q u e r . In the i n d u s t r y the Deep e t c h p l a t e s have found wider usage i n view of t h e i r m e c h a n i c a l s t a b i l i t y towards p r i n t i n g more copies. From the m a t e r i a l s p o i n t of view, the p r i n t i n g i n d u s t r y has used a l l p o s s i b l e c o m p o s i t i o n s of photopolymer i z a b l e p r e p o l y m e r s . Commonly used systems a r e m a t e r i a l s w i t h u n s a t u r a t e d m o i e t i e s as e s t e r s , k e t o n e s , e t h e r s , and m o d i f i c a t i o n s of t h e s e . Of these c l a s s e s , b r o a d e s t p a t e n t coverage seems to c e n t e r around a c r y l i c e s t e r s and d e r i v a t i v e s o f a c r y l a t e systems, e s p e c i a l l y i n the a r e a s of s u r f a c e p r i n t i n g p l a t e s . The major emphasis i n t h i s type of l i t h o g r a p h i c p l a t e s seems to be on the m e c h a n i c a l s t a b i l i t y of the p o l y m e r i z e d image to s u s t a i n r e p e a t e d impacts w i t h o u t s e r i o u s l o s s of r e s o l u t i o n and d e f o r m a t i o n of the images. From t h i s p o i n t of view, i n c r e a s e d m e c h a n i c a l s t r e n g t h ob t a i n e d by extraneous c r o s s l i n g i n g agents have found the w i d e s t a p p l i c a b i l i t y to o b t a i n r i g i d s t r u c t u r e s . On the other hand, i n the areas of Deep e t c h p l a t e s , the emphasis seems to be on the s t a b i l i t y and e t c h r e s i s t a n c e of the images ( t o w i t h s t a n d c h e m i c a l a g e n t s ) n e c e s s i t a t e the use of c h e m i c a l l y s t a b l e c o m p o s i t i o n s (such as c y c l i z e d i s o p r e n e , e p o x i d i s e d e s t e r s ) . Ad d i t i o n a l f a c t o r s such as a d h e s i o n of the p h o t o r e s i s t


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to the m e t a l p l a t e a f f e c t the q u a l i t y of the f i n a l image. The s e n s i t i z e r s and i n i t i a t o r s used i n the p r i n t i n g p l a t e i n d u s t r i e s are b a s i c a l l y w e l l known s e n s i t i z e r s i n the p h o t o c h e m i c a l f i e l d . Most of these have s t r o n g a b s o r p t i o n i n the near U V - v i s i b l e s o u r c e s , Hg lamps occupying the major r o l e i n p r e s e n t - d a y t e c h n o logy. T a b l e V i s a comprehensive summary of some im p o r t a n t s e n s i t i z e r s and prepolymers used i n the p h o t o lithographic f i e l d . From p r o c e s s i n g p o i n t of view, emphasis has been p l a c e d h e a v i l y on s u b s t i t u t i n g hard p l a s t i c s i n the p l a c e of metals used i n the p a s t , w i t h the major ad vantage of reduced c o s t s i n v o l v e d i n f a b r i c a t i o n . Al most every p o s s i b l e p h y s i c a l and c h e m i c a l p r o p e r t y has been experimented w i t h to o b t a i n the n e c e s s a r y c o n t r a s t between the exposed and unexposed areas of the l i t h o g r a p h i c p l a t e s . P h y s i c a l p r o p e r t y d i s t i n c t i o n s based on a d h e s i o n , thermal p r o p e r t i e s ( m e l t i n g p o i n t , f o r example) have paved the way f o r the long sought a f t e r dry p r o c e s s i n g t e c h n i q u e s . The t r e n d seems to be i n the d i r e c t i o n of d e p a r t i n g from wet c h e m i c a l p r o c e s s i n g (such as s o l u b i l i t y methods) techniques. I t should a l s o be noted t h a t emphasis seems to be i n the d i r e c t i o n of a v o i d i n g p r o c e s s i n g s t e p s under darkroom c o n d i t i o n s . S e v e r a l commercial c o m p o s i t i o n s a r e o b t a i n a b l e i n the l a m i n a t e form, u s u a l l y w i t h an i n e r t oxygen i n s e n s i t i v e top f i l m to a v o i d quenching e f f e c t s caused by oxygen d u r i n g expo sures · To improve the m e c h a n i c a l s t r e n g t h and hardness c a l l f o r the use of c u r i n g agents ( u s u a l l y a c t i v a t e d by a bake step a f t e r development) i n the p h o t o p o l y merizing composition. T y p i c a l c u r i n g agents mentioned have been a l k y l h y d r o p e r o x i d e s , b i f u n c t i o n a l amines, a c i d s , e t c . The l a s t c l a s s of compounnds have been the most p o p u l a r a d d i t i v e s to the epoxy based polymer compositions. A d d i t i o n a l l y , the use of i n e r t f i l l e r s i s a l s o known. A r e c e n t example of newer c l a s s e s of compounds i n p h o t o s e n s i t i v e p r i n t i n g p l a t e i n d u s t r y i s the use of s u l f e n y l c a r b o x y l a s e s and a r y l s u l f o n y l d i a z o methane d e r i v i t i v e s I n the same r e p o r t , examples of n e g a t i v e working d i a z o i n d o l e based polymers are shown.


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The concepts of p o s i t i v e or n e g a t i v e systems are by no means a s t r i c t c l a s s i f i c a t i o n s i n c e m o d i f i c a t i o n s of the o r i g i n a l pre-polymer w i t h a d d i t i v e s , w i t h s u i t a b l e changes i n the development p r o c e s s e s can be u t i l i z e d to o b t a i n p o s i t i v e r e s i s t s w h i l s t the unmodified material i s a negative r e s i s t . For example, t e t r a c h l o r o d i a z o c y c l o p e n t a d i e n e s i n g e n e r a l are p o l y m e r i s a b l e by UV l i g h t to o b t a i n n e g a t i v e r e s i s t systems. By u s i n g these m a t e r i a l s w i t h novolak type ( p h e n o l formaldehyde f o r example) r e s i n s , and u s i n g aqueous d e v e l o p e r s , one can o b t a i n a p r e f e r e n t i a l s o l u b i l i t y of the exposed a r e a s , a p o s i t i v e working system. In t h i s c o n t e x t i t s h o u l d be mentioned t h a t most of the newer m a t e r i a l s s u f f e r from the d i s a d v a n t a g e of h a v i n g a b s o r p t i o n peaks around 3000A , a r e g i o n r e s t r i c t e d f o r use o n l y w i t h q u a r t z o p t i c s and d e a r t h of h i g h i n t e n s i t y l i g h t s o u r c e s . The photo- and e l e c t r o n ( i o n and x - r a y ) beam r e s i s t s a r e d i v i d e d i n t o two c l a s s e s ( c f . , F i g . 1) of s o l u b i l i z a t i o n i n d e v e l o p i n g an image. A p o s i t i v e r e s i s t i s more s o l u b l e i n the d e v e l o p e r i n r e g i o n s i r r a d i a t e d w h i l e a n e g a t i v e r e s i s t i s i n s o l u b i l z e d i n the exposed r e g i o n s . The s o l u b i l i t y d i f f e r e n c e s between the exposed and unexposed r e g i o n s are used to form t h r e e d i m e n s i o n a l images which p r o t e c t d e s i r e d m a t e r i a l s d u r i n g the e t c h i n g of the s u b s t r a t e . The p h o t o r e s i s t s are exposed through a mask by c o n t a c t or p r o j e c t i o n p r i n t i n g w i t h the 3650, 4050, and 4350A mercury l i n e s . E l e c t r o n beams 5-30KV and x-rays 210A° have a l s o been employed. The p o s i t i v e r e s i s t r e produces the mask or image i n a d i r e c t p o s i t i v e r e p l i c a t i o n w h i l e a n e g a t i v e r e s i s t reproduces an image r e v e r s a l of the opaque p o r t i o n s of the mask. 0

0

The r e s i s t a r e about one to two microns t h i c k f o r s i l i c o n semiconductor f a b r i c a t i o n of two to ten micron d e v i c e s and about t h r e e to t w e n t y - f i v e micron t h i c k f o r p r i n t e d c i r c u i t board p a c k a g i n g . The r e s i s t s are m a i n l y used i n s u b t r a c t i v e masking f o r e t c h i n g by c h e m i c a l s o r , most r e c e n t l y , by plasmas of f l u o r i d e ions. S i n c e p o s i t i v e r e s i s t s are s o l u b l e i n o r g a n i c s o l v e n t s , they a r e e x c l u s i v e l y used i n an a d d i t i v e c y c l e to d e f i n e metal lands i n a l i f t - o f f scheme. Most of the r e s i s t s are i n the l i q u i d form, but r e c e n t l y s o l v e n t l e s s dry f i l m r e s i s t f o r t h i c k f i l m ap plications have been i n t r o d u c e d . 2 Dry f i l m s p r o t e c t p l a t i n g h o l e s i n c i r c u i t boards.

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TABLE V List

o f commonly used p r e p o l y m e r s i n p h o t o l i t h o g r a p h y

Acrylates Acrylonitriles A l l y l Esters Cellulose esters Epoxies E t h y l e n i c (unsaturated) hydrocarbons Poly-hydroxy e s t e r s ( e g : P e n t a e r y t h r i t o l tetramethacrylate,Ethylene Glycol diacrylate) Polyolefins V i n y l compoundêsters,acids) V i n y l i d e n e compounds. Common

Sensitizers


Class

Typical

Examples

Benzoin,Quinones,Benzophenone Diazonium compounds, Mercaptans , D j ^ u l f i ^ ç s Per a c i d s Fe - Fe Silver Halides, CBr ,HgBr Eosin/Amine,Cyanine dyes Methylene b l u e Metal a l k y l s M(CO) ZnO Benzophenone,Benzothiazoles Nitroaromatics E o s i n dyes.

C a r b o n y l compounds Azo compounds O r g a n i c S u l f u r Cpds, Redox systems Halogen compounds Dyes

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The k e t e n e i n t e r m e d i a t e r e a c t s w i t h w a t e r p r e s e n t i n the p h e n o l i c polymer base or i n subsequent a l k a l i n e developer to form a s o l u b l e indene c a r b o x y l i c a c i d . R e c e n t w o r k by R u s s i a n W o r k e r s h a s r e v e a l e d a more r e a s o n a b l e e x p l a n a t i o n o f t h e p h o t o i n s o l u b i l i z a t i o n o f a p h e n o l i c r e s i n by t h e d e c o m p o s e d d i a z o q u i none. The k e t e n e i n t e r m e d i a t e was f o u n d t o be g r a f t e d onto the p h e n o l i c r e s j n through the f o r m a t i o n of a c a r b o x y l i c a c i d ester-

The g r a r t e d p o l y m e r i s s o l u b l e i n t h e a l k a l i n e ph-12 developer. The u n e x p o s e d p h e n o l i c r e s i n p r o t e c t e d by t h e i n s o l u b l e n a p t h o q u i n o n e d i a z i d e i s u n a f f e c t e d by the d e v e l o p e r . Since the polymers are p h o t o s e n s i t i v e t o 5500A°, t h e y c o u l d e v e n t u a l l y be u s e f u l s p i n o f f s as photodegradable polymers s o l u b l e i n a l k a l i n e s o i l s . In c o n t r a s t to dry f i l m p l a t i n g or e t c h i n g nega t i v e r e s i s t s s u c h as D u p o n t s R i s t o n ^ n o d r y f i l m p o s i t i v e r e s i s t s h a v e b e e n m a r k e t e d . — The p o s i t i v e resists offer versatile processing alternatives. In F i g u r e 1, t h e a d d i t i v e l i f t - o f f o r s u b t r a c t i v e e t c h process i s o u t l i n e d . Since the p o s i t i v e r e s i s t s are s o l u b l e i n o r g a n i c s o l v e n t s , m e t a l f i l m s c a n be p a t t e r n e d and " l i f t e d o f f " i n u n d e s i r e d r e g i o n s . Nega t i v e r e s i s t s are not s o l u b l e i n o r g a n i c s o l v e n t s . H a r s h s t r i p p e r s s u c h as a l k a l i o r s t r o n g a c i d s a r e used. T h e s e a g e n t s w o u l d a t t a c k m e t a l s s u c h as c o p p e r or aluminum d e p o s i t e d over n e g a t i v e r e s i s t s . N e g a t i v e r e s i s t s have been e x c l u s i v e l y used i n lithographic plate production. They have been a d o p t e d to e l e c t r o n i c manufacture of p r i n t e d c i r c u i t boards and m i c r o s c o p i c s e m i - c o n d u c t o r d e v i c e s . Micro e l e c t r o n i c grade n e g a t i v e r e s i s t s f r e e of p a r t i c l e s or m e t a l i o n s have been e x c l u s i v e l y marketed. Negative r e s i s t s h a v e b e e n f o r m u l a t e d on a l l a s p e c t s o f p h o t o p o l y m e r i z a t i o n , c r o s s l i n k i n g , and i n s o l u b l i z i n g r e a c t i o n s o f u n s a t u r a t e d monomers o r p o l y m e r s . Since h i g h molecular weight polymers or c r o s s l i n k e d i n s o l u b l e polymers are d e s i r e d to produce a " n e g a t i v e " (image area exposed i s i n s o l u b l e i n d e v e l o p e r ) , v a r i o u s !

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s e n s i t i z e r s and v i n y l o r a l l y l based p o l y m e r s have been used. Three s u c c e s s f u l r e s i s t s e x e m p l i f y b a s i c p a t h s t o o b t a i n an i n s o l u b l e h i g h p o l y m e r . These a r e based on t r i p l e t — s e n s i t i z e d c r o s s l i n k i n g o f p o l y v i n y l c i n n a m a t e . O v e r 300 p a t e n t s h a v e b e e n d i s c l o s e d on t h i s r e a c t i o n . CH 6

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In t h i n f i l m s ^ 2 j i , t h e s y s t e m i s l i m i t e d t o 3650A e x posure and i s n o t s u i t a b l e f o r p r o j e c t i o n p r i n t i n g . R e c e n t l y , t h e s p e c t r a l s e n s i t i v i t y of" t h e c o n j u g a t i o n a s s o c i a t e d w i t h a z i d e chromophore has been extended to 4350A for projection exposure.— In very thin layers 5000A , t h e p r e s e n c e o f oxygen w i l l quench t h e n i t r e n e c r o s s l i n k i n g by c o n v e r s i o n i n t o a n i t r o s o derivative. — P r e p o l y m e r s e s p e c i a l l y d i , t r i , and t e t r a f u n c t i o n a l a c r y l a t e s o r d i a l l y l p t h a l a t e s s e n s i t i z e d by t r i p l e t o r r a d i c a l photos e n s i t i z e r s form t h e t h i r d c l a s s o f n e g a t i v e r e s i s t s . These r e s i s t s a r e a l s o u s e f u l as examples o f r a d i a t i o n c u r a b l e c o a t i n g s , paints, or inks. The b e n z o i n - s e n s i t i z e d i n s o l u b i l i z a t i o n of d i a l l y p t h a l a t e p r e p o l y m e r s and t h e t - b u t y l a n t h r a q u i o n e sensitized gelation of pentaerythritol acrylates - t Q examples o f t h i c k f i l m wet and d r y n e g a t i v e r e s i s t s . — The s y s t e m s a r e o x y g e n s e n s i t i v e . Rapid 0

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i n s o l u b i l i z a t i o n i s achieved through s p a t i a l poly merization i n a l l dimensions. I t i s e x p e c t e d t h a t t h e r m o s e t epoxy p r e p o l y m e r s w i l l be e v e n t u a l l y u s e d as h i g h s p e e d n e g a t i v e r e s i s t s o n c e t h e s h e l f l i f e c a n be e x t e n d e d . Heat s e n s i t i v e p o l y m e r s c o u l d be a c t i v a t e d by a d d i n g t h e p h o t o s e n s i t i z e r immediately before exposure. H i g h l y s e n s i t i v e n e g a t i v e r e s i s t s a r e more d i f f i c u l t t o p r o cess s i n c e i n s o l u b l e r e s i d u e s a r i s i n g from "dark" p o l y m e r i z a t i o n c a n n o t be t o l e r a t e d i n t h e d e v e l o p e d (unexposed) r e g i o n s . Solventless compositions w i l l e v e n t u a l l y be r e q u i r e d f o r h i g h s p e e d n e g a t i v e resists. I d e a l l y , t h e r m a l l y s t a b l e (up t o 300°C) b u t r a d i a t i o n s e n s i t i v e polymers are needed i n the f i e l d of t h r e e d i m e n s i o n a l i m a g e r e c o r d i n g . I n c o m p a r i s o n o f p o s i t i v e and n e g a t i v e r e s i s t s , t h e n a t u r e o f t h e s u b s t r a t e and e t c h i n g c o n d i t i o n s e v e n t u a l l y d i c t a t e the c h o i c e of r e s i s t s . In T a b l e VI i s a comparison of a commercial p o s i t i v e d i a z o q u i o n e r e s i s t with a commercial negative r e s i s t . The p o s i t i v e r e s i s t s have h i g h e r r e s o l u t i o n but s l o w e r s p e e d and l e s s e t c h r e s i s t a n c e . S i n c e g l a s s and m e t a l f i l m s w i t h a m u l t i t u d e of e t c h a n t s or processes are used, b o t h r e s i s t s are w i d e l y used i n s e m i c o n d u c t o r manufacture. H y d r o p h i l l i c and h y d r o p h o b i c s u r f a c e s are e n c o u n t e r e d . A u n i v e r s a l r e s i s t with adhesion and h i g h e t c h r e s i s t a n c e b u t s o l u b l e i n o r g a n i c s o l v e n t s c a n be e n v i s i o n e d as t h e n e x t g e n e r a t i o n material. In t h e s e m i c o n d u c t o r t e c h n o l o g y , a major e f f o r t t o p r o d u c e m i c r o c o m p u t e r c i r c u i t s w i t h enormous memories has been u n d e r t a k e n i n the l a s t decade. T h i s has i n c r e a s e d t h e demand f o r t h e d e v e l o p m e n t o f c i r c u i t s w i t h d i m e n s i o n s n e a r t h e w a v e l e n g t h o f l i g h t 0.5/1. H i g h e r r e s o l u t i o n e x p o s u r e u s i n g X - r a y s , i o n beams and e l e c t r o n beams h a v e b e e n i n v e s t i g a t e d . S i n c e the e l e c t r o n beam c a n be a c c u r a t e l y moved by d e f l e c t i o n i n e q u i p m e n t s u c h as i n a s c a n n i n g e l e c t r o n m i c r o s c o p e , v a r i o u s r e s i s t s b a s e d on d e g r a d i n g o r c r o s s l i n k i n g polymers have been i n v e s t i g a t e d . The n e g a t i v e e l e c t r o n beam r e s i s t s as shown i n T a b l e V I I a r e b a s e d on v i n y l p o l y m e r s —

with

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TABLE V I

Comparison o f P o s i t i v e and N e g a t i v e Positive Diazoquinone

Negative Bis-Azide 0

S p e c t r a l Sens. 3650-5000A Oxygen Sens. no Exposure ergs/cm 2 χ 10 Resolution/mm 800 l i n e s Etch Resistance a c i d but n o t alkali· Strip organic solvents Lift-Off Yes


2

Resists

β

0

3650A yes 4 χ 10 300 l i n e s acids& a l k a l i s 5

acids No

TABLE V I I Electron

Beam

Polymers

POSITIVE - DEGRADING TYPE Min. Dose Coul/cm 2

POLYMETHYL METHACRYLATE POLY T-BUTYL METHACRYLATE POLYMETHYL ISOPROPENYL KETONE CROSSLINKED POLYMETHYL METHACRYLATE POLY OLEFIN SULFONES POLY HEXYL ISOCYANATE DIAZOQUINONE PHOTORESIST

5

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NEGATIVE - INSOLUBILIZING TYPE

AZIDE PHOTORESIST POLY DIALLYL ORTHO PHTHALATE POLYSILOXANES EPOXIDIZED RUBBERS POLYGLYCIDYL METHACRYLATE

-6

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C r o s s l i n k i n g s i d e c h a i n s o f R s u c h as e p o x y , o l e f i n , or c o n j u g a t e d k e t o n e s have a l s o been i n v e s t i g a t e d . Traditionally a l l c o a t i n g s c u r e d b y h e a t , MEV e l e c t r o n s , gamma r a y s , o r UV l i g h t may f i n d u s e f u l a p p l i c a t i o n s as h i g h s p e e d ( t h i n f i l m ( O . l - l . O / i ) ) e l e c t r o n beam r e s i s t s . The r e s o l u t i o n o f t h e n e g a t i v e e l e c t r o n beam r e s i s t s a t l-2/ι r a n g e i s n o t a s g o o d as t h e e q u i v a l e n t p o s i t i v e e l e c t r o n beam r e s i s t s . Since no p h o t o s e n s i t i z e r s a r e n e e d e d , i t c a n be e x p e c t e d t h a t t h o u s a n d s o f v i n y l p o l y m e r s c o u l d be f o r m u l a t e d into negative r e s i s t s . Since polymers w i t h t e t r a s u b s i t u t e d centers are d i f f i c u l t t o s y n t h e s i z e , two m a i n t y p e s o f p o s i t i v e r e s i s t s have been d i s c l o s e d . P o l y m e t h y l m e t h a c r y l a t e ^ o r p o l y m e t h y l i s o p r o p e n y l k e t o n e ^ h i c h undergo s i d e c h a i n e l i m i n a t i o n and s u b s e q u e n t main c h a i n f r a c t u r e have b e e n u s e d as p o s i t i v e e l e c t r o n beam r e s i s t s ( c f . Table V I I ) . The i n c o r p o r a t i o n o f a weak l i n k i n t h e m a i n c h a i n o f a o l e f i n s u l f o n e c o p o l y m e r s u c h as p o l y - 1 butene s u l f o n e i s a r e c e n t example o f polymers w i t h high G value ( ând high s e n s i t i v i t y 10" coul/sq. cm. needed t o r e a d and w r i t e p a t t e r n s i n m i n u t e t h r o u g h p u t times.— The l o w c e i l i n g t e m p e r a t u r e (y*+25° C) o f some highly radiation sensitive sulfones severely limits the p r a c t i c a l p r o c e s s i n g of r e s i s t s . Polymer f i l m s w h i c h a r e d e p o s i t e d as c o l d f i l m s a t l o w t e m p e r a t u r e by gas p h a s e r e a c t i o n s i n c l u d i n g r a d i a t i o n p r o c e s s i n g s u c h as p l a s m a o r UV, c a n be e n v i s i o n e d as t h e n e x t generation of s o l v e n t l e s s r e s i s t s of high s e n s i t i v i t y . The e q u i p m e n t f o r p r o c e s s i n g p o l y m e r s p e r f o r m s t h r e e m a j o r s t e p s : c o a t - d r y , e x p o s e , and d e v e l o p - r e a d out. C o a t i n g f i l m s g r e a t e r t h a n a m i c r o n i s accom p l i s h e d b y p a i n t t e c h n i q u e s s u c h as r o l l , d i p , s p r a y , or laminate. T h i n c o a t i n g s s u c h as i n m i c r o e l e c t r o n i c s a r e a p p l i e d by s p i n n i n g t h e o b j e c t and applying the r e s i s t solution. U l t r a t h i n films are a p p l i e d b y p o l y m e r i z a t i o n o f a c t i v a t e d monomers o n t o the s u b s t r a t e . S o l v e n t c o a t i n g s c a n b e d r i e d by c o n d u c t i o n , c o n n e c t i o n o r o t h e r f o r m s o f e n e r g y s u c h as i n f r a r e d , microwave, or i n d u c t i o n h e a t i n g . After d r y i n g t h e f i l m s c a n be f l o o d e x p o s e d b y m e r c u r y v a p o r l a m p s o r s p r a y e d w i t h KV e l e c t r o n s . T h i n r e s i s t f i l m s are contacted p r i n t e d w i t h l i g h t or d i g i t a l l y patterned w i t h s u b m i c r o n e l e c t r o n beams. The i m a g e s a r e u s u a l l y d e v e l o p e d by o r g a n i c o r a l k a l i n e s o l v e n t s by s t a t i c o r spray processing. The e f f i c i e n c y o f t h e e x p o s u r e s o u r c e i s a c r i t i c a l c o s t and t h r u p u t f a c t o r . U l t r a v i o l e t sources output up t o 1 0 % o f t h e i n p u t e n e r g y w h i l e 150 KV e l e c t r o n c u r e s o u r c e s can be 70% e f f i c i e n t . To r e c o r d h i g h l 0

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121

r e s o l u t i o n images, p o i n t s o u r c e s are used but at a s a c r i f i c e o f low o u t p u t i n t e n s i t y and e f f i c i e n c y . E l e c t r o n beams a r e t h e most a c c u r a t e l y c o n t r o l l e d t h a n d e f l e c t e d l i g h t , i o n , or x - r a y s . The p r o c e s s i n g e q u i p m e n t i n r e s i s t t e c h n o l o g y has advanced from the t r a d i t i o n a l b a t c h tanks f o r d e v e l o p ing, e t c h i n g , and s t r i p p i n g t o f u l l y a u t o m a t e d c o a t i n g , b a k i n g , e x p o s i n g d e v e l o p i n g , e t c h i n g , and s t r i p p i n g systems. The a u t o m a t i o n o f t h i c k r e s i s t f o r p r i n t e d c i r c u i t s was a c h i e v e d i n t h e l a t e s i x t i e s and t h e automation of t h i n f i l m s r e s i s t f o r semiconductor p r o c e s s i n g has b e e n made c o m m e r c i a l l y a v a i l a b l e . — The e x p o s u r e o f r e s i s t s has s w i t c h e d from c o n t a c t t o p r o x i m i t y to p r o j e c t i o n p r i n t i n g i n a e f f o r t to extend mask l i f e . M o r e d u r a b l e masks o f chrome o r i r o n o x i d e on g l a s s h a v e o v e r c o m e t h e l i m i t e d l i f e o f s i l v e r e m u l s i o n masks. I n t h e f u t u r e e l e c t r o n beams a r e e n v i s i o n e d t o r e p l a c e masked e x p o s u r e . Besides u s i n g the r e s i s t s to e t c h copper boards, m e t a l p a r t s , o r s i l i c o n s u r f a c e s , new a p p l i c a t i o n s have been d i s c l o s e d . S p a t i a l images have been h o l o g r a p h i c a l l y recorded i n r e s i s t s . — The r e s i s t s y s t e m s of s i l o x a n e s c a n be c o n v e r t e d a f t e r i m a g i n g by o x i d a t i o n i n t o p a s s i v a t e d g l a s s f o r d i r e c t f o r m a t i o n of insulated c i r c u i t s . — The r e s i s t c a n be f i l l e d w i t h glass£2 o r m e t a l s — and f i r e d t o f o r m c i r c u i t s directly. I n one a p p l i c a t i o n , t h e r e s i s t s a r e f i l l e d w i t h e l e c t r o l e s s p l a t i n g s e n s i t i z e r f o r d e p o s i t i o n of metal f i l m s . — The r e s i s t s c a n be u s e d i n t h e l i t h o g r a p h i c s e n s e as r e l i e f p r i n t i n g p l a t e s . — The c h e m i c a l m i l l i n g of s m a l l machine p a r t s i s a economical means o f mass p r o d u c t i o n . The g e n e r a l i m p r o v e m e n t s i n v a r i o u s p r o p e r t i e s s u c h as c h e m i c a l and t h e r m a l s t a b i l i t y are d e s i r a b l e f o r e x t e n s i v e f u t u r e a p p l i cations. The c o m m e r c i a l s u p p l i e r s o f r e s i s t h a v e e s t a b l i s h e d e n g i n e e r i n g l a b s to e x p e r i e n c e the customer a p p l i c a t i o n s on m u l t i f a r i o u s i n s u l a t o r o r c o n d u c t i v e s u b s t r a t e s and t o i m p r o v e by e x p e r i e n c e t h e performance of r e s i s t f o r m u l a t i o n s . In the l a s t c a t e g o r y o f r a d i a t i o n s e n s i t i v e p o l y m e r s , the development of e c o l o g i c a l l y sound p h o t o d e g r a d a b l e p o l y m e r s has been a n o t e w o r t h y application. Polymers have been t r a d i t i o n a l l y f o r m u l a t e d w i t h u l t r a v i o l e t absorbers or a n t i - o x i d a n t s to prevent o x i d a t i v e , p h o t o c h e m i c a l , t h e r m a l , o r biodégradation d u r i n g p r o c e s s i n g or i n consumer p r o d u c t s . Consumer p a c k a g i n g p r o d u c t s have been s e n s i t i z e d to photod e g r a d e by t h e i n c o r p o r a t i o n o f k e t o n e g r o u p s t o a b s o r b 3000A s u n l i g h t e x p o s u r e . ^ The N o r r i s h e l i m i n a t i o n of t h e a d j a c e n t a l k a n e g r o u p s i n e t h y l e n e c a r b o n mon o x i d e c o p o l y m e r s w i t h c h a i n s c i s s i o n i s p r o p o s e d as a 0


122

UV


l i k e l y d e g r a d a t i o n mechanism. In a r e a l i s t i c sense, v e r y l i t t l e 2537 A° r a d i a t i o n i s a v a i l a b l e f r o m t h e p o l l u t e d atomsphere f o r photodecomposition. The p o l y m e r i c a d d i t i v e s i n c o m m e r c i a l p l a s t i c s h a v e t o be eliminated i f reasonable degradation i s to occur. The b i o d e g r a d a b l e s y s t e m s a r e more e f f e c t i v e t h a n p h o t o degradation s i n c e uniform exposure i s d i f f i c u l t t o achieve f o r waste m a t e r i a l s . Perhaps, a combination o f b i o - , p h o t o - , o r h e a t d e g r a d a b l e p l a s t i c s w i l l be produced f o r food p r o d u c t s . Up t o t h i s p o i n t we h a v e surveyed the v a r i o u s a p p l i c a t i o n s of r a d i a t i o n s e n s i t i v e p o l y m e r s , t h e c o m p o s i t i o n s and c h e m i c a l p a r a m e t e r s and b r i e f l y e x a m i n e d t h e p r o c e s s i n g s equences. F i n a l l y , the comparison of the e f f i c i e n c y of v a r i o u s r a d i a t i o n sources i s of imminent i n t e r e s t . Each a p p l i c a t i o n r e q u i r e s a p a r t i c u l a r c o m b i n a t i o n o f t h e m a t e r i a l a b s o r p t i o n and t h e r a d i a t i o n s o u r c e . I n o r d e r t o a s s e s s t h e e f f i c i e n c y o f each o f t h e s y s t e m s , s e v e r a l f a c t o r s h a v e t o be t a k e n i n t o c o n s i d e r a t i o n . N o t a b l e among t h e s e a r e t h e t h i c k n e s s o f t h e f i l m , t h e a b s o r p t i o n c r o s s s e c t i o n , and t h e s u b s e q u e n t c h e m i c a l r e a c t i o n s i n t h e exposed phase. As n o t e d b e f o r e i n T a b l e I I , t h e s p e c t r u m o f e n e r g i e s used r a n g e f r o m h i g h e n e r g y (MeV) e l e c t r o n s t o l o w e r energy (KeV) e l e c t r o n s and x - r a y s t o t h e l o w e s t e n e r g y (eV) u l t r a v i o l e t r a y s . I n some c a s e s t h e a b s o r p t i o n c r o s s s e c t i o n s a r e d e p t h d e p e n d e n t f u n c t i o n s as t y p i f i e d by e l e c t r o n p e n e t r a t i o n i n t o p o l y m e r s ( c f . , f i g u r e 2 ) . H e n c e t h e c h o i c e o f e l e c t r o n beam e n e r g i e s i s s t r i c t l y a m a t t e r o f the p e n e t r a t i o n range d e s i r e d i n t h e e x p o s e d medium. F o r t h i n r e s i s t f i l m s ( l - 4 j i ) , 5-30kV e l e c t r o n beams a r e u s e d i n s e m i c o n d u c t o r fabrication. The c u r i n g o f t h i c k e r f i l m s (lO-1000/ι) r e q u i r e s a h i g h e r e n e r g y ( 0 . 0 5 - 2 M e V ) e l e c t r o n beam. The i m a g e r e a d o u t e f f i c i e n c y ( a s i m p l e c o n t r a s t f u n c t i o n f o r l i t h o g r a p h i c r e s i s t ) c a n be r e l a t e d t o the r a t i o of the c h e m i c a l or p h y s i c a l p r o p e r t i e s of the exposed t o unexposed m o l e c u l a r w e i g h t s . A r e l a t i v e comparison i s given i n Table V I I I of v a r i o u s exposable compositions. The g r e a t e s t c h a n g e o r c o n t r a s t i n m a t e r i a l p r o p e r t i e s o c c u r s b e t w e e n a monomer (M) a n d a p o l y m e r . ( P z ) The n M — t P n ^ P z — » P*, r a d i a t i o n i n t i a t e d p o l y m e r i z a t i o n has t h e h i g h e s t e f f i c i e n c y s i n c e a m p l i f i c a t i o n (up t o 1 0 ^ ) o c c u r s a f t e r i n i t i a t i o n by c o n v e n t i o n a l p o l y m e r i z a t i o n a t l o w ( ^ 5 k c a l . ) a c t i v a t i o n e n e r g i e s . As t h e r e a c t i o n proceeds to high c o n v e r s i o n , the propagating i n t e r m e d i a t e s a r e d e a c t i v a t e d by t h e v i s c o u s m e d i a .


9.

MOREAU

AND

viswANATHAN


S e c o n d l y , l o w v i s c o s i t y monomers c a n n o t f o r m f i l m s u s e f u l i n c o a t i n g o r i n k c u r i n g a n d as r e s i s t s . Thus, the a m p l i f i c a t i o n and c o n t r a s t g a i n i s l o s t by u s i n g p r e p o l y m e r s Pn t o o b t a i n f i l m f o r m i n g p r o p e r t i e s . From a photochemical viewpoint, a polymerizable composition would have a h i g h speed b u t f o r p r a c t i c a l i m a g i n g c o n t r a s t , t h e c r o s s l i n k i n g r e a c t i o n i s t h e second b e s t . Although c r o s s l i n k i n g (Pz—•Poo) o r s c i s s i o n i n g ( P o o — * P z ) h a v e a b o u t t h e same G v a l u e s ( 0 . 1 - 1 0 ) a n d a c t i v a t i o n e n e r g i e s , (10-40 k c a l / m o l e ) , a g r e a t e r change i n p h y s i c a l p r o p e r t i e s f o r e q u i v a l e n t doses occurs i n going from a uncross l i n k e d t o a c r o s s l i n k e d polymer i n s t e a d o f l o w e r i n g t h e m o l e c u l a r w e i g h t by degradation. O n l y two c r o s s l i n k s p e r c h a i n a r e n e e d e d to achieve i n f i n i t e m o l e c u l a r weight. By t h e same reasoning, the selective degradation of crosslinks i n a polymer network would form a h i g h speed " p o s i t i v e " r e s i s t or photodegradable f i l m . The c r o s s l i n k i n g o f p o l y m e r s c a n b e a c h i e v e d b y t h r e e major r e a c t i o n s : ( 1 ) p o l y m e r i z a t i o n o f m u l t i f u n c t i o n a l monomers ( e . g . , t e t r a a c r y l a t e s ) , ( 2 ) s e l f c r o s s l i n k i n g p o l y m e r s ( e . g . , p o l y v i n y l c i n n a m a t e s ) and (3) c r o s s l i n k i n g a g e n t s ( e . g . , b i s a z i d e s ) . For the c h o i c e o f e x p o s u r e s o u r c e and c o s t , i t a p p e a r s t h a t h i g h r e s o l u t i o n imaging f o r s m a l l exposed areas i s a c h i e v a b l e b y u l t r a v i o l e t o r l o w KV e l e c t r o n s f o r s u b micron dimensions. I n f i g u r e I I I , the exposure costs f o r l a r g e r a r e a s a r e c o n s i d e r e d t o be l o w e r t h a n thermal curing. I n t h i n f i l m s f o r m i c r o e l e c t r o n i c s (lyu) t h e e x p o s u r e s y s t e m s c a n b e c o m p a r e d i n T a b l e I X . As c a n be s e e n , t h e t y p i c a l u l t r a v i o l e t ( 3 6 5 0 A ) e x p o s u r e i s a b s o r b e d much more e f f i c i e n t l y t h a n e l e c t r o n o r x - r a y s . H o w e v e r , t h e h i g h e r e n e r g y e l e c t r o n beam c a n be f o c u s e d to submicron dimensions w h i l e t h e x-ray exposure t h r o u g h a mask h a s a l a r g e d e p t h o f f o c u s . From t h e m a t e r i a l s v i e w p o i n t , i t i s o f i n t e r e s t t o examine t h e thermodynamics o f r a d i o l y s i s o f p o l y m e r s . S i n c e gamma r a d i o l y s i s d a t a i s r e a d i l y a v a i l a b l e , p o l y m e r s c a n be c o m p a r e d as i n T a b l e X. N o t e t h a t m a t e r i a l s having a l a r g e heat of p o l y m e r i z a t i o n tend to c r o s s l i n k under r a d i o l y s i s . T h e same p o l y m e r s a r e t h e r m a l l y r e s i s t a n t t o d e g r a d a t i o n . Degrading polymers h a v e l o w c e i l i n g t e m p e r a t u r e s (^150°C) a n d l o w h e a t s of p o l y m e r i z a t i o n . The G v a l u e s ( c h e m i c a l e v e n t s / 1 0 0 e v a b s o r b e d ) a r e a l s o i n d i c a t i o n s of the tendency of a polymer to degrade o r i n s o l u b i l i z e under any form o f r a d i a t i o n i n e x c e s s o f bond e n e r g i e s . In Table X I , a compilation of t h e G v a l u e s i s n o t e d a l o n g w i t h t h e expended energy f o r each bond b r o k e n o r formed. I n the case of 0


124


TABLE Image IMAGING PROPERTY S o l u b i l i t y Rate Volatilization Rate Tackiness Adhe s i o n Adsorption Viscosity Solidification Permeabili ty Transmi ssion Re f r a c t i o n

VIII

Differentiation TYPE OF Positive

IMAGE Negative Dec Dec Dec Inc Dec Inc Inc Dec Dec Inc

Inc inc

Inc Dec Dec Inc Inc Dec

COST COMPARISON OF CURING i—

I Figure S. Cost comparison of curing

ι ι ι ι ι 50 100 130 200 PROCESSED AREA SQUARE METERS 1 METER Wl DTH

9.

MOREAU

AND


viswANATHAN

TABLE Efficiency


E-Beams,Ion beams(20-50KV) X-Rays,Gamma (50KV-1MEV) UV ( t y p . 3 6 5 0 A ° )

Typical absorption percent 4 - 1 1

0.5-1

50

25

Radiation Effect "~

Overall efficiency percent 0.01 0.0005 12.5

X

of Crosslinking

Poly ethylene Crosslink -Propylene -Methyl acrylate -Isobutene Scission -Methyl methacrylate -alpha-Methyl styrene from

Reaction efficiency percent 1

- 0.01

TABLE

Polymer

IX

of Radiation Reactions i n Films

Energy System

Comparison

125

Vs

Scission

of

Polymers

Heat o f PolymnΓ Kcal/mole

Monomer y i e l d on pyrolysis

22 16.5 19

0.025 2.0 2.0

10 13

20.0 100.0

9

100.0

D.E.Roberts,J.Res.Natl.Bur.Std.,

%

44,221,(1950)

UV


126


POLYMERS

d e g r a d i n g p o l y m e r s , t h e s u l f o n e s expend the l e a s t energy (12.5ev/bond) which i s s t i l l i n excess of t h e b o n d e n e r g y ( 4 - 5 e v ) . C o m p e t i n g r e a c t i o n s s u c h as e m i s s i o n , c h a r g i n g , and t h e r m a l d i s s i p a t i o n o f t h e absorbed e n e r g i e s reduce the e f f e c t i v e n e s s of absorbed u l t r a v i o l e t e l e c t r o n , x - r a y , o r i o n beams. F r o m T a b l e X I , i t c a n be s e e n t h a t p o l y m e r s h a v e s p e c t r u m o f d o s e s n e c e s s a r y t o o b t a i n maximum r e a d o u t efficiency. We w i l l e x a m i n e i n t h e f o l l o w i n g e x a m p l e t h e i m a g i n g o f a p o s i t i v e e l e c t r o n beam r e s i s t as a f u n c t i o n o f dose and m o l e c u l a r w e i g h t - s o l u b i l i t y changes· A p o s i t i v e r e s i s t s y s t e m c a n be o f e i t h e r two types. The c l a s s i c a l d i a z o q u i n o n e s y s t e m r e p r e s e n t s a photochemical rearrangement r e a c t i o n which i s the basis of commercial p h o t o r e s i s t s . Scissioning or d e g r a d a t i o n o f a p o l y m e r c h a i n by l i g h t o r e l e c t r o n s i s a l a t e r example of s o l u b i l i t y i n d u c e d change. We w i l l examine t h i s change i n d e t a i l . A p o s i t i v e e l e c t r o n beam r e s i s t i m a g e i s d e v e l o p e d by i m m e r s i o n i n a s o l v e n t w h i c h d i s s o l v e s the e x p o s e d r e g i o n a t a r a t e ( S f ) w h i c h i s f a s t e r ( a p p r o x . 10X) t h a n t h e u n e x p o s e d r a t e ( S i . The r a t e of d i s s o l u t i o n o f a l i n e a r p o l y m e r i s r e l a t e d t o i t s molecular weight M by t h e U b e r r e i t e r f u n c t i o n ( 2 6 ) :

s

*

k

M"

0

[1 ]

1

where q i s t h e s o l v e n t - p o l y m e r s o l u b i l i t y p a r a m e t e r . I n f i g u r e 4, t h e i n c r e a s e i n s o l u b i l i t y o r d e c r e a s e in viscosity i s i l l u s t r a t e d f o r a corresponding decrease i n molecular weight. In a p o s i t i v e r e s i s t t h e image r e a d o u t e f f i c i e n c y f o r a p a r t i c u l a r e x p o s u r e dose i s d e f i n e d as t h e r a t i o of s o l u b i l i t y r a t e s :

The exposure

image r e a d o u t c a n be d e r i v e d d o s e by t h e e q u a t i o n ( 2 7 ) : SR

= ( 1 + |

so ci 2

(TX)o

+

φ5 0®C 1 J

Sooci "excited acceptor Transfer 2

Energy

+ Clso « free r a d i c a l products 2

0



142

POLYMERS

In t h i s type of energy t r a n s f e r r e a c t i o n , t h i o xanthone (TX) i s the o n l y a c t i v e l i g h t a b s o r b i n g s p e c i e s (cut o f f f i l t e r s or t i t a n i u m pigments s t r o n g l y absorb r a d i a t i o n below 360 nm) c o n t a i n e d i n the r e a c t i v e p a i n t system. Thioxanthone alone does not p h o t o i n i t i a t e p o l y m e r i z a t i o n of v i n y l u n s a t u r a t e d monomers. Q u i n o l i n e s u l f o n y l c h l o r i d e (QSC) absorbs l i g h t at a p p r o x i m a t e l y 310-330 nm which r e s u l t s i n h o m o l y t i c cleavage of the s u l f o n y 1 - c h l o r i d e bond to produce i n i t i a t i n g f r e e r a d i c a l s p e c i e s ( c l e a r r e a c t i v e systems). In f i l t e r e d or pigmented c o a t i n g systems a l l l i g h t t r a n s m i t t e d at 310-330 nm i s absorbed by the f i l t e r s or pigment so d i r e c t a b s o r p t i o n or e x c i t a t i o n by QSC i s not a l l o w e d . The o n l y way a p h o t o c h e m i c a l i n i t i a t i o n can take p l a c e i s through the l i g h t a b s o r p t i o n of energy (370-380 nm) by TX which can then t r a n s f e r i t s absorbed energy to a ground s t a t e QSC m o l e c u l e and r e s u l t i n f r e e r a d i c a l f o r m a t i o n (see diagram ) . < i l ) The t r i p l e t energy f o r TX i s 65 k c a l / m o l e and the t r i p l e t energy f o r QSC i s 60 k c a l / m o l e (ΔΕ = 5 k c a l / mole e x c e s s ) which meets one of the b a s i c r e q u i r e m e n t s f o r e f f i c i e n t energy t r a n s f e r i . e . Et donor (TX) > E a c c e p t o r (QSC). (±1> t

Experimental The same e x p e r i m e n t a l c o n d i t i o n s were used as p r e v i o u s l y d e s c r i b e d i n r e f e r e n c e 13 f o r the photo p o l y m e r i z a t i o n of m e t h y l m e t h a c r y l a t e (MMA) with 4,4 b i s ( d i e t h y l a m i n o ) benzophenone (DEABP) l x l 0 ~ M i n com b i n a t i o n w i t h 3X10~2M benzophenone. The d i l a t o m e t r i c i r r a d i a t i o n equipment i s shown i n F i g u r e 5. This apparatus a l l o w s the f o l l o w i n g of r a t e s of p h o t o p o l y m e r i z a t i o n (Rp) of MMA i n the presence of v a r i o u s con c e n t r a t i o n s of pigments and a l s o a l l o w s measurement of changes i n l i g h t i n t e n s i t y ( I ) as % t r a n s m i t t a n c e ( Τ ) . The pigment used i n these experiments was u n t r e a t e d c h l o r i d e process r u t i l e titanium dioxide (Glidden P i g ments D i v i s i o n , B a l t i m o r e , Md.). T h i s was o b t a i n e d as a water s l u r r y (pH 1-3) and was screened b e f o r e u s i n g . U l t r a v i o l e t s p e c t r a of v a r i o u s p h o t o s e n s i t i z e r s ( i n THF) were taken w i t h a P e r k i n Elmer 350 spectrophoto meter . f

2

0

Results

and

Discussion

One of the u n u s u a l f e a t u r e s of the U V - c u r i n g of pigmented c o a t i n g s i s t h a t the apparent r a t e of p o l y m e r i z a t i o n (Rp) appears to be somewhat f a s t e r than the


10.

MCGiNNiss

UV

Curing of Pigmented Coatings

143

conventional clear coatings. One example i n the l i t e r a t u r e i s the thermal and p h o t o c h e m i c a l p o l y m e r i z a t i o n s of MMA i n the p r e s e n c e of s i l i c a g e l pigments. These r e s u l t s show t h a t the a d d i t i o n of c o l l o i d a l s i l i c a to MMA markedly d e c r e a s e s the time r e q u i r e d to complete the p o l y m e r i z a t i o n . These e f f e c t s c o u l d be caused by the change i n v i s c o s i t y of the d i s p e r s i o n due to the s i l i c a because an i n c r e a s e i n v i s c o s i t y would be expected to reduce the r a t e c o n s t a n t ( k t ) f o r t e r m i n a t i o n and e f f e c t the o v e r a l l r a t e of p o l y m e r i z a t i o n (Rp k t - 1 / 2 ) . The a c c e l e r a t i o n of the r a t e of p o l y m e r i z a t i o n or c o n v e r s i o n was shown to occur at c o n c e n t r a t i o n s of s i l i c a where t h e r e was l i t t l e e f f e c t on v i s c o s i t y of the d i s p e r s i o n . Another mechanism i s the s t a b i l i z a t i o n of f r e e r a d i c a l s on the pigment s u r f a c e which reduces mutual t e r m i n a t i o n r e a c t i o n s of growing r a d i c a l c h a i n s but the Authors found no e v i dence of polymer g r a f t i n g onto the s i l i c a s u r f a c e . The s i l i c a pigment was thought to e x e r t some s o r t of c a t a l y t i c e f f e c t on the r a t e of p o l y m e r i z a t i o n . (jJi) In the p h o t o p o l y m e r i z a t i o n of MMA i n the presence* of r u t i l e T i 0 i t was n o t i c e d ( F i g u r e 6) t h a t as one i n c r e a s e s pigment c o n c e n t r a t i o n t h e r e i s a c o r r e s p o n d ing d e c r e a s e i n l i g h t t r a n s m i s s i o n ( I ) through the reaction c e l l . The R f o r the r e a c t i o n shows a s l i g h t i n c r e a s e up to a l i m i t i n g v a l u e of pigment c o n c e n t r a t i o n at which p o i n t the R d e c r e a s e s c o r r e s p o n d i n g to the d e c r e a s e i n I . T h i s r e l a t i v e i n c r e a s e of Rp at d e c r e a s i n g Io c o u l d be a measure of c a t a l y t i c a c t i v i t y or l i g h t r e f l e c t a n c e and s c a t t e r i n g of the pigment. C l e a r or pigmented l i q u i d p h o t o c u r a b l e coatings can be a p p l i e d to a f l e x i b l e s u b s t r a t e ( m e t a l , p o l y e t h y l e n e or f i l t e r paper) and the r e l a t i v e r a t e of cure or network f o r m a t i o n as a f u n c t i o n of changes i n dynamic m e c h a n i c a l p r o p e r t i e s v e r s u s exposure time can be determined. F i g u r e s 7 and 8 c o n t a i n a diagram and the energy c a l c u l a t i o n s f o r a simple f l a s h lamp c i r c u i t . This xenon f l a s h lamp ( L l ) was used to cure f i l m s on a t o r s i o n pendulum ( F i g u r e 9 ) . The theory and manufact ure of c o n v e n t i o n a l t o r s i o n a l pendulums have been des c r i b e d elsewhere but a simple diagram and b a s i c c a l c u l a t i o n s f o r the pendulum are shown i n F i g u r e s 9 and 1 0 . ? ' Both pigmented and c l e a r c o a t i n g s were cured by t h i s t e c h n i q u e e i t h e r w i t h f l a s h or steady s t a t e i l l u m i n a t i o n from a 450watt medium p r e s s u r e mer cury lamp ( L i ) . F i g u r e 11 shows a sample t r a c e of the t o r s i o n a l pendulum b e f o r e and a f t e r i r r a d i a t i o n . Be f o r e i r r a d i a t i o n the c o a t i n g i s a f l u i d v i s c o u s l i q u i d and e x h i b i t s a h i g h degree of damping (peak to peak 2

0

p

p

0

1


144

^DILATOMETER WITH STIRRING BAR 1

TO RECORDER

-PHOTOCELL


CONSTANT TEMPERATURE BATH MAGNETIC STIRRER Figure 5.

I

3

5

10

[PIGMENT] X

Figure 6.

3 I0 (g) 2

5

10

POLYMERS

MCGiNNiss

UV Curing of Pigmented Coatings

115 VAC

TRIG. WIRE 0 TESLA COIL

R, — L A B RHEOSTAT T| — 1 5 KV, 30MA (neon sign trans.) D | . - R C A CR-IIO , 10 KV R — 2 5 0 K , H I G H VOLTAGE R — ( 1 0 ) 2 5 K . I 0 WATT 4


2

3

145

V —VOLTMETER S 3 - HIGH VOLTAGE SWITCHES S - SPARK GAP C — 20 KV 10 )JF CAPACITOR L - XENON FLASH LAMP XENON CORP h

4

Figure 7.

C - S T O R A G E CAPACITY IN FARADS V - V O L T A G E ON C IN VOLTS C= IO]JF V* 5 0 0 0 VOLTS Ε = 125 J

P - A V E R A G E POWER DELIVERED TO FLASH TUBE F - F L A S H E S PER SECOND FLASH DURATION TO 5 0 % PEAK s 2 0 4 0 JJ SEC. QUANTA FLASH ( 2 0 0 - 4 0 0 nm) S 10 QUANTA Figure 8.



LOWER SAMPLE HOLDER Figure 9.

K

s

38.54 LI CD p P 3

2

G-SHEER MODULUS IN DYNES/CM L - SAMPLE LENGTH IN INCHES C- SAMPLE WIDTH IN INCHES D-SAMPLE THICKNESS IN INCHES I -POLAR MOMENT OF INERTIA BAR IN G - C M P-PERIOD OF OSCILLATION IN SEC. μ-SHAPE FACTOR C/D 2

2

A-KX)

•'«(%)

Δ-LOG DECREMENT Figure 10.

POLYMERS


10. MCGiNNiss


4,

~~5 FLASHES Figure 12.

10

147

148


IN POLYMERS

d i s t a n c e s a r e l a r g e and t h e r e i s a l a r g e r a p i d d e c r e a s e i n a m p l i t u d e ) . A f t e r i r r a d i a t i o n the c o a t i n g under goes c u r i n g which i n f l u e n c e s the r e l a t i v e r i g i d i t y of the sample and the degree of damping becomes lower (peak t o peak d i s t a n c e s a r e s h o r t e r and t h e r e l a t i v e change i n amplitude of the waveform i s s m a l l e r or shows l e s s damping).(19) In F i g u r e 12 i s a t y p i c a l c u r i n g curve f o r a c o a t ing as a f u n c t i o n of r e l a t i v e r i g i d i t y (K V p ) , l o g decrement (£n Σ /Am+1) and f l a s h i r r a d i a t i o n . The c o a t i n g (O.lg sample), p e n t a e r y t h r i t o l t r i a c r y l a t e (PETA) s e n s i t i z e d w i t h 2% η-butyl e t h e r of b e n z o i n (BEB) or 2% benzophenone (BP)/3% m e t h y l d i e t h a n o l a m i n e (MDEOA), was cured between t h i n p o l y e t h y l e n e s h e e t s and as t h e c u r i n g r e a c t i o n took p l a c e , r e l a t i v e r i g i d i t y stayed f a i r l y constant u n t i l a f i n a l breaking point was reached a f t e r 5 i r r a d i a t i o n f l a s h e s (5,000 v o l t s / flash). A damping i n c r e a s e o c c u r r e d a f t e r i n i t i a l r a d i a t i o n then d e c r e a s e d r a p i d l y at t h e onset of further gelation. In pigmented c o a t i n g s the r e l a t i v e i n c r e a s e of change i n r i g i d i t y or d e c r e a s e i n damping as a f u n c t i o n of exposure time f o r d i f f e r e n t pigments a t e q u a l PVC f o l l o w t h e o r d e r : S 1 O 2 > anatase Ti02 > r u t i l e T i 0 2 . The r e l a t i v e changes i n r i g i d i t y (K 1/p ) or l o g de crement a r e a measure of cure f o r t h e c o a t i n g . The d a t a a t t h i s time i s o n l y q u a l i t a t i v e but e f f o r t s a r e b e i n g made to develop t h e t o r s i o n a l pendulum-UV-curing t e c h n i q u e s and to q u a n t i f y r e l a t i o n s h i p s between r e l a t i v e r i g i d i t y , damping, g e l - p o i n t , v i t r i f i c a t i o n , and i r r a d i a t i o n i n t e n s i t y or exposure time f o r c l e a r as w e l l as pigmented p h o t o c u r a b l e c o a t ings . F u r t h e r work i s planned t o determine t h e exact parameters a s s o c i a t e d w i t h p h o t o c u r i n g of pigment c o a t i n g s and the development of more e f f i c i e n t p h o t o i n i t i ators. 2


An

2

References 1. 2. 3.

4.

K i n s t l e , J . , Paint Varnish Prod., 17, June (1973). Special Radiation Cure, Issue Paint Varnish Prod., August (1974). Bassemir, R. W. and Bean, A. J . , paper presented at 26th Annual Meeting of TAGA, St. Paul, Minn., May 13-15, 1974; to be published i n TAGA Pro ceedings. Wicks, Z. W., paper presented at 14th Annual Coatings Symposium, North Dakota State u n i v e r s i t y , Fargo, North Dakota, June 3-5, (1974).

10. MCGiNNiss

5. 6. 7. 8. 9. 10. 11. Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch010

12. 13. 14. 15. 16. 17. 18. 19.


149

Vanderhoff, J . W., i b i d . Huemmer, T. F., Journal of Radiation Curing, 9, July (1974). A l l e n , Ν. S., et al, Polymer Letters, 12, 241 (1974). Patterson, D., "Pigments", E l s e v i e r Publishing Co., New York, (1967). McGinniss, V. D., and Dusek, D. Μ., J . Paint Tech., 46, 23 (1974). Turro, N. J . , "Molecular Photochemistry", W. A. Benjamin, Inc., New York, (1967). McGinniss, V. D., U. S. Patent 3,827,956; 3,827,957; 3,827,958; 3,827,959; 3,827,960 (1974). Sander, M. R., Osborn, C. L., and Trecker, D. J . , J. Polymer S c i . , 10, 3173 (1972). McGinniss, V. D., and Dusek, D. M., Am. Chem. Soc. Div. Polymer Chem. Preprints 15, 480 (1974). McGinniss, V. D., Paper presented at 14th Annual Coatings Symposium, North Dakota State University, Fargo, North Dakota, June 3-5, (1974). McGinniss, V. D., U. S. Patent 3,857,769 (1974). Manley, T. R., and Murray, B., European Polymer Journal, 8, 1145 (1972). Pierce, P. E., and Holsworth, R. Μ., J . Paint Tech., 38, 263 (1966). Nielsen, L. E., "Mechanical Properties of Poly mers", Reinhold Publishing Corp., New York, (1962). Gillham, J . Κ., J . Macromol. S c i . Phys., 89 (2), 209 (1974).

11 Oxygen Insensitive Resins and Coatings for High Energy Curing


D. D. PERRY, W. ROWE, A. CIRIGNANO, and D. S. DAVIS Polychrome Corp., Clark, N. J. 07066

For the past several years, Polychrome Corporation has had under development a family of energy-curable resins which have been given the trade name of Uvimer .* Several of these materials have reached the commercial stage while others are in an advanced state of development. The present paper describes representative members of the Uvimer™ family of resins in terms of their basic physical and chemical properties and how these relate to specific end-uses. TM

Basic Considerations and Approaches In approaching the development of an energy-curable resin system, we have attempted to solve a number of problems with which the ultimate user of these types of materials is faced, namely: 1. Oxygen inhibition of cure which exists in many energy-curable resins necessitating the use of inert blanketing. 2. The desire to minimize the number of handling and formulating operations. 3. The necessity for good shelf-life, both in sensitized and unsensitized formulations. 4. The desire to limit the number of items in the product line, while permitting the tailoring of properties for a variety of end-uses. 5. The need to minimize costs without sacrificing essential properties. * Patents Applied For. 150

11.

PERRY E T A L .

Oxygen Insensitive Resins and Coatings

151

We w i l l attempt to show in this paper* how these goals have been approached* Much has already been published concerning the composition of energy-curable resin systems for use in coatings and inks, and the basic approach to the development of such vehicles i s well known (Refs. 1, 2, 3)* A typical formulation contains an oligo mer, which may or may not contain reactive functional groups (e.g., double bonds), a cross-linking agent, a reactive diluent (for viscosity control), and a photosensitizer or photoinitiator. Control over the properties of the cured resin vehicle may be exercized in a number of ways:


1.

Via the structure of the oligomer backbone, including such factors as: (a) (b) (c) (d)

Degree of chain branching Types of functional groups Number and type of double bonds Molecular weight

2.

Functionality and level of cross-linking agent.

3.

Nature and level of reactive diluent.

4.

Sensitizer or photoinitiator.

It was our i n i t i a l aim to develop a basic oligomer or family of oligomers that, i n suitably formulated systems, were r e l a tively unaffected by the usual inhibiting action of molecular oxygen. It was fortunate that in the early stages of the syn thetic investigations such materials were obtained. Consequently, these newly synthesized oligomer structures became the basis for the Uvimer™ resins. Variants on the basic structures were later developed to obtain specific end-use properties. Description of Typical Uvimers™ TM AA

The principal members of the Uvimer series are uniquely designed unsaturated urethanated oligomers. One method of i n corporating unsaturated functionality i s by reaction of pendant isocyanate groups with unsaturated compounds containing active hydrogen (See, e.g., Ref. _1). 11 ι ? 11 R-NCO + H-R-C=CHR > çii R-NHCO-R -0=CHR (1) ±

1

111

152

UV

Where R and R

and

R R


POLYMERS

Oligomer backbone s t r u c t u r e 1

11

-0CH CH 00O, 2

2

- O Ç H - C H 2 O O C - , -OCH , or CH3 2

- N H C H 2 - , f o r example = H- o r CH = H- o r C 6 H 5 3

1 1 1


P o l y o l and isocyanate components may be r e a c t e d to a f f o r d isocyanate-terminated oligomers which can then be subjected to the g e n e r a l r e a c t i o n i l l u s t r a t e d i n equation 1 above. Together w i t h other s y n t h e t i c m o d i f i c a t i o n s , t h i s permits the p r e p a r a t i o n of a wide v a r i e t y of oligomers. One may v a r y , f o r example: (1) (2) (3) (4)

The h y d r o x y l f u n c t i o n a l i t y of the p o l y o l . The molecular weight of the p o l y o l . The backbone s t r u c t u r e of the p o l y o l . The type of other f u n c t i o n a l groups present i n the p o l y o l . S i m i l a r m o d i f i c a t i o n s may be made i n the s t r u c t u r e of the p o l y i s o c y a n a t e component.

The oligomers employed i n our work are based on commercially a v a i l a b l e m a t e r i a l s . However, they embody unique s y n t h e t i c modi f i c a t i o n s made on the s t a r t i n g m a t e r i a l s p r i o r to t h e i r being converted to oligomers. In regard to the r e l a t i v e insens i t i v i t y of the Uvimer™ ma t e r i a l s to oxygen i n h i b i t i o n , the mechanism of t h i s i s not f u l l y understood a t present. However, we b e l i e v e t h a t i t r e l a t e s to a common s t r u c t u r a l f e a t u r e i n a l l of these r e s i n s , namely the occurrence of the e t h y l e n i c double bonds i n a p a r t i c u l a r p o s i t i o n r e l a t i v e to the other f u n c t i o n a l groups i n the oligomer. S e l e c t e d r e a c t i v e d i l u e n t s are employed t o reduce the v i s c o s i t y of uvimer™ f o r m u l a t i o n s and t o impart a g r e a t e r f l e x i b i l i t y t o the cured product. The choice of a monomer o r r e a c t i v e d i l u e n t f o r a Uvimer™ i s based on a number of f a c t o r s , i n c l u d i n g c o m p a t i b i l i t y w i t h the base r e s i n , odor, v o l a t i l i t y , t o x i c i t y , and i t s e f f e c t on s p e c i f i c p r o p e r t i e s such as adhesion. Changing the d i l u e n t monomer does not d r a s t i c a l l y a l t e r the b a s i c proper t i e s of the r e s i n so t h a t i t i s p o s s i b l e to supply a g i v e n Uvimer™ w i t h a choice of d i l u e n t s i n order to comply w i t h par t i c u l a r user requirements. The Uvimer™ d e s i g n a t i o n thus represents a g e n e r i c s t r u c t u r a l type, w h i l e the d i l u e n t may be v a r i e d f o r the reasons e x p l a i n e d above. For the purposes of t h i s paper, a Uvimer may be shown w i t h a p a r t i c u l a r r e a c t i v e d i l u e n t o r combination of d i l u e n t s , although t h i s composition i s not c u r r e n t l y o f f e r e d com m e r c i a l l y . We consider the Uvimer™ type to be independent of the p a r t i c u l a r d i l u e n t or d i l u e n t s employed. M

11.

PERRY E T

AL.


153

We w i l l describe typical members of the Uvimer™ series in terms of their composition and physical properties. UvimerTM 530, the f i r s t member of the series, i s a relatively hard, b r i t t l e and fast-curing resin which i s suitable for use as an ink vehicle and as a coating on r i g i d substrates. The table below gives basic formulation and cured film properties of this Uvimer™:

TABLE I - UVIMER™ 530 A.

Composition


Component Urethane Oligomer A PETA B.

Parts 60 40

Physical Properties Viscosity, poises Color, Gardner Weight/vol., lbs./gal. Non-volatile cure loss Sward Hardness Abrasion Resistance*

375-600 2-3 9.80 ± 0.05 0% >50 No v i s i b l e effect

C.

Cured Film Characteristics: Hard, glossy, b r i t t l e , fastcuring resin; highly chemically resistant.

D.

End-Uses : Ink vehicles, particle board coatings. * (ASTM Method D-968-51, Falling Sand Method)

Uvimer™ 530 exhibits outstanding chemical resistance as evi denced by the data presented i n Table I I . Uvimer™ 530 and the other members of the Uvimer™ series may be cured with benzoin ether type photoinitiators and with d i ethoxyacetophenone.** Table III presents data on the cure speed of Uvimer™ 530 formulated with butyl benzoin ether. These data were obtained in a i r without the use of inert blanketing. Films of 3 mil thickness were irradiated on a moving belt using a 5000 watt Addalux uv lamp (Berkey Photo, Inc.) focussed on one linear inch. This i s a low pressure lamp incorporated i n an experimental unit. Faster cure speeds (up to 200 f t . per minute/lamp) were obtained in later studies with a unit employing two medium pres sure, 200 watts/linear inch Hanovia lamps.

Union Carbide Corporation

154


POLYMERS

TABLE I I - CHEMICAL RESISTANCE OF UVIMER™ 530 Test Conditions; 3.5 m i l f i l m cured at 0.6 sec. per l i n e a r i n c h of t r a v e l using 5,000 watt Addalux lamp (400 w a t t s / l i n e a r inch) a t 3" above work s u r f a c e . Tests were done under watch g l a s s where a p p l i c a b l e . Substrate was p a r t i c l e board. Duration of Test (Hours)

Test L i q u i d

S u l f u r i c a c i d (50%) S u l f u r i c a c i d (98%)

48 144

35% N i t r i c a c i d 70% N i t r i c a c i d

48 144

Glacial acetic acid 20% Sodium hydroxide 40% Sodium hydroxide

144 48 144

Ammonium hydroxide, 26° Baume' F e r r i c n i t r a t e (50%) (cone, copper etching solution) 10% Iodine i n Xylene

144

No e f f e c t Very s l i g h t d i s coloration No e f f e c t Moderate d i s coloration No e f f e c t Heavy d i s coloration No e f f e c t No e f f e c t Very s l i g h t d i s coloration No e f f e c t

144

No e f f e c t

144

Very s l i g h t d i s coloration Slight discoloration No e f f e c t No e f f e c t No e f f e c t No e f f e c t No e f f e c t No e f f e c t No e f f e c t No e f f e c t No e f f e c t

70% H y d r o f l u o r i c 70% H y d r o f l u o r i c Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch011

Condition

acid acid

48 144

10% Iodine aqueous 48 solution Dimethyl formamide 144 Dimethyl s u l f o x i d e 144 Phenol (90%) 144 l-Methyl-2-pyrrolidone 144 Trichlorethylene 144 Aromatic solvents 144 A l i p h a t i c solvents 144 Oxygenated s o l v e n t s 144 Methyl e t h y l ketone 1000 rubs

Remarks

Film intact

Film i n t a c t

Film intact

Film i n t a c t

Film intact Disappears i n 2 hrs.

11.


PERRY E T A L .

155

TABLE III - CURE SPEED OF UVIMER™ 530 Formulation: Uvimer™ 530 Butyl Benzoin Ether Speed

Effective Exposure Time


16 fpm 36 fpm 50 fpm 65 fpm

0·30 0.14 0.10 0.08

secs. secs. secs. secs.

100 parts 4 parts Condition of Film* No effect - film No effect - film Slight dulling Slight dulling -

intact intact film intact film intact

* Determined by rubbing 100 times with cloth saturated with methyl ethyl ketone.

A pigmented Uvimer™ 530 formulation coated on particle board was successfully cured i n an electron beam accelerator using one-half the normal nitrogen flow. In the complete absence of nitrogen blanketing, p a r t i a l curing was obtained. The formu lation used was as follows: Uvimer™ 530 2-Ethylhexyl acrylate Styrene Ti0 CaS04 Stabilizer 2

51.5 5.0 5.0 15.4 23.0 0.1

parts parts parts parts parts parts

Another member of the Uvimer™ series i s Uvimer™ 545 whose properties are summarized i n Table IV. A third member of this series i s Uvimer™ 580 whose proper ties are summarized i n Table V. The above resins gave hard, r i g i d , highly cross-linked films, suitable for use on r i g i d substrates or as ink vehicles. They are also relatively high i n viscosity i n the uncured state. We recog nized early the need for softer, more flexible film-formers, as well as for lower viscosity formulations for certain applications. This was achieved by significant modification of the oligomer structure. This approach i s i l l u s t r a t e d by four members of the Uvimer™ 700**series: 720, 740, 765, 775. Uvimer™ 720 (Table VI) i s softer and more flexible than 530, 540, and 580 and exhibits improved adhesion to most substrates.

** Patents applied for.


156

TABLE

A.

IV -

UVIMERTM

Composition Component

Parts

Urethane Oligomer Β Reactive Diluent Β·

40 60

Properties Viscosity, poises Color, Gardner Weight/vo1., lbs./gal. Non-volatile cure loss Chemical resistance


545

400-600 3 max. 10.00 ± .05 0% (60 sees.) Same as Uvimer™ 530, but less resistant to strong oxidizing acids.

General Characteristics and End-uses; overprint varnish (offset printing)·

Same as 530;

TABLE V - UVIMER™ 580 Composition Acrylate Oligomer A-l Reactive Diluent B.

Properties Viscosity, poises Color, Gardner Weight/vol., lbs./gal.

C.

83.5 16.5

150-300 2-3 9.63 ± .05

General Characteristics ; Fast curing, hard, b r i t t l e .

11.

PERRY E T A L .


157

TABLE VI - UVIMER™ 720 A.

Composition 91 9

Urethane Oligomer C RD-30 B.

Properties 500-700 5 max. 9.10 ± 0.05 0% (60 sees.)


Viscosity, poises Color, Gardner Weight/vol., lbs./gal. Non-volatile cure loss C.

General Characteristics; Flexible, tough, abrasion resistant, moderately fast cure.

D #

End-Uses : T i l e coatings, photopolymer liquid plate.

Because of i t s superior f l e x i b i l i t y and adhesion to many substrates, Uvimer™ 720 was considered a prime candidate for a floor t i l e coating. Table VII presents data on two formulations developed for this application. The primary consideration i n this instance was abrasion resistance, with resistance to curl running a close second. Both formulations showed good abrasion resistance, but the formula containing the increased level of diluent was significantly better as shown by comparison of the results with the 1000 gram weight. Data on Uvimer™ 740 i s presented i n Table VII, while Uvimers™ 765 and 775 are compared i n Table VIII. Uvimer™ 740 is somewhat s t i f f e r and harder than either 765 or 775, primarily due to the difference i n the structures of oligomers D and E. It TM 1

should also be noted that Uvimers 765 and 775 have better color and do not exhibit yellowing on aging. Since the latter two resins employ the same basic oligomer, i t i s evident that on the basis of viscosity reduction 2-ethoxyethyl acrylate i s a more effective diluent than the 2-ethylhexyl acrylate/hydroxyethyl acrylate combination. Thus, it- i s possible i n Uvimer™ 775, to employ a lower level of reactive diluent and thereby obtain a harder, s t i f f e r , faster-curing formulation. The use of Uvimer™ diluent RD-30 i n a 740 formulation gives a higher viscosity than 2-ethoxyethyl acrylate. However, the odor level i s significantly reduced.


TABLE VII - FLOOR TILE COATINGS BASED ON UVIMER™ 720 Formulation Uvimer™ 720 Reactive diluent DEAP Accelerator Leveler Slip Additive

Β 100 10 3, 1. 3, 2.2

A 100 5 3 1 3 2


Properties Viscosity, poises, Brook200 f i e l d , RVT, #5/20 rpm/80° F. Taber Abrasion*, 500 g. 8 Taber Abrasion, 1000 g. 25 Curl, 12 hrs./120° F. None

155

16 None

* CS-17 wheel for 1,000 cycles, tested on 2 mil coatings

TABLE V n i - UVIMER™ 740 FORMULATIONS A.

Composition Urethane Oligomer D 2-Ethoxyethyl Acrylate RD-30

B.

C. General Characteristics: soft, flexible. *

65 35

Physical Properties Viscosity, poises Color, Gardner Weight/vol., lbs./gal. Hardness, Shore A Hardness, Shore D

D

66 34 —

22-27 4-5 9.27 ± 0.05 90 45

100-140 4-5 9.47 ± 0.05 — —

Moderately fast cure, medium

End-Uses: Coatings for flexible substrates; insulating tape.

11.

PERRY E T

AL.


TABLE IX - COMPARISON OF UVIMERS™ 765 AND A.

Composition Urethane Oligomer E, parts 2-Ethylhexyl acrylate Hydroxyethyl acrylate 2-Ethoxyethyl acrylate

159

775

765

775

52 34 14

67

33


B. Properties Viscosity, poises 22-27 Color, Gardner 1 Weight/vol., lbs./gal. 8.65 ± 0.05 Hardness, Shore A 68 Cure Speed* ft./min./lamp 15

18-22 1 9.10 ± 0.05 90 37

* Determined on 3.5-4.0 mil film using two 200 watts/linear inch Hanovia lamps over moving belt 3 inches from work surface. Sensitized with 3% by wt. diethoxyacetophenone (DEAP). C e

End-Uses: F l e x i b i l i z i n g additive; strapping tape; fabric coatings; and paper coating.

Pressure Sensitive Adhesive Recently, Dowbenko et a l (Réf. 4) of PPG Industries des cribed two approaches to the development of energy-curable pressure sensitive adhesives. One involved the incorporation of low molecular weight acrylic polymers into hot melt formulations; the second employed radiation polymerization of monomer-polymer syrups. We have taken a somewhat different approach. By employing mixed molecular weight oligomers, we have been able to develop an extremely promising uv-curable adhesive system. Data on the composition and properties of this adhesive are presented i n Tables X and XI. In Table X, two formulations with different reactive diluents are shown. In Table XI, data are shown on the properties of the cured adhesive with and without a plasticizer type additive.

S h e l f - l i f e of Uvimer™ Resins The Uvimers™ exhibit excellent s h e l f - l i f e , as demonstrated by room temperature and 60° C. aging experiments. A l l of them are indefinitely stable i n a i r at room temperature, and for three

160

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POLYMERS

TABLE X - PRESSURE SENSITIVE ENERGY-CURABLE ADHESIVES

A.

Composition

Uvimer.TM 1793

Uvimer™ 1818

90 10

90

Oligomer F RD-31 RD-30


B.

10

Physical Properties Viscosity, poises, Brookfield, RVT Viscosity, Gardner-Holdt Color Wt./vol., lbs./gal. Cure Speed, ft./min.

1106

520

Zq-h Water White 8.60 ± .05 35

Ζη Water White 8.61 ± .05 35

TABLE XI - PROPERTIES OF ADHESIVES BASED ON UVIMER™ 1793 A. Formulation Uvimer™ 1793 Sensitizer Accelerator Plasticizer B.

100 3 1 35

100 3 1

Cured Adhesive Properties * 1

2

Shear strength/ * i n . , min. 135° hold, sec. 180° peel strength, oz./ in. Quickstick Adhesive Offset

>120 88 > 30

>72 60 25-30

2

Very Good None

Very Good None

* 1.5 mil adhesive films cured on paper or Mylar backing at 70 ft./min. with two 200-watts/linear inch Hanovia mercury vapor lamps.

11.

PERRY E T A L .


161

u

months or more at 60 C. In addition, formulations sensitized with 3% DEAP have shown v i r t u a l l y unlimited s h e l f - l i f e i n the dark at room temperature. Specifically, Uvimer M 775 sensitized with 3% DEAP, has been aged for 23 days at 60° C. without appreciable change. Upon longer aging (48 days), there was a significant increase i n v i s cosity, but no gelation. Formulations containing RD-30 and RD-31 have s t a b i l i t y at least equivalent to those containing other d i l u ents. Both 530 and 545 are stable i n sensitized form at 60° C. for 30+ days. It i s , therefore, possible to supply sensitized formulations to the user i n order to minimize handling of the materials i n their uncured state. T


f

Summary The Uvimers™ comprise a useful new group of energy-curable resins suitable for a wide variety of applications. Special features which should be of interest to potential users include: 1.

The low degree of oxygen inhibition of cure.

2.

The wide range of properties available from the members of the series. These can be augmented by additional formulation by the user.

3.

Excellent s h e l f - l i f e .

4.

Good s t a b i l i t y of sensitized formulations.

Literature Cited 1. Carden, C. H. andO.W. Smith "Coatings and Plastics Preprints", 34 (1), 664-668 (1974). 2. Koch, S. D. and J. M. Price, Ibid, 34 (2), 432-434 (1974). 3. Fussa, A. D. and S. V. Nablo, Ibid, 34 (2), 422-431 (1974). 4. Dowbenko, R; R. M. Christenson; C. C. Anderson; and R. Maska, "Chemtech", 1974 (September) 539-543. Acknowledgement : The contribution of Mr. Joseph Lifland of these laboratories to some of the evaluation studies described i n this work i s gratefully acknowledged.

12 Ultraviolet Light Cured Inks—A Review


J. W. VANDERHOFF National Printing Ink Research Institute, Lehigh University, Bethlehem, Penn. 18015

The printing ink industry with annual sales of about $0. 7 billion is a small but vital part of the $37-billion-a-year graphic arts and communications industry (1). This industry is founded upon technical service because the ink formulations are tailored to the specific printing application. It is estimated that one million new ink formulations are produced each year, with another 4-5 million older formulations still available (2). Printing Processes This very large number of formulations is necessitated by the several different printing processes and the varying requirements of each printing application. There are three main printing processes: 1. letterpress; 2. lithography; 3. gravure. Figure 1 shows schematic representations of these three processes. In letterpress or relief Letterpress

^rink

Gravure

Figure 1. Schematic of printing processes 162


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163

printing, the image area i s raised above the plate surface. The ink i s applied only to the image area (type-face) and then transferred to the paper substrate i n the type-face plane. Flexography i s a special type of letterpress printing i n which the plate i s rubber instead of metal. In l i t h ography or planographic printing, the image i s essentially co-planar with the non-image area. The image area is hydrophobic (i. e., ink-receptiv0 while the non-image area is hydrophilic (i. e . , not receptive to ink). The hydrophobic ink f i l m i s applied to the image area and aqueous fountain solution to the non-image area; the ink f i l m is transferred to flie blanket r o l l and then to the paper substrate. In gravure o r intaglio printing, the image area i s recessed below the plate surface. Ink is applied to the plate and forced into the image cavities; the remainder is "doctored" off the plate surface. During printing, the paper substrate i s forced into the cavities, picking up the ink. Preparation and Characteristics of Printing Inks Printing inks consist of a dispersion of a pigment o r dye in a fluid vehicle. Most inks also contain a polymeric o r resinous component that forms a continuous f i l m upon drying. T h e pigments may be black (carbon black, furnace black, lampblack), white (opaque or transparent), i n o r ganic colors, or organic colors; dyes may also be used. The vehicles may consist of non-drying o i l s , drying o i l s , resin-solvent solutions, r e s i n - o i l combinations, o r resin-wax combination. In addition to the p i g ment and vehicle, the ink may also contain d r i e r s , waxes, compounds, greases, lubricants, reducing o i l s , body gums, binding varnishes, anti oxidants, antiskirining agents, o r surface-active agents. The mode of drying is as varied: absorption (non-drying oils); oxida tion (drying oils); evaporation of solvent (resin-solvent solutions); c o l d set (resin-wax combinations); quick-set (resin-oil combinations); heat-set (resin-oil-solvent combinations). Printing inks a r e usually prepared by dispersing powdered pigment in a vehicle by mechanical mixing. The powdered pigment consists of agglo merates and aggregates of p r i m a r y particles, e. g . , some carbon blacks consist of 30-100u-diameter agglomerates and aggregates of the 0. 020.03 ί ΐ - d i a m e t e r p r i m a r y particles. The dispersion process usually c o n sists of premixing and mixing steps during which the large pigment agglomerates and aggregates a r e broken down to a colloidal dispersion of s m a l l aggregates and p r i m a r y particles. A much finer degree of d i s p e r sion i s required for printing inks than for other types of coatings because the ink films are much thinner (perhaps only 1-2^ thick). Moreover, for a l l pigments, there is an optimum degree of dispersion (or p a r t i c l e - s i z e distribution) for maximum opacity or color strength (3), and ink makers devote much effort to dispersing pigment agglomerates and aggregates to


164

this desired size range. Table I (4) shows that the pigment content and viscosity of printing inks vary widely according to the printing process and application. Table I


Composition and Viscosity of Printing Inks Printing Process

% Pigment

Viscosity, poises

lithography letterpress

20-80 20-80

100-800 10-500

flexography gravure news-ink

10-40 10-30 8-12

1-100 0.5-10 2-10

The paste inks used i n letterpress and lithography have relatively high concentrations of pigment, while the low-viscosity liquid inks used i n flexography and gravure have much lower concentrations. The d i f f e r ences i n the viscosities a r e due to the different pigment and solvent c o n centrations. Figure 2 shows a schematic representation of the ink transfer procesa

Figure 2.

Schematic of ink transfer process

12.

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165

The ink transfer rolls apply a uniform film of ink to the printing plate or roll; this ink film is transferred directly to the paper substrate, or first to the printing blanket and then to the paper substrate. During printing, the ink film penetrates into the porous paper substrate, and that part of the ink film remaining between the substrate and the plate splits, leaving part of the film on the substrate and part on the plate. This ink film splitting is described by the Walker-Fetsko equation (5):


y = b + f(x - b) where y_ is the amount of ink transferred per unit area of the print, χ the amount of ink originally on the plate, b the amount of ink immobil ized by penetration into the substrate, and f_ the fraction of ink film between the substrate surface and the plate that is transferred to the substrate. For a wide variety of printing processes and conditions, ^f is about 0.4 for non-absorbent substrates and about 0.2 for absorbent substrates (6). New Developments in Printing Inks Stimulus for Development of New Inks. Many inks are formulated with solvents to control the viscosity of the formulation, and during printing these solvents evaporate into the atmosphere. For example, the rapiddrying heat-set inks widely used in webfed printing contain 35-45% of vol atile hydrocarbon solvents, which must be removed quickly in the dryer if fast printing speeds are to be maintained. However, recently-enacted or impending legislation by the federal, state, and local governments administered by the Environmental Protection Agency restricts the amount of solvent that can be vented to the atmosphere in all types of coating operations. Moreover, increased concern has been expressed by individuals and unions over the possible toxic effects of prolonged expo sure to solvents, and the establishment of the Office of Safety and Health Administration has given the organizational means to monitor and control such exposures. For printers and converters, this means that all but the smallest companies must reduce sharply the amounts of solvent vented to the atmosphere. There are three possible approaches to elimination of solvent emis sion: 1. incineration of solvents emitted from solvent-based inks; 2. recovery of solvents emitted from solvent-based inks; 3. use of solventless inks. The choice that a printer or converter must make is influenced by the energy shortage, which makes procurement of addi tional natural gas or fuel oil difficult, and the raw material shortage, particularly the solvents used in printing inks. The incineration of solvents is an approach worth considering for


166

UV LIGHT INDUCED REACTIONS IN P O L Y M E R S

printers and converters who use letterpress or lithographic printing processes. The solvents contained in the heat-set inks can be burned at 1500° C to produce carbon dioxide and water; however, a supply of natural gas or fuel oil is required. Therefore, although this approach is tech nically and economically feasible, the energy shortage is certain to limit its adoption. The recovery of solvents is an approach worth considering for printers and converters who use gravure or flexographic printing processes. These inks contain high concentrations of low-boiling solvents, and the usual practice is to dilute them further in the press room. The large volume of solvents emitted makes incineration impractical, but these solvents may be absorbed on activated charcoal, and removed by steam distillation when the charcoal is saturated. The recovered solvents may be re-used or sold; however, it may be impractical to fractionate some recovered solvent blend that boil in the same temperature range, which lessens their value considerably. Therefore, although this approach is also technically and economically feasible, the difficulty in procuring the solvents used for dilution may restrict the continued use of these solventbased inks and, hence, the adoption of solvent recovery approach. The use of solventless inks avoids the emission of solvents during drying. The development of such inks has been a major goal of the printing ink industry for the past several years and has resulted in the development of several different types: 1. ultraviolet light-cured inks; 2. electron beam-cured inks; 3. thermally-catalyzed inks; 4. waterbased inks; 5. solventless oil-based inks plus overcoat; 6. low-smoke low-odor inks. Radiation Curing Methods. There are three types of radiation curing systems that are adaptable to the rapid cure of polymer films: 1. micro wave; 2. electron beam; 3. ultraviolet light. Microwave radiation is most useful for drying aqueous or highly-polar systems, e. g., waterbased gravure or flexographic inks, but not for the non-polar letterpress or lithographic inks. Electron beam radiation because of its high inten sity cures both thick and thin films rapidly, and it is readily adaptable to paste and liquid inks; however, its capital cost is high, and it requires safety shielding for the x-rays produced and an inert atmosphere to pre vent the formation of ozone. Ultraviolet light radiation is a good compro mise in that its capital cost is relatively low and it is easily shielded, yet it gives a reasonably high radiation intensity. At present, this latter sys tem is the most advanced of the various approaches to produce solventless inks. Ultraviolet Light-Cured Inks Ultraviolet light-cured inks are a distinct departure from conventional printing inks in that their mechanism of


12.

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UV

Light Cured Inks

167

curing is vinyl addition polymerization. The vehicles consist of mixtures of polyfuntional vinyl monomers and photoinitiator s, which upon exposure to ultraviolet light form free radicals that initiate a rapid crosslinking polymerization to form a three-dimensional polymer network. An important advantage of these ultraviolet light-cured inks is that they give excellent film properties at fast printing speeds and thus have potential for further technical and economic development. Also, the non volatile vinyl monomers and prepolymers used in their formulation com pletely eliminate the emission of solvents. Moreover, since the ink film is cured at temperatures only slightly above room temperature, the paper substrate is not degraded, and therefore lower-quality paper may often be used. Anti-offset spray is not needed, and wet-trapping can be elimin ated because of the rapid curing; in multi-color printing, the colors may be dried between stations. In addition, the ultraviolet lamps occupy less space than the heat-set ink dryers they replace. The disadvantages are that the present cost of the ultraviolet lamps is relatively high, and the cost of lamp replacement can be greater than that of oven maintenance. Moreover, oxygen inhibition of the photoinitiated vinyl polymerization must be overcome, and paper printed with these inks cannot be recycled in the processes currently used (8), thus lowerig the return on the sale of scrap (9). Electron Beam-Cured Inks . Electron beam-cured inks are similar in principle to ultraviolet light-cured inks except that no photoinitiator is needed. Vinyl polymerizations may be initiated by any form of ionizing radiation, e.g., neutrons, α-particles, y-rays, and x-rays, as well as by high-energy electrons ( jS-rays). The mechanism of initiation is more complex than that of photochemical initiation in that radiation of vinyl monomers gives cations and anions as well as free radicals; however, most radiation-initiated polymerizations are radical-initiated because the cations and anions formed are not stable at the temperature of poly merization and therefore dissociate to form radicals. Although similar in principle to ultraviolet light-curing, electron beam curing offers higher intensities and, hence, faster rates of curing (at least for thick ink films). However, the impingement of high-energy electrons upon metals gives secondary x-radiation, a more highly pene trating form that requires elaborate shielding for safely. The lowerintensity electron beam generators currently offered (10) may lessen the safely problems somewhat, but probably at the expense of reduced inten sity (although this reduced intensity is sufficient to cure ink films). Also, as with the ultraviolet light-cured inks, paper printed with these electron beam-cured inks cannot be recycled in the processes currently used. Thermally-Catalyzed Inks Thermally-catalyzed inks are stable at room temperature, but crosslink rapidly when heated to dryer tempera-


168


POLYMERS

tures. There are two types: 1. acid-crosslinkable prepolymers; 2. prepolymers that become reactive at dryer temperatures. The acid-crosslinkable prepolymers are polyester-alkyd, ureaformaldehyde, or melamine-formaldehyde resin adducts combined with a "blocked" acid catalyst that becomes "unblocked" at the dryer temper atures (7, 11, 12), e. g., a melamine-polyester resin adduct with "blocked" £-toluenesulphonic acid catalyst (12). These acid-cross linkable prepolymers are stable on the press distribution system but are converted to three-dimensional polymer networks by heating to 275325° F. The prepolymers that become reactive at dryer temperatures are styrenated-alkyd adducts (11). Inks formulated with these vehicles con tain small concentrations of high-boiling solvents, which are not evapor ated during drying and presumably remain in the film. Letterpress and lithographic inks can be formulated from these thermally-catalyzed vehicles. Their main advantage is that they can be cured with the present flame or hot-air dryers, to give carbon dioxide and water plus small amounts of alcohols and formaldehyde. Moreover, the properties of these inks films are very good, particularly their resis tance to scuffing, abrasion, or scratchirg. In addition, they handle well on the press distribtion system or on the printing plate, and their shelf life is adequate (3-6 months). The main disadvantage is that web temperatures of 275-325° F are required to cure the film. These temperatures affect the paper proper ties adversely, and therefore higher-quality, more-expensive paper must be used. Even so, blistering or cracking in the folder may be a problem. Moreover, the solvent emission, although reduced signifi cantly, is not eliminated, and, as with the ultraviolet light-cured inks, paper printed with these thermally-catalyzed inks cannot be recycled in the currently used processes (8, 9). Water-Based Inks. In this context, the term "water-based" means a preponderance of water as the solvent rather than an absence of organic solvents. For example, water-based flexographic inks may contain as much as 20% alcohol. However, since Rule 66 and other governmental regulations allow emission of these quantities of alcohols (at least in certain geographic areas), these inks are a suitable approach to solve the printers' problems. These water-based inks use water-based polymer vehicles. There are three main types of water-based polymer systems: 1. latexes; 2. water-soluble polymers; 3. water-solubilized polymers. The latexes are colloidal dispersions of submicroscopic polymer spheres; the latex viscosity is dependent primarily upon the interactions between the colloidal particles and is independent of the molecular weight of the


12.

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169

polymer. Thus, the concentration of polymer in a low-viscosity latex may be 50% or more. Aqueous solutions of water-soluble polymers are true solutions, i . e., molecular dispersions of polymer molecules; the viscosity of these solutions is dependent upon both the concentration and molecular weight of the polymer. High-molecular-weight polymers can be used only at the expense of polymer concentration, and vice-versa. The water-solubilized polymers lie between the latexes and polymer solutions; i . e., between the colloidal and molecular sizes. These watersolubilized polymers are often prepared by the neutralization of carboxylcontaining latex particles with base, to give swelling and partial disinte gration of the particles. A review (13) of water-based inks points out that ink vehicles must have "tack" for printàbility and that high solution viscosities are required to give this tack. The viscosity of most latexes is too low to give this tack, so water-soluble or water-solubilized polymers are usually used. Upon drying, however, these polymers must no longer be soluble in water. Therefore, most water-based ink vehicles are ammonium hydroxide-neutralized alkaline solutions of carboxyl-containing polymers; when the solution is dried, the ammonia evaporates, leaving the polymer in its water-insoluble carboxyl form. The main advantage of these water-based inks is the elimination of non-exempt solvent emission without resorting to solvent recovery or incineration. In addition, microwave radiation can be used to dry these inks rapidly because of the high dielectric constant of water. Their main disadvantage is that the heat of vaporization of water is greater than that of the organic solvents usually used, e.g., 1043 BTU/ lb as compared with about 180 BTU/lb for toluene, methyl ethyl ketone, and ethyl acetate. Also, the utility of these inks is limited: they cannot be used in letterpress printing because they dry too rapidly on open roller systems, and they cannot be used in web-offset lithography because they are miscible with the aqueous fountain solution. Moreover, the water may swell the paper substrate and give poor register, and paper printed with some water-based inks cannot be recycled in the presently-used processes. These inks can be used in gravure printing on absorbent substrates, where their slow drying speed, low gloss, and limited adhesive qualities are offset by their non-flammability, easy wash-up, and dilution with water. These inks are also used in flexographic printing on absorbent paper and paperboard substrates; alcohols and glycol ethers are often added to these formulations to improve printàbility and drying speed. Solventless Oil-Based Inks Plus Overcoat. The principle of this approach developed some years ago (14) is to protect the wet ink film on a printed surface by applying a thin, transparent, film-forming polymer


170


POLYMERS

solution while the ink is still wet. In present practice, solventless oilbased inks, which require several hours to dry by oxidation, are overcoated with a thin, transparent coating that protects the ink film while it dries (15). These solventless oi-tased inks are the same types used before the development of heat-set and quick-set inks. The polymer coatings are oxygen-permeable abrasion-resistant alcohol-soluble pro pionate resins (16) or water-soluble polymers. The main advantage of this approach is that solvent emission (except that from the coating solution) is eliminated. In addition, the printed films show high gloss and good abrasion resistance. Moreover, paper printed by this process can be recycled without complication, and both alcohol-based and water-based coatings are available. The main disadvantage is the capital cost of the additional coating operation. Also, the whole sheet must be coated (not a disadvantage in some applications), and the complete drying of the ink film under the protective coating requires a long time. Low-Smoke Low-Odor Inka Low-smoke low-odor inks are refine ments of the solvent-based heat-set inks currently used. These inks contain lower concentrations of solvents (e.g., 20% instead of 40%), with a smaller proportion of saturated hydrocarbon solvents and none of the photochemically-reactive solvents, yet have printing and ink-film pro perties typical of heat-set inks. Some of these inks contain exempt solvents (e.g., low-boiling azeotropic mixtures), some contain highboiling co-solvents (e.g., long-chain alcohols), others contain emulsified water. The main advantage of these inks is that they can be used in the pres ent drying systems without installing new equipment. Moreover, some can be dried at temperatures as low as 220°F instead of the usual 275° F. Their main disadvantage is that the solvent emission, although reduced significantly, is not eliminated. Nevertheless, they can serve as satis factory interim replacements for the heat-set inks until the next gener ation of solventless inks is fully developed. Composition of Ultraviolet Light-Cured Inks The formulations of ultraviolet light-cured inks are proprietary, and at present, the best source of information as to their composition is the rapidly-growing patent literature. However, it is risky to infer the composition of inks used in the field from those described in the patent literature because only a small proportion of patented compositions ever become commercial products, and the compositions used in the field may be the subject of patent applications not yet granted, or may be held as technical secrets without applying for patents. Nevertheless, the patent


12.

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171

literature is presently the only available source of information on the composition of ultraviolet light-cured inks. The wide variety of compo sitions described in the patent literature are difficult to classify, but can be divided into five main types: 1. esters of unsaturated acids and polyhydric alcohols combined vith a photoinitiator (17-25); 2. acrylate- or methacrylate-ester derivatives of conventional ink vehicles combined with a photoinitiator (26-39); 3. acrylate- or methacrylate-ester derivatives of epoxy resins combined with a photoinitiator (40-43); 4. unsaturated polyester prepolymers combined with a photoinitiator (44-46); 5. conventional ink vehicles (i. e., whose polymerization mechanism is not vinyl polymerization) combined with a photoinitator (47-51). The first type of composition is exemplified by: 1. A mixture of an acrylic acid ester of a di- or trihydric alcohol (e. g., trimethylolpropane triacrylate) and a £-toluenesulfonamide-formaldehyde resin combined with a halogenated photoinitiator (17); 2. A mixture of an acrylic acid ester of pentaerythritol dipentaerythritol, or polypentaerythritol with an aryl sulfonamide-formaldehyde resin combined with a halogenated photoinitiator (18); 3. An acrylic acid ester of pentaerythritol, dipenta erythritol, or polypentaerythritol combined with a halogenated photo initiator (19); 4. An acrylic acid ester of a di-, t r i - , or tetrahydric alcohol combined with a halogenated photoinitiator (20); 5. An acrylic, methacry lie, or itaconic acid ester of pentaerythritol combined with a halogenated photoinitiator (21); 6. A mixture of pentaerythritol tetramethyacrylate and a 70:30 acrylic acid-styrene copolymer esterified with glycidyl methacrylate combined with a mixture of 1,2-benzanthraquinone and benzoin methyl ether photoinitiators (22); 7. A 63:37 0-(vinyloxy) ethyl methacrylate-trimethylolpropane trimethacrylate adduct combined with a 2-ethylanthraquinone photoinitiator (23); 8. A mixture of penta erythritol tetraacrylate and a 63:37 monobutyl maleate-styrene copolymer combined with a photoinitiator (24); 9. Poly-/?-[(2-fiiryl)acryloyloxy] ethyl methacrylate combined with a 5-nitroacenaphthene photoinitiator (25). The second type of composition is exemplified by a wide variety of acrylate- or methacrylate-ester derivatives of conventional ink vehicles combined with a photoinitiator: 1. The reaction product of tung oil fatty acids, glycidyl methacrylate, £-benzoquinone, and 2-methyl-imidazole mixed with tung oil and treated with tolylene diisocyanate, combined with benzoin methyl ether (26); 2. Glycerol-linseed oil-isophthalic acid alkyd reacted with isocyanate-containing prepolymer (formed by reaction of tolylene diisocyanate, p_-henzoquinone, 2-hydroxypropyl acrylate in ethyl acetate solution) using dibutyltin diacetate catalyst, combined with tung oil, synthetic varnish, and benzoin methyl ether (27); 3. Epoxidized


172


POLYMERS

soybean oil acrylates or their methyl isocyanate or tolylene diisocyanate derivatives, trimethylolpropane-tall oil fatty acid-adipic acid alkyd, and the reaction product of tolylene diisocyanate-2-hydroxyethyl acrylate adduct combined with a photoinitiator having a triplet energy of 42-85 kcal/mole (28,29); 4. Maleic acid-modified tung oil-4,4-isopropylidenebis (3,5-dichlorophenolpropylene oxide) adduct and trimethylolpropane triacrylate-pentaerythritol tetraacrylate-1,4-butanediol diacrylate copolymer combined with Aroclor 5460, hydroquinone, and stannous chloride (30); 5. Co Resyn I unsaturated polyester-2-hydroxyethyl acrylate-neopentyl glycol diacrylate mixture with 2,2-dimethoxy-2phenylacetophenone (31); 6. Tolylene diisocyanate-2-hydroxyethyl meth acrylate-α , α-bis (hydroxymethyl)propionic acid adduct combined with benzoin methyl ether (32); 7. Unsaturated polyester-methyl methacrjiatetetrahydrofurfuryl methacrylate-2-ethylhexyl acrylate adduct combined with benzoin methyl ether and zirconium oxide (33); 8. Adduct of tolylene diisocyanate, 2-hydroxyethyl methacrylate, and £-benzoquinone with a 15:15:35 :35 acrylic acid-2-hydroxyethyl methacry late-ethyl acrylatemethyl methacrylate copolymer combined with benzoin ethyl ether (34); 9. A mixture of nitrocellulose, a hydroxyethyl acrylate-tolylene diiso cyanate adduct, and a tripolycaprolactone Triol A-tolylene diisocyanatehydroxyethyl acrylate monoisocyanate adduct in toluene-2-ethoxyethyl acetate-ethyl acetate-butyl acetate solution combined with benzophenone (35); 10. A mixture of a DER 332(bisphenol A diglycidyl ether)-acrylic acid-pelargonic acid adduct, a DER 332-polyethylene glycol diglycidyl ether adduct, pentaerythritol tetraacrylate, and polyethylene glycol diacrylate combined with l-chloro-2-methylanthraquinone (36); 11. A castor oil fatty acid-trimethylolpropane-triethanolamine-phthalic anhy dride alkyd-glycidyl methacrylate adduct combined with benzoin methyl ether, cobalt octanoate, and a rare earth octanoate (37). The third type of composition is exemplified by acrylate- and methacrylate-ester derivatives of epoxy resins combined with a photo initiator: 1. Epoxy prepolymers (e.g., glycidyl methacrylate-allyl glycidyl ether copolymers or Ciba ECN 1299) combined with a photo sensitive aryldiazonium compound (e.g., £-nitrobenzenediazonium hexaflourophosphate) (38,39); 2. Epoxy prepolymers (e.g., a diglycidyl ether of disphenol A-(3,4-epoxycyclohexyl)-methyl-3,4-epoxycyclohexanecarboxylate-alkyl glycidyl ether mixture) combined with a photo sensitive aryldiazonium compound (e.g., £-chlorobenzenediazonium hexaflourophosphate) and l-methyl-2-pyrrolidone gelation inhibitor (40); 3. A mixture of GY-280 epoxy resin, ethylene glycol diacrylate, and crotonic acid combined with hydroquinone monomethyl ether and N, Ndimethylaniline (41). The fourth type of composition is exemplified by unsaturated polyester prepolymers combined with a photoinitiator 1. A mixture of a tolylene diisocyanate-modified maleic anhydride-phthalic anhydride-propylene :


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glycol copolymer with styrene combined with a benzoin-propylene oxide addition product(42); 2. A mixture of maleic acid-phthalic acid-1, 2~ propanediol copolymer and styrene combined with benzoin ethyl ether, hydrogen peroxide (color stabilizer), and diphenyl disulfide (hydrogen peroxide stabilizer) (43); 3. A wood oil-maleate resin adduct diluted with spindle oil and combined with Fanal Pink Β and benzoin ethyl ether (44); 4. A fumaric acid-propylene glycol-diethylene glycol-trimethylolpropane-trimethylolpropane diallyl ether copolymer combined with 4benzoybenzal chloride (45). The fifth type of composition is exemplified by formulations which are not usually considered to polymerize by a free radical mechanism: 1. A mixture of acid-hardening resins, physically-drying resins, solvents boiling at less than 150°, and pigment dyes containing acidreleasing photoinitiators (e. g., halomethylated benzophenones or a -(sulfonyloxymethyl)benzoins) (46); 2. Pigmented drying oil mixtures combined with azides, diazo compounds, naphthoquinone azides, or diaminodiphenyl ketones (47); 3. Conventional fast-drying ink comprised ofalkyds, polyesters, aluminum compounds, driers, carbon black, and solvent combined with an 8:1 benzophenone-Michler's ketone mixture(48); 4. Air-drying alkyd resin lacquer without photoinitiator (49); 5. A mix-ture of short oil alkyd, butanolic urea, and melamine urea resins com bined with 2-chloro-4-tert-butylacetophenone (50); 6. A plasticized urea-formaldehyde resin in butanol-xylene-dimêthylcarbinol solvent mixture combined with 2,2-dichloro-4-tert-butylacetophenone (51). Curing of Ultraviolet Light-Cured Inks Theoretical Aspects. Ultraviolet light-cured inks are cured by free radical-initiated vinyl addition polymerization. The photochemical initiation of vinyl polymerization has been the subject of many investi gations dating back more than 30 years, to the first systematic studies of polymerization kinetics. Photochemical initiation offered a reprodu cible source of radicals that is not dependent upon temperature, as is the thermal decomposition of free radical initiators. However, despite these early studies of its mechanism and kinetics, only recently has photochemical initiation become of practical interest, mainly because of the recent development of ultraviolet lamps suitable for production curing of printing inks and coatings. Other authors (52,53) have described the mechanism and kinetics of photochemically-initiated polymerization in detail; therefore this subject will be reviewed only briefly here. Free radicals can be initiated by ultraviolet irradiation of pure mono mer or monomer containing initiators or photcesnsitizers. For a pure monomer M, the reaction can be expressed as:

174

UV

M + hv

L I G H T INDUCED REACTIONS I N P O L Y M E R S

T

*(M)*

2

>R* + -R

()

The identity of the activated compound (M)* has not been well-established, even for such standard monomers as styrene. However, the rate of initiation and the rate of polymerization Rp can be expressed as: B ^ f ^ ^ c

I

0

[m|

3/2

R = kp[M] {C H -C- + · C H - C H »

benzoin

6

I

5

6

5

6

5

6

(9)

5

In systems that contain an initiator In_, the rates of initiation and polymerization can be expressed as: Ηΐ = 2φ€ΐ [Ιη]

(10)


0

Rp = k [Μ] [ φ I (1 - e ^ p

I n l b

0

)/k } t

1 / 2

(H)

In this case, the rate of initiation varies directly with the incident light intensity and initiator concentration, and the rate of polymerization varies directly with monomer concentration and the 0. 5 power of inci dent light intensity, but varies in a complex manner with initiator con centration. A photosensitizer 2^ is a compound that brings about the reaction of monomers or initiators that do not undergo sufficient excitation at the available frequencies of light. The reactions can be expressed as: Ζ + hv—>(Z)* (Z)* + C — > Z + (C)* (C)*—*-R* + · R

T

(12) (13) (14)

In this case, the activated compound (C)* cannot be produced by direct irradiation with light of frequency ν . Instead, the activated photosensi tizer (Z)* transfers energy to the monomer or initiator £ at an appro priate frequency that can be absorbed by C. Typical photosensitizers used in photochemically-initiated polymerizations are benzophenone, fluorescein, and eosin. Practical Aspects. The foregoing theoretical relationships are difficult to apply to the curing of ultraviolet light-cured printing inks for several reasons. First, the polymerization kinetics of the complex monomers and prepolymers used in the ink formulations have not yet been investigated under conditions that can be correlated with theory (i. e., low concentrations of reactants, constant temperature, simple model formulations). Second, the ultraviolet light-cured inks are poly merized in bulk to high conversion; the polyfunctional nature of the

176

UV



monomers and prepolymers and the autoacceleration of the polymerization rate almost always observed are difficult to correlate with theory. TlinJ* the ink is applied to the substrate as a thin film, which is almost immedi ately exposed to a high-intensity ultraviolet light lamp for a very short time and then is moved down the line out of the light path; during this brief exposure, sufficient radicals must be generated to complete the polymerization despite the oxygen inhibition (the conditions are ideal for inhibition by oxygen diffusing from the air into the very thin ink films). Finally, most inks are pigmented, and the dispersed pigment particles absorb part of the ultraviolet light energy and thus mask off part of the film. Nevertheless, the present ultraviolet light-cured inks cure very rapidly upon exposure to ultraviolet light despite the presence of pigment and inhibition by oxygen. Intensity of Irradiation. The intensity of irradiation varies with posi tion relative to the ultraviolet light lamp (55, 56). The relative intensity of irradiation was determined by scanning along the three coordinate axes Χ, Ύ , and Ζ shown in Figure 3 (55). For a Hanovia 200 watt/in quartz

X

Journal of Paint Technology

Figure 3. Schematic of coordinate system used to determine intensity of ultraviolet light irradiation (55)

lamp with a Hanovia reflector of semi-elliptical cross-section, the rela tive intensity along the vertical jZ) axis dropped off rapidly with increasiag distance to a minimum at about 3 cm, then increased to a maximum at about 8 cm and then dropped off rapidly once more (Figure 4); this curve was interpreted (55) as a composite of the two types of irradiation: 1. direct emission from the lamp; 2. elliptically-reflected from the reflector. Figure 5 shows the variation of relative intensity with distance along the horizontal £ Q axis as a function of the vertical distances ^Z}_

12.

UV Light Cured Inks

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177

1.2 μ

1.0

S 0.8 ω

α Β

>0.6

>

α ΡΗ


0.4 h

0.2 μ

2

4 6 8 10 12 Ζ, inches

-4-3-2-1 0 1 2 3 Υ, inches Journal of Paint Technology

Journal of Paint Technology

Figure 4. Variation of relative intensity of Figure 5. Variation of relative intensity of ultraviolet light irradiation with distance ultraviolet light irradiation with distance along Z axis ( 55 ) along Y axis as a function of Ζ ( 55)

corresponding to the arrows in Figure 3 (55). The asymmetric shape of the radiation pattern was attributed to minor misalignment of the lamp relative to the reflector. The areas under these curves were found to be the same, so that a printed substrate moving at a given speed would be exposed to the same overall intensity of irradiation independent of the value of 21 Figure 6 shows that the variation of relative intensity along

£ l 2 S i.o Φ

0.8

I 0.6 « 0.4 0.2 -15 -10 -5 0 5 10 X, inches Journal of Paint Technology

Figure 6. Variation of relative intensity of ultra violet light irradiation along X axis (55)


178

IN

POLYMERS


the axis of the lamp £ Q was approximately the same independent of the value of X throughout the 25. 5-in arc length (55); the slight asymmetry was attributed to misalignment of the lamp relative to the reflector. These results were confirmed by another study (56) using a Hanovia 200 watt/in quartz lamp with a 100A. Jband centered at 2537A., equipped with a Sun Graphics elliptical reflector. Figure 7 shows that the rela tive intensity decreased, then increased to a maximum at the focal distance, and then decreased again with vertical distance in a similar manner (compare with Figure 4). Figure 8 shows that the variation of

TAGA Proceedings

Figure 8. Variation of relative intensity of ultraviolet light irradiation with dis tance along Y axis as a function of Ζ (56)

TAGA Proceedings

Figure 7. Variation of relative inten sity of ultraviolet light irradiation with distance along Ζ axis (56)

relative intensity with lateral, distance from the centerline was also similar (compare with Figure 5), i . e., two-peaked at distances smaller than the focal distance, single-peaked and hgi at the focal distance, and single-peaked, low, and broad at distances greater than the focal dis tance. Both of these studies (55, 56) emphasize the importance of optimizing the position of the moving web of printed substrate relative to the ultra violet light lamp. Experimental Measurement of Rate of Cure. The rate of cure was measured experimentally in several studies (54-58). Three of these (54, 57, 58) used the dilatometric method to determine the rate of


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179

polymerization, which, although of great value for mechanistic studies and exploratory development of new coating formulations, is not directly applicable to the curing of very thin printed ink films. Only one study, (56) has examined in detail the curing of very thin ink films containing a colorant. In this work (56), three empirical methods were used to evaluate the rate of cure of a given formulation: 1. the maximum rate of cure (in ft/min/lamp) that gave tack-free (TF) films; 2. the maximum rate of cure that gave thumb-twist-free (TTF) films (rotary twisting motion vith downward pressure of 5 kg); 3. the maximum rate of cure that gave scratch-free (SF) films. Ink films of about 3μ thickness were applied to glass slides and cured at various rates. Figure 9 shows the variation of log cure rate in ft/min/lamp with log relative intensity as a function of the colorant used: magenta, yellow, cyan, or black (56). In no case did the rate of cure vary with the 1. 0 or 0. 5 power of the intensity as pre dicted by theory, but instead with a higher power according to the color ant: the fastest rates of cure were observed with the magenta and the slowest with the black. Figure 10 compares the maximum rates of cure (ft/min/lamp) that gave tack-free and thumb-twist-free films for the same four colorants (56) 500r 500 1 1 1 1 1 MM J

-

UJ200

magenta—^/ Π

-

-

1 #100 u

-

u

/ yellow-y

/

~

ο 50

-

1

/ /

20 -

/ black^

/

1

1 /

1

Zrcyan

2

/>

1/ 1 1/11 1 1

5 10 Relative Intensity TAGA Proceedings

magenta yellow cyan black TAGA Proceedings

Figure 9. Variation of log rate of cure Figure 10. Maximum rate of cure that with log relative intensity of ultravioletaave thumb-twist-free and tack-free films (56) light irradiation (56)

U V LIGHT

180

INDUCED

REACTIONS IN

POLYMERS

Again, the magenta gave the fastest rate of cure and the black the slowsst. Moreover, the maximum rates of cure for a given colorant were always greater for the tack-free than for the thumb-twist-free test. These differences in rates of cure observed for the different coloiants were attributed to different absorbances of these colorants over the wave length range 220-350 nytf (56). Figure 11 shows the percent transmissionwavelength curves of 10% suspensions of these colorants in mineral oil. 1 '—* 60


50

I

magenta^

40

••H

S

m

S

3 0

H § 20 ο u ο

β* 10

260

240

280 300 Wavelength, mu

-L 340

320

TAGA Proceedings

Figure 11.

Variation of % transmission with wavelength (56)

Although the percent transmission curves vary with wavelength, that for magenta generally showed the highest percent transmission at a given wavelength, followed by yellow, cyan, and black in that order. The presence of colorant was also shown to affect the behavior of the photosensitizer (56). Figure 12 compares the curing efficiency of photosensitizers A, B, and C^with and without 10% black pigment. In both Τ IJ I Mill 1 1 I I I Mir 0) Clear Film

-

-

.§3

H

Η

II 2

5

10 i Percent Photosensitizer 1

1

Figure 12.

1

1 1m

Percent Photosensitizer TAGA Proceedings.

Variation of relative cure time with log photosensitizer ( 56 )

12.

VANDERHOFF

UV Light Cured Inks

181

cases, the fastest rates of cure were obtained with photosensitizer C_, but the relative efficiencies of photosensitizers and Β were reversed when 10% black pigment was added to the clear film. As expected, the rate of cure decreased slightly with increasing film thickness (56). Figure 13 shows that the decrease of log rate of cure with film thickness was linear and about the same for the four colorants; ^ 1 ! ι I - ^ ^ " ^ ^ ^ ^ ^-magenta ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ yellow

a

1

•i-l


—

U

cyan-^^~

^ ^ ^ ^ ^

501

-

^^^---^..^^-black

u

10 0

i 4

2

1 6

1 8

.110

12

Film Thickness, μ TAGA Proceedings

Figure IS.

Variation of log rate of cure withfilmthickness ( 56 )

in the film-thickness range of 2-12/t , the rate of cure was approxi mately halved for each 5/( increase in film thickness. The effect of wavelength was also studied by substituing a Hanovia "ozone-free" lamp for the standard type (56). Figure 14 shows the ratio

magenta yellow

cyan

black

white

silver

TAGA Proceedings

Figure 14.

Ratio of "ozone-free"/standard cure rate ( 56 )

UV LIGHT

182

INDUCED



of cure rates for the "ozone-free" lamp relative to the standard lamp for films containing various colorants. In all cases, the rate of cure was slower for the "ozone-free" lamp, but the decrease was greatest for cyan and yellow and least for magenta. This difference in cure rates was attributed to the different spectral output of these lamps (56). Figure 15 shows that the relative emission-wavelength curves for the τ

180

200

240 280 320 Wavelength, ταμ

360

400

TAGA Proceedings

Figure15. Variation of relative emission with wavelength for Hanovia "ozone-free" and standard lamps (56)

"ozone-free" lamp was lower than that of the standard lamp in the wave length range 200-280 mjt, particularly near the peak at 257nyi The foregoing results illustrate the complexity of these systems and emphasize the importance of studying the ultraviolet light-cured ink sys tem in addition to its individual components. Cost of Ultraviolet Light-Cured Inks Inevitably, the cost of ultraviolet light-cured inks is higher than that of the conventional solvent-based inks they are designed to replace. Part of the increased cost is offset by their higher concentrations of vehicle and pigments (volatile solvents are replaced by reactive mono mers and prepolymers) so that the potential volume of ink film per lb of ink is greater; however, in practice, only part of this difference is realized as greater mileage. It is difficult to specify the increased cost of thse ultraviolet lightcured inks with any accuracy. For example, in one case, the cost of the ink was estimated to be 30-50% higher (7), but an estimate made in 1971 cannot be expected to apply today when Taw material costs are increased on a monthly or even weekly basis. In another case, the overall cost of

12.

VANDERHOFF

UV Light Cured Inks

183

operating a web-offset lithographic press using ultraviolet light-cured inks was estimated to be 88% higher than for the conventional solventbased heat-set inks plus incineration; however, this cost analysis was also made 2-3 years ago and is probably not applicable today. In any event, although the cost is higher, the rate of increase in cost of the ultraviolet light-cured inks is often less in today's rapidly changing market than the corresponding increase of the conventional solventbased inks.


Summary Ultraviolet light-cured inks have been formulated and used success fully in letterpress, lithographic, gravure, and flexographic printing as a substitute for the solvent-based paste and liquid inks currently used. These new ultraviolet light-cured inks offer advantages other than the elimination of solvent emission, notably, a combination of excellent film properties with fast printing speeds. The great interest in their development is shown by the large number of patent citations describing various compositions that can be cured by ultraviolet light. Although the future of these new inks must be determined by commercial trial, it appears that their most promising future is in the paste inks used in letterpress and lithographic printing rather than in the liquid inks used in gravure or flexographic printing.

References 1. 1972 U. S. Census of Manufacturers; cited in American Ink Maker 52(5), 19 (1974). 2. National Association of Printing Ink Manufacturers, 101 Executive Boulevard, Elmsford, New York 10523. 3. P. D. Mitton, "Opacity, Hiding Power, and Tinting Strength, " in Printing Ink Handbook Vol.III,T. C. Patton, editor, John Wiley & Sons, New York, 1973, p. 289-339; J. W. Vanderhoff, "Rheology and Dispersion of Printing Inks, " ibid., p. 409-416. 4. A. C. Zettlemoyer and R. R. Myers, "The Rheology of Printing Inks, " in Rheology Vol. 3, F. R. Eirich, editor, Academic Press, New York, 1960, p. 146. 5. W. C. Walker and J. M. Fetsko, American Ink Maker 33(12), 38 1955. 6. J. M. Fetsko, Tappi 41(2), 49 (1958).

184

UV LIGHT INDUCED REACTIONS IN POLYMERS 7. R. W. Bassemir, Preprints, 8th TAPPI Graphic Arts Conference, Miami, Fla., Oct. 19-22, 1971, p. 21. 8. J. W. Vanderhoff, American Ink Maker 51(4), 38 (1973). 9. J. W. Matheson, "Tech Talk #5, " American Paper Institute, 1971.


10. S. V. Nablo, B. S. Quintal, and A. D. Fussa, paper presented at the Annual Meeting, Federation of Societies for Paint Technology, Chicago,Ill.,Nov. 1973. 11. M. H. Bruno, "State of the Art of Printing in the U. S. A., " paper presented at the 11th Intematicnal Ccrference of IARIGAI, Rochester, N.Y., May 12-19, 1971. 12. A. Morano, "Catalytic Thermo-Cured Web Inks, " talk presented at the New York Printing Ink Production Club meeting, Sept. 20, 1972. 13. H. Dunn, paper presented at the 8th TAPPI Graphic Arts Conference, Miami, Fla., Oct. 19-22, 1971; published in Penrose Annual. 14. J. P. Costello (to Fred'k H. Levey Co., Inc.), U.S. 2, 696, 168, Dec. 7, 1954. 15. W. A. Rocap, Jr., Meredith Printing Co., Des Moines, Iowa, private communication, October 1972. 16. Eastman Chemical Products Publication X-214, "Alcohol-Soluble Propionate (ASP) in Printing Inks, " and Customer Service Report 217-1, "Alcohol-Soluble Propionate in Flexographic and Gravure Printing Inks." 17. R. W. Bassemir, R. Dennis, and G. I. Nass (to Sun Chemical Corp ), U.S. 3, 551, 235, Dec. 29, 1970. 18. R. W. Bassemir, D. J. Carlick, and G. E. Sprenger (to Sun Chemical Corp. ), U.S. 3, 551, 246, Dec. 29, 1970. 19. G. I. Nass, R. W. Bassemir, and D. J. Carlick (to Sun Chemical Corp.), U.S. 3, 551, 311, Dec. 29, 1970. 20. R. W. Bassemir, R. Dennis, and G. I. Nass (to Sun Chemical Corp), U.S. 3, 552, 986, Jan. 5, 1971.

12. VANDERHOFF

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185

21. R. W. Bassemir, D. J. Carlick, and G. I. Nass (to Sun Chemical Corp.), U.S. 3, 661, 614, May 9, 1972. 22. K. Hosoi, H. Sagami, and I. Imai (to Ashai Chemical Industry Co., Ltd.), Ger. Offen. 2, 164, 518, July 20, 1972; CA 78, 45224c (1973).


23. T. Ichijo, T. Nishibuko, and K. Maki (to Japan Seal Oil Industry Co., Ltd.), Japan Kokai 72 22, 490, Oct. 7, 1972; CA 78, 30778n (1973). 24. R. E. Beaupre, M. Ν. Gilano, and M. A. Lipson (to Dynachem Corp.), Ger. Offen. 2, 205, 146, Nov. 23, 1972; CA 78, 59876d (1973). 25. C. Osada, M. Satomura, and H. Ono (to Fuji Photo Film Co., Ltd.), Ger. Offen. 2, 222, 472, Nov. 23, 1972; CA 78, 45221z (1973). 26. Y. Nemoto, M. Shiraishi, and T. Syuto (to Dainnippon Ink and Chemicals, Inc.), Ger. Offen, 2, 157, 115, June 22, 1972; CA 77, 90224t (1972). 27. Y. Nemoto and S. Takahashi (to Dainippon Ink and Chemicals, Inc. ), Ger. Offen. 2, 158, 529, June 22, 1972; CA 77, 90225u (1972). 28. J. F. Ackerman, J. Weisfeld, R. G. Savageau, and G. Beerli (to Inmont Corp.), U.S. 3, 673, 140, June 27, 1972. 29. J. F. Ackerman, G. Beerli, R. G. Savageau, and J. Weisfeld (to Inmont Corp.), U.S. 3, 713, 864, Jan. 30, 1973. 30. G. Pasternak (to Continental Can Co., Inc.), U.S. 3, 712, 871, Jan. 23, 1973. 31. M. R. Sandner and C. L. Osborn (to Union Carbide Corp.), Ger. Offen 2, 261, 383, June 20, 1973; CA 79, 127478y (1973). 32. T. Kakawa and T. Kato (to Kansai Paint Co., Ltd. ), Japan Kokai 73 43, 729, June 23, 1973; CA 78, 93572g (1973). 33. S. Nakabayashi and I. Sumiyoshi (to Nippon Paint Co., Ltd. ), Japan Kokai 73 47, 534, July 6, 1973; CA 79, 106212s (1973). 34. H. Morishita (to Kansai Paint Co., Ltd.), Japan Kokai 73 54, 135, July 30, 1973; CA 80, 5033z (1974).

186


35. J. E. Gaste and R. E. Ansel (to De Soto, Inc.), U.S. 3, 749, 592, July 31, 1973. 36. S. B. Radlove, A. Rawe, and Κ. H. Brown (to Continental Can Co., Inc.), Ger. Offen 2, 317, 522, Oct. 31, 1973; CA 80, 84858y (1974). 37. M. Shiraishi, Y. Yanagida, and H. Hisamatsu (to Dainippon Ink and Chemicals, Inc.), Japan Kokai 73 85, 628, Nov. 13, 1973; CA 80,97460n(1974).


38. S. I. Schlesinger (to American Can Co. ), Ger. Offen. 2, 035, 890, Jan. 27, 1972; CA 77, 68565h (1972). 39. S. I. Schlesinger(to American Can Co.), U.S. 3, 708, 296, Jan. 2, 19 73. 40. W. R. Watt (to American Can Co.), U.S. 3, 721, 617, Mar. 20, 1973. 41. T. Nishibuko, S. Ugai, and T. Ichijo (to Japan Seal Oil Industry Co., Ltd.), Japan Kokai 73 55, 280, Aug. 3, 1973; CA 80, 72185p (1974). 42. E. Guertler, R. Kiessig, P. Herte, H. Schuelert, R. Zenker, and P. Noekel, Ger. (East) 81, 929, Oct. 5, 1971; CA 78, 59875c (1973). 43. H. Seidl, H. Meyer, and H. -J. Twittenhof (to Peroxid-Chemie G.m.b.H.), Ger. Offen. 2, 102, 366, Aug. 3, 1973; CA 78, 31569q (1973). 44. A. Bassl, Ger. (East) 91, 706, Aug. 5, 1972; CA 78, 59891e (1973). 45. K. Fuhr, H. J. Traencker, H. J. Rosenkrant, H. Rudolph, M. Patheiger, and A. Haus (toFarbenfabrikenBayer A.-G. ), Ger. Offen. 2, 221, 335, Nov. 15, 1973; CA 80, 49390t (1974). 46. H. Rosenkranz, A. Haus, and H. Rudolph (to Farbenfabriken Bayer A. -G. ), Ger. Offen. 2, 105, 179, Aug. 10, 1972; CA 78, 31603p (1973). 47. V. S. Lapatukhin, L. P. Savanov, and V. P. Tikhonov, U. S. S. R. 355, 203, Oct. 16, 1972; CA 78, 86137v (1973). 48. R. G. Savageau and P. J. Whyzmuzis (to Inmont Corp. ), Ger. Offen. 2, 216, 154, Oct. 19, 1972; CA 78,45254n(1973).

12. VANDERHOFF

UV Light Cured Inks

187

49. C. Sander (Robert Hildebrand MaschinenenbauG. m. b. H.), Ger. Offen. 2, 213, 831, Sept. 27, 1973; CA 80, 16596m (1974). 50. AKZON.V.,Neth. Appl. 72 05, 394, Oct. 23, 1973; CA 80, 72142x (1974). 51. R. Havinga and P. D. Swaters (Akzo G.m.b.H. ), Ger. Offen. 2, 317, 846, Oct. 31, 1973; CA 80, 49372p (1974). 52. J. J. Bulloff, Polymer Eng. Sci. 11(5), 405 (1971).


53. J. Kinstle, Paint Varnish Prod., June 1973, p. 17. 54. V. D. McGinniss and D. M. Dusek, J. Paint Tech. 46(589), 23 (1974). 55. H. Rubin, J. Paint Tech. 46(588), 74 (1973). 56. R. W. Bassemir and A. J. Bean, paper presented at 26th Annual Meeting of TAGA, St. Paul, Minn., May 13-15, 1974; to be published in TAGA Proceedings. 57. V. D. McGinniss and D. M. Dusek, Am. Chem. Soc. Div. Polymer Chem. Preprints 15(1), 480 (1974) 58. V. D. McGinniss, R. M. Holsworth, and D. M. Dusek, Am. Chem. Soc. Div. Org. Coatings Plastics Chem. 34(1), 667 (1974).

13 Use of the Interaction of Ultraviolet Light With Chain Molecules to Study the Rate of Conformational Transition


H. MORAWETZ Polytechnic Institute of New York, Brooklyn, Ν. Y. 11201 In the study of the solution properties of flexible chain polymers, a great deal of emphasis has been placed on methods by which the mean extension of the chain molecule can be characteri zed. The mean square radius of gyration of the molecular chain, <s>, is obtained rigorously from the angular dependence of light scattering intensity (1) and a less rigorous but experimentally more convenient estimate of this quantity can be derived from the intrinsic viscosity (2). As is well known, <s> may be represent ed by <s20>α , where the "unperturbed" mean square radius of gyra tion <s20>depends on the conformational distribution in the chain backbone determined by short-range interactions, while the "expansion coefficient" α arises from long-range interactions which lead to an excluded volume effect depending on the polymer solvent interactions and the length of the chain molecules (3). Relatively much less is known about the rate at which confor mational transitions take place. In comparing such a process in a long chain molecule and in an analogous molecule of low molecular weight, we are faced with a conceptual difficulty: If the confor mational transition involves rotation around a single bond in the backbone of the polymer, a long chain would have to swing rapidly through a viscous medium and such a process would lead to a pro hibitively large dissipation of energy. It was, therefore, sugge sted that conformational transitions take place by a coordinated rotation around two bonds, so that only a short section of the chain is involved in a "crankshaft-like motion" (4)(5). This concept is illustrated in Figure 1. However, the requirement of such a coordinated motion would necessarily lead to a pronounced increase of the free energy of activation and one should, there fore, expect conformational transitions in long chain molecules to be slower than in their low molecular weight analogs. We have looked for such an effect by using an NMR technique for the study of conformational transitions in piperazine polyamides and their low molecular weight analogs, but found that there is no signifi cant difference between the rates of their conformational transi tions (6). 2

2

2

188


13.

Interaction of UV Light with Chain Molecules

MORAWETZ

189

Photochemical and Thermal Isomerization Rates of Polymers Containing Azobenzene Residues. The NMR technique for studying rates of conformational transitions suffers from two limitations. (1) High resolution spectra are obtainable only i n systems of low viscosity aid the technique cannot, therefore, be applied to a study of conformational transitions i n concentrated polymer sys tems. (2) Analysis of the data leads only to a mean rate cons tant and the method i s , therefore, not suitable for detecting differences between the conformational mobility of chemically similar groups. Both these limitations are eliminated i f we follow by ultraviolet spectroscopy the photochemical or thermal isomerization of azobenzene residues i n the backbone or the side chains of polymeric molecules. The photochemical trans-cis isomerization leads to a shift of the absorption peak to shorter wavelength and the thermal reverse reaction to the more stable trans form takes place at convenient rates i n the neighborhood of ambient temperatures. A Nylon 66-type polyamide containing a small number of azodianiline residues -CO (CH ) CONH-^^-N«N-^H^ -NHCO(CH >CONH(CH ) ^NH2

4

2 4

2

was prepared and i t s behavior was compared with that of the analog C H C0NH-^^N=N-^^-NHC0 6

i:L

It was found that i n 95% formic acid solution the ratio of the dark reaction, following photoisomerization to the cis form, was the same, within experimental error, for the polymer and the small molecules (7). Moreover, this equivalence held even for polymer solutions containing 15 weight % of Nylon 66 and 15 weight % of i t s low molecular weight analog, N-propylpropionamide, although at this concentration the polymer molecules would be heavily intertwined. These results seem incompatible with the assumption of a "crankshaft-like motion". We believe that they can be understood i f we assume that the transition from the cis to the trans form does not take place i n a single step but rather by a large number of oscillations around the bond angle by which the transition state i s approached. If we then impede these oscillations by incorporating the azobenzene group into a polymer chain, we reduce equally the rate at which the transition state i s approached and the rate at which a strained bond relaxes to i t s i n i t i a l shape. The transition state i s then s t i l l i n equilibrium with the ground state and incorporation of the azobenzene residue into a polymer chain has no effect on i t s rate of isomerization. It may be noted that this argument i s similar as i n the case of bimolecular reactions, which have reaction rates independent of the viscosity of the medium as long as they are characterized by

190


high activation energies. This i s so since a large number of collisions are required for the reaction and an increasing visco sity slows down both the approach of the reagents towards each other and their diffusion apart. The behavior of the polyamides described above could not be studied in bulk because the polymer i s crystalline and i s not transparent. However, with a methyl methacrylate copolymer carrying a small number of azobenzene residues in side chains CH CH CH I 3 3 ,3 -CH -C-CH.-C-CH -C0

Q

0

t

10

ι

^ 1

CO

CO

0CH NH 3 ^


o

0

^

I

CO ScH 3 0

Copolymer ABA-MMA H

Ν

data could be collected over the entire concentration range, from dilute solution to the glassy polymer i n bulk (8). The results for the thermal reaction following photoisomerization are shown in Figure 2. It may be seen that the rate of isomerization i s almost identical for a dilute polymer solution i n butyl acetate and a polymer film plasticized with 30% of dioctyl phthalate. Even more surprising is the behavior of the unplasticized polymer: Here the isomerization deviates from first-order kinetics exhibi ting an anomalous component faster than in dilute solution! We believe that this i s due to azobenzene residues which are trapped during the photoisomerization i n high energy conformations from which they may return more easily to the stable trans form. This concept i s supported by the observation that samples irradiated above the glass transition temperature and cooled under i r r a d i a tion do not exhibit this anomaly. In any case, the results consti tute an interesting chemical demonstration of the micro-hetero geneity of the glassy state. Similar results were obtained with the isomerization of azonaphthalene and 4-ethoxyazobenzene dis solved i n a glassy polymer (9). A comparison of the photochemical behavior of azobenzene residues i n the backbone and the side chains of polymers i s of special interest since the short lifetime of the excited state makes i t more d i f f i c u l t for the chain to undergo long-range con formational changes favoring the trans-cis isomerization process. In fact, the photochemical process i s much less sensitive to the r i g i d i t y of the system i f the azobenzene i s attached as a side chain. Copolymer ABA-MMA exhibited a quantum efficiency only 4-5 times lower in a glassy film then i n dilute solution (8). By contrast, Figure 3 shows the relative quantum efficiencies for the trans-cis isomerization of a copolyamide of isophthalic acid with 4,4 -diaminodiphenyl sulfone containing a small proportion of ,


MORAWETZ


191

Figure 1. Schematic of a conformational transition in a polymer chain. Top, rotation around a single bond; bottom, correlated rotations around two bonds in a crankshaft like motion.

Time (min.) Figure 2. Medium-dependence of the rate of cis-trans isomerization of copolymer ABA-MMA. •, dilute solution in butyl acetate; Δ , polymer in bulk; O , polymer plasticized with 30% OOF.



192

10

20

Weight 7· DM SO Figure 3. Photoisomerization of isophthalic acid copolyamide with 4,4'-diaminodiphenylsulfone and 4,4 -diaminoazobenzene f

13. MORAWETZ


193

4,4'-diaminoazobenzene residues -NH-^^-N=N-^^NHCO-^|^CONH-^^S0 -^^-NH2


as a function of the dilution of the polymer with dimethylsulfoxide (10). It may be seen that with the azobenzene residue i n the chain backbone, there i s a very sharp drop i n the quantum e f f i c i e ncy of the photoisomerization i n solutions containing about 15 weight % of polymer. In glassy films containing small proportions of DMSO, the quantum efficiency i s extremely low. Unfortunately, we were not able to determine i t i n the completely unplasticized polymer, since i t i s soluble only i n non-volatile solvents which cannot be completely removed. Intramolecular Excimer Fluorescence Studies i n Polymers Carrying Aromatic Side Chains. Some years ago, i t was shown that certain excited aromatic molecules may form a complex with a simi l a r molecule i n the ground state, which i s characterized by a structureless emission band red-shifted relative to the emission spectrum of the monomer. The formation of such complexes, called "excimers", requires the two chromophores to l i e almost parallel to one another at a distance not exceeding about 3.5A° (11). Later, i t was found that intramolecular excimer formation i s also possi ble. In a series of compounds of the type CÛîC^)^^^ , excimer fluorescence, with a maximum at 340nm, was observed only for n-3 a l l the other compounds had emission spectra similar to toluene, with a maximum at about 280nm (12). Similar behavior was observed in polystyrene solutions, where the phenyl groups are also separ ated from one another by three carbon atoms (13). It i s obvious, that the conformation of a section of poly styrene which i s required to make two neighboring phenyl residues l i e parallel to one another (at a distance of about 2.5A°) would be characterized by a prohibitive potential energy and such a conformation would not be expected to be significant i n the ground state. This conclusion i s also supported by the fact that polystyrene exhibits the normal UV spectrum of alkylbenzene, while benzene rings constrained to l i e parallel to one another at such a short separation have characteristically distorted absorption spectra (14). We must, therefore, conclude that excimers are formed by a conformational transition during the lifetime of the excited state. This transition i s assisted by the large exothermicity of excimer formation which i s characterized by ΔΗ values of -5.1 and -3.9 kcal/mole for benzene and toluene, and by -5.8 and -6.9 kcal/mole for naphthalene and the methylnaphthalenes (11). Recent studies i n our laboratory were aimed at defining more closely the conditions governing intramolecular excimer formation in dilute polymer solutions (15). An alternating copolymer of styrene with maleic anhydride or methylmethacrylate showed no excimer emission, confirming that interactions of other than neighboring phenyl residues made no significant contribution to

194

UV


POLYMERS

the effect. Poly(α-methylstyrene) had an even larger fraction of i t s emission i n the excimer band than polystyrene; presumably, this polymer i s highly strained i n a l l i t s conformations and the additional energy requirement for the conformation i n which nei ghboring phenyl groups are parallel to each other i s smaller than in the less hindered polymer containing no quaternary carbons i n the chain backbone. It was, however, quite enexpected that the alternating copolymer of stilbene and maleic anhydride


(-CH-CH-CH-CH-) ι ι ι ι η

had an excimer emission band, since this had been found absent i n both 1,2-diphenylethane and 1,4-diphenylbutane (12). We see here, that the peculiar conformational restrictions of a polymer chain can produce effects which are not easily simulated i n low molecu lar weight analogs. An interesting problem arises with polyacenaphthylene (-CH-CH-) where the r i g i d i t y of the chain clearly precludes a face-to-face orientation of two neighboring naphthalene residues. Yet, this polymer does exhibit excimer fluorescence and this has been assumed to arise by interaction of next-to-nearest neighbors (16). We have confirmed this interpretation by showing that the a l t e r nating acenaphthylene-maleic anhydride copolymer behaves i n a very similar manner as the homopolymer. Table I l i s t s the ratio i ^ / ^ of the quantum yields of dimer and monomer fluorescence of alternating acenaphtylene copolymers with maleic anhydride and other comonomers. There i s a striking contrast between the co polymers with acrylonitrile or methyl acrylate, where there i s very l i t t l e excimer formation, and copolymers with methacrylon i t r i l e or methyl methacrylate, where excimer emission i s much more pronounced than i n polyacenaphthylene. It i s seen that excimer emission i s a sensitive indicator of a change i n the conformational distribution of the polymer chain. Table I. Ratio of Quantum Efficiencies of Excimer and Normal Emission i n Acenaphthylene Homopolymer and Equimolar Copolymers Comonomer 1.2 None 1.2 Maleic Anhydride 0.3 Methyl Acrylate 0.3 Acrylonitrile 2.2 Methyl Methacrylate 3.3 Methacrylonitrile


13.

MORAWETZ


195

Since a conformational transition during the short lifetime of the excited state i s required for excimer formation, excimer fluorescence i s potentially a powerful tool for studying confor mational changes involving relatively low energy barriers. The f e a s i b i l i t y of this approach has recently been demonstrated on l,3-di(a-naphthyl)propane by following the time-dependence of the excimer fluorescence intensity (17). In ethanol solution at 25°C, a rate constant of 1,2x10** see"* was measured and this rate cons tant was found to decrease with an increase i n the viscosity of the medium. We have prepared the monomer Ν,Ν'-dibenzylacrylamide and copolymerized i t with a large excess of methyl methacrylate to study the effect of the r i g i d i t y of the medium on excimer formati on i n a group attached to a polymer chain. As would be expected, excimer fluorescence was observed i n dilute solution, but not i n the glassy polymer film. However, a trace of dibenzyl ether dissolved in poly(methyl methacrylate) did exhibit some excimer emission. We intend to expand studies of this type of compare the results with experiments on the viscosity dependence of excimer emission requiring conformational transitions i n the chain backbone. Data of this kind should c l a r i f y the v a l i d i t y of theories dealing with the role of potential energy barriers and the viscosity of the solvent medium i n determining rates of con formational transitions (18). Acknowledgement I am indebted to the National Institutes of Health, Grant GM-05811 and to the National Science Foundation, Grant GH-33134, for financial support of this work.

Literature Cited 1. 2. 3.

Zimm, B.H. J.Chem. Phys. (1948)16,1099. Flory, P.J. J.Chem.Phys. (1949) 17, 303. Flory, P.J. "Statistical Mechanics of Chain Molecules", Interscience Publ., New York (1969). 4. Schatzki, T. Polymer Prepr., Am.Chem.Soc., Div.Polym.Chem. (1965)6,646. 5. Helfand, E. J.Chem.Phys. (1971)54,4651. 6. Miron, Y., McGarvey, B.R., and Morawetz, H. Macromolecules (1969) 2, 154. 7. Tabak,D.,and Morawetz, H. Macromolecules (1970)3,403. 8. Paik, C.S., and Morawetz, H. Macromolecules (1972) 5, 171. 9. Priest, W.J., and Sifain, M.M. J.Polym.Sci. (1971) A-1, 9, 3161. 10. Chen, D.T., and Morawetz, Η.,unpublished results. 11. Birks, J.B. Progr.Reaction Kinetics (1970)5,181. 12. Hirayama, F. J.Chem.Phys. (1965) 42, 3163. 13. Vala, Jr., M.T., Haebig, J., and Rice, S.A. J.Chem.Phys. (1965) 43, 886.

196



14. Cram, D.J., Allinger, V.L., and Steinberg, H. J.Am.Chem.Soc. (1954) 76, 6132. 15. Wang, Y.C., and Morawetz, H. Macromol.Chem., 176 (in press). 16. David,C.,Lempereur, M., and Geuskens, G. Eur.Polym.J. (1972) 8, 417. 17. Avouris, P., Kordas, T., and E1-Bayoumi, M.A. Chem.Phys.Lett. (1974) 26, 373. 18. Peterlin, A. J.Polymer Sci. (1972) B10, 101.

14 Photochemical Reaction of Monosubstituted Aminoazidobenzoic Acid Derivatives in Novolak


TAKAHIRO TSUNODA and TSUGUO ΥΑΜΑΟΚΑ Faculty of Engineering, Chiba University, Japan

We found that when 4.4'-diazidodiphenylamine in novolak was irradiated by UV light, novolak was insolubilized in alkali aqueous solution and colored in black1) But it is impossible to use it as photopolymer for plate-making because 4.4'-diazidodiphenylamine is very dangerous causing explosion during storage. We investigated the photosensitive azide compounds that was stable and made a deep color image by UV irradiation. We synthesized monosubstitutedamino azidobenzoic acid derivatives which made a deep color image and caused insolubilization of novolak in alk ali aqueous solution by UV irradiation. These photo sensitive azide compounds including some couplers gave more deep color image by UV irradiation. 2.4Dichloronaphthol was the most important coupler to photodecomposed monosubstitutedamino-azidobenzoic acids. The photosensitive layer ( thickness: about 3 . 7 μ ) containing 2-(p-methylphenylamino)-5-azidobenzoic acid, 2.4-dichloronaphthol and novolak had the color density 1.48 by UV irradiation. The polyaster film coated with this photocolored polymer was exposed by UV light through a negative film and then 197


198

washed out by d i l u t e a l k a l i move the unexposed a r e a . polyester

s o l u t i o n to

The p r o d u c e d c o l o r

f i l m may be u s e d as n e g a t i v e

intermediate salt

aqueous

or

re-

image on

positive

f i l m f o r plate-making instead of

silver

emulsion f i l m .

Experimental 1) S y n t h e s i s o f benzoic a c i d


a)

2-monosubstitutedamino-azido-

derivatives

2-Monosubstituted(R)amino-5-nitrobenzoic A mixture

of

acid(I)

2-chloro-5-nitro-

b e n z o i c a c i d and amine d e r i v a t i R-NH-^^-N0

2

uOOH

ves

i n g l y c e r o l was s t i r r e d

heat adding potassium

I

After reaction, filtered

the

carbonate.

s o l u t i o n was

and a c i d i f i e d w i t h h y -

d r o c h l o r i c a c i d . The p r e c i p i t a t i o n was t h e product.

with

T a b l e 1 shows t h e r e a c t i o n

reaction

condition,

pro-

d u c t s and y i e l d . b)

2-Monosubstituted(R)amino-5-aminobenzoic

( HCl-salt

acid

)(II) A mixture

of I ,

m e t h a n o l and

h y d r o c h l o r i c a c i d was R - N H - ^ - î î H - HOI 2

COOH

refluxed

w i t h t i n powder f o r 3-5

hours

and t h e n f i l t e r e d . Prom the dified filtrate ric

acid,

with hydrochlo-

the reduced

product,

amine d e r i v a t i v e was s e p a r a t e d as h y d r o c h l o r i c salt. and c)

T a b l e 2 shows the r e a c t i o n

aci-

condition,

acid

products

yield. 2-Monosubstituted(R)amino-5-azidobenzoic

acid

(III) II

was d i a z o t i z e d w i t h n i t r o s y l s u l f u r i c a c i d

sodium n i t r i t e

i n aqueous

or

s o l u t i o n of a c e t i c a c i d

or

14. TSUNODA & YAMAOKA

hydrochloric II

Amino-azidobenzoic Acid Derivatives

199

acid. After filtration,

the d i a z o t i z e d

was a z i d o t i z e d w i t h sodium a z i d e .

The p r e c i p i t a -

t i o n was a z i d e compound I I I . R-NH--^^—Table

3 shows t h e r e a c t i o n c o n d -

ition,

COOH

products

and y i e l d .

Ill Table 1


R-

Table 2

Temp. Reac- Y i e l d M e l 'C tion % ting hour point G hrs 4

®CH3O-®

S R,-

Reac- Y i e l d M e l tion % ting lour point hrs •c

140

5

46.1

251-3

3

63.9

232-5

130

6

94.4

246.5 -7.5

4

61.9

216-7

HO

5

81.9

146-8

5

97.2

207-8

140

4

80.8

186-7

5

69.8

226-9

140

5

13.0

258-9

3

45.7

214-5

Compound

Reaction Yield Melhour(hrs) % ting Diazo Azido point •c 1

1

74.4

over 250

1

1

90.1

253-4 2 - phenylamino - 5 azidobenzoic acid

2-(p-chlorophenylamino ) - 5 - a z i d o b e n zoic acid

b 257-9 2 - ( p - m e t h y l p h e n y l amino)-5-azidobenzoic acid IIII 3 C I

CH 0-@3

1

4

46.8

1

1

56.5

156 2-(p-methoxyphenyldecom. a m i n o ) - 5 - a z i d o b e n zoic acid I H ^


200

162 2- (of - n a p h t h y l a m i decoml n o ) - 5 - a z i d o b e n z O ' i c acid III

51.7

2)

Novolak r e s i n

m - C r e s o l t y p e n o v o l a k , A l n o v o l 429K tferke 3)

A l b e r t W i e s b a d e n - B i e b r i c h t ) was u s e d .

Couplers

2 . 4 - D i c h l o r o n a p h t h o l ( m p . 107°C ) , (mp. 1 2 5 ° C ) , Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch014

(Chemische

4-methoxynaphthol

l-phenyl-3-methylpyrazolone(mp. 129°C)

and 2 . 5 - d i c h l o r o a c e t a c e t a n i l i d e ( m p . 9 5 C )

were u s e d

e

as c o u p l e r t o

photodecomposed

-5-azidobenzoic 4)

Measurement o f I n f r a r e d

Infrared type 5)

spectrophotometer.

Measurement o f s p e c t r a l

Spectral non a r c

sensitivity

s e n s i t i v i t i e s were measured by Narumi RMe q u i p p e d w i t h g r a t i n g and 450W X e -

lamp.

Measurement o f o p t i c a l d e n s i t y o f c o l o r e d

Macbeth d e n s i t o m e t e r transmittance 7)

spectra

s p e c t r a were measured by H i t a c h i EPI-S2

35 monochromator 6)

2-monosubstitutedamino

acids.

Exposure

image.

was u s e d f o r measurement

of

density.

equipment

Vacume p r i n t e r e q u i p e d w i t h t e n 40W c h e m i c a l lamps ( D a i n i p p o n S c r e e n P - 1 1 3 - B ) was u s e d . The f r o m lamp t o n e g a t i v e 8)

cm.

P h o t o d e c o m p o s i t i o n and s e p a r a t i o n o f

osed

photodecomp-

products

I n 500 m l o f b e n z e n e , solved. the

f i l m was 4 . 4

distance

0.5g

o f a z i d e compound was

I n a f l a s k e q u i p p e d w i t h UV l i g h t

source,

a z i d e i n benzene was photodecomposed p a s s i n g

nitrogene

gass.

The photodecomposed p r o d u c t s

were

separated

by column chromatography w i t h developement


solvents Result


o f c h l o r o f o r m and a c e t o n e .

and

discussion

T a b l e 4 and 5 show the c o l o r o f photodecomposed zoic

acid.

and o p t i c a l

i n t a b l e 4 were p r o d u c e d by UV

a filter

a n o l s o l u t i o n of azide

paper a b s o r b e d w i t h 30% compounds.

The c o l o r s

5 were p r o d u c e d by UV i r r a d i a t i o n t o Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch014

inum p l a t e o r p o l y e s t e r of mixture

o f n o v o l a k and a z i d e compounds

I

X

m

I

I

b

J

I

ni

Ι

a

τ

1

c

d

β

meth

i n table

a grained

f i l m c o a t e d w i t h 10%

alum

solution

(10:3

)

so

methylethylketone.

T a b l e 4 The c o l o r o f p h o todecomposed a z i d e s c o a t e d on f i l t e r paper w i t h o u t novolak

Compound

density

2-monosubstitutedamino-5-azidoben-*

The c o l o r s

i r r a d i a t i o n to

lved i n

201

Exposure time (min.)

1 3 5 1 3 5 1 3 5 1 3 5 1 3 5

Color

l i g h t G. l i g h t G. l i g h t G. (G: gray) l i g h t G. black black

T a b l e 5 The d e n s i t y o f photodecomposed a z i d e s c o a t e d on p o l y e s t e r f i l m w i t h novolak

Expos Color ure time (min.)

Optical density coating thickness (micron) a f t e r was* out

2

black

0.30

l i g h t G. black black

2

black

0.30


2

green black

0.32


1

black

0.36


202

The t r a n s m i t t a n c e

densities

were measured on p o l y e s t e r

of the c o l o r e d

image

f i l m a f t e r UV i r r a d i a t i o n

o r a f t e r i r r a d i a t i o n and washing out

i n 2% aqueous

s o l u t i o n o f sodium p h o s p h a t e . Pig.

1 shows t h e

spectral

sensitivities

of

a z i d e compounds. The mechanism o f t h e c o l o r i s not c l e a r ,

but i t

c t i o n products

is

assumed t h a t

of nitrenes

these reaction

the v a r i o u s

f r o m photodecomposed

rea azi


de compounds p r o d u c e t h e mixed c o l o r i n g m a t t e r .

300

400 III

300 III

a

300

400 I I ] C

300

b

300

400 d

400

m

e

400 Ι

ΐ

τ

ο

Figure 1. Spectral sensitivities of 2-monosubstituted-amino-5-azidobenzoic acids

In t h i s

photocolor reaction

amino-5-azidobenzoic a c i d ,

of

2-monosubstituted-

t h e i m i n o and c a r b o x i l i c

groups i n the molecules are v e r y i m p o r t a n t .

The com

pounds (IV and V) w h i c h h y d r o g e n o f t h e s e g r o u p s s u b s t i t u t e d w i t h a l k y l group are when t h e y a r e

are

very hypsochromic

photodecomposed. The r e a s o n why t h e

photodecomposed I V and V a r e h y p s o c h r o m i c may be due to

the f a c t

ture l i k e VI

t h a t t h e y c a n not have i n n e r s a l t .

struc


14. TSUNODA & YAMAOKA Amino-azidobenzoic Acid Derivatives

203


204

When t h e m i x t u r e

of

2-monosubstitutedamino-5-azido-

b e n z o i c a c i d and n o v o l a k was i r r a d i a t e d by UV l i g h t , p h o t o c o l o r and p h o t o c u r e o c c u r e d a t t h e same t i m e . I n this case,

i t i s considered that

quinonimine d e r i v a t -

i v e s a r e a l s o p r o d u c e d by t h e p h o t o r e a c t i o n

between

a z i d e compounds and novolakr-^

C00H

C00H


R-HH m-cresol novolak

nitrene

type

000H

RNH-^^-NH

2

quinonimine compound Pig.

2 shows t h e I n f r a r e d s p e c t r a o f 2 - p h e n y l a m i n o -

5-azidobenzoic acid I I I duct i n K B r t a b l e t

b

and t h e photodecomposed pro-

by UV i r r a d i a t i o n .

Figure 2. Infrared spectra of 2-phenylamino-5-azidobenzoic acid and the photodecomposed product

14.


TSUNODA & YAMAOKA

2-Phenylamino-5-azidobenzoic

a c i d III^

decomposed i n benzene w i t h o u t o x y g e n ,

was

205 photo-

and t h e n

the

photodecomposed p r o d u c t s were s e p a r a t e d by column chromatography. the

P i g . 3 shows the

Infrared

spectrum

photodecomposed component o b t a i n e d f r o m t h e

of

deve-


loped l a y e r of the column.

Wave ^ i a U ^ ft**""') Figure 3.

Infrared spectrum of the one component from photodecomposed 2-phenyU amino-5-azidobenzoic acid

A mixture

of

2-monosubstitutedamino-5-azidobenzoic

a c i d and c o u p l e r gave deep c o l o r image by UV i r r a d i ation.

It

seems t h a t t h e dye p r o d u c e d by c o u p l i n g

photodecomposed a z i d e w i t h c o u p l e r added t o

the

pho-

t o c o l o r of monosubstitutedamino-azidobenzoic P o r example,

the c o u p l i n g r e a c t i o n

COOH

acid. 3) i s as f o l l o w s ^ - ' ;

COOH

dye

OH

UV L I G H T INDUCED REACTIONS I N

206 The f o l l o w i n g example

i s the c o a t i n g

solution

a i n i n g a z i d e compound, n o v o l a k and c o u p l e r . rage t h i c k n e s s g i v e n are

of coating

by w e i g h t

percent.

5 32

novolak(alnovo1) coupler

4

methylcellosolve


6

59

shows the t r a n s m i t t a n c e

c o l o r images

cont

The a v e

l a y e r was 3·7ρ. A l l p a r t s

a z i d e compound

Table

POLYMERS

densities

of

the

p r o d u c e d by UV i r r a d i a t i o n t o the

coat

ing l a y e r o f the m i x t u r e

o f a z i d e compound, n o v o l a k

and c o u p l e r on p o l y e s t e r

film,

out

and t h e n by w a s h i n g

i n 2% aqueous s o l u t i o n o f sodium p h o s p h a t e .

color

image f r o m t h e m i x t u r e

ino)-5azidobenzoic

acid(III

of

The

2-(p-methylphenylam-

) and

2.4-dichloronaphth-

c o l gave the h i g h e s t

d e n s i t y 1.48

.

Conclusion 2-Monosubstitutedamino-5-azidobenzoic ivatives

acid

der

are v e r y u s e f u l f o r p h o t o c o l o r i n g m a t t e r .

The d e n s i t y o f t h e

p h o t o c o l o r image i n c r e a s e s by a d d

i n g a proper coupler to

the

a z i d e compound. I t

p o s s i b l e t o f i x the c o l o r e d image u s i n g t h e

is

photo-

polymers c o n t a i n i n g t h e s e a z i d e compounds and n o v o lak.

In future,

it

may be p o s s i b l e t o use t h i s

c o l o r f i l m f o r p l a t e making i n s t e a d o f s i l v e r emulsion

photohalide

film.

Acknowledgment The a u t h o r s

are

deeply indebted to M r . Shigeo

Maeda and M r . Susumu Suzuka i n Hodogaya C h e m i c a l I n dustry Co. f o r t h e i r various

compounds.

assistance with synthesis

of


\

Table

Ε

I : C I

C-CH3 •H

CI

00H

Pi]Lter R G

Pi]Lter G R

3

°d1 Pi].ter R G

1 2 3 4 5

0.42 0.58 0.58 0.65 0.62

0.59 0.75 0.80 0.95 0.88

0.63 0.63 0.56 0.63 0.62

0 . 8 8 0.57 0 . 5 4 0.89 0.81 0.55 0.52 0.90 0 . 8 6 0 . 4 9 0.48

1 2 3 4 5

0.96 0.97 1.00 1.07 1.10

1.19 1.20 1.33 1.48 1.48

0.54 0.57 0.56 0.58 0.57

0.67 0.64 0.69 0.68 0.69

1 2 d 3 4 5

0.79 0.82 0.74 0.76 0.84

1.05 1.12

1 2 3 4 5

0.61 0.60 0.62 0.65 0.60

0.90 0.89 0.92 0.91 0.88

1.05

1.17 1.18 0.56 0.51 0.59 0.58 0.55

207

6

OH

0

A \



0.38 0.40 0.40 0.41 0.39

0.25 0.29 0.33 0.32 0.34

0.45 0.48 0.51 0.58 0.56

0.42 0.43 0.44 0.55 0.52

NHC0CH C0CH, 2

0

Pi].ter R G

0.53 0.57 0.67 0.69 0.62

0.38 0.40 0.98 0.47 0.41

0 . 6 6 0.26 0.24 0.56 0.69 0.39 0.33 0.73 0.67 0.38 0.34

C:Coupler, A:Azide compound, E:Exposure time(min.) R:Red f i l t e r , G:Green f i l t e r

Literature cited 1) T.Tsunoa, T.Yamaoka and Y.Yamaguchi: Japanese Patent pending 2) Takahiro Tsunoda: Technical Papers of Photopolymer Conference (1973) 20 3) J.J.Sagura and J.A.Van Allan ( Kodak Co.): U.S. Pat. 3.062.650 (1962)

15 Photodegradation of Polyvinyl chloride


ERYL D. OWEN Chemistry Department, University College, Cardiff, Wales

1. Introduction The mechanisms of the thermal and photochemical degradation of poly(vinyl chloride) (PVC) continue to be active areas of research in polymer chemistry mainly because its high chemical resistance, comparatively low cost and wide variety of application make PVC one of the most widely used thermoplastic materials. The wide variety of forms which the material can take includes pastes, lattices, solutions, films, boards and moulded and extruded pieces and depends to a very large extent on the good electrical and mechanical properties of the polymer. In spite of these advantages the even wider application of the material has been restricted by its low thermal and photochemical stability. Thermal instability is a problem since processing of the polymer is carried out at about 200 C and the photochemical instability places a limit on the extent of the outdoor applications which can be developed. Both thermal and photochemical processes take the form of a dehydrochlorination reaction which leads to discolouration as well as extensive changes in the internal structure of the polymer which has an unfavourable effect on the desirable electrical and mechanical properties. It has become apparent that considerable similarity exists between the two degradation processes and that it is neither easy nor desirable to make a vigorous distinction between the two. Information gained from experiments on thermal degradation are often directly relevant to the analogous photochemical process. The reaction involved may be written:isopropyl CI but i n this case nineteen hydrocarbon products were detected i n addition to various organic chlorides. The alkyl aryl ketone sensitised photolysis (313 nm; ot hexane solutions of t-butyl CI reported by Harrimanl2 also gave HC1 as a major product together with 2-methyl propene. Quantum yields ranged from 0.19 when benzophenone was the sensitiser to 0.313 for EtC=0 and the involvement of the t r i p l e t state was confirmed by the quenching of the reaction by 2,5-dimethyl-hexa2,4 diene. Comparison of the range of E values (280 - 330 kJ mole"l) of the sensitisers with the dissociation energy of the C-Cl bond (327 kJ mole~l) led these authors to suggest that some charge transfer interaction was involved since there was ample evidence for such a process occuring between t r i p l e t ketone acceptors and amineiS or sulphideM. donors. In contrast,Golub— has concluded that the photolysis of 1,4 DCB sensitised by a variety of aliphatic ketone sensitisers i s a singlet state reaction. There i s ample evidence that this i s also the case when aromatic hydrocarbons act as sensitisers i n a variety of processes. Finally Whittle has shown that hexafluoroacetone i s able to sensitise the elimination of HC1 from a variety of alkyl chlorides. I t appears that the t r i p l e t or singlet state may be involved and there i s a strong possibility that the actual elimination process occurs from a vibrationally excited ground state. Clearly then different groups of workers favour the singlet, t r i p l e t or vibrationally excited ground state as the precursor for the elimination process. There i s l i t t l e evidence to c l a r i f y the extent to which catalyst end groups arising from i n i t i a t i o n or termination of the polymer T

i.e. O R

R-

+

CH =CHC1

> R-CH^CHCl

2

-Â^CHCl

+

R-

VV

>~ CHC1R


15.

OWEN

Photodegradation of Polyvinyl chloride.

211

influence the photochemical s t a b i l i t y but this w i l l clearly be determined by the nature of R and i t s absorption and photo chemical characteristics. Effects of R on the thermal s t a b i l i t y are contraversial since a l l of the studies carried out so far have been plagued by the presence of traces of unreacted catalyst present not as a polymer end group but trapped within the polymer. These traces of free catalyst are undoubtedly much more effective than those present as end groups particularly since transfer reactions mean that only 20-40% of the polymer have R ends. Similar conclusions can be drawn regarding end group unsaturation since the nature of the chromophore i s such that absorptions would be i n the far ultraviolet unless the unsaturation was an extended system of conjugation. Thermal s t a b i l i t y of these systems however i s much more clearly defined and i t may be concluded that end group unsaturation contributes to the i n s t a b i l i t y of low molecular weight polymers only, while the i n s t a b i l i t y of high molecular weight fractions is more strongly influenced by some other factor, probably the presence of branch points or internal double bonds. Many of the attempts which have been made to implicate particular structural features as a cause of thermal or photochemical i n s t a b i l i t y have u t i l i s e d low molecular weight model compounds and there are several considerations which severely limit any extrapolations which can be made from such studies. F i r s t l y although small model compounds undergo unimolecular homogeneous reactions, the introduction of a surface component could cause a dramatic reduction i n the energy of activation and so any possible surface effect could be very important. Secondly many of the studies which have been carried out i n solution are complicated by factors such as solvent participation or the catalytic effect of HC1. Thirdly, as well as requiring information on the effect of particular structural features, the frequency of their occurrence i n the polymer i s also essential. Before enough information of this type i s available however much more information on the kinetics of vinyl chloride polymerisation are required, b) Chromophores. It i s possible to imagine many chromophores which could arise during processing of the polymer but the most effective i n affecting photostability are probably /C * 0, -00H and metal ions Μ^ . Ketones and hydroperoxides have low intensity absorptions i n the ultraviolet region and there is no reason why these should be much affected by attachment to a polymer chain. Some comments have already been made regarding the ketone sensitised photolysis of model compounds which occurs by transfer of energy from the ketone to the alkyl chloride. In addition, several other possible mechanisms of i n i t i a t i o n are possible. For example the ketone may decompose by the Norrish Type I mechanism with the formation of free radicals thus:+


212

-C-C-CI I

N

>

N

-C-C-+-CI I

.

>

-C-+C+-CI I

Alternatively the Norrish Type II process may operate since the p o s s i b i l i t y of a Y hydrogen being available i n the polymer i s high g i l l h* - C - C - C - C — N


,M

f i -C*C

v

I I + C * C

i l " >

Y

I A further and very important p o s s i b i l i t y i s the transfer of energy from the ketone sensitiser to molecular oxygen forming one of the two low lying singlet states of oxygen ketone + 0 ( £~) * ketone + 0 or Δ ) . 3

3

3

X

2

1

2

Reactions of singlet oxygen with isolated olefins having a l l y l i c hydrogen atoms are extremely rapid and i n this sense isolated double bonds at any point i n the polymer are a source of photochemical i n s t a b i l i t y „ i

Λ Λ

·

UUn

ι -c=ç-ç- + o ( A ) I

* > -c-ç>ç

1

1

2

Yet another possible mode of i n i t i a t i o n i s hydrogen abstraction thus:3

ketone + RH

> ketone *-H + R'

so that at least four well documented mechanisms exist by which a t r i p l e t ketone sensitiser can i n i t i a t e photodegradation namely i ) decomposition with the formation of free radicals, i i ) energy transfer to the polymer, i i i ) energy transfer to molecular oxygen, iv) hydrogen abstraction with consequent formation of free radicals. Alkyl hydroperoxides which may also be formed i f oxygen is present during processing have an absorption which extends to about 320 nm which may lead to the formation of free radicals thus : R00H —

R

0

- + -OH

Metal ions act as efficient sensitisers for this process as well as forming free radicals by the process:Λχ"

>

(n

M -

1)+

( n - l ) X - • Χ*

The nature of X may effect the position of \ considerably.

m a x

very

15. O W E N

Photodegradation of Polyvinyl chloride

213


c) Impurities* Two main classes of impurities affect the photostability of PVC. The f i r s t type, traces of catalyst, have already been discussed i n section (a) while the second type, traces of solvent, are important when the polymer i s i n the form of thin films which have been cast from a solvent. Such traces are extremely d i f f i c u l t to remove completely and are undoubtedly responsible for much of the irreproducibility and disagreement which i s apparent i n the literature. One of the best solvents for PVC is tetrahydrofuran (THF) but this solvent i s particularly d i f f i c u l t to remove. Its removal i s very necessary however since i t reacts very readily with dissolved oxygen to form a photolabile peroxide whose decomposition products are very effective i n i n i t i a t i n g the photodegradation reaction. Mechanism of the Reaction It i s probable that once the i n i t i a t i o n has been accomplished, the mechanisms of the thermal and photochemical reactions have much i n common. For that reason i t i s profitable to make some comments about the mechanism of the reaction, most of which result from work on the thermal process. Until about ten years ago most workers i n the f i e l d believed that a radical mechanism operated. This view depended to a large extent on analogy with the results of the thermal decomposition of other polymers. ^5 1959 two mechanisms were put forward simultaneously by Winkler — and by Strombergi§ which differed i n the details of the initiation process but agreed that propagation occurred by a "Zipper" process which may be written In

CHCHCl^—^ / ~ N ^ CH=CH

>

^^^CH=CH^^

+ CI*

+C1 · — > ~CH=CH«CH* C H C l ^ ^ +HC1

and was supported to some extent— by earlier work on the gas phase decomposition of alkyl chlorides. At about the same time i t was found that the presence of small quantities of salts like FeCl^ or ZnClj had a strong accelerating effect on the rate of the dehydrochlonnation reaction which led some authors to believe that an ionic mechanism could be operative at least i n the absence of oxygen. Support for the idea that at least a p a r t i a l separation of charge i n the transition state was possible began to grow and gained impetus from the following observations:a) typical radical inhibitors like hydroquinone did not affect the rate of dehydrochlorination i n a nitrogen atmosphere, b) chlorine has never been detected as a reaction gfoguct, c) the catalytic effect of HC1 i n the absence of oxygen i s d i f f i c u l t to explain by means of a radical mechanism but easier to explain i n terms of a non-radical process proceeding v i a a c y c l i c , six membered transition state, d) the effect of solvent, i n particular nitrobenzene, f i r s t

214


mentioned by Goto and F u j i i l 5 and later confirmed by BengoughlSjJJ as well as the relation between the rate of dehydrochlorination and the dielectric constant of the s o l v e n t is typical of non-radical processes, e) the low molecular weight model compounds which undergo thermal decomposition i n the gas phase by a homogeneous, f i r s t order, monomolecular process, probably do so v i a a polar transition state. In spite of the increasing support for an ionic mechanism and the evidence which has been advanced for the presence of carbonium ion species i n Pwff8,29 there are s t i l l many workers who support a free radical mechanism. The work of Bamford2£ is typical and involves carrying out the degradation i n labelled C H CH T or C H C ^H3 toluene at 180°C where i t is found that both Τ and are incorporated into the polymer chain. The evidence for both types of mechanism i s so convincing and the conditions so d i f f i c u l t to standardise that i t i s tempting to speculate that both mechanisms may operate given the right conditions. Indeed Guyot^l h suggested that the i n i t i a l step has an ionic mechanism and that this might i n i t i a t e a radical process which predominates when high concentrations of HC1 are present. Papko and Pudov2£ have gone one step further and suggested that radical, molecular and ionic mechanisms may make contributions which depend on the conditions. In particular they suggest that the presence of HC1 may induce an ion-molecule reaction. I t i s becoming apparent from results such as these that the presence of HC1 may be very important and i t s presence may dictate which mechanism predominates. 1


6

5

2

6

5

a s

Termination and Extent of Dehydrochlorination Various estimates have been made of the length and distribution of the polyene sequences which result from the dehydrochlorinat ion reaction. GuyotJL suggested a range of 5-12. Geddes-22 thought that 13-15 was more typical while Braun£-t believed that values as high as 25-30 were possible. More recent work*5^3P favours the shorter range ^ 12 while the more sophisticated ^ computer based spectrum matching procedure of Daniels and Rees— shows that the range 3-14 applies to the sample degraded under their carefully defined conditions but that the temperature and duration of degradation may be c r i t i c a l . The method used by most workers to analyse the distribution of polymers i s ultraviolet spectroscopy and this i s undoubtedly the main reason for the considerable disparity i n the results obtained by different investigators. Polyene sequences are very reactive and the spectra may be considerably affected by interaction with a) the solvent, b) solvent derived peroxides, c) molecular oxygen, d) HC1, or by undergoing intramolecular or intermolecular Diels-Alder type reactions. a) Russian authors— have shown that the NMe group of dimethylformamide causes a bathochromic shift of polyene 2


15.

OWEN


215

absorptions while Daniels and ReeslZ have had to assume shifts i n order to obtain agreement between their experimental and synthetic spectra. b) Many workers suspected that the behaviour of PVC films which had been cast from solution depended on the nature of the solvent used. Some authors^ attributed the improved resistance to discoloration of films prepared from THF to the fact that oxygen containing impurities i n the solvent retarded the formation of poly exes. In a very careful study however, Daniels^ has shown that of the various products of both dark and photo chemical reactions of oxygen with tetrahydrofuran only the oL -hydroperoxytetrahydrofuran reacts with the polyene sequences. This reaction takes the form of an addition to the terminal double bond of the polyene sequence. Films cast from THF and containing trace amounts of this solvent w i l l therefore be subject to variable behaviour unless oxygen i s rigorously removed. c) As well as reacting with the solvent to form unstable peroxidic products, molecular oxygen i n the gas phase or i n solution may have a very profound effect on the degradation process. The mechanisms by which i t reacts are undoubtedly very complex and have been summarised i n several good reviews of the photooxidation of polymers . > . Its accelerating effect on the degradation (measured by HC1 evolution) i s due at least i n part to the formation of)C 0 and -00H chromophores which sensitise further decomposition. It also undergoes a very rapid photochemical reaction with polyene sequences which make the measurement of the extent of degradation in the presence of oxygen by spectroscopic measurements i n the v i s i b l e and ultraviolet regions meaningless. d) The effect of HC1 on the rate and extent of the degradation reaction has long been a matter of controversy. Technologists involved with the processing of PVC have long interpreted the stabilising effect of HC1 acceptors as evidence that HC1 i s a catalyst for the degradation process. The opposite view held by Arlmani and Druesdow" however was widely accepted for many years. The definate catalytic effect found by Rieche— and TalaminiâS however has been amply confirmed by more recent work=. and i s now generally accepted. Our own work has shown z2 conclusively that a rapid photochemical addition of HC1 to the polyene sequences of degraded PVC results i n a bleaching reaction which i s the reverse of the photodehydrochlorination. The reaction, which i s wavelength dependent reaches a photo stationary state i n which a new polyene distribution i s formed which depends on the wavelength of the irradiating light and the concentration of HCl. Results for the photo addition of HC1 to the model compound diphenyloctatetraene (DPOT) indicate that the reaction i s slower i n ethanol than i n hexane and therefore do not support an ionic mechanism. The thermal dehydrochlorination reaction i n the liquid phase on the other s

U V LIGHT


216

INDUCED

REACTIONS IN

POLYMERS

hand i s undoubtedly accelerated by the presence of free HCIÂZ o-nitrophenol— or strong electrophiles such as ZnC^ or SnCl^ 4? especially i n polar solvents while non-polar solvents inhibit. In these systems, the rate of reaction depends on the dielectric constant and on the electron accepting power of the medium, results which lead to the inevitable conclusion that an ionic mechanism or at least a strongly polarised transition state is involved. Clearly the role played by HC1 i s not completely understood. Its presence may f a c i l i t a t e ionic mechanisms which may not be significant when i t s concentration i s low. Since i t s concentration may vary very considerably from one experimental arrangement to another the mechanisms involved may vary accordingly. e) It is necessary to mention one other factor which effectively terminates the dehydrochlorination reaction when the PVC i s i n the form of thin films. This i s the formation of the polyenes in a thin strongly absorbing surface layer which effectively protects the bulk of the film from further degradation^. When this layer i s separated from the bulk of the film i t i s found to be highly insoluble and therefore cross linked to some extent. Any spectrophotome t r i e analysis of degraded films therefore should take this fact into account since the polyenes are not homogeneously distributed throughout the film as i s often assumed. In order to overcome this problem the films described in our work£2 were cast from solutions using PVC which had been thermally degraded and were homogeneous so that the surface layer did not present a problem. Stabilisation of PVC At least four types of additives have been used to stabilise PVC towards photodegradation. The f i r s t type are merely strong ultraviolet absorbers which function by dissipating as thermal energy light which would otherwise be absorbed by a sensitiser and i n i t i a t e photodegradation. Some of the most effective are of the hydroxybenzophenone or 2 -hydroxybenzotriazole type which u t i l i s e internal hydrogen bond formation i n the dissipative mechanism. A second type of stabilisers are quenchers which can accept the energy of an excited sensitiser molecule and convert i t harmlessly into heat. One molecule which can do this very effectively i s oxygen but the singlet oxygen so formed may be more damaging than the original sensitiser. For this reason quenchers should be as non selective as possible i e . they should quench as large a range of excited states as possible. One very useful class of molecules i n this respect are Ni(II) chelates which quench both t r i p l e t sensitiser and singlet oxygen. I t i s interesting though that dLamagnetic square planar complexes are more effective i n stabilising polypropylene ,


15. O W E N


217

than paramagnetic ones. One explanation which has been suggested is that energy transfers to the higher energy t r i p l e t ligand states of the paramagnetic compares are endothermic while transfers to the lower energy ligand f i e l d states of the diamagnetic complexes are exothermic though s t e r i c a l l y hindered. Essentially similar results have been obtained for polystyrene!! The efficiencies of this type of quencher are generally higher than those of the absorber type discussed above but the fact that they are coloured may be unacceptable. The third class of stabilisers are included only for the sake of completeness but the mechanism by which they operate are probably the best understood. They may be described as antioxidants and are effective because they intercept the radical chain carriers or decompose the peroxides or hydroperoxides which are potential radical i n i t i a t o r s . The fourth class which are peculiar to PVC have one thing in common namely an a b i l i t y to react wifti HC1. They include such diverse substances as inorganic lead salts, heavy metal soaps, organo-tin compounds, epoxides and many others. Although their a b i l i t y to react with HC1 may be a contributory factor their main function i s undoubtedly to react with and deactivate potential sites of i n i t i a t i o n such as branch points or areas of unsaturation.u2


218


References 1. Arlman E.J., J.Polymer.Sci., (1954), 12, 547. 2. Guyot Α., Benevise J.P. and Trambouze Υ., J.Appl. Pol. Sci., (1962), 6, 103. 3. Suzuki T., and Nakamura Μ., Japan Plast., (1970), 4, 16. 4. Mayer Z., Obereigner B. and Lim D., J.Polym.Sci., (1971),C33,289. 5. ν Erbe F., Grewer T. and Wehage Κ., Angew.Chem., (1962), 74, 988. 6. Asahina M. and Onozuka Μ., J.Polym.Sci., (1964),2,3505. 7. Chytry V., Obereigner B. and Lim D., Eur.Pol.J.Suppl., (1969), 379. 8. Golub M.A., J.Amer.Chem.Soc., (1969), 91, 4925. 9. Golub M.A., J.Amer.Chem.Soc., (1970),92,2615. 10. Golub M.A., J.Phys.Chem., (1971), 75, 1168. 11. Benson H.L., and Willard J.E., J.Amer.Chem.Soc, (1961), 83, 4672, (1966), 88, 5689. 12. Harriman Α., and Rockett B.W., J.Chem.Soc., Perkin 2, (1974), 5, 485. 13. Cohen S.G. and Parson G., J.Amer.Chem.Soc., (1970), 92, 7603. 14. Davidson R.S., and Steiner P.R., J.Chen.Soc., (C), (1971), 1682. 15. Winkler D.E., J.Polym.Sci., (1959), 35, 3. 16. Stromberg R., Strauss S. and Achhammer B.G., J.Polym.Sci., (1959), 35, 355. 17. Barton D.H.R., and Howlett K.E., J.Chem.Soc., (1949), 155. 18. Imoto Μ., Mem. Fac. Eng. Osaka City Univ., (1960),2,124. 19. Baum B., SPEJ., (1961), 17 71. 20. Hartmann, Α., Kolloid-Z., (1956), 149, 67. 21. Rieche Α., Grimm A. and Mucke Η., Kunststoffe, (1962), 52, 265. 22. Braun D., and Bender R.F., Eur.Pol.J.Suppl., (1969), 269. 23. Marks G.C., Benton J.L., and Thomas CM., Soc.Chem.Ind., (London) Monograph, (1967), 26, 204. 24. Geddes W.C., Eur.Pol.J., (1967), 3, 267. 25. Koto Κ., and Jujii Η., Nippon Gomu Kyokaishi, (1959), 32, 90. 26. Bengough W.I., and Sharpe H.M., Makromol.Chem., (1963), 66, 31,45. 27. Bengough W.I., and Varma I.K., Europ.Polym.J., (1966), 2, 49, 61. 28. Scalzo E., Mater.Plast.,(1962), 28, 682. 29. Schlimper R., Plaste Kautschuk, (1966), 13, 196. 30. Bamford C.H., and Fenton D.F., Polymer,(1969),10, 63. 31. Guyot Α., and Bert Μ., J.Appl.Polym.Sci., (1973), 17, 753. 32. Papko R.A., and Pudov V.S., Vyoskomol.Soedin., (1974), A16, 1409. 33. Geddes W.C., Eur.Polym.J., (1967),3,747. 34. Braun D. and Thallmaier Μ., Makromol.Chem., (1966), 99, 59. 35. Smirnov L.V. and Popov K.R., Vyoskomol.Soedin., (1971), A13, 1204.


15. OWEN


36. Minsker K.S., and Krac E.O., Vyoskomol Soedin., (1971), A13, 1205. 37. Daniels V. and Rees N.H., J.Polym.Sci., (1974), 12, 2115. 38. Smirnov L.V. and Popov K.R., Int.Symp.Makromol.Chem., Prepr.(1969), 5, 461. 39. Daniels V. Ph.D. thesis Univ.of Wales, (1974). 40. CicchettiO.,Adv. Polymer Sci., (1970), 7, 70. 41. KarspukhinO.N.and Slobodetskaya E.M., Russ.Chem.Revs., (1973), 42. 42. Druesdow D. and Gibbs C.F., Nat.Bur.Stds Circ., (1953), 525. 43. Talamini G., Cinque G. and Palma G., Mat.Plast., (1964), 30, 317. 44. Corsato-Arnaldi Α., Palma G. and Talamini G., Mat.Plast., (1966), 32, 50. 45. Mayer Z. and Obereigner Β., Eur.Polym.J., (1973),9,435. 46. Kimura G. Yamamoto K. and Sueyoski Κ., Yuki Gosei Kagaku Kyokaishi, (1965), 23, 136. 47. Leprince P. Rev. Inst. Fr. Petroli Ann Combust. Liquides (1959), 14, 339. 48. Bailey R.J., Ph.D. thesis, Univ.of Wales (1972). 49. Owen E.D. and Williams J.I., J.Polym.Sci., (1974), 12, 1933. 50. Adamczyk A. and Wilkinson F., J.Appl.Polym.Sci., (1974), 18, 1225. 51. Harper D.J. and McKellar J.F., J.Appl.Polym.Sci., (1974), 18, 1233. 52. Anderson D.F. and McKenzie D.A., J.Polym.Sci., A-1, (1970), 8, 2905.

16 Dielectric Studies of the Photochemistry of Polystyrene Films R. GREENWOOD and N. A. WEIR


Chemistry Department, Lakehead University, Thunder Bay, Ontario, Canada

The photochemistry of polystyrene has been the subject of a considerable amount of research during the past twenty-five years. Two main approaches have been employed: (a) analyses of gaseous products liberated or absorbed during photolyses, and (b) analyses of the polymer residue after the absorption of a given amount of radiation. Analyses of the photolysis products has been achieved by mass spectrometry(1,2) and by gas chromatography(3), and absorption of O2 during photo-oxidation has been indicated by differential ma nometry(4,5). Structural changes in the polymer residue have been monitored spectroscopically (I.R.(6,7,8), U.V.( ), N.M.R.(7), e.s.r.( ) and fluorescence( ) and by molecular weight determinations( . These studies have shown that the mech anisms of the photolyses reactions are very sensitive to experi mental conditions, in particular the wavelength of the radiation and presence of oxygen critically control both primary and secon dary photochemical processes. It is possible to distinguish four principle systems: 1) short-wave U.V. (λ=254nm) photolysis under high vacuum conditions, 2) short-wave U.V. photolysis in presence of O2, 3) long-wave U.V. photolysis (λ>300nm) under high vacuum, and 4) long-wave photolysis in presence of O2. Absorption of short-wave radiation by the phenyl chromophores results in the formation of the vibrationally excited singlet state, S*1, which undergoes rapid vibrational relaxation to S1, which rapidly becomes deactivated to form the excimer (18). The very small excimer-triplet (T1) splitting facilitates intersystem crossing from the excimer to Τ1. Excimer fluorescence is also observed, but no singlet (S1) fluorescence has been observed for the solid polymer at room temperature(19). The primary photo chemical process involves fission of the tertiary C-H bond(3,12), and this step presumably involves both singlet and triplet states, since each have associated with them degrees of excitation in ex cess of the C-H bond dissociation energy (estimated to be 80±3 kcal. mole ), provided that this energy can be concentrated in the C-H bond. The tertiary radicals subsequently interact with 39,1,0

11,12

13,14)

15,16,17)

-1

220

16.

GREENWOOD AND WEIR

Photochemistry of Polystyrene Films

themselves and add to benzene rings to form cross-links (—) and molecular H2 i s formed by combination of Η-atoms and by inter- and intra-molecular abstractions of hydrogen from the polymer, the latter process being responsible for the formation of conjugated sequences i n the polymer c h a i n ( · 1 i 2 1 ) . Recent studies of pulsed photolysis of the polymer indicate that main chain C-C bonds are also broken, but the quantum yield i s much lower than that for C-H f i s s i o n d ) . In presence of O2, the tertiary radicals produce peroxy radi cals, which in turn yield the hydroperoxides v i a inter- and intra molecular Η-atom a b s t r a c t i o n s ) . Hydroperoxides may undergo further photolytic decomposition or decompose as a result of ener gy transfer from the excimerOit), presumably by an exchange inter action, to yield chain-end carbonyl compounds with concomitant chain scission. Acetophenone, which has recently been observed as a reaction product, could conceivably be formed from the carbonyl compound by a Norrish type II reaction. Polystyrene has no long-wave U.V. (A>300nm) chromophores, and the i n i t i a l formation of radicals during long-wave irradiation i s d i f f i c u l t to reconcile with the structure of the pure polymer. However, i t has been shown that long-wave irradiation under vacuum produces small extents of random chain s c i s s i o n ^ > ) , which has been attributed to the photolysis of 0-0 bonds which are incorpo rated into the chains as a result of the free radical copolymerization of small amounts of O2, and in support of this hypothesis, i t has been shown that anionically prepared polymers (which should not contain such impurities) show considerably greater s t a b i l i t y when treated under identical conditions(—). Ketonic impurities, resulting from small extents of purely thermal oxidation of the polymer, have also been invoked(ii) as possible chromophores for long-wave U.V., and acetophenone has been identified as such an oxidation product Initiation of the long-wave photodegradation can be achieved by photolysis of such impurities into radicals which can, by means of Η-atom abstraction produce radical centers on the polymerCSit). It would appear that the small radicals produced from, for exam ple, acetophenone would, because of their high mobilities and reactivities, act as efficient i n i t i a t o r s . In addition, excited triplet states of such aromatic ketones, which can result from intersystem crossing following absorption of long-wave U.V., are capable of Η-atom abstraction from the polymer(ϋ). In the pres ence of O2, the polymer radicals w i l l undergo oxidation, admitted ly at a lower rate, to the hydroperoxide in a similar sequence to that outlined above. Hydroperoxide decomposition i s qualitatively similar to that outlined above, however, because of the broad absorbance in the long-wave region, direct photolysis i s possi ble (1^). The resulting ketonic products are similarly photolab i l e . The general kinetic characteristics of the long-wave photooxidation are significantly different from those of the 254nm initiated reaction, in that appreciable induction periods are 3

1 3

2 0

2


221

17

2Lf


222

POLYMERS

observedC ), and i t would seem that these times are associated with the production of macro-radicals. Also, since the absorption characteristics of the two types of radiation are completely d i f ferent, the long-wave reaction i s l i k e l y to be homogeneous throughout the films. Most of the results obtained by the above techniques inevita bly refer to relatively advanced stages of the photo-reactions, as i t i s often very d i f f i c u l t to detect with any degree of accuracy, the small structural modifications of the polymer which occur in the very early stages of the reaction. Since polystyrene has a very low d i e l e c t r i c loss (typically ε"=4.5χ10"^ at 1 0 H z ) , and since photodegradation introduces polar groups, e.g. C=0 groups and polarizable groups (conjugated C=C sequences) into the polymer, i t can be predicted that the dielectric characteristics w i l l be significantly modified by the presence of relatively low concentrations of polar impurity. Hence, measurements of changes in d i e l e c t r i c properties as a function of time of degradation appear to offer a sensitive monitor of the incorporation of polar and polarizable groups into the polymer. Dielectric properties depend largely on the surface characteristics (e.g. resistance) of the sample and, where thin films are used, the surface layers constitute a major proportion of the total volume of the sample. Since most of the photo-reaction occurs in the surface layers, d i e l e c t r i c measurements reflect f a i r l y accurately the effects of photolysis. It can also be readily predicted that such modifica tions of non-polar polymeric insulators and dielectrics w i l l be reflected as less desirable insulating and d i e l e c t r i c properties of these polymers. However, despite the consequences of deterio rating e l e c t r i c a l and dielectric properties brought about by the inevitable exposure of these materials to potentially photodegradative environments, few systematic studies have been made. The effect of γ-radiation in a i r on the dielectric properties of polyethylene has been studied CSZ) and i t was shown that both the d i e l e c t r i c constant and the dissipation factor increased grad ually with increasing radiation dose. The effects of the radia tion were frequency dependent and more pronounced changes i n both ε' and tan ό were observed in the 10 -10 Hz region. Outdoor aging of polyvinyl chloride for a period of six months results in an enhanced dielectric dispersion, and d i e l e c t r i c constant and increased a.c. conductivity at low frequencies (30-50Hz) has been used to determine the kinetics of the HC1 elimination reaction Irradiation of polyvinylidine chloride with 254nm U.V, results in increased conductivity(£2.), and similar treatment of polyethylene rapidly increases the d i e l e c t r i c constant, and power factor ( s i n o ) . Photo-oxidation has also been used as a tracer in the assignment of d i e l e c t r i c relaxations. The assignment of the d i electric relaxations. The assignment of the dielectric a-relaxa tion i n polyethylene has been the subject of considerable contro versy, and this was resolved by introducing C=0 groups, by means of photo-oxidation, into crystalline p o l y e t h y l e n e ( i , ) . 23


5

5

6

3 1

3 2

3 3


16.

GREENWOOD AND WEIR


223

Results demonstrated that the α-relaxation i s associated with re orientations occurring i n the crystalline region. The object of the present work was to investigate the kinet ics of the various photo-decompositions of polystyrene, in partic ular the early events i n these reactions, using d i e l e c t r i c tech niques, and to compare the results with those previously obtained. The results w i l l , i n addition, provide information regarding the effects of such photo-degradations on the d i e l e c t r i c properties of the polymer. Dielectric measurements were made using a Hewlett-Packard Model 4342-A Q-Meter for the high frequency range (22kHz-70MHz) and a 1620-AP General Radio Capacitance Bridge for the lower f r e quency range (10 Hz-20kHz)· The Q-meter was used i n conjunction with a 1620-AP G. R. capacitance measuring assembly mounted on a 4342 mounting j i g (Rutherford Research) without with which reproducible results were not obtained. The test c e l l for the G. R. Bridge was a 3-terminal spring-loaded p a r a l l e l plate capacitance c e l l . Good e l e c t r i c a l contact was ensured by attaching metal f o i l to the surfaces using thin grease films. In order to reduce the effect of variable atmospheric humidity, the c e l l s were flushed with dry nitrogen and a dry N2 atmosphere was maintained i n the c e l l s dur ing measurements. Polystyrene was prepared from carefully purified monomer both thermally without i n i t i a t o r and anionically using an established method ( _ ) . Purification consisted of repeated precipitation of the toluene solutions from pure methyl alcohol. Precipitates were dried at 65°C under high vacuum for at least 100 hours before use. Characteristics of the polymers used are as in Table 1. Dielec t r i c measurements were made i n 5 cm circular discs 0.075 mm thick which were made by solvent evaporation from toluene solutions or were moulded from powdered polymer using a press and die. U.V. irradiations were carried out i n the presence of oxygen and i n vacuum, the discs being mounted i n a fused s i l i c a vessel. The U.V. sources used were a General Electric High Pressure Hg Lamp f i t t e d with a Pyrex f i l t e r for long-wave irradiation and a P.C.Q. low pressure Hg arc (Ultra Violet Products) which emitted predom inantly 254nm radiation, the incident flux of which was 7.8x10" Ecm" sec~ . The t o t a l flux of long-wave U.V. i . e . the flux inte grated over the wavelength range 300-366nm was 2.4xlO" Ecm"' sec" . When a voltage i s applied to an "ideal" d i e l e c t r i c , polariza tion of the dipoles occurs instantaneously, and there i s no lag between the orientation of the molecules.and the variation of the electric f i e l d . Under these conditions the current has a dis placement of π/2 relative to the e.m.f., and no loss of e l e c t r i c a l energy occurs. This i s illustrated i n (a). For a real d i e l e c t r i c the situation i s quite different. F i r s t l y , there i s an inertia associated with the orientation polarization of a dipole, and a f i n i t e time i s required for the dipole to become oriented, and secondly, the dipoles tend to undergo a relaxation to the original 9

2

1

8

2

1

224

UV


Table 1 Characteristics of Polystyrenes Polymer

Polymerization Temperature (°C) 50 50 50 50 50 50 50 60 72 80 90

Anionic tl tt tt tt tt tt

Radical tt


It tt

4

MnxlO" 2.1 4.8 10.1 24.2 58.0 87.0 200.0 160.0 94.2 51.0 18.7

random orientations which they would adopt i n the absence of the f i e l d . As the frequency of the applied f i e l d , E, increases the. polarization, P, lags behind i t . The applied f i e l d which varies as E=EoSinu)t, and the polarization takes the form P=P Sin(u)t-o). Because of the phase lag and the existence of a f i n i t e period of d i e l e c t r i c relaxation, part of the e l e c t r i c a l energy i s dispersed as heat. It can be seen from i l l u s t r a t i o n (b) that the current i s no longer out of phase with the e.m.f. by π/2, but by (π/2-ό) where 6 i s the phase displacement or loss angle. The loss can be equated with the product of the e.m.f. and the in-phase component which the current acquires, i . e . isino. Another consequence of the lag i s that the permittivity decreases at higher frequencies where the loss angle i s appreciable. The permittivity may be represented by the complex function: 0

f

ε* = e - j " e

(1)

where ε' i s the real permittivity or d i e l e c t r i c constanf, and ε" is the d i e l e c t r i c loss (imaginary component) and j=(-l)^, i s the algebraic operator. From Figure 1(c) i t can be seen that: tano = ε'νε'

(2)

in which tano i s the dissipation factor. The Q values obtained from the Q-Meter are related to tano as Q=(tan6)" . Removal of the external f i e l d results i n a reorientation of dipoles and the polarization decays with time according to the equation, 1

P

( t )

= P exp(-t/T) 0

(3)

where P i s a constant and τ i s the relaxation time for the polar ization, i . e . the time required for the polarization to be reduced 0

16.

GREENWOOD A N D WEIR


225

to 1/e of i t s o r i g i n a l v a l u e . Since r e l a x a t i o n i n v o l v e s r o t a t i o n a l rearrangements, there are energy b a r r i e r s a s s o c i a t e d w i t h t h e process, τ i s r e l a t e d t o the energy (H) by the equation, τ = T exp(-H/kT)

(4)

0

where τ i s a constant and k i s Boltzmann's constant, τ i s f u r ther r e l a t e d t o t h e frequency a t which maximum l o s s occurs, ( f ) , f o r a g i v e n r e l a x a t i o n by the equation, 0

m


τ = l/2uf

(5)

m

The frequency dependence of the complex d i e l e c t r i c constant can be expressed by the Debye equation, ε* = el + ô " " 1 + ίωτ £

ε

(6)

where ε^ and ε£ a r e t h e d i e l e c t r i c constants a t very h i g h and very low ( s t a t i c ) f r e q u e n c i e s . I t can be shown t h a t ε* and ε" are r e l a t e d t o frequency by combining (1) and ( 6 ) : ε

1

=

ε'

+

£

Q

~

£

° ° — 2

1 + ω τ ε» = ° " " ^ 1 + ω τ £

and

£

2

(7) 2

(8) 2

In t h e present work ε* and ε£ a r e not r e a d i l y determinable, p a r t i c u l a r l y s i n c e the chemical nature of the polymer i s changing. However, f o r a given r e l a x a t i o n , i f i t can be assumed t h a t there i s a symmetrical d i s t r i b u t i o n of r e l a x a t i o n times ε^ and ε£ can be r e l a t e d by the Kramers-Kronig e q u a t i o n ( — ), or by t h e f o l l o w i n g alternative e x p r è s s i o n , OB

Δε» = ε J - ε· = 2AH/RirJ(e")d(l/T) .... (9) Ο where ΔΗ i s the a c t i v a t i o n energy and R i s the gas constant. Δε' i s a measure of the i n t e n s i t y of the d i e l e c t r i c r e l a x a t i o n spectrum (the r e l a x a t i o n strength) and i t i s i n t u r n r e l a t e d to t h e number of r e l a x i n g d i p o l e s (N) per u n i t volume by t h e equation( i ), 3 7

3 8

4ffNgy

Δε ' =

:

3kT

where μ i s the d i p o l e moment, η i s the r e f r a c t i v e index, g i s the Kirkwood g f a c t o r and k i s Boltzmann's constant. The v a r i a t i o n of

UV L I G H T

226

REACTIONS

IN

POLYMERS

Δε' with degradation i s a useful measure of the accumulation of polar groups i n the polymer. However, there appears to be consid erable overlapping of multiple peaks, and i n the early stages of the reaction, Δε', which i s determined from areas under curves i n the ε" vs 1/T plots i s not readily accessible. The relaxation peaks are f a i r l y symmetrical and ε" values at the frequency of maximum loss have been used as alternative data to follow reactions. Dielectric constants of the radically and the anionically prepared polymers are comparable over the frequency range investi gated (Fig. 2) and the frequency dependence shown i s similar to that previously reported(—)· Values of the corresponding dielec t r i c losses are shown i n Figs. 3 and 4, and the frequency depen dence of each i s qualitatively very similar to what has already been demonstrated( % )« However, the actual losses, i n partic ular those i n the 10 -10 Ηζ region, are numerically lower than those previously determined, and these values may reflect the lower concentrations of polar impurities that are introduced into the chains during the present polymerizations. It i s significant that anionic polymers, i n which incorporation of polar groups i s much less probable than i n free radical polymerization, have lower losses over the entire frequency range. Dissipation factors (tano) are frequently used as a measure of d i e l e c t r i c loss, and these data, shown for the anionic polymer (Fig. 3), are qualita tively similar to the ε" data. However, during degradation ε values vary i n a complex manner, and the use of tan6 values to monitor degradation tends to obscure the real effects of degrada tion on d i e l e c t r i c losses. Since molecular weight changes are frequently associated with photochemical reactions, the effects of molecular weight or d i electric properties of both types of polymer were investigated i n order to avoid possible complications arising from varying molec ular weights. The data shown i n Figs. 5 and 6 indicate that polymers with low molecular weights (10^) have relatively higher losses, particularly at low and very high frequencies, and these figures perhaps reflect the relative importance of impurities i n the low molecular weight polymers, the effective ratio of polar to non-polar material being higher. The maxima i n the loss curves at around 8xl0 are not immedi ately explicable, however, dynamic mechanical loss moduli (E ) show a similar variation with molecular weight (Jl£). The smaller losses associated with higher molecular weight material (Hh>10 ) are perhaps a consequence of chain entanglement which increases with increasing molecular weight (ill.), and which would inhibit to some extent group rotations and other co-operative movements which are prequisites for d i e l e c t r i c dispersion. The following work has been carried out using the polymers (MnvLO ) which have the lowest losses over a wide frequency range, i n order to extend the sensi t i v i t y of the technique. Photo-oxidation of polystyrene (254nm, 400torr oxygen) 36


INDUCED

4

39

6

1

5

M

6

5

GREENWOOD AND WEIR


Ε

Ε

(α)

(b)

IDEAL DIELECTRIC DIELECTRIC LOSS


Figure 1.

30

2-5

20 h

log f Figure 2. Dielectric constants (e'j as a function of fre quency (f). O, anionic; ·, radical polymers (Mn = 10 and 1.87 X JO , respectively). s

5



log f Figure 4. Dielectric loss (t") as a function of frequency. Radical polymer (Mn = 1.87 X 10 ). s

6h

Ο

3

* I 0 Hz

5μ

κ±>

7

Hz

HO

5

Hz

4

log Figure 5.

Ο χ

MO

Mn

Dielectric loss (ί') as a function of molecular weight (Mn). Anionic polymer.

6h

log Figure 6.

Mn

Dielectric loss (t") as a function of Mn. Radical polymer.

POLYMERS

16.

GREENWOOD AND WEIR


229

results i n increased d i e l e c t r i c constants and losses (Figs, 7 and 8) over the entire frequency range. Three distinct areas of maxi mum loss appear in the d i e l e c t r i c spectrum with increasing degra dation, a low frequency band around 2.5xl0 Hz, another centered on lO^Hz and a pronounced peak around 2xl0 Hz. Radically prepared polymers show identical behaviour. It i s obvious that the general increase i n d i e l e c t r i c parameters results from overlapping of mul t i p l e relaxations of a range of oxidation products, however, the three areas of more pronounced dispersion were further investiga ted. The variation of d i e l e c t r i c constant with increasing extents of degradation i s shown in Figure 9 and i t can be seen that the actual pattern of increase i n ε i s complex and also frequency dependent. The apparent "levelling o f f " of the ε' at high f r e quencies could be indicative of some type of inhibition occurring, but i t i s probably the result of decreasing d i e l e c t r i c constants, the orientation polarization decreasing as the relaxing dipoles interact with the high frequency f i e l d . It would therefore appear that interpretation of such data cannot be made unequivocally. Increases in d i e l e c t r i c losses occurring at the frequency regions of highest loss are shown as a function of degradation time (short-wave U.V. in O2) i n Figure 10, and i t can be seen that the losses increase with increasing degradation in a similar fash ion i n each of the frequency ranges. However departures from the linearity with time of degradation occur at around 180 minutes degradation, and this i s probably attributable to the photodecomposition of some of the polar entities which contribute to the dielectric losses, small molecules forming and diffusing out of the films. It i s possible also that mobilities of polar groups are restricted somewhat by cross-linking. The three main dispersion regions are discussed in more d e t a i l as follows: 2

6


1

2

(a) The 2.5xl0 Hz region. It i s l i k e l y that the lowest f r e quency loss i s associated with D.C. conductivity and possibly also Maxwell-Wagner i n t e r f a c i a l polarization. Dielectric losses are due to d i e l e c t r i c relaxation and to a number of other processes related to D.C. conduction by the sample. The total loss ε^ can be equated with these component losses as follows ( J i ) . 2

ε£ = ε" + ka/f

(11)

where σ i s the D.C. conductivity and k i s a constant. It can be seen that D.C. conductivity w i l l contribute significantly only at low frequencies. Interfacial scattering of radiation also occurs at phase boundaries when particles of one material are suspended in another which has a different d i e l e c t r i c constant (MaxwellWagner p o l a r i z a t i o n ) ^ ) , and d i e l e c t r i c losses similar to those due to dipolar relaxations are observed, often at low frequenciesCu). Small highly cross-linked matrices formed in the



I 2

ι

ι

ι

1

1

3

4

5

6

7

POLYMERS

1—

8

log f Figure 7. Changes in dielectric loss as a function of f with irradiation at 254 nm in 0 (400torr) (Mn = 2.4 X 10 ). Anionic polymer. s

2

I

1

2

3

1

4

I

I

I

5

6

7

L_

8

log f Figure 8. Changes in c' as a function of f on irradiation (254 nm) in 0 (400torr). Anionic polymer Μ η = 2.4χ 10 . ·, no irradiation; O, after 60 min. £

s

GREENWOOD AND WEIR


I 0 Hz 2xl0 Hz Ι Ο Hz 4

29 r

3

5

28-

2xl0 Hz 6

2726251


24b 60

120

240

180

T i m e (min) Figure 9. Increases in c' with time of irradiation (254 nm) in 0 (400torr). Anionic polymer (Mn = 2.4 Χ 10 ). 2

5

2xl0

6

Hz

2xl0 Hz 3

l0 Hz 5

Time

(min)

Figure 10. Dielectric loss as a function of time of irradiation (254 nm) in 0 (400torr). Anionic poly mer (Mn= 2.4 χ J O ; . 2

5

232

UV

LIGHT

INDUCED REACTIONS IN

POLYMERS

surface layers i n particular could constitute the suspended p a r t i cles, which, by extensive modification due to degradation, would have completely different d i e l e c t r i c properties from those of the polymer. The magnitudes of such losses depend c r i t i c a l l y on the geom etry of the particles, and since i t was not possible to obtain such information for the cross-linked polymer, the effect was not further investigated.


4

(b) The 10 Hz region. The low temperature dependence of the lO^Hz d i e l e c t r i c loss of an oxidized film (254nm in O2) was inves tigated and the results are shown in Figure 11. Oxidation results in the formation of a broad loss peak with a maximum at 232°K, which overlaps the very small loss peak at around 220°K i n the pure polymer. The latter peak, designated as the γ-relaxation p e a k ^ — h a s been ascribed to oscillatory motion of the phenyl groups around the C-CgHs bond accompanied by a small amplitude twist of the main chain of the polymer. It has been shown(ilD that thermal degradation of polysty renes containing peroxide linkages leads to a similar enhancement of the γ-relaxation peak, and this has been attributed to the presence of small degradation products of these peroxides. That the large loss peak i n Figure 11 i s associated with small, vola t i l e polar photo-oxidation products (e.g. ketones) was demonstra ted by heating the oxidized sample to 75°C at 10"" torr and deter mining the d i e l e c t r i c loss as a function of heating time. It can be seen (Figure 11) that after 16 hours heating, the loss peak has largely disappeared and after 30 hours heating, the losses i n the oxidized film are very similar to those in pure sample. It would appear that the enhancement of the γ-peak i s related to the co operative motions of the small molecules and the phenyl groups, orientation of the small occluded molecules being made possible by phenyl rotations. 6

6

(c) The high frequency relaxation at 2xl0 Hz. The loss peak at 2xl0 Hz was unaffected by heat treatment i n vacuum, and i t can be concluded that i t i s due to the relaxations of dipoles which are chemically bound to the polymer chain. The temperature varia tion of this loss peak i s shown i n Figure 12 and the activation energy (E) associated with the relaxation was determined from the variation i n relaxation time (τ) with temperature (T) using the equation, 6

τ = T exp(-E/RT)

(12)

0

where, τ i s a constant. A value Ε = 12.4±1.5kcal mole" was obtained. Since this value i s comparable with the values of energy barriers to rotation observed for aromatic ketones (measured in a polystyrene ma trix) which also show loss maxima around 5xl0 Hz, i t i s 0

1

6

GREENWOOD AND WEIR


PHOTO OXIDIZED


PURE

170

POLYSTYRENE

POLYSTYRENE

180 190 200 210 220 230 240 250 260 Temperature (°K)

Figure 11. Dielectric loss of oxidized anionic polystyrene (1.5 hr at 254 nm in 400 ton Og)_at low temperatures (f = 2.4XlO*Hz)Mn = 10 s

Figure 12. High frequency relaxation of oxidized anionic polystyrene (1.2 hr at 254 nm in 400torr O ). Effect of temperature. g

234


reasonable to associate the high frequency loss peak with relaxa tions of ketonic groups i n the polymer. Terminal ketonic groups have been found i n oxidized polysty rene (JJÏJLIË) and the relaxation process could be envisaged as rota tion of the C6H5-CO group around the 1-2 C-C bond. 0

C H Kinetically both anionically and radically prepared polymers conform to the following relationships, the rate of photo-oxida tion being expressed as the rate of increase of the high frequency loss (de"/dt). Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch016

6

5

m

Rate = de"/dt = ^ ( I ) ( 0 ) 0

n

(13)

2

where Κ i s a constant, φ i s the quantum yield, I i s the incident U.V. intensity, and m and η are exponents. Two limiting cases can be distinguished: (a) at low O2 pres sures, Po 150torr, n=l and m=0. The variation of rate with increasing oxygen pressure i s shown i n Figure 13. These results can be rationalized i n terms of O2 s o l u b i l i t i e s and diffusion coefficients. At low pressures, both of these quan t i t i e s are small and the oxidation i s essentially a diffusion controlled process, and since the radical flux i s relatively higher than the local concentration of O2 within the films, the rate i s independent of I. However, beyond about 150torr, the diffusion coefficient i s independent of the O2 pressure(£Z), and consequently the rate of oxidation becomes independent of oxygen pressure. Diffusion coef ficients are subject to further reductions due to concomitant cross-linking, which i s more probable when the O2 scavenger con centrations are lowCitJL). The f i r s t order dependence on I i s explained as follows: 0

2

2

0

Rate of radical formation = φΙ {1-exp(-$£)}..(14) 0

where I i s the thickness of film and 3 i s the effective absorption coefficient. For the films under study, exp(-3£)«l, hence Rate - φΙ Short-wave irradiation of the polymer under high vacuum con ditions following rigorous degassing at 10" torr for 24 hours, gives rise to significant losses i n the low frequency region (at 10 -10 Hz) only (Figure 14). However, i f the degassing procedure is not observed, small, but detectable losses also occur i n the 10 Hz region. It has been shown that such treatment results in the forma tion of conjugated sequences i n the polymer chains, and i t i s 0

6

2

6

3

GREENWOOD AND WEIR



50

100

150

200

250

300

350

400

Pressure ( T o r r ) Figure 13. Effect of 0 pressure on rate of increase of loss at 2.25 X JO Hz (254 nm, anionic polymer Mn = 2.4 X 10 ) 2

6

s

2

3

4

log

5

6

7

8

f

Figure 14. Irradiation (254 nm) of anionic polymer at 10~ torr. Effect of photodegradation on loss. 6



236

possible that these have semi-conducting characteristics similar to these exhibited by polyenes, and the low frequency dispersion could be related to D.C. conductivity. Under these conditions the polymer becomes highly cross-linked and there may also be formed sufficient cross-linked particles containing polyenes to give rise to some i n t e r f a c i a l polarization. Long-wave (X>300nm) irradiation i n O2 results i n much less rapid increases i n d i e l e c t r i c losses, however i t would appear from the ε" versus log f data (Figure 15) that the products of oxida tion are very similar to those formed on short-wave irradiation. The more rapid oxidation of the radically prepared sample perhaps reflects i t s high concentration of photo-labile impurities which can act as i n i t i a t i o n centers (ID. The higher reactivity of the radically prepared sample i s further demonstrated by the shorter induction period (than that for the anionic polymer) shown i n Figure 16. The general behaviour shown i n Figure 16 i s aualitatively i n agreement with however, the induction periods are significantly shorter than those previously determined. Since i t i s highly unlikely that the present polymers contain higher concentrations of photolytic impurities, the discrepancy i s not a function of the polymer, but is rather more l i k e l y to be related to the relative s e n s i t i v i t i e s of the d i e l e c t r i c and manometric techniques for determining the induction periods. The present technique would appear to be a sensitive monitor of polymer modification. Kinetically, long-wave oxidation after the induction period, may be represented by the expression, ,

1

m

Rate = dε 7dt = Κ ' φ ( I ) ( 0 ) 0

(15)

n

2

in which K' i s a constant, φ' the quantum yield for photolysis (in the linear region) and m and η are exponents. (a) at low O2 pressures, n=l and m»l (b) at higher O2 pressures (>60torr) n=0,

m»l

It would appear that after the induction period, i n i t i a t i o n i s brought about by photolysis of the i n i t i a l l y formed oxidation products, and the O2 pressure dependence (Figure 17) i s a t t r i b utable to the very low radical concentrations i . e . only at very low O2 pressures i s the radical flux higher than the available O2 concentration. Because of the low reaction rates, intensity expo nent data cannot be accurately measured, however, a low chromophore concentration (C), would lead to f i r s t order dependence on I , since 0

iQÎl-exp-ÎCft'A)}

f

I C3 £ 0

for low extents of absorption.

The relaxation strength (ε^-ε^) of the dipole which shows the high frequency absorption i s related to the dipole moment, μ,

GREENWOOD AND WEIR



Ο

log f Figure 15. Irradiation of anionic (Mn = 2.4 Χ 10 ) and radical (Mn = 10 ) polystyrenes at λ > 300 nm in Oft (400 torr). Losses as a function of time degradation. 5

s

225xlO

Ο χ

6

Hz Radical

6

5h

225 xlO Hz Anionic 6

10

20

30

40

50

60

70

80

9 0 100

Time ( h r s ) Figure 16. Characteristics of loss increases for anionic (Μη = 2.4χ 10 ) and radical (Mn^lO ) polystyrenes. Reactions at λ > 300 nm in O (400 torr). s

5

t


238

IN

POLYMERS

and the number of dipoles per unit volume, N, by the following form of the Onsager equation, Δε' = e'

0

- ; = ε

< \ ..(16)

3

where ξ i s a constant (£=^g/3k) . For polymers used i n this study, Δε'/3εÂ-A-A-

X

9

—-A-A-A -A-A-A—8 The l i f e t i m e of migrating exciton i s 10 sec. During that time the exciton may " v i s i t " several molecules over a long

264


distance of 10*2-10^ JL Such migrating exciton may be captured by impurities or traps (e.g.physical defects i n the c r y s t a l ) and loses i t s energy which i s then transformed into the v i b r a t i o n a l energy of an atom or a molecule. The exciton energy transfer can occur i n many polymers with an adequate symmetry. Photosensitized reactions of polymer degradation


The c l a s s i f i c a t i o n of photosensitized reactions according to the polymer structure seems to be a resonable approach,but unfortunately the quantitative evidence necesary f o r such c l a s s i f i c a t i o n i s s t i l l inadequate. In practice i t i s convenient to group the s e n s i t i z e d reactions according to the photochemical reactions of the i n i t i a t o r s and s e n s i t i z e r s , e . g . Benzophenone The t r i p l e t state i s the photochemically reactive state of t h i s compound (9-12,91)»

BENZOPHENONE

L

J

I t has been found that only ketones,in which the lowest t r i p l e t state has the n, p o l y ( v i n y l alcohol) (94)» polyisoprene (95,96),polyurethane (97) and polyadenylic acid (98)· In s o l i d state benzophenone also produce an extensive c r o s s l i n k i n g of polyethylene (99-103)· I t has also been found that benzophenone and i t s derivatives caused an i n i t i a l rapid oxidation,increasing with ketone concentration (70)· Currently the r e l a t i v e importance of singlet oxygen formation i n energy transfer reaction between excited benzophenone and molecular oxygen i s discussed

(92,104): \θ

θ ] * ° » -~ Ο 3

2,3ς

ί

Ό *V' +1(

18. RABEK

Photosensitized Degradation of Polymers

265

Quinones


On exposure to UV i r r a d i a t i o n the reactions of quinones involve t h e i r carbonyl groups and the r i n g double bonds, Quinones arecapable of abstracting hydrogen atoms from various hydrogen donors. The s i n g l e t and t r i p l e t states of quinones are considered to be the i n i t i a l reactive i n t e r mediate i n the reactions

Quinones are e f f e c t i v e compounds f o r the photodegradation of polystyrene (105-107)tPolyisoprene (108 109)•polypropylene (110).polyamides (111),polyurethanes (97) and c e l l u l o s e (112). lîuring the quinone-sensitized photooxidative degradation of polystyrene f i l m and i t s solution i n benzene,an i n i t i a l rapid decrease of average molecular weight has been observed by v i s c o s i t y measurement (Pig.8) and GPC (Pig.9) (107)· t

Figure 8.

Figure 9. (Legends on following page)

266


Figure 8. (page 262, left) Change of viscosity number in benzene solution during uv irradiation in the presence of air and (Φ) p-quinone, (O) duroquinone, ( V ) anthraquinone, (*ψ) chloranil, and (X) without quinone. Motor ratio of styrene units to quinone = 88:1.


Figure 9. (page 262, right) Gel permeation chromatograms of (—) undegraded polymer and polystyrene degraded after 5 hr uv irradiation with (--) p-quinone, ( ) duro quinone, (" -) chloranil, (—) anthraquinone, and without quinone ( ). (A), polysty rene as film; (B), polystyrene in benzene solution. Molar ratio of styrene units to qui none = 88:1.

The reaction rates are strongly increased by quinone such as p-quinoneιduroquinone,anthraquinone,and c h l o r a n i l . l t has been suggested ( l O j ) that t h i s photosensitized degradation of polystyrene occurs by a s i n g l e t oxygen reaction which might be related to an energy transfer mechanism from excited t r i p l e t states of quinone to molecular oxygen. Peroxides They are e s p e c i a l l y important i n the process o f thermal and photodegradation of polymers. The primary photod i s s o c i a t i o n of a l k y l and a r y l peroxides takes place i n the f i r s t absorption region. The weak RO-OR* bond i s disrupted! RO-OR* + hv — - RO* + 'OR' Below 2 2 0 nm a l k y l peroxides dissociate i n another way:

sometimes very important differences of the natures of the r a d i c a l s produced i n thermal and photochemical d i s s o c i a t i o n are found. I t was shown ( 1 1 ? ) that the peroxide molecules are dissociate by heat to pairs of benzyloxy radicals,some of which may dissociate f u r t h e r to phenyl r a d i c a l s and carbon dioxide. Photochemical d i s s o c i a t i o n leads to the d i r e c t production of some phenyl r a d i c a l s (114). I t has been found that the photodegradation of several polymers such as p o l y ( v i n y l chloride)(115)> polyisoprene ( I 0 9 t 1JjS),bisphenol A polycarbonate ( 1 1 7 ) i s s e n s i t i z e d by peroxides. P o l y c y c l i c aromatic hydrocarbons They are well known as compounds which are photochemically very r e a c t i v e . The excited s i n g l e t and t r i p l e t states are the photochemically reactive states,and they also can p a r t i c i p a t e i n energy transfer reactions ( 2 , 3 , 1 0 , 1 1 ) . Some of these compounds such as anthracene,phenanthrene,pyrene added to polyethylene effect the photodegradation of polymer ( 1 1 8 ,

18. RABEK


267

1 1 9 ) · They can s e n s i t i z e the abstraction of the a l l y l i c hydrogen of unsaturated groups i n polyethylene* Photoexeited t r i p l e t state transfers i t s excess energy to the unsaturated bond to excite it,and the excited unsaturated groups release t h e i r a l l y l i c hydrogen atom,producing an a l l y l i c r a d i c a l . Hexahydropyrene s e n s i t i z e d chain s c i s s i o n of polypropylene and polyisobutylene during l i g h t i r r a d i a t i o n ( J 2 0 ) . P o l y c y c l i c hydrocarbons have a important role i n s e n s i t i z e d photo oxidation of polyisoprene ( 1 2 2 ) , p o l y s t y r e n e ( I 2 3 ) t poly(methyl methacrylate) ( 1 2 3 - 1 2 6 ) « I t i s quite probable that these reactions can also occur with p a r t i c i p a t i o n of s i n g l e t oxygen.


Polymers r e a d i l y degraded by l i g h t The photosensitizing degradation may considerable contribu te to the solution of the p l a s t i c - l i t t e r problem. In order to improve the standards of our environment the amounts of p l a s t i c l i t t e r should be reduced and t h i s aim may e a s i l y be achived by the use of p l a s t i c s which would r e a d i l y degradable by sunlight. This p r a c t i c a l problem has been f u l l y reviewed by the following publications ( 3 , 1 2 7 ) . Conclusions I t has been demonstrated i n t h i s short review that the understanding of the photosensitization mechanism i n polymers may be applied f o r the improvement of t h e i r p h o t o s t a b i l i t y and f o r the development of polymers with c o n t r o l l e d l i f e t i m e .

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269

270



82. A.C.Somersall and J.E.Guillet,Macromolecules,5, 410(1972). 83. M.Kato and Y.Yoneshige,Makromol.Chem.,167, 159 (1973). 84. M.Kato,in Conference on "Degradability of Polymers and Plastics",Inst.Electrical Engineers,London,Nov.1973,p.5/1. 85.E.Dan.A.C.Somersall and J.E.Guillet,Macromolecules,6, 228 (1973). 86. E.Dan,and J.E.Guillet,Macromolecules,6, 230 (1973). 87. E.Cernia,W.Marconi.N.Palladino and P.Bacchin,J.Appl.Polym. S c i . , 1 8 , 2085 (1974). 88. J.E.Guillet and R.G.W.Norrish,Proc.Roy.Soc.,A235, 172 (1955). 89. K.P.Wissbrun,J.Am.Chem.Soc.,81, 58 (1959). 9 0 . Y.Amerik and J.E.Guillet,Maoromolecules,4, 375 (1971). 9 1 . R.S.Cooke and G.S.Hammond,Chem.Soc.Spec.Publ.London,No.24, 1970. 9 2 . D.J.Harper and J . F . M c K e l l a r , J . A p p l . P o l y m . S c i . , 1 7 , 3503 (1973). 93.E.D.Owen and R . J . B a i l e y , J . P o l y m . S c i . , A 1 , 1 0 , 113 (1965). 9 4 . Y . T r u d e l l e and J . N o e l . C . R . A c a d . S c i . , S e r . C . , 2 6 0 , 1950(1965). 9 5 . J.F.Rabek,Chem.Stosow.,11, 53 ( 1 9 6 7 ) . 9 6 . J . F . R a b e k , J . P o l y m . S c i . , S e r . c , N o . 1 6 , 949 (1967). 9 7 . H.C.Beachell and I.L.Change,J.Polym.Sci.,A1,10, 503 (1972). 98. M.Charlier,C.Helene and W.L.Carrier,Photochem.Photobiol., 15, 527 ( 1 9 7 2 ) . 9 9 . A.Charlesby,C.S.Grace and F . B . P i l k i n g t o n , P r o c . R o y . S o c . , A268, 205 (1962). 100. G.Oster,J.Polym.Sci.,22,185 ( 1 9 5 6 ) . 101. G.Oster,G.K.Oster and H.Moroson,J.Polym.Sci.,34, 671(1959). 102. Tsao-Gun,Chian-Pin-Chen,Hoy En-Chian,Vysokomol.Soedin., 1, 635 ( 1 9 5 9 ) . 103. H.Wilski,Kolloid-z.z-Polym.,188, 4 (1963). 104. A.M.Trozzolo,Adv.Chem.Ser.,No.77, 167 (1968). 105. K.Nakamura,T.Yamada and K.Honda.Chem.Lett.(Japan),1975, 35. 106. J.F.Rabek and B.Rånby i n IUPAC Symposium on "Photochemical Processes i n Polymer Chemistry",Leuven,June 1 9 7 2 . 107. J.F.Rabek and B.Rånby,J.Polym.Sci.,A1,12, 295 (1974). 108. J.F.Rabek, Chem.Stosow.,11, 89 (1967). 109. J.F.Rabek i n XXIIIrd IUPAC Congress, Boston, USA, 1971, ed. Butterworths,London, v o l . 8 , p . 2 9 . 110. M.Kitano,Japana Pat.71 24,796 (1971). 111. M . V . L o c l and F.B.Sugar,Proc.Chem.Soc., 1960, 358. 112. C.Kujirai,S.Hashiya,K.Shibuya and K.Nishio,Kobunski Kagaku,25, 193 (1968). 115. J.C.Bevington,Proc.Roy.Soc.,259, 420 (1957). 114.P.K.Sengupta and J.C.Bevington.Polymer,14, 527 (1973). 115.W.H.Gibb and J.R.McCallum,J.Polym.Sci.,Symp.,No.40, 9 (1973). 116. J.P.Rabek,Chem.Stosow.,11, 183 (1967). 117. G.B.Pariskii,D.Ya.Toptygin,E.Ya.Davydov,O.A.Ledneva,Yu.A. Mikheev,and V.M.Karasev.Vysokomol.Soedin.,B14, 511 (1972).

18. RABEK


5,

Photosensitized Degradation of Polymers271

118. T.Takeshita,K.Tsuji and T.Seiki,J.Polym.Sci.,A1,10, 2315 (1972). 119. K.Tsuji,T.Seiki and T.Takeshita,J.Polym.Sci.,A1,10, 3119 (1973). 120. L.M.Baider,M.V.Voevodkaya and N.V.Fok,Khim.Vys Energ., 422 (1971). 121. J.F.Rabek,Scientific Report,Wroclaw Technical University, 1971. 122. J.F.Rabek,Scientific Report,Uppsala University, 1968. 123. H.Mönig and H.Kriegel,Z.Naturfosrch.,15, 333 (1960). 124. H.Mönig and H.Kriegel,Proc.Intern.Congr.Photobiol., 3rd.Copenhagen, 1960,p.618. 125. H.Mönig and H.Kriegel,Biophysik,2, 22 (1964). 126. R.B.Pox and T.R.Price,J.Appl.Polym.Sci.,11, 2373 (1967). 127. B.Baum and R.D.Deanin,Polym.Plast.Techn.Eng.,2, 1 (1973).

19 Photodegradation of Vinyl Chloride-Vinyl Ketone Copolymer M. HESKINS EcoPlastics Ltd., 201 Consumers Rd., Willowdale, Ontario, Canada M2J 4G8


W. J. REID,* D. J. PINCHIN,** and J. E. GUILLET DepartmentofChemistry,UniversityofToronto,Toronto,Ontario, Canada M5S 1A1

Recently it has been shown that photodegradable polymers suitable for use in packaging applications can be prepared by copolymerization of ethylene or styrene with vinyl ketone monomers (1). Since PVC is the third important plastic used for packaging, we have investigated the possibility of using the same method to develop photodegradable PVC compositions. Experimental Synthesis. The copolymers were prepared by suspension polymerization using a 1 gal stainless steel reactor operating at the autogenous pressure of vinyl chloride at 50° C. The vinyl chloride (Matheson) was purified by passing the gas over KOH pellets. A 9:1 ratio of water to vinyl chloride was used, the suspending agent being methyl cellulose (Methocel 25 cps, Dow Chemical). Percadox 16 (Noury Chemical Corp.) was the catalyst. Because the calculated reactivity ratios indicated that the vinyl ketone would be used up faster than the vinyl chloride, the methyl vinyl ketone was added at frequent intervals throughout the run to maintain an approximately constant feed composition. Yields of 7085% copolymer were obtained for the copolymers in 5-6 hr. The properties of the resulting copolymers are given in Table I. Film Preparation. The PVC homopolymer and the copolymers were reprecipitated from THF solution with methanol. Experimental compositions were made up containing 100 parts PVC homopolymer or PVC copolymer, 3 parts Ferro 75-001 (Ba-Cd stabilizer), 1 part Mark C (organic phosphite chelator), and 4 parts Drapex 3.2 (Epoxy plasticiPresent address: *Uniroyal Limited, Research Laboratories, Guelph, Ontario, Canada. ** Cavendish Laboratories, Cambridge University, Cambridge, England. 272

19. HESKiNS E T A L .

Vinyl Chloride-Vinyl

Ketone Copolymer

273

T A B L E I. Copolymers of Vinyl Chloride and Methyl Vinyl Ketone

Ketone content mol-%


6 1.6 0

[η] dl/g 0.72 0.95 0.83

Tensile Elongation strength, psi at break, %

M ν

4400 4900 4900

54,000 78,000 65,000

130 130 110

zer). This stabilizer system was found to be more effective for the co polymers than other systems tried, including the tin salts. The above ingredients were dissolved in THF (15 ml/gm) and subsequently cast onto glass plates. The glass plates rested on mercury in order to en sure constant film thickness. Films of thickness 0.08, 0.20 and 0.25 mm were prepared in this manner. The presence of Drapex 3.2 im parts a slight plasticization to the film. Degradation. The films were weathered either on wooden racks placed on the roof of the Chemistry Department (University of Toronto), facing south at 45°, or in an American Ultraviolet Accelerometer (Model NS-1200). The temperature in the weatherometer was 37° C. Molecular Weight. The viscosity average molecular weight of the homopolymer and copolymers was determined from single point deter minations of viscosity of a 0.25% solution in Fisher purified cyclohexane at 30.0+0.01° C. The molecular weight is calculated from the intrinsic viscosity, [η], using the relationship (2) [η] = 1.63x10"* M 0.77 l

v

Results Preliminary experiments showed that when thermally stabilized thin films (80 μ thickness) of vinyl chloride-methyl vinyl ketone copoly mers were subject to UV radiation they became brittle without color for mation. When the ketone concentration in the copolymer was approxi mately 6 mol-%, the time to brittleness in the Accelerometer was less than 20 hr, whereas 40 to 50 hr were required if the ketone concentra tion was only 2%. Samples of PVC homopolymer, identically stabilized, were photolyzed along with the copolymers. No color or chemical change was apparent in these samples, even after 100 hr photolysis. Quantitative information was then obtained on the degradation pro cess by irradiating 0.2 mm thick film samples in the Accelerometer for varying periods of time and subsequently measuring their elongation at

UV LIGHT


274

INDUCED

REACTIONS

IN

POLYMERS

break. The results are shown in Fig. 1, while Fig. 2 shows how the molecular weight varied with photolysis time for both the homopolymer and copolymer. The molecular weight results are plotted in the form of [(l/M) - (1/MQ)] which gives the number of breaks per chain, corrected for differing initial molecular weight. As expected, the molecular weight and percent elongation remaining decreased with time for the copolymer, while little change was noted for the homopolymer. Outdoor weathering of the various samples confirmed the high susceptibility to UV degradation of the copolymer relative to the homo polymer. The elongation, tensile strength and molecular weight all de creased rapidly outdoors. The results are shown in Figs. 3-5. Figure 3 shows that the copolymer with 6% ketone degrades rapidly for the first three weeks outdoors. The film rapidly turns a brown color thereafter, possibly because the barium soap has been exhausted and the cadmium chloride is catalyzing the formation of color centers. The same dis coloration was noticed after 30 hr in the Accelerometer. The homopoly mer stabilized in the same way and exposed to the same conditions r e mains colorless for much longer periods of time. The stabilizers appear to be depleted in the copolymers at a higher rate than in the homopolymer. Moreover, the depletion rate increases with ketone con tent as the films with approximately 1. 5% present were still colorless after six weeks outdoor exposure. Infrared spectra of the crude copolymer showed a main carbonyl peak at ~1710 cm"* with a small peak at 1770 cm" . Reprecipitation of the copolymer removed the 1770 cm" peak completely. Upon irradia tion of the purified copolymer, the 1710 cm" peak immediately began to decrease while the 1770 cm" peak together with another peak at 1725 cm" , reappeared. These effects are shown in Fig. 6, which shows the IR spectrum of the irradiated film in the carbonyl region using an un irradiated film as reference. However, when the degraded polymer was reprecipitated at an early stage of photolysis, the 1770 cm" band again disappeared. Continued photolysis leads to a broadening of the 1770 cm" peak and the eventual disappearance of the 1710 cm" peak. The rates of appearance and disappearance of these bands were unaffected by the stabilizer system since a copolymer sample without stabilizer showed similar rates of change. 1

1

1

1

1

1

1

1

1

Discussion Previous studies by Hartley and Guillet (3) and by Amerik and Guillet (4) have shown that polymers containing ketone groups undergo photolysis by the Norrish type I and type II reactions. The relative proportions of type I and Π depend on the polymer concerned and the particular environment of the carbonyl group. These reactions are

HESKINS E T A L .

Vinyl Chloride-Vinyl Ketone Copolymer


Τ

Hours in Figure 1.

Accelerometer

Percent elongation remaining with irradiation in the accel erometer;filmthickness, 0.2 mm

Hours

in

Accelerometer

Figure 2. Normalized chain breaks with irradia tion in accelerometer;filmthickness, 0.2 mm


U V L I G H T INDUCED REACTIONS I N

Outdoor Figure 3.

POLYMERS

exposure, weeks

Normalized chain breaks on irradiation (outdoors)



HESKINS E T A L .

pvc/MVKTî 6% ketone I Time (weeks) Figure 5.

Unirradiated

2

Percent residual tensile strength, outdoor weather ing; film thickness, 0.2 mm

Λ Ι

I5i/ hr irradiation 2

32 hr irradiation

JL

2000

JL 1700 Wavelength,

cm'

1

Figure 6. Photodegradation of PVC copolymer, differential infrared spec tra

278


POLYMERS

represented in the scheme below : CI

CI

I C H - C H - C H


2

I - C H - C H

2

J c=o I

~CH

wwv

2

Partly because it is known that thermal stabilizers for PVC also inhibit UV degradation, it was thought that the photodegradation of PVC followed a mechanism similar to that established for its thermal degra dation. This mechanism is based on the free radical initiated dehydrochlorination reaction of alkyl chloride which, in PVC, results in conju gated unsaturation. This unsaturation is responsible for the rapid discoloration of PVC on heating. In photooxidation studies it is found, however, that HC1 begins to be produced only at a relatively late stage and that ketonic structures are observed at a relatively early stage. Kwei (5) has shown that these ketones are present as β chloroketones in the chain, i.e., — CH —C(=0)— CHg-CHCl— . A simple model for this type of ketone is 4-chloro-2-butanone which undergoes type I scission. Although this type of ketone can also undergo chain scission by type Π, this mode was not considered because no unsaturation was observed during photolysis. In the MVK/PVC copolymers the type I reaction does not cause chain scission so that, for chain scission to occur, the type II or some secondary mechanism is necessary. In order to minimize the effects of the secondary mechanisms which, in these copolymers also cause crosslinking and discoloration, the polymers were irradiated in the presence of stabilizers. Under these conditions the copolymers did indeed undergo chain scission. Only a very small amount of vinyl double bond absorption was observed in IR or the irradiated films, but 2


19.

HESKiNS E T A L .


279

even in the thermal degradation, where conjugated unsaturation is ob served in the UV spectrum, it is difficult to observe unsaturation in the IR. Photolysis of chloro ketones of structure similar to the copolymers has not been reported so we have examined the photolysis of 5 chloro2-hexanone, the simplest compound of similar structure, to see if it can undergo type II reaction. Irradiation of 5 chloro-2-hexanone in hexadecane at room temperature with 313 nm radiation results in the Norrish type II reaction, producing acetone and 2 chloropropene. Under the conditions of the experiment, the type I product observed would be expected to be 2 chlorobutane, but this was not detected. The study of the photolysis of 5 chloro hexanone is being continued and will be r e ported later. Although no type I product could be observed in this model compound it is probable that the type I would be significantly higher in the copolymer since in this case the carbonyl is attached to a secondary carbon instead of primary carbon and in simple ketones this increases the type I yield. Infrared studies of the carbonyl absorption of the copolymers as photolysis proceeds indicates the appearance of two new peaks and the loss of the original peak. The new absorption at 1725 cm" can be ex plained by several processes. One is the change from a secondary ketone to a primary ketone as the type II reaction proceeds. A second explanation is the oxidation of free radical sites generated by type I. This is similar to the mechanism postulated by Kwei for the production of the β chloro ketones. It is also possible that this absorption is due to the acetyl fragments split off by type I photolysis. We have observed that the carbonyl absorption at 1770 cm" can be removed by reprecipitation from THF solution by methanol. This indicates that the group responsible for this absorption is not chemically attached to the mole cule. The acetyl radical produced by the type I can abstract hydrogen, or chlorine, either from the polymer or the stabilizer to form either acetaldehyde or acetyl chloride which may hydrolyze to the acid. Both acetic acid and acetaldehyde have an absorption in the 3100 cm" region of the infrared and are unlikely since no absorption in this region was observed. When the IR spectrum of homopolymer PVC im pregnated with acetyl chloride was measured, two absorptions in the carbonyl region are observed at 1776 and 1722 cm" . These two absorp tions correspond quite well with those of the degraded copolymer. It does however indicate that the radical has abstracted a chlorine from the polymer, a process generally considered unlikely. 1

1

1

1

280


Summary Copolymers of vinyl chloride and methyl vinyl ketone undergo chain scission with concomitant rapid decreases in tensile strength and elongation when exposed to near ultraviolet light and solar radiation. Free radicals formed by the homolytic scission of the acyl group apparently deplete the stabilizers used and lead to rapid discoloration of the polymer, presumably by the usual radical chain reaction involv ing the production of HC1 and conjugated double bonds.


Acknowledgements The authors wish to acknowledge the financial support for this r e search by EcoPlastics Limited and from the National Research Council of Canada under a P.R.A.I, grant number S-22. We also wish to thank Dr. H. G. Troth and members of the staff of Van Leer Research Labora tories, Passfield, England, for helpful discussions.

Literature Cited 1. Guillet, J. E., in "Polymers and Ecological Problems", J. Guillet, ed., Plenum Publishing Corp., New York, 1973. 2. Dailer, K. and Vogler, K., Makromol. Chem.,(1952), 6, 191. 3. Hartley, G. H. and Guillet, J. E., Macromolecules, (1968) 1, 165. 4. Amerik, Y. and Guillet, J. E., Macromolecules, (1971) 4, 375. 5. Kwei, K.-P. S., J. Polym. Sci., Part A-1, (1969) 7, 1975.

20 Photodegradation of Styrene-Vinyl Ketone Copolymers


M. HESKINS EcoPlastics Ltd., 201 Consumers Rd., Willowdale, Ontario, Canada M2J 4G8 T. B. McANENEY* and J. E. GUILLET Department of Chemistry, University of Toronto, Toronto, Ontario, Canada M5S 1A1

The photolysis of copolymers containing ketone groups has both academic and practical interest since the way in which the polymeric environment affects the photochemical pathways leads to an understand ing of the photodegradation of polymers in which the ketone group is present as an adventitious or intended impurity. Copolymers with vinyl ketones also provide a practical means for preparing plastics with controlled lifetimes as a means of combatting litter problems (1-3). In this report we examine the effects of several vinyl ketone monomers on the photodegradation of polystyrene in the solid phase. Previous work (4, 5) has indicated that copolymers containing vinyl ketones undergo photolysis by the Norrish type I and type II primary reactions. Studies by Golemba and Guillet (6) and by Kato and Yone shiga (7) have shown that these processes also occur in copolymers of styrene with methyl vinyl ketone and with phenyl vinyl ketone.

It can be seen that the type Π reaction will break a C—C bond in the chain backbone, while the type I reaction will produce two free radicals without primary main chain scission. In the presence of air these radi cals initiate photooxidation and therefore cause chain scission by sec* Present address: INCO Research, Sheridan Park, Ontario, Canada. 281

282


ondary reactions. The effect of ketone structure on the relative rates of these reactions is the subject of this investigation.


Experimental Preparation of Monomers. Methyl vinyl ketone (MVK) was ob tained from Pfizer Chemical Division, New York, and distilled to r e move the inhibitor. Methyl isopropenyl ketone (MIPK) was prepared by the aldol condensation of methyl ethyl ketone and formaldehyde, accord ing to the method of Landau and Irany (8). The major impurity in this monomer is ethyl vinyl ketone (5%). The monomer was redistilled be fore use. 3 Ethyl 3 buten 2 one (EB) was prepared by the aldol conden sation of methyl propyl ketone and formaldehyde. Ethyl vinyl ketone (EVK) was prepared by a Grignard synthesis of the alcohol, followed by oxidation to the ketone. t-Butyl vinyl ketone (tBVK) was prepared from pinacolone and formaldehyde by the method of Cologne (9). Phenyl vinyl ketone (PVK) was prepared by the dehydrochlorination of β chloro propiophenone (Eastman Kodak). Phenyl isopropenyl ketone (PPK) was prepared by the Mannich reaction using propiophenone, formaldehyde and dimethylamine HC1. These monomers have a tendency to dimerize on standing at room temperature and were kept at 0°F. A l l the monomers were at least 95% pure, except the MIPK which contained 5% EVK. The purity of this monomer was 93%. Copolymers. The copolymers were prepared by emulsion polym erization in a "pop bottle" polymerizer at 80° C using sodium aryl alkyl sulphonate (Ultrawet K) as emulsifier and ammonium persulphate as catalyst. The polymerizations were carried to greater than 95% con version (4-8 hr) and the feed concentrations of the vinyl ketones were taken as their concentrations in the polymer. Copolymers containing 1% and 5% by weight of vinyl ketones were prepared. The copolymers were not homogeneous in ketone concentration since the reactivity ratios indicated that the vinyl ketones would be used up before the end of the polymerizations. Thin films were prepared for irradiation by compres sion molding in a Carver Press at 150° C at 20,000 psi. The films were usually 0.22 mm thick. Blends were prepared by solution blending the copolymers with a Dow general purpose molding grade (667) polystyrene, casting from solution and then compression molding film as for the copolymer. Irradiation. The films were irradiated in air in an American Ultraviolet Co. UV Accelerometer. This uses a medium pressure mercury arc lamp enclosed in Pyrex and its major emission is at 313 nm together with less intense light at 298 and 366 nm. The lamp runs at about 40° C. The amount of near ultraviolet light absorbed by the

20.

HESKiNS E T A L .

Styrene-Vinyl Ketone Copolymers

283

films during the tests is estimated to be in the order of 4 χ 10"' einstein/ sq cm/hr. The degradation of the copolymers or blends was followed by the change in their solution viscosity in toluene at 30° C. The viscosity average molecular weights were calculated from the equation (10), 0

7 2 5

[η]= l . l x l O ^ M The number of breaks/chain is given by [(M )/(M )] - 1, where M ^ is the initial number average molecular weight and M the number average molecular weight after irradiation. The total number of chain breaks is then n

n

n


n

where g is the weight of irradiated sample. The results are reported here as [(1/M ) - (1/M ^)] against time of exposure. Although a stand ard size sample is used, there is an error involved in using viscosity average rather than number average. However, since it was not pos sible to measure the light absorbed, we have not attempted to measure quantum yields. The expression is used to counteract the effect of dif fering initial molecular weights of the copolymers. V

V

Results and Discussion The photodegradation of 0. 22 mm thick films of styrene/1% MVK and styrene/1% MIPK copolymers in the Accelerometer are shown in Fig. 1. The MVK copolymer degrades significantly faster than the MIPK copolymer. The difference in structure in the ketone units of the copolymer is that the MIPK copolymer has the acetyl group attached to a tertiary carbon on the chain, while in MVK this carbon is secondary. Studies with simple ketones of precisely this structure, i . e., acetyl group attached to secondary or tertiary carbon which can also undergo a type H reaction, have not been made. However, studies with related methyl ketones (Π, 12) lead to the expectation that the MIPK copolymer will have a somewhat higher type I yield and lower type II. In the i r radiation with high intensity artificial UV sources it is expected that any photooxidation will be minimized and chain scission will be principally due to the type II reaction. Figure 2 shows the effect of outdoor ex posure where, although the breakdown of the MVK copolymer is initially faster, it does not maintain its rate and the MIPK copolymer eventually achieves a greater degree of chain scission. This can be ascribed to the continuing effect of photooxidation initiated by the type I free radicals in the latter case.


ι


τ

Hrs

UV

ι

Γ

Accelerometer

Figure 1. Photolysis of PS containing 1% MVK and 1% MIPK

τ

3

1

1

Γ

6 9 Exposure, weeks

12

Figure 2. Weathering of copolymer of 5% MVK and 5% MIPK with styrene in Arizona


20.


HESKINS ET AL.

285

tert-Butyl vinyl ketone copolymers would be expected to show a low type Π yield since this is observed for simple ketones of analogous struc ture. The 1% tBVK copolymer does indeed give a low rate of degradation compared to a similar MVK copolymer, as shown in Fig. 3. The other two monomers chosen have an ethyl group replacing a methyl group, either in the acyl group (EVK) or on the a carbon (EB). The photolysis of these comonomers, when copolymerized at the 1% level with styrene, are intermediate between the methyl substituted analogs MVK and MIPK, as shown in Fig. 4. The photodegradation of a 1% PVK copolymer is compared with the 1% MVK copolymer in Fig. 5. Previous studies (6,1) have indicated that the PVK copolymer has a significantly higher quantum yield for type II reaction than MVK and the ketone group has a higher extinction coefficient in the UV region. It would therefore be expected that a faster rate of chain scission would be obtained for the PVK copolymer than the MVK. This is indeed true during the early stages of the reac tion, but the rate of degradation slows down rapidly at longer exposure times. The reactivity ratios for PVK-styrene are not favourable for a random distribution of the phenone groups. In fact, calculations show that, with all monomer charged initially, nearly all the PVK is incor porated in the first 20% of polymer produced. Since chain scission is concentrated in this fraction of the copolymer, viscosity will give a poor indication of the total number of chain scissions in this case. The PIPK copolymer showed a somewhat similar effect. At high degrees of de gradation there is also a color-forming reaction (related to photooxida tion) which makes the polymer opaque to UV radiation. This also con tributes to the retardation of the photolytic process. Blends. The type I reaction produces free radicals which, in the presence of oxygen, initiates photooxidation which also results in a de crease in the polymer molecular weight. An indication of the relative importance of the type I reaction in these systems can be estimated from the amount of chain scission induced in a blend of the copolymers with homopolymer polystyrene. For these experiments, one part of 5% vinyl ketone copolymer was blended with four parts of styrene homopolymer to retain an overall ketone monomer concentration of 1%. Figure 6 shows the rates of degradation for four of the copolymers. This figure shows that the overall rate is greater for MIPK than for MVK copolymer blends. It should be noted that for all these blends, the rate of degradation is greater than can be attributed to the break down of the copolymer in the blend. Under the conditions of the test the value of [(1/M) - (1/MQ)] for the MVK copolymer blend would not rise above 1 χ 10" if no sensitization had taken place. This was de termined by irradiating the 5% MVK copolymer and the homopolymer and then preparing a solution of the degraded polymers in the correct 6



Hrs

UV

Accelerometer

Figure 3. Photolysis of 1% PVK copolymer andl% MVK copolymer

Figure 4. Photolysis of 1% tBVK copolymer andl% MVK copolymer



HESKiNs E T A L .

Figure 6. Photolysis of 4:1 blends of polystyrene with polystyrene 5% vinyl ketone copolymers


288


proportions prior to measuring the solution viscosity. The greater total degradation in the case of the MIPK copolymer is attributed to the i n creased photooxidation initiated by a higher type I in this system than in the MVK system. This affect is even more marked for the tBVK co polymer where the rate is comparable to that of MVK, even though the molar concentration of ketone groups is only one-half as great. The PVK copolymer blend shows a lower rate and less sensitization of degradation in the homopolymer. The PVK copolymer is known to have a low yield of type I reaction (6) and this supports the idea that the sensitization of photooxidation is due to free radical forma tion rather than more elaborate mechanisms such as via formation of singlet oxygen. It also appears that the hydrogen abstraction reaction of excited phenone groups (13) which is the basis of the use of benzophenone as a prodegradant in polystyrene (14), is not as important in this system. The determination of quantum efficiency of the photochemical reactions has not been attempted because it is difficult to separate definitely the effects of the type I and type II reactions. However, on the assumption that the type II dominates in copolymer photolyses and that the type I dominates in the photolyses of blends, the results appear to be in accord with expectations based on the photochemistry of related simple molecules. Acknowledgements The authors wich to acknowledge the financial support for this r e search by EcoPlastics Limited and from the National Research Council of Canada under a P.R.A.I, grant number S-22. We also wish to thank Dr. H. G. Troth and members of the staff of Van Leer Research Labora tories, Passfield, England, for helpful discussions.

Literature Cited 1. Guillet, J. Ε., in "Polymers and Ecological Problems", J. Guillet, ed., Plenum Publishing Corp., New York, 1973. 2. Guillet, J. E. and Troth, H. G., U.S. Patent 3, 860, 538. 3. Guillet, J. Ε., U.S. Patent 3, 753, 953. 4. Guillet, J. E. and Norrish, R. G. W., Proc. Roy.Soc.,(1955) A233, 153. 5. David, C., Demarteau, W. and Geuskens, G., Polymer, (1967) 8, 497. 6. Golemba, F. J. and Guillet, J. E., Macromolecules, (1972) 5, 212. 7. Kato, M. and Yoneshige, Υ., Makromol. Chem., (1973) 164, 159. 8. Landau, E. F. and Irany, E. P., J. Org. Chem., (1947) 17, 422. 9. J. Colonge, Bull. Soc. Chim., (1936) 3, 2116.

20. HESKINS ET AL.

Styrene-Vinyl Ketone Copolymers 289


10. Danusso, F. and Moraglio, G., J. Polym. Sci., (1957) 24, 161 11. Golemba, F. J. and Guillet, J. E., Macromolecules, (1972) 5, 63. 12. Nicol, C. H and Calvert, J. G., J. Amer. Chem.Soc.,(1967) 89 1790. 13. Turro, N. J., "Molecular Photochemistry", W. A. Benjamin, New York, 1965, Ch. 6. 14. Ohnishi, Α., Japanese Patent 71 38, 687.

21 Novel Additive System for Photodegrading Polymers DONALD E. HUDGIN and THOMAS ZAWADZKI


Princeton Polymer Laboratories, Inc., Plainsboro, N.J. 08536

Introduction and History Many organizations i n the world have been con cerned with the problem of disposing of plastics which have no further use. P a r t i c u l a r l y those plastics giv en intense consideration are the so-called disposable products such as wrapping films, non-reuseable con tainers, 6-pack holders and drinking cups and lids. Also of consideration i s the area of agricultural mulch films, which are designed to degrade at the proper time for maximum crop y i e l d . A number of solutions have been forthcoming as evidenced by the large number of publications and patents concerned with methods for environmentally de grading polymers. In a publication of the Plastics Technical Evaluation center at Picatinny Arsenal en titled "Environmentally Degradable P l a s t i c s : A Re view" and authored by Mrs. Joan Titus, much of the past history up to about February 1973 has been cov ered. Also papers presented at the conference on "Degradability of Polymers and P l a s t i c s " i n London i n November, 1973 have summarized many of the fundamental and p r a c t i c a l aspects of polymer degradation. By far the greatest volume of work has involved photodegradation. Although biodegradable has become a popular, overused and much misused word with plas tics, the experts know that very few p l a s t i c products are t r u l y biodegradable and certainly none of the socalled commodity resins. Several years ago, Princeton Polymer Labs, con tracted with a c l i e n t to develop a degradable a g r i cultural mulch f i l m . A first generation additive system was developed and some 12,000 sq. f t . of poly ethylene film containing the additive was t r i e d out i n 14 states i n the U.S.A. Both black and clear films 290


21. HUDGiN AND ZAWADZKi

Additive System for Photodegrading Polymers 291

were used. The a v e r a g e t i m e t o b r e a k u p was 1 1 d a y s f o r t h e c l e a r f i l m a n d 13.9 days f o r t h e b l a c k f i l m . A second g e n e r a t i o n , and f a r more e f f e c t i v e p h o t o d e g r a d i n g a d d i t i v e s y s t e m was l a t e r d e v e l o p e d a n d e x t e n s i v e i n d o o r a n d o u t d o o r t e s t i n g was d o n e . With s u c h a n e f f e c t i v e s y s t e m i t was o b v i o u s t h a t i t s u s e c o u l d g o much f u r t h e r t h a n j u s t a g r i c u l t u r a l m u l c h films. I t i s w e l l known t h a t i n d i v i d u a l l y , p h o t o i n i t i a t o r s a n d c e r t a i n m e t a l l i c o r g a n i c compounds c a u s e hydrocarbon polymers, such as p o l y e t h y l e n e o r p o l y s t y r e n e , t o photodegrade f a s t e r than the o r i g i n a l polymer n o tc o n t a i n i n g such an a d d i t i v e . We h a v e d i s c o v e r e d t h a t by combining both t y p e s o f a d d i t i v e s in a plastic a striking synergistic effect occurs. Up t o a 10 f o l d i n c r e a s e i n t h e r a t e o f d e g r a d a t i o n has been o b s e r v e d . Testing

Method

The s t a n d a r d f i l m u s e d f o r s c r e e n i n g c a n d i d a t e c o m p o u n d s was a 1.5 m i l LDPE f i l m s o l d b y t h e U n i o n C a r b i d e C o r p . u n d e r t h e name " Z e n d e l P o l y e t h y l e n e Film." S e v e r a l t e s t s were c a r r i e d o u t on t h e u n t r e a t e d f i l m i n t h e UV d e g r a d a t i o n a p p a r a t u s a n d showed t h i s f i l m t o f a i l i n 1100 h o u r s . T h i s was t h e c o n t r o l t o w h i c h a l l t h e t r e a t e d f i l m s were compared. In order t o screen c a n d i d a t e s as r a p i d l y as pos s i b l e t h e f o l l o w i n g p r o c e d u r e was e v o l v e d : P l a s t i c f i l m ( u s u a l l y L D P E ) 1 " χ 3" a n d 1.5 m i l s t h i c k was d i p p e d i n t o a s o l u t i o n o f t h e c a n d i d a t e compound o r m i x t u r e o f c o m p o u n d s i n c h l o r o f o r m a t 60°C f o r 10 s e c o n d s . A f t e r dipping i n the chloroform s o l u t i o n , t h e f i l m was d i p p e d i n t o f r e s h c o l d c h l o r o f o r m f o r 1 s e c ond t o w a s h o f f a n y r e s i d u a l m a t e r i a l f r o m t h e s u r f a c e of t h ef i l m . The f i l m was t h e n a l l o w e d t o d r y i n t h e o p e n a i r a t a m b i e n t t e m p e r a t u r e f o r a t l e a s t 15 m i n u t e s , a f t e r w h i c h i t was m o u n t e d i n c l a m p s o n a h o l d e r as d e s c r i b e d under " T e s t Equipment." T e s t Equpment ( F i g u r e 1 ) Samples o f f i l m ( 1 t o 5 m i l s ) a n d s h e e t (5-50 m i l s ) w e r e e x p o s e d b y m o u n t i n g a 1" χ 3" s p e c i m e n i n c l a m p s o n a h o l d e r s o t h a t i t w a s s u s p e n d e d a b o u t 1/4" above t h e s u r f a c e . E a c h s p e c i m e n t h u s mounted was p l a c e d on an 18" d i a m e t e r h o r i z o n t a l round t u r n t a b l e , w h i c h was r o t a t e d a t 16 rpm. Above t h e t a b l e a t a p p r o x i m a t e l y 1]L i n c h e s w e r e M- G e n e r a l E l e c t r i c RS


292

UV


POLYMERS

sunlamps. A l l o f t h i s e q u i p m e n t was h o u s e d i n a m e t a l box a p p r o x i m a t e l y 21" w i d e , 21" deep and 24" h i g h . On o p p o s i t e s i d e s o f t h e b o x a t t h e t u r n t a b l e l e v e l w e r e t w o o p e n i n g s 1 7 " w i d e b y 7" h i g h . A f a n of ad j u s t a b l e s p e e d was u s e d t o b l o w t h r o u g h t h e o p e n i n g s t o c o n t r o l t h e t u r n t a b l e t e m p e r a t u r e (37° + 2 ° C ) . The l i g h t i n t e n s i t y was c h e c k e d p e r i o d i c a l l y t o make s u r e t h a t i t r e m a i n e d f a i r l y c o n s t a n t . When a d r o p i n i n t e n s i t y was o b s e r v e d , new s u n l a m p s w e r e u s e d to r e p l a c e the used lamps. The s p e c i m e n s w e r e p e r i o d i c a l l y t e s t e d b y a p p l y i n g p r e s s u r e on t h e suspended f i l m o r s h e e t . A s p e c i m e n was c o n s i d e r e d t o h a v e f a i l e d when i t b r o k e o n a p p l i c a t i o n of a s l i g h t pressure (zero elongation). The s u n l a m p e x p o s u r e s w e r e a l s o c o r r e l a t e d w i t h t e s t s i n outdoor sunlight. D u r i n g t h e months f r o m A p r i l t h r o u g h S e p t e m b e r a n o u t d o o r r a c k was u s e d t o e x p o s e s a m p l e s o f p o l y m e r f i l m a n d s h e e t . T h e s e sam p l e s were mounted i n h o l d e r s s i m i l a r t o t h o s e used f o r sunlamp exposure as p r e v i o u s l y d e s c r i b e d . The r a c k f a c e d S o u t h a n d was s e t a t a 4 5° a n g l e t o t h e h o r i z o n tal. Although outdoor sunlight i n t e n s i t y i s - v a r i a b l e d e p e n d i n g on h a z e , c l o u d s and r a i n , o v e r a p e r i o d o f t i m e r e a s o n a b l y g o o d c o r r e l a t i o n was o b t a i n e d . I n d i v i d u a l Compounds T e s t e d S e v e r a l h u n d r e d compounds were s c r e e n e d by t h e t e s t method d e s c r i b e d above and f r o m t h i s t h e most p r o m i s i n g c a n d i d a t e s were d e r i v e d . These f e l l i n t o two c l a s s e s : m e t a l l i c o r g a n i c compounds and p h o t o chemical activators. Examples o f m e t a l l i c o r g a n i c compounds a r e l i s t e d in Table 1 along with t h e i r times to f a i l u r e . Like w i s e examples o f p h o t o a c t i v a t o r s a r e l i s t e d i n T a b l e 2. The t i m e s t o f a i l u r e o b v i o u s l y v a r y w i d e l y a n d s i n c e o n l y s e l e c t e d samples were a n a l y z e d f o r t h e a c t u a l amounts o f d e g r a d i n g a g e n t s p r e s e n t , t h e t r u e amounts i n c o r p o r a t e d i n t h e polymer f i l m a r e n o t known i n m o s t c a s e s . O b v i o u s l y t h e amount i n c o r p o r a t e d w i l l v a r y w i t h the p a r t i c u l a r compound. O n l y by m e l t b l e n d i n g w i l l t h e r e be p r e c i s e c o n t r o l o f t h e a m o u n t i n c o r p o r a t e d i n t o the polymer. N e v e r t h e l e s s , f o r an initial screening this method p r o v e d q u i t e a d e q u a t e , and i t was d e t e r m i n e d t h a t o n c e c o n d i t i o n s w e r e w o r k e d o u t , the r e s u l t s p r o v e d t o be q u i t e r e p r o d u c i b l e .


21. HUDGiN AND ZAWADZKI


TURN TAB'wF. Figure 1. Apparatus for polymer degradation

Table

l_

T i m e to Failure of Polyethylene F i l m Containing a metallic-organic compound Metallic-Organic Compound

T i m e to F a i l u r e (hrs. )

Zirconium Neodecanoate

234

Cobalt Pentanedione

311

F e r r i c Pentanedione

141

Ferrous Dicyclopentadienyl

311

F e r r o u s Pentanedione

281

Titanium Pentanedione

94

Cobalt Dicyclopentadienyl

102

Vanadium Neodecanoate

448

Ferrous S t é a r a t e

115

F e r r i c Naphthenate Control

91 1100


Table

2


Time to Failure of Polyethylene Film Containing a Photo activator Type Compound

Photoactivator

Time to Failure (Hrs. )

4-Chlorobenzophenone

214

Βenzophenone

236

4-Methyl benzophenone

165

4, 4 -Dichlorobenzophenone

165

Phenyl benzoate

214

4 -M ethoxyb enz ophenone

207

Acetophenone

214

O-Dibenzoyl benzene

240

5-Chlorobenzotriazole

214

Benzil

109

l

4~Phenyl-2-pyridyl ketone

84

21. HUDGiN AND ZAWADZKi

Combinations


Tested

A l t h o u g h s e v e r a l o f t h e o r g a n i c - m e t a l l i c com pounds a n d t h e p h o t o a c t i v a t o r s showed good p r o m i s e i n d i v i d u a l l y , i t was f e l t t h a t a more a c t i v e s y s t e m was d e s i r e d . S e l e c t e d m i x t u r e s w e r e t r i e d a n d amaz i n g l y and q u i t e unexpectedly, a pronounced s y n e r g i s t i c e f f e c t was d i s c o v e r e d . One v e r y g o o d e x a m p l e i s a s follows :


Compound(s) Iron(+2)Pentanedione (A) 4 - C h l o r o b e n z o p h e n o n e (Β) (A) + (B)

Time t o F a i l u r e

(hours)

281 214 21

T h u s t h e a b i l i t y t o d e g r a d e t h e LDPE h a s b e e n i n c r e a s e d t e n f o l d o v e r t h e s i n g l e compounds. Table 3 l i s t s d i f f e r e n t combinations showing t h i s remarkable s y n e r g i s t i c e f f e c t . Some c o m b i n a t i o n s gave b e t t e r r e s u l t s t h a n o t h e r s , b u t s i n c e t h e a m o u n t s a c t u a l l y i n c o r p o r a t e d i n t o the p o l y m e r w a s n o t known, i n most o f t h e s e examples t h e b e s t s y n e r g i s t i c e f f e c t was n o t a l w a y s o b t a i n e d . Optimum R a t i o s a n d Amounts A l l t h e p r e v i o u s l y r e p o r t e d t e s t s were c a r r i e d o u t u s i n g t h e same s t r e n g t h d i p s o l u t i o n s . T h e s t r e n g t h o f s o l u t i o n s w a s now v a r i e d t o d e t e r m i n e i f t h e r a t i o o f t h e t w o i n g r e d i e n t s w a s i m p o r t a n t a n d how important. I n a f i r s t attempt, zirconium neodecanoate was s e l e c t e d a s t h e m e t a l l i c o r g a n i c c o m p o u n d a n d 4chlorobenzophenone as t h ep h o t o a c t i v a t o r . I t was d i s c o v e r e d t h e r a t i o was q u i t e i m p o r t a n t a s i s shown i n F i g u r e 2. A m i n i m u m o r s h o r t e s t t i m e t o f a i l u r e i s c l e a r l y shown. F i g u r e 2 shows a n e x a m p l e i n w h i c h the c o n c e n t r a t i o n o f the a d d i t i v e , z i r c o n i u m neo decanoate, i s h e l d constant w h i l e t h e second a d d i t i v e , 4-chlorobenzophenone, i s v a r i e d . F i g u r e 3 shows t h e r e v e r s e o f F i g u r e 2, n a m e l y t h a t the 4-chlorobenzophenone i s held constant i n s o l u t i o n c o n c e n t r a t i o n , w h i l e t h e z i r c o n i u m neo d e c a n o a t e i s v a r i e d . A g a i n a s w a s shown o n F i g u r e 2, a m i n i m u m c u r v e i s s h o w n , w h i c h shows t h a t a n o p t i mum r a t i o i s n e c e s s a r y t o g e t t h e f a s t e s t d e g r a d a t i o n . I n F i g u r e M- t h e c o n c e n t r a t i o n o f z i r c o n i u m n e o d e c a n o a t e was p l o t t e d a g a i n s t t h e c o n c e n t r a t i o n of t h e 4-chlorobenzophenone and c o n t o u r l i n e s drawn for s e l e c t e d f a i l u r e time u s i n g i n t e r p o l a t i o n and

4- chlorobenzophenone 4- chlorobenzophenone 4* chlorobenzophenone 4-M ethylbenz ophenone 4-Bromobenzophenone 4, 4-Dichlorobenzophenone 2-Phenylacetophenone


Ferric Naphthenate

Ferrous Neodecanoate

Ferrous Pentanedione




,

4-chlorobenzophenone

Cobalt Pentanedione

94

281

281

281

141

91

94

236

165

236

165

214

214

214

44

47

47

76

96

49

21

85

98 236 115

Benzophenone

Ferrous Stéarate

214

36 84 281

Phenyl 2-pyridyl ketone


311

49 214

281

Phenyl benzoate


42 236

281

Benzophenone


42

311

2 -Phenylacetophenone

Cobalt Pentanedione

236

21

236

94

2-Phenylacetophenone


85

214

311


Cobalt Pentanedione

142

Time to Failure(hrs) Β A+ Β 214



Α 234

Photoactivator (B)

Metallic Organic Compound (A)

Time to Failure of a Polyethylene Film Containing Both a Metallic-Organic Compound and a Photoactivator Type Compound

Table 3


HUDGiN AND ZAWADZKi

21.


140L

120L 100L 80 L 60 L


2 ο

40L

20 L

4

6

8

10

12

14

PER CENT 4-CHLOROBENZOPHENONE

Figure 2. Chloroform dip solution contained plus 4-chlorobenzophenone as indicated

100L

80 if) a

χ

—

60

ui oc

3

-J

2 ο H UJ

40L

20

L

0.2

0.3 PER

CENT

ZIRCONIUM

0.4 NEODECANOATE

Figure 3. Chloroform dip solution contained 3.2% 4-chlorobenzophenone plus zirconium neodecanoate as indicated


298

UV


IN

POLYMERS

extrapolation. T h i s t y p e o f p l o t shows c l e a r l y t h e m o s t e f f e c t i v e a r e a . A l s o t h i s p l o t c a n n o t o n l y be useful i n selecting the best r a t i o of ingredients f o r a d e s i r e d f a i l u r e t i m e , b u t c a n be e m p l o y e d t o d e t e r mine t h e most e c o n o m i c a l c o m b i n a t i o n o f d e g r a d i n g additives. F u r t h e r e v i d e n c e i s shown i n F i g u r e 5 i n w h i c h two l e v e l s o f b e n z o p h e n o n e a r e u s e d a s t h e c o n c e n t r a t i o n o f f e r r o u s stéarate i s v a r i e d . The t w o curves cross over rather abruptly. By t h e same t o k e n i n F i g u r e 6 t h e f e r r o u s stéarate i s u s e d a t t w o l e v e l s w h i l e t h e benzophenone i s v a r i e d . A s i m i l a r c o n t o u r p l o t i s shown i n F i g u r e 7 a s was shown i n F i g u r e 4 , e x c e p t b e n z o p h e n o n e a n d f e r r o u s stéarate a r e t h e t w o i n g r e d i e n t s u s e d . An i d e a l i z e d p l o t i s shown i n F i g u r e 8. T h i s p l o t i l l u s t r a t e s t h e i n t e r a c t i o n s between t h e two t y p e s o f d e g r a d i n g a d d i t i v e s a n d shows how t h e s e i n t e r a c t i o n s c a n a f f e c t the r a t e o f d e g r a d a t i o n as w e l l as t h e economics. A t p o i n t Ζ t h e most e f f e c t i v e c o m b i n a t i o n w o u l d be expected. I f the r a t i o i s held constant but the t o t a l amount o f a d d i t i v e i s d e c r e a s e d ( g o i n g f r o m Ζ t o w a r d 0 ) t h e t i m e t o f a i l u r e i n c r e a s e s a s w o u l d be e x p e c t e d ; h o w e v e r , i f t h e t o t a l amount i s i n c r e a s e d ( g o i n g f r o m Ζ t o w a r d P) t h e t i m e t o f a i l u r e a l s o i n c r e a s e s . I n a s i m i l a r manner i f Component Β i s h e l d c o n s t a n t a n d C o m p o n e n t A i s i n c r e a s e d ( g o i n g f r o m R t o S\. t h e t i m e t o f a i l u r e d e c r e a s e s t o a minimum, t h e n rises. T h e same e f f e c t i s o b s e r v e d b y h o l d i n g A c o n s t a n t and i n c r e a s i n g Β ( g o i n g f r o m X t o Y ) . Once a p l o t o f t h i s t y p e i s a c c u r a t e l y e s t a b l i s h e d , i t c a n be u s e f u l i n d e t e r m i n i n g t h e amounts of components A and Β t o b r i n g about f a i l u r e i n t h e desired time at the lowest a d d i t i v e cost. For ex a m p l e , i f c o m p o n e n t Β i s 10 t i m e s a s e x p e n s i v e a s component A and a 100 h r . f a i l u r e t i m e i s d e s i r e d , t h e l e a s t a m o u n t o f c o m p o n e n t Β t o g i v e a 100 h r s . f a i l u r e t i m e w o u l d b e a t p o i n t C. On t h e o t h e r h a n d i f com p o n e n t A i s 10 t i m e s a s e x p e n s i v e a s c o m p o n e n t B, t h e m o s t e c o n o m i c a l c o m b i n a t i o n w i l l b e a t p o i n t D. Analysis of Ingredients i n Films A l l o f t h e w o r k r e p o r t e d up t o t h i s p o i n t h a s l e f t a b i g unknown and t h a t i s t h e a c t u a l amounts o f compounds a b s o r b e d i n t o t h e p o l y m e r w e r e n o t known. F i l m s were p r e p a r e d u s i n g i n d i v i d u a l s o l u t i o n s o f t h r e e m e t a l l i c o r g a n i c compounds a n d one p h o t o a c t i v a t o r a n d t h e a m o u n t o f compound d e t e r m i n e d f o r e a c h film. The c o n d i t i o n s f o r f i l m p r e p a r a t i o n and t h e



H U D G i N AND ZAWADZKI

sol Ο

I

I

I

0.1

0.2

0.3 %

1 0.4 FERROUS*

1

1

0.5

0.6

Ι 0.7

STEARATE

Figure 5. Chloroform dip solutions contains amounts of compounds as indicated



300

t 0.2

f OA

I

I

0.6

0.8

I

t

1.0

1.2

t M

%

1

J

I

I

I

I

I

I

1.6

1.8

2.0

2.2

2A

2.6

2.8

3.0

1 3.2

I

I

!

3.4

3.6

3

BENZOPHENONE

Figure 6. Chloroform dip solutions contains amounts of compounds as indicated

Figure 7. Contour plot showing time to failure

21.

H U D G i N AND

ZAWADZKI


a n a l y t i c a l r e s u l t s a r e shown i n T a b l e 4. T h e s e r e s u l t s showed l e v e l s t h a t o f f e r e d a g u i d e f o r t h e n e x t s t e p , w h i c h i n v o l v e d b l e n d i n g w i t h p r e c i s e amounts o f ingredients.


Blending

Studies

Two i n g r e d i e n t s , z i r c o n i u m n e o d e c o n o a t e a n d benzophenone, were b l e n d e d i n t o a l o w d e n s i t y f i l m g r a d e p o l y e t h y l e n e a s i n d i c a t e d o n T a b l e 5. T h e t o t a l amount o f i n g r e d i e n t s was k e p t a t 1 0 0 mg. p e r k i l o g r a m o f p o l y e t h y l e n e o n a r o l l m i l l a t 325°F. T h e p o l y e t h y l e n e was s h e e t e d o f f t h e r o l l m i l l a n d a p o r t i o n pressed between p o l i s h e d photographic p l a t e s i n a p r e s s t o g i v e a 1 1/2 m i l f i l m . The f i l m s were e x posed a s d e s c r i b e d under T e s t i n g Method above and t h e time t o f a i l u r e recorded. The r e s u l t s a r e g r a p h i c a l l y shown o n F i g u r e 9. T h u s , a s w i t h t h e s o l u t i o n d i p s t u d i e s , i t h a s b e e n e m p h a t i c a l l y shown t h a t t h e r a t i o o f t h e two types o f i n g r e d i e n t s i s important f o r o b t a i n i n g t h e maximum r a t e o f d e g r a d a t i o n . Comparison o f Indoor and Outdoor

Testing

The m e t h o d s a n d p r o c e d u r e a r e d e s c r i b e d u n d e r T e s t i n g Method and Test Equipment. Ten f i l m s p e c i mens w e r e a r b i t r a r i l y c h o s e n f o r c o m p a r i s o n o f i n d o o r and o u t d o o r t e s t i n g . R e s u l t s a r e shown i n T a b l e 6. On a n a v e r a g e t h e t i m e t o f a i l u r e f o r i n d o o r e x p o s u r e was a b o u t o n e - t h i r d f o r o u t d o o r e x p o s u r e . T h i s seemed t o v a r y somewhat w i t h t h e p a r t i c u l a r s y s t e m u s e d . Types and Forms o f P l a s t i c s

Tested

A number o f p l a s t i c t y p e s a s w e l l a s f o r m s w e r e t e s t e d by t h e d i p technique d e s c r i b e d e a r l i e r and e x p o s e d i n t h e i n d o o r UV t e s t i n g e q u i p m e n t . The t i m e t o f a i l u r e f o r t h e t r e a t e d a n d u n t r e a t e d s a m p l e s i s shown i n T a b l e 7. Continuing

Research

A l t h o u g h t h e work r e p o r t e d i n t h i s paper has been concerned w i t h combinations o f two degrading a d d i t i v e s , s y s t e m s i n v o l v i n g t h r e e o r more a d d i t i v e s a r e b e i n g investigated. Even f a s t e r r a t e s t h a n were o b t a i n e d w i t h d u a l s y s t e m c a n be a c h i e v e d . With three addi t i v e s and u s i n g a t y p i c a l t e r n a r y composition graph, c o n t o u r l i n e s c a n b e p l o t t e d a n d t h e a r e a o f maximum e f f e c t i v e n e s s determined q u i c k l y . P r i n c e t o n Polymer

U V L I G H T INDUCED REACTIONS I N

302

POLYMERS


α

X

0

C O N C E N T R A T I O N OF M E T A L L I C ORGANIC C O M P O U N D ( C O M P O N E N T A )

Figure 8.

Idealized graph showing failure times at various concentrations

Table

4

Analysis of Films

Compound in Film

Analytical Results

Cobalt Pentanedione

124 ppm Co


139 ppm Fe


1000 ppm Z r

4-Chlorobenzophenone

140 ppm CI

21.

HUDGIN A N D Z A W A D Z K I


Table

5

Synergistic Effect


Mg. Benzophenone per Kg. Polyethylene

Mg. Zirconium Neodecanoate per Kg. Polyethylene

Time to Failure

0

100

242 hours

20

80

119 hours

40

60

46 hours

60

40

26 hours

80

20

106 hours

0

224 hours

100

300,-

200 3 -J 2

ο 3

g 100

_1_

-L

20 40 60 80 % COMPONENT A IN TOTAL ADDITIVE MX ITURE Figure 9.

100


POLYMERS

Table 6


Comparison of Indoor and Outdoor Tests»

Sample

Days to Failure Indoor Outdoor

1

2

9

2

3

14

3

2

7

4

3

14

5

2

7

6

4

10

7

10

20

8

9

20

9

4

9

10 Total Average

3_ 42

9_ 119

4

12

•Based on 24 hrs./day exposure to sunlamos findoorsï «nH 24 hrs./day sunlight and darkness (outdoors).

21.

HUDGiN AND ZAWADZKi


Tabic

7

T y p e e and F o r m e


Polymer

Type

of P l a s t i c s

Tested

T i m e to F a i l u r e (Days) Untreated Treated

Form

Polyethylene (Low Density)

Clear Film Black Film 20 M i l Sheet 40 M i l Sheet

45 45 790 >100

1 to 4 2 to 5 21 10

Polyethylene (High Density)

Bottle 20 M i l Sheet

?100

7 20

Polyethylene (Medium Density)

Bottle

>22

Polypropylene

Film

Polystyrene

F i l m (1 1/2 M i l ) S h e e t (5 M i l ) Jar

>32 13 > 100

Polyvinyl

Sheet

>

Chloride

;ioo

100

42

MethacrylonitrileStyrene Copolymer

Bottle

Polyethylene Terephthalate

M y l a r F i l m (1 Mylar Film.(2

Polyamide (Nylon)

S h e e t (10

Mil)

Polymethyl Methacrylate

Sheet (30

Mils)

Cellulose

Film

(3

Mils)

Polyacetal

Film

(2

Mils)

13

AcrylonitrileEthyl Acrylate Copolymer

S h e e t (20

Mil)

>100

Acetate

100

>100

3

22

2 >100

306

UV


POLYMERS

L a b o r a t o r i e s i s n o t p r e p a r e d t o p r e s e n t d a t a on t h i s work a t t h i s t i m e , however, a paper c o v e r i n g t h i s ex t e n d e d w o r k m i g h t be g i v e n some t i m e i n t h e f u t u r e .


Summary P r i n c e t o n P o l y m e r L a b o r a t o r i e s has d i s c o v e r e d t h a t m i x t u r e s o f a m e t a l l i c o r g a n i c compound a n d a p h o t o a c t i v a t o r produce a degradative e f f e c t i n c e r t a i n p o l y m e r s t h a t may be a s much a s t e n f o l d g r e a t e r t h a n t h a t p r o d u c e d by t h e i n d i v i d u a l compounds. The r a t i o o f t h e c o m p o n e n t s , t y p e o f p l a s t i c and t o t a l amount o f a d d i t i v e a r e some o f t h e i m p o r t a n t f a c t o r s t h a t a f f e c t the time to f a i l u r e . Because o f t h i s s t r o n g syner g i s t i c e f f e c t , t h e amount o f a d d i t i v e r e q u i r e d i s q u i t e s m a l l , thus r e s u l t i n g i n a v e r y low c o s t , which h a s b e e n e s t i m a t e d a t l e s s t h a n 0.1 c e n t p e r p o u n d o f finished plastic.

22 Sensitizers for Photodegradation Reactions J. D . C O O N E Y and D . M . W I L E S


Division of Chemistry, National Research Council of Canada, Ottawa, Canada K 1 A 0R9

Intensive research in the past eight years on sensitizers for the photodegradation of thermoplastics has led to the new concept of photodegradable plastics or Degradable P l a s t i c s . The term Degradable Plastics is commonly used to describe plastics which undergo more rapid than normal deterioration on exposure to unfiltered (although not necessarily direct) sunlight while maintaining long-term indoor stability. Owing to the action of outdoor weathering, Degradable Plastics disintegrate into tiny fragments and it is frequently claimed that these fragments are further degraded by micro-organisms (i.e., fungi, bacteria). Realistic uses for Degradable Plastics include those applications where materials are used outdoors for a limited time only and it is not economically desirable to collect the residual materials after use. Examples are agricultural mulch, films and cordage, twine, etc. Another application is that of packaging which is stored and used indoors and discarded outdoors, in other words litter for which manual collection is not practicable. Degradable Plastics are not inherently biodegradable but, for the most part, are s l i g h t l y modified conventional resins which are intentionally made more sensitive to the erythemal (sunburn) region of the sun's spectrum. One common approach to the required modification involves the addition of a photosensitive additive, hopefully with just the right UV absorption spectrum and subsequent excited state reaction mechanisms to produce early embrittlement. The other approach involves the use of a copolymer where the conventional resin monomer is copolymer!zed with a small amount of another monomer containing 307

308

UV

L I G H T INDUCED REACTIONS

IN

POLYMERS


a f u n c t i o n a l group which absorbs UV r a d i a t i o n . I d e a l l y the l i g h t - s e n s i t i v e groups absorb s t r o n g l y i n t h a t p a r t o f the sun's spectrum which passes through the atmosphere but i s not t r a n s m i t t e d by window g l a s s , i . e . , absorb i n the r e g i o n 2 9 0 t o 3 2 0 nm. The use o f a m a s t e r b a t c h i s p o s s i b l e w i t h b o t h approaches, and i t i s c l a i m e d t h a t t h e r e are no d e l e t e r i o u s e f f e c t s on p r o c e s s i n g c h a r a c t e r i s t i c s or on f i n a l p h y s i c a l properties. T h i s paper d e s c r i b e s the e f f e c t s o f UV l i g h t , the most s i g n i f i c a n t a s p e c t of outdoor w e a t h e r i n g , on a v a r i e t y o f Degradable P l a s t i c s and a l s o d i s c u s s e s the e x t e n t of b i o d e g r a d a b i l i t y o f the photodegraded samples. Examples o f Degradable

Plastics

(a) E c o l y t e Res ins : E c o P l a s t i c s L t d . , T o r o n t o , Canada. E c o l y t e r e s i n s Π Π a r e c o p o l y m e r s c o n t a i n i n g a few per cent of v i n y l ketone comonomer and are based on the i n v e n t i o n s o f P r o f e s s o r James E. G u i l l e t o f the U n i v e r s i t y of T o r o n t o . Quantum y i e l d c o n s i d e r a t i o n s account f o r the e f f i c a c y o f h a v i n g the u n i t s d i s t r i b u t e d randomly (and s p a r s e l y ) i n the backbone of ~CRi~ polymer c h a i n s . P l a s t i c s o f both the a d d i t i o n ^ and c o n d e n s a t i o n polymer types can be made (" s u i t a b l y p h o t o s e n s i t i v e i n t h i s way a l t h o u g h R2 polystyrene (Ecolyte PS), polyethylene (Ecolyte PE) and p o l y p r o p y l e n e ( E c o l y t e PP) are the f i r s t t o be developed. C o n t r o l l e d outdoor l i f e t i m e s over a v e r y wide range of time p e r i o d s (days t o months) are p o s s i b l e w i t h apparent l o n g term s t a b i l i t y i n d o o r s . The p e r s i s t e n c e of outdoor m e c h a n i c a l i n t e g r i t y depends on the c o n c e n t r a t i o n o f the ketone comonomer and i t s s p e c i f i c s t r u c t u r e , e.g., the n a t u r e of Ri and R2. The t i m e - t o - e m b r i t t l e m e n t o f E c o l y t e PS and PE i n our Weather-Ometer exposures are l i s t e d i n T a b l e s I and I I . = 0

(b) Sty-Grade: B i o - D e g r a d a b l e P l a s t i c s , I n c . , B o i s e Idaho ( 2 ) . The approach i n t h i s case i s the i n c o r p o r a t i o n of a p h o t o s e n s i t i v e a d d i t i v e with c o n v e n t i o n a l r e s i n , e.g., benzophenone b l e n d e d w i t h polystyrene. The r a t e o f d e g r a d a t i o n outdoors can be v a r i e d by changing the amount o f a d d i t i v e used, a l t h o u g h t h e r e i s e x p e c t e d to be an upper l i m i t beyond which s e n s i t i v i t y t o a r t i f i c i a l l i g h t becomes a problem. Benzophenone has been g i v e n FDA a p p r o v a l f o r c e r t a i n p a c k a g i n g end-uses . Time-to-embrittlement s

22.

cooNEY

Sensitizers for Photodegradation Reactions

A N D WILES

TABLE I E m b r i t t l e m e n t Times

o f Degradable

Polystyrenes

Time t o E m b r i t t l e m e n t Xe Arc (hr )


Samples Ecolyte Ecolyte Ecolyte Ecolyte

"Fast" "Fast" "Slow" "Slow"

foam, 2β00μ foam, " foam, 2000μ foam, "

Ecolyte Ecolyte

"Fast" "Slow"

film film

275 T0 2^00 600° b b

23 55

Outdoors (months ) 1.0

-

>6.5

-

-

6

Sty-Grade r e s i n cone, f i l m Sty-Grade r e s i n cone. let-down 5:1 f i l m Der W i e n e r s c h i t z e l c o f f e e cup l i d , a Sty-Grade p r o d u c t , r e s i n cone, let-down 50:1

22

-

112

C o n t r o l , S t y r o n 666

U95

-

a

film

Foams are e x p e c t e d t o be slower than f i l m s owing t o t h i c k n e s s and o p a c i t y . The UV-degradable f o r m u l a t i o n s l i s t e d here are not n e c e s s a r i l y those which are t o be used i n commercial p r o d u c t s . In a d d i t i o n , comparison o f e m b r i t t l e m e n t times i s not i n t e n d e d t o r e l a t e t o the r e l a t i v e m e r i t s o f the v a r i o u s mater ials .

Îrradiated °Turned

35 hr on each

over every 2k h r .

side.

309

#3

#2

LDPE, E c o P l a s t i c s L t d . ,

LDPE, E c o P l a s t i c s L t d . ,

#1 L D P E , E c o P l a s t i c s L t d . ,

C o n t r o l L D P E , 12μ

C o n t r o l L D P E , E c o P l a s t i c s L t d . , 6k\i

6ΐμ

Ecolyte

63μ

Ecolyte

53μ

Ecolyte

B i o - D e g r a d a b l e P l a s t i c s I n c . , LDPE, k2]A E N D E - p l a s t LDPE w i t h FAM-1, A k e r l u n d a n d R a u s i n g , 58μ

1820

2075

1^65

1180

>18

>h.5

2.5

>k.5

1550

820

U.o

6.5 .11

Outdoors (months)

750

2210

Ecoten

Xe A r c (hr) 1175

L D P E , 51μ

Polyolefin

Time t o E m b r i t t l e m e n t

Times o f D e g r a d a b l e

C o n t r o l LDPE, Dupont, l l O f l

Samples

Embrittlement

TABLE I I


kQO

315

230

85

375

215

k90

koo

Xe A r c (hr)

T i m e t o 1/10

Films o f %E

22.

COONEY AND WILES



data f o r samples o f t h i s Tables I and I I .

311

t y p e o f p r o d u c t a r e shown i n

(c) E c o t e n . The a p p r o a c h u s e d b y P r o f e s s o r G e r a l d S c o t t ( U n i v e r s i t y o f A s t o n , B i r m i n g h a m , U.K.) i n v o l v e s t h e u s e o f m u l t i p l e a d d i t i v e s such as f e r r i c d i b u t y l d i t h i o c a r b a m a t e [Fe ( I I I ) DBC] o r f e r r i c stéarate, p o s s i b l y w i t h a c o b a l t , c o p p e r , chromium, manganese o r cerium s a l t . This type o f system which p r o v i d e s s t a b i l i t y d u r i n g t h e p r o c e s s i n g a n d i n i t i a l UV exposure p e r i o d s , has been used i n p o l y o l e f i n s and p o l y s t y r e n e , f o r example. A f t e r an i n d u c t i o n p e r i o d , F e ( l l l ) DBC o x i d i z e s a n d d e c o m p o s e s g i v i n g p r o d u c t s that c a t a l y z e the photodegradation o f the polymer. I t has b e e n n o t e d (3.) t h a t , a l t h o u g h i r o n d i a l k y l d i t h i o c a r b a m a t e s r e p r e s e n t d e l a y e d a c t i o n UV a c t i v a t o r a d d i t i v e s , s u l f u r - c o n t a i n i n g ligands are not e s s e n t i a l . Advantages c l a i m e d i n a d d i t i o n t o t h e i n d u c t i o n p e r i o d are t h e s m a l l q u a n t i t y o f a d d i t i v e r e q u i r e d [0.01 t o 0.05$ F e ( l l l ) D B C ] a n d t h e c o n t i n u i n g o x i d a t i v e degradation i n t h e dark f o l l o w i n g i n i t i a l photoo x i d a t i o n ( d u r i n g which t h e a d d i t i v e i s changed from s t a b i l i z e r t o p r o - d e g r a d a n t ) . I t appears t o be n e c e s s a r y , however, t o s e l e c t an a d d i t i v e l e v e l which i s i n a f a i r l y n a r r o w r a n g e f o r minimum e m b r i t t l e m e n t time. The p e r f o r m a n c e o f a n E c o t e n l o w - d e n s i t y p o l y e t h y l e n e sample i n our Weather-Ometer exposure i s shown i n T a b l e I I . The p r o d u c t s d e s c r i b e d a b o v e w e r e s e l e c t e d i n o r d e r t o i l l u s t r a t e some o f t h e p r i n c i p l e s a s s o c i a t e d w i t h t h e enhancement o f s u n l i g h t s e n s i t i v i t y . There are, i n a d d i t i o n , o t h e r e x a m p l e s (2) o f t h e c o p o l y m e r a p p r o a c h , e.g., e t h y l e n e / c a r b o n monoxide, and t h e a d d i t i v e approach. (d) E x p e r i m e n t a l M o d i f i c a t i o n o f C o n v e n t i o n a l Plastic Films. We h a v e e x a m i n e d n u m e r o u s m o d i f i c a t i o n procedures p o t e n t i a l l y capable o f reducing the embrittlement time o f commercial p l a s t i c f i l m s (mainly the polyolefins). P a r t i a l thermal o x i d a t i o n , corona d i s c h a r g e i n d i f f e r e n t e n v i r o n m e n t s , ozone t r e a t m e n t a n d γ-irradiation w e r e some o f t h e p r o c e d u r e s t h a t p r o v e d t o be i m p r a c t i c a l . T h e m a i n e f f o r t t h e n was t o f i n d a n a d d i t i v e t h a t w o u l d make l o w - d e n s i t y p o l y ethylene a photodegradable p l a s t i c . A fewo f the a d d i t i v e s employed were t i t a n i u m d i o x i d e , benzophenone, ^-nitrobenzophenone, U-hydroxybenzophenone, **-( 3 - h y d r o x y e t h o x y ) b e n z o p h e n o n e , a n t h r a q u i n o n e , anthrone, benzanthrone, xanthone, deoxybenzoin,

312


a-bromo-£-phenylacetophenone, ( 2 - b e n z o y l v i n y l ) f e r r o cene, f e r r o c e n e c a r b o x y a l d e h y d e , and 1 , 1 - d i b e n z o y l f e r rocene. The d a t a are summarized i n T a b l e s I I I , IV and V. 1

Experimental Outdoor exposures were conducted i n the summers of 1972-7 * at a U5 angle f a c i n g s o u t h , l a t i t u d e h$ Ν i n Ottawa, Canada. A 6000W xenon a r c A t l a s Weather-Ometer (k) equipped w i t h Corning No. 77^0 b o r o s i l i c a t e i n n e r and o u t e r f i l t e r s was used as a l a b o r a t o r y l i g h t source because i t s e m i s s i o n spectrum i s s i m i l a r t o t h a t o f s u n l i g h t , as i l l u s t r a t e d i n F i g . 1. The WeatherOmeter was o p e r a t e d w i t h the lamp on c o n t i n u o u s l y and the exposure chamber was m a i n t a i n e d at a b l a c k p a n e l temperature o f lU5±5°F and a r e l a t i v e h u m i d i t y o f


1

30±5%.

Polymer e m b r i t t l e m e n t was determined by l80° f o l d and/or p e n c i l puncture t e s t s . E l o n g a t i o n i n the t r a n s v e r s e d i r e c t i o n o f the LDPE samples was measured on an I n s t r o n TT-C t e n s i l e t e s t e r at 70°F and 65% r e l a t i v e humidity. The time t o break was 25-50 s e c . f o r 2 mm s t r i p s u t i l i z i n g a gauge l e n g t h o f 0.5" and jaw-type clamps. The l a b o r a t o r y treatments l i s t e d i n T a b l e s I I I , IV and V were performed by s o a k i n g the f i l m s i n s o l u t i o n s c o n t a i n i n g l.Og o f p o t e n t i a l p h o t o s e n s i t i z e r i n 250 ml o f methylene c h l o r i d e or e t h a n o l f o r 3 days at 22°C. The T 1 O 2 t r e a t m e n t s , however, i n v o l v e d a T i C l i t ( g ) i n N2 treatment f o l l o w e d by a H2O soak f o r conversion to T1O2 . Pis cussion Degradable P o l y s t y r e n e s . The E c o l y t e " F a s t " p o l y s t y r e n e foam has a h i g h e r c o n c e n t r a t i o n o f photo s e n s i t i v e v i n y l ketone comonomer than the E c o l y t e "Slow" foam and as a r e s u l t has a s h o r t e r i r r a d i a t i o n time t o e m b r i t t l e m e n t as l i s t e d i n T a b l e I . The " F a s t " foam was b r i t t l e a f t e r one month outdoor ex posure or 275 h r xenon a r c i r r a d i a t i o n . The outdoor exposure was f a c i l i t a t e d by wind and r a i n e r o s i o n . T u r n i n g t h e sample over a f t e r 35 hr xenon i r r a d i a t i o n lowered the e m b r i t t l e m e n t time t o a remarkably s h o r t p e r i o d o f 70 hr c o n s i d e r i n g the t h i c k n e s s and o p a c i t y of the foam. S i m i l a r l y the "Slow" foam had i t s xenon e m b r i t t l e m e n t time reduced from 2k00 hr t o o n l y 600 hr by t u r n i n g the foam over a f t e r each 2k h r i r r a d i a t i o n p e r i o d . To o b t a i n a b e t t e r i d e a o f the r a t e of

22. COONEY AND WILES


313

TABLE I I I Embrittlement

times

o f Low-Density Polyethylene

Hours t o Embrittlement


Treatments LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE LDPE

Films

, ΙΙΟμ, c o n t r o l + Ti0 (2.0%) , lOOu, c o n t r o l + 1,1 -dibenzoylferrocene (1.3%) + b e n z o p h e n o n e (10%) , 110y, c o n t r o l + ferric dibutyldithiocarbamate + 2-methylanthraquinone + 1,3,5-triacetylbenzene + 9-fluorenone + flavone + Uchromanone + dibenzanthrone + carbazole + benzophenone 2

1

1520 330 1350 288

660 1225 825 920 920 1030 1030 1030 1030 1030 1030

TABLE I V Embrittlement

Times o f H i g h - D e n s i t y

Polyethylene

Treatments HDPE, 2 3 y , c o n t r o l HDPE + T i 0 (2.3%) HDPE + a n t h r a q u i n o n e HDPE + a n t h r o n e HDPE + b e n z a n t h r o n e HDPE + 2 - m e t h y l a n t h r a q u i n o n e HDPE + 1,3 , 5 - t r i a c e t y l b e n z e n e HDPE + a - b r o m o - p _ - p h e n y l a c e t o p h e n o n e HDPE + 9 - f l u o r e n o n e - U - c a r b o x y l i c a c i d HDPE + f l a v o n e 2

Films

Hours t o Embrittlement 925 ^ 385 ^20 ^20 ^50 ^50 ^75 575 650

UV

314

p h o t o - o x i d a t i o n , f i l m s were p r e p a r e d from t h e foams. F i l m s c a s t from C C 1 s o l u t i o n s o f t h e " F a s t " and "Slow" foams e m b r i t t l e d i n 23 a n d 55 h r x e n o n i r r a d i a t i o n , r e s p e c t i v e l y , whereas t h e c o n t r o l p o l y s t y r e n e f i l m r e q u i r e d 1*95 h r i r r a d i a t i o n t o p r o d u c e e m b r i t t l e m e n t . The p h o t o s e n s i t i v e v i n y l k e t o n e comonomer c o n c e n t r a t i o n c a n be v a r i e d t o c o n t r o l t h e l i f e t i m e o f t h e D e g r a d able polystyrene t o s u i t the desired use. B i o - D e g r a d a b l e P l a s t i c s I n c . recommend t h a t t h e i r S t y - G r a d e r e s i n c o n c e n t r a t e b e l e t - d o w n 10:1 o r 20:1 f o r u s e i n f o r m u l a t i n g a D e g r a d a b l e p o l y s t y r e n e (2_) . A f i l m produced by employing t h e r e s i n c o n c e n t r a t e l e t - d o w n 5:1 e m b r i t t l e d i n 22 h r w h i l e a c o m m e r c i a l p r o d u c t b e l i e v e d t o b e l e t - d o w n 50:1 (5.) e m b r i t t l e d w i t h a 112 h r x e n o n i r r a d i a t i o n . A film of the resin concentrate embrittled i n 6 hr; therefore, v a r i a t i o n of t h e let-down r a t i o can c o n t r o l and vary t h e l i f e t i m e of t h e p o l y s t y r e n e d u r i n g outdoor u s e . D e g r a d a b l e L o w - D e n s i t y P o l y e t h y l e n e (LDPE) F i l m s . T y p i c a l xenon a r c e m b r i t t l e m e n t times f o r u n s t a b i l i z e d l o w - d e n s i t y p o l y e t h y l e n e a r e l600-2U00 h r s . The c o n t r o l s chosen f o r t h i s r e s e a r c h a r e l i s t e d i n Table I I ; t h e t h i n LDPE g a r m e n t b a g e m b r i t t l e d i n 1820 h r a n d t h e E c o P l a s t i c s LDPE c o n t r o l became b r i t t l e a f t e r 2075 h r xenon i r r a d i a t i o n . The t h i n g a r m e n t b a g was n o t b r i t t l e , a s d e t e r m i n e d b y a f o l d t e s t , a f t e r 18 m o n t h s outdoor exposure. The D u p o n t " S c l a i r " LDPE c o n t r o l l i s t e d i n T a b l e I I was u n u s u a l s i n c e i t h a d s h o r t e m b r i t t l e m e n t t i m e s o f 1175 h r i n t h e W e a t h e r - O m e t e r a n d 6.5 m o n t h s o u t d o o r s . T h i s f i l m was u s e d as a y a r d s t i c k t o m e a s u r e t h e D e g r a d a b l e LDPE f i l m s s i n c e a D e g r a d a b l e P l a s t i c s h o u l d n o t l a s t more t h a n 6 m o n t h s , p e r h a p s l e s s , o u t d o o r s a n d £1200 h r i n t h e xenon a r c Weather-Ometer. E c o t e n LDPE e m b r i t t l e d i n 2210 h r s i n t h e W e a t h e r O m e t e r a n d -11 m o n t h s o u t d o o r s . T h i s sample o f Ecoten does n o t a p p e a r t o be a s a t i s f a c t o r y D e g r a d a b l e P l a s t i c because o f t h e h i g h l e v e l o f T1O2 added t o t h e f i l m . C l e a r Ecoten f i l m w i t h o u t t h e t i t a n i u m d i o x i d e pigment was t e s t e d o u t d o o r s i n I s r a e l a s a g r i c u l t u r a l m u l c h and f o u n d t o e m b r i t t l e i n l e s s t h a n f o u r m o n t h s (6). B i o - D e g r a d a b l e P l a s t i c s I n c . b r o w n LDPE f i l m was b r i t t l e i n 750 h r x e n o n i r r a d i a t i o n a n d f o u r m o n t h s outdoors. T h i s l i f e t i m e c o u l d p r e s u m a b l y be d e c r e a s e d or i n c r e a s e d by c h a n g i n g t h e m a s t e r b a t c h r a t i o . E N D E - p l a s t LDPE w i t h f o r t i f i e d " a d d i t i v e m i x t u r e o n e " ( F A M - l ) i s m a r k e t e d as a D e g r a d a b l e P l a s t i c b y A k e r l u n d a n d R a u s i n g i n Sweden. A c l e a r f i l m o f ENDEp l a s t was b r i t t l e i n 1550 h r x e n o n i r r a d i a t i o n a n d >1*.5 m o n t h s o u t d o o r s a n d a p p e a r s t o h a v e a l o n g e r H




22. COONEY AND WDLES


315

l i f e t i m e t h a n w a r r a n t e d f o r most u s e s o f D e g r a d a b l e Plastics. T h i s l i f e t i m e p r e s u m a b l y c a n be d e c r e a s e d by l o w e r i n g t h e m a s t e r b a t c h l e t - d o w n r a t i o . The E c o l y t e LDPE f i l m s a r e n u m b e r e d i n r e l a t i o n to the c o n c e n t r a t i o n o f p h o t o s e n s i t i v e v i n y l ketone comonomer p r e s e n t i n t h e p o l y m e r i e . 1>>2>3. T h e E c o l y t e #1 LDPE c o n t a i n i n g t h e m o s t v i n y l k e t o n e c o monomer was b r i t t l e i n 820 h r i n t h e W e a t h e r - O m e t e r and 2.5 m o n t h s o u t d o o r s . The E c o l y t e #2 was b r i t t l e i n l l 8 0 h r a n d t h e E c o l y t e #3 r e q u i r e d lU65 h r w h i l e t h e E c o P l a s t i c s L t d . LDPE c o n t r o l r e q u i r e d 2075 h r xenon arc i r r a d i a t i o n t o reach e m b r i t t l e m e n t . This s e r i e s o f E c o l y t e s demonstrates t h e v a r i e d and con t r o l l e d l i f e t i m e s t h a t c a n be d e s i g n e d i n t o D e g r a d able P l a s t i c s by c o n t r o l l i n g c o n c e n t r a t i o n o f t h e photosensitizer. The d i s c r e p a n c y b e t w e e n x e n o n a r c a n d o u t d o o r weathering data f o r theBio-Degradable P l a s t i c s Inc. LDPE a n d t h e E c o l y t e #1 LDPE m o s t l i k e l y o c c u r s b e c a u s e t h e x e n o n a r c l a m p was o p e r a t e d w i t h a n UV e n ergy s l i g h t l y below, r a t h e r than s l i g h t l y above, t h a t o f n o o n summer s u n l i g h t a s d e p i c t e d i n F i g . 1. The lower xenon a r c o u t p u t would g r e a t l y reduce t h e energy i n t h e 285-315 nm r e g i o n w h e r e t h e E c o l y t e #1 a b s o r b s r a d i a t i o n b u t w o u l d n o t have as g r e a t an e f f e c t i n t h e 315-3^0 nm r e g i o n w h e r e B i o - D e g r a d a b l e P l a s t i c s I n c . LDPE a p p e a r s t o a b s o r b r a d i a t i o n . In a second experiment t h e f u n c t i o n a l l i f e t i m e o f t h e D e g r a d a b l e LDPE P l a s t i c s was e x a m i n e d . The f u n c t i o n a l l i f e t i m e was d e f i n e d a s t h e x e n o n a r c i r r a d i a t i o n time r e q u i r e d t o reduce t h epercent elongation a t b r e a k b y 9/10. T h e s e v a l u e s l i s t e d i n T a b l e I I a r e d e t e r m i n e d from t h e e l o n g a t i o n as a f u n c t i o n o f i r r a d i a t i o n t i m e c u r v e s i n F i g s . 2 a n d 3. The o r d e r o f i n c r e a s i n g t i m e b y w h i c h f i l m s r e a c h 10$ o f t h e i r o r i g i n a l elongation i squite s i m i l a r t o t h eorder o f embrittlement times. T h u s , t h e f o r m e r c a n be u s e d as a convenient i n d i c a t o r o f r e l a t i v e s u s c e p t i b i l i t y t o b r i t t l e n e s s. A f e w o f t h e p h o t o s e n s i t i v e a d d i t i v e s t h a t we h a v e t r i e d a r e l i s t e d i n T a b l e I I I , I V a n d V. T a b l e I I I h a s t h e h o u r s o f x e n o n a r c i r r a d i a t i o n t o em brittlement of treated low-density polyethylene, Table IV contains r e s u l t s f o r t r e a t e d h i g h - d e n s i t y polyethylene while Table V contains r e s u l t s for treated polypropylene films. I t s h o u l d be n o t e d t h a t t h e s e a d d i t i v e s w e r e d e s i g n e d t o i n c r e a s e t h e number o f c h r o m o p h o r e s on a n d n e a r t h e f i l m s u r f a c e s . I n principle the "optimal" concentration of the "right" chromophores s h o u l d enhance t h e a b s o r p t i o n o f l i g h t


316

IN POLYMERS

TABLE V E m b r i t t l e m e n t Times o f P o l y p r o p y l e n e F i l m s Hours t o Embrittlement

Treatments PP, 27y, c o n t r o l PP + T i 0 (5.9*) PP , acetone e x t r . PP , acetone e x t r . PP, acetone e x t r . PP , acetone e x t r . acetophenone PP , acetone e x t r . PP , acetone e x t r . f e r r o c e n e (0.2%) PP , acetone e x t r . quinone PP, acetone e x t r . benzene PP , acetone e x t r .

2k0


2

+ anthraquinone + anthrone + a-b romo-]D-phenyl+ xanthone + 1,1* d i b e n z o y l -

16 155 50 65 65 75 85

+ 2-methylanthra-

C=0

PET

PP

Photo-initiation


324

UV LIGHT

INDUCED


i r r a d i a t e d PET has t e e n measured by IR s p e c t r o s c o p y (at 3290 cm"* ) and by non-aqueous t i t r a t i o n on s o l u b l e samples Ç6) . PmPiPA p r o b a b l y undergoes p h o t o - c l e a v a g e at t h e C(=0)-NH bond t o g i v e a caged r a d i c a l p a i r . Some CO e l i m i n a t i o n from ~φ-00 ( r e a c t i o n l ) o c c u r s b e f o r e t h e r a d i c a l s recombine. R a d i c a l combination can r e f o r m the o r i g i n a l amide l i n k , or produce 2-aminobenzophenone backbone u n i t s - a P h o t o - F r i e s rearrangement ( r e a c t i o n 2) (T ) · These 2-aminobenzophenone groups have been d e t e c t e d by t h e i r c h a r a c t e r i s t i c e m i s s i o n Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch023

1

0

0

OH

Η

_

0

10

Η

Η

s p e c t r a (U60 nm e m i s s i o n from ~UlO nm e x c i t a t i o n i n PmPiPA) i n PmPiPA f i l m s which had been i r r a d i a t e d i n the absence o f O 2 · From t h e quantum y i e l d d a t a i n T a b l e I and t h e e x t e n t o f s u n l i g h t a b s o r p t i o n by each polymer ( T a b l e I and F i g u r e l ) i t i s apparent t h a t a l t h o u g h PET and PmPiPA absorb s t r o n g l y , they have v e r y low quantum y i e l d s f o r t h e i r p r i m a r y r e a c t i o n s . PP on t h e o t h e r hand absorbs weakly due t o low c o n c e n t r a t i o n s o f chromophores b u t t h e s e chromophores g i v e r a d i c a l p r o d u c t s by e f f i c i e n t p r o c e s s e s . In a d d i t i o n photoo x i d a t i o n l e a d s t o a r a p i d b u i l d up o f a d d i t i o n a l UV a b s o r b i n g -00H groups ( d i s c u s s e d i n S e c t i o n I I I ) .

23.

CARLSSON AND WILES

III· Oxygen

UV Light on Fiber Forming Polymers

325

Reactions

Oxygen can permeate amorphous r e g i o n s i n polymers and i n t e r c e p t some of the r a d i c a l s produced i n the p h o t o - i n i t i a t i o n s t e p s (U, 5_, 6 , 8) ( T a b l e I ) . In PP no more than one i n s i x of the p h o t o - i n i t i a t i o n s t e p s can be expected t o r e s u l t i n a f r e e t e r t . peroxy r a d i c a l ( P P O 2 . r e a c t i o n 3 ) ; the remainder of the i n i t i a t i o n s t e p s CH 3 ÇH 3 PP ' R , » H2-C-CH ^ ^ „ C H - Ç - C H ~ (ΡΡ0 ·)(3) etc · £ Q

R #


. y i e l d r a d i c a l p a i r s which t e r m i n a t e i n the cage ( 9 . ) . The f r e e peroxy r a d i c a l ( P P O 2 · ) can propagate i n t r a m o l e c u l a r l l y by hydrogen a b s t r a c t i o n from a n e i g h b o r i n g t e r t - C - H s i t e on the same backbone ( r e a c t i o n h) or undergo an i n t e r m o l e c u l a r t e r t - H - a b s t r a c t i o n ( r e a c t i o n 5) w i t h i n t r a m o l e c u l a r a t t a c k b e i n g f a v o r e d r o u g h l y t h i r t e e n f o l d over the i n t e r m o l e c u l a r p r o c e s s (_L0 ). Up t o 100 p r o p a g a t i o n s can then o c c u r b e f o r e t e r m i n a t i o n w i t h another peroxy r a d i c a l (£) . The o v e r a l l r e s u l t of a p h o t o - i n i t i a t i o n s t e p i s then the g e n e r a t i o n of up t o r o u g h l y 30 f r e s h -00H groups, ,CH -Ç — C H - C 2

\>H ?H

— CH ~—£*-ΡΡ0 · etc

2

2

2

(

'

U

)

ÇÏÏ3

3

.CH2-Ç-CH2-Ç-CH2v

° n

H

"PPNÉ.

9 3

Ç 3

H

2

0

ÇH3

H

—CH -Ç-CH ~+~CÏÏ2-Ç-CÏÏ2~

^CH -Ç

2

2

H

χ

0

H

io

2

PP0 . etc (5) 2

each of which can e f f i c i e n t l y p h o t o - c l e a v e and so i n i t i a t e f u r t h e r PP o x i d a t i o n . Hydroperoxide groups have a l s o been s u g g e s t e d t o be i n t e r m e d i a t e s i n PET p h o t o - o x i d a t i o n (6). The b u i l d up of d e t e c t a b l e q u a n t i t i e s of -00H groups has not been d i r e c t l y o b s e r v e d , but the presence of h y d r o x y t e r e p h t h a l a t e groups a f t e r UV i r r a d i a t i o n i n a i r has been suggested t o r e s u l t from ·OH (from -00H p h o t o - c l e a v a g e ) a d d i t i o n t o the PET backbone Ç6). The

UV

326


POLYMERS

h y d r o x y t e r e p h t h a l a t e groups were d e t e c t e d by f l u o r e s c e n c e s p e c t r o s c o p y ( e m i s s i o n a t k60 nm, f r o m an e x c i t a t i o n maximum a t 3^0 nm) (l_L ) . T h i s f l u o r e s c e n c e b u i l d up was o n l y o b s e r v e d d u r i n g i r r a d i a t i o n i n the presence of 0 (6.) · Hydro p e r o x i d e groups c o u l d r e s u l t from r a d i c a l a b s t r a c t i o n from a backbone methylene group i n the presence of O2 as shown i n T a b l e I . The phot©decomposition o f these hydroperoxide intermediates to give macro-alkyl r a d i c a l s c a n be e x p e c t e d t o s u b s e q u e n t l y c a u s e f u r t h e r backbone s c i s s i o n v i a r e a c t i o n IV ( T a b l e i ) , f o l l o w e d b y CO2 e l i m i n a t i o n f r o m t h e ~<J>CC>2 · r a d i c a l . A p p r e c i a b l e CO2 e v o l u t i o n i s o n l y o b s e r v e d d u r i n g t h e p h o t o d e g r a d a t i o n o f PET i n a i r , when t h e C0 q u a n t u m y i e l d r e a c h e s -1. 5xl0~ m. e i n s t " , as c o m p a r e d t o - 2 x 1 0 " m . e i n s t " i n t h e a b s e n c e o f O2 [ r e c a l c u l a t e d v a l u e s f r o m (6_)]. I n t h e p r e s e n c e o f O2, t h e p h o t o l y s i s o f PmPiPA g i v e s c a r b o x y l i c a c i d g r o u p s , p o s s i b l y b y O2 i n s e r t i o n a f t e r NH-C(=0) c l e a v a g e t o g i v e t h e t r a n s i e n t p e r o x i d e shown i n T a b l e I ( r e a c t i o n V l ) (f$ ) . C a r b o x y l i c a c i d f o r m a t i o n [ q u a n t u m y i e l d ~8xl0~ m einst" (]3)] i s a much more e f f i c i e n t p r o c e s s t h a n CO e l i m i n a t i o n o r P h o t o F r i e s r e a r r a n g e m e n t o b s e r v e d d u r i n g vacuum i r r a d i a t i o n ( T a b l e i ) . Carboxylic acid f o r m a t i o n i n P m P i P A f i l m s and f i b e r s h a s b e e n o b s e r v e d b y IR s p e c t r o s c o p y ( a t -1720 cm" ) and b y p o t e n t i o m e t r i c t i t r a t i o n s on d i s s o l v e d s a m p l e s ( j B ) . In f a c t 2-aminobenzophenone u n i t s were not d e t e c t e d a f t e r t h e i r r a d i a t i o n o f P m P i P A f i l m s i n a i r . The i r r a d i a t i o n o f PmPiPA f a b r i c s i n a i r however r e s u l t e d i n t h e b u i l d up o f b o t h c a r b o x y l i c a c i d g r o u p s and 2-aminobenzophenone u n i t s (12). I n a l l c a s e s , O2 e n h a n c e s t h e d e g r a d a t i v e e f f e c t s o f UV i r r a d i a t i o n e i t h e r b y g e n e r a t i n g new p h o t o u n s t a b l e s p e c i e s , o r as i n t h e c a s e o f P m P i P A b y i n t e r c e p t i n g p r i m a r y r a d i c a l s which i n the absence of O2 w o u l d l a r g e l y r e c o m b i n e t o r e f o r m t h e a m i d e l i n k b e c a u s e o f t h e r i g i d i t y o f t h e PmPiPA m a t r i x ( T g ~275°C). P h o t o - o x i d a t i o n r e s u l t s i n a p r o g r e s s i v e s h i f t o f t h e UV a b s o r p t i o n s p e c t r u m o f e a c h p o l y m e r towards l o n g e r w a v e l e n g t h , even i n t o t h e v i s i b l e i n t h e c a s e o f PmPiPA, w h i c h becomes y e l l o w - b r o w n . UV i r r a d i a t i o n of a l l t h r e e polymers i n the absence of O2 c a u s e s r e l a t i v e l y l i t t l e c h a n g e i n m e c h a n i c a l p r o p e r t i e s w h e r e a s UV e x p o s u r e i n t h e p r e s e n c e o f O2


2

2

lf

5

1

1

5

1

1

23. CARLSSON AND WILES

leads t o a and a l e s s difference a i r can be


IV.

Surface


327

r a p i d d e t e r i o r a t i o n i n e l o n g a t i o n at break marked r e d u c t i o n i n t e n s i l e s t r e n g t h . The between i r r a d i a t i o n under vacuum and i n seen i n F i g u r e 2. Reactions

Some o f t h e p r o d u c t s from t h e i n i t i a t i o n and p h o t o - o x i d a t i o n s t e p s l i s t e d i n T a b l e Î can be d e t e c t e d ( o f t e n w i t h d i f f i c u l t y ) by t r a n s m i s s i o n IR spectroscopy. T r a n s m i s s i o n IR and s t r e s s - s t r a i n changes a r e shown i n F i g u r e s 2, 3 and k f o r PmPiPA ( 8 ) , PP (13.) and PET (lk) f i l m s and f i b e r s i r r a d i a t e d i n a i r . In a l l cases i t i s immediately apparent t h a t t h e decrease i n m e c h a n i c a l p r o p e r t i e s i s not p a r a l l e l e d by t h e observed i n c r e a s e i n d e g r a d a t i o n p r o d u c t s (-00H groups i n PP, at 3^00 cm"" , -C(=0)0H groups i n PET at 3290 cm"" and -C(=0)0H groups i n PmPiPA at 1720 cm" ) as measured by t r a n s m i s s i o n IR. T h i s d i s c r e p a n c y i s removed i f s u r f a c e IR s p e c t r a o f the degraded f i l m s a r e measured by i n t e r n a l r e f l e c t i o n s p e c t r o s c o p y (1RS). T h i s t e c h n i q u e employs an IR beam r e f l e c t e d from t h e i n t e r f a c e between t h e polymer and an IR t r a n s p a r e n t r e f l e c t i o n element (Ge or a T l l / T l B r c r y s t a l , KRS-5) at an angle o f i n c i d e n c e g r e a t e r than t h e c r i t i c a l angle f o r t h e i n t e r f a c e . The observed IR spectrum r e s u l t s o n l y from t h e s u r f a c e l a y e r (~0.1-2u) o f t h e polymer, t h e a c t u a l t h i c k n e s s m o n i t o r e d depending on t h e o p t i c a l c o n d i t i o n s ( 1J5 ) . The 1RS s p e c t r a o f PmPiPA a f t e r v a r i o u s p e r i o d s o f UV exposure i n a i r are shown i n F i g u r e 5. P h o t o - o x i d a t i o n l e a d s t o a r a p i d b u i l d up i n c a r b o n y l a b s o r p t i o n ( a t -1720 cm" ) i n t h e s u r f a c e IR s p e c t r a , but t h i s change i s v e r y much l e s s prominent i n t h e t r a n s m i s s i o n s p e c t r a ( F i g u r e 2, c f . n o r m a l i z e d 1RS and t r a n s m i s s i o n d a t a , ODi η2 ο / O D 1 1 » ι ο ) · A l l q u a n t i t a t i v e IR data i n F i g u r e s 2-U have been n o r m a l i z e d t o u n i t t h i c k n e s s o f f i l m i n each case. Normalization involves r a t i o i n g the o p t i c a l density (OD) o f t h e product a b s o r p t i o n t o t h e o p t i c a l d e n s i t y of an i n v a r i a b l e r e f e r e n c e band, ( i . e . , a band un a f f e c t e d by d e g r a d a t i o n ) measured under i d e n t i c a l r e f l e c t i o n or t r a n s m i s s i o n c o n d i t i o n s ( l 6 ) . 1

1

1

1

The 1RS n o r m a l i z e d o p t i c a l d e n s i t y d a t a shown i n F i g u r e s 3 and h f o r each i r r a d i a t i o n time were measured w i t h s e v e r a l angles of i n c i d e n c e (30°, ^ 5 ° , 6 θ ° ) and on two r e f l e c t i o n elements (Ge and KRS-5)



IRRADIATION TIME (hr) Figure 2. IR and tensile changes: PmPiPA photodegradation. Xenon arc Weather-Ometer stress-strain data: 44 tex, 2 ply yarn Nomex. Α Δ , vacuum irradiated; # 0 , air irradiated. Initial elonga tion at break, 30%. Initial tensile strength, 9.0 X JO g cm' . IR data: ΙΟμ.films.Trans, +; 1RS, x(d — 0.55μ). 7

p

2


CARLSSON AND WILES

0


20

40 IRRADIATION

60 ( hr )

TIME

80


Figure 3. IR and brittleness changes: PP photooxidation (24). 21 μ Eastmanfilm,irrad. 320 nm (filtered Hg arc). 0 hr tensile strength: 3.0 Χ Ί0 g cm*. Trans, ir: O. 1RS ir (d ): + (0.19μ), Δ (0.42^, Ο (0.22μ), · (0.59μ). 5

p

IRRADIATION TIME (hr)

Figure 4. IR and tensile changes: PET photooxida tion. 21 μ Mylar film, irradiated Xenon arc WeatherOmeter. Initial elongation at break, 190%. Initial tensile strength, 7.3 χ 10 g cm' . d = depth of penetration for 1RS ir spectra. 5

2

p


330

so t h a t s p e c t r a c o r r e s p o n d i n g t o s e v e r a l d i f f e r e n t depths of p e n e t r a t i o n ( d ) of the r e f l e c t e d beam c o u l d he measured f o r each f i l m sample (l6.) . From the 1RS d a t a , d e g r a d a t i o n takes p l a c e almost e n t i r e l y i n the s u r f a c e l a y e r (up t o ~ l y ) f o r each polymer, and l i t t l e i n the h u l k of the sample (as shown by the t r a n s m i s s i o n IR d a t a ) . For PET and PmPiPA, only the s u r f a c e d i r e c t l y f a c i n g the l i g h t source was e x t e n s i v e l y degraded, the obverse s i d e s b e i n g l i t t l e changed. On the other hand, f o r PP, both the d i r e c t l y i r r a d i a t e d and obverse s i d e s of each f i l m showed v i r t u a l l y i d e n t i c a l e x t e n t s of s u r f a c e oxidations. S i m i l a r s u r f a c e e f f e c t s were observed f o r the g e n e r a t i o n of c a r b o n y l p r o d u c t s (at -1715 cm"" ) and the change i n backbone h e l i c a l content (997 cm" a b s o r p t i o n ) of PP (13.). P r o f i l e s of n o r m a l i z e d o p t i c a l d e n s i t y as a f u n c t i o n of dp c l o s e to one PP f i l m s u r f a c e are shown i n F i g u r e 6. These data show t h a t e x t e n s i v e r e l a x a t i o n of the amorphous PP chains i n t o the p r e f e r r e d h e l i c a l conformation has o c c u r r e d i n the s u r f a c e zone as the photoo x i d a t i o n proceeds. I n i t i a l l y the s u r f a c e (from 1RS) and i n t e r i o r (from t r a n s m i s s i o n IR) h e l i c a l contents were i d e n t i c a l (OD9 g 7/ODg 71> =0 . 8k ) . S i m i l a r s u r f a c e " c o n c e n t r a t i o n " p r o f i l e s can a l s o be c o n s t r u c t e d f o r the d i r e c t l y i r r a d i a t e d s u r f a c e s of PET and PmPiPA. The n o r m a l i z e d OD-dp curves can be c o n v e r t e d i n t o t r u e p r o f i l e s of product c o n c e n t r a t i o n as a f u n c t i o n of depth i n t o the samples, but s e v e r a l assumptions are r e q u i r e d (16.) . The s t r o n g a b s o r p t i o n of s h o r t wave UV by PET and PmPiPA f i l m s ( F i g u r e 1 and Table I) means t h a t the f i r s t few microns ( f o r PET) and t e n t h s of microns ( f o r PmPiPA) w i l l absorb a l l of the damaging s h o r t wave UV from the Xe arc ( s i m i l a r t o s u n l i g h t ) and s h i e l d the remainder of the sample. Only l o n g e r wavelengths (weakly absorbed, and lower energy) w i l l r e a c h the obverse s u r f a c e of each sample. In f a c t i r r a d i a t i o n of PET or PmPiPA f i l m s w i t h l o n g e r wave l e n g t h UV (>315 nm and >36θ nm r e s p e c t i v e l y ) r e s u l t s in v i r t u a l l y uniform c a r b o x y l i c a c i d generation throughout the f i l m s (8^, 17.) . For PP, the f r o n t and r e a r s u r f a c e p h o t o - o x i d a t i o n p r o b a b l y stems from s u r f a c e t h e r m a l o x i d a t i o n d u r i n g p r o c e s s i n g when t r a c e amounts of >C=0 and -00H groups are g e n e r a t e d i n the f i l m s u r f a c e s d u r i n g the b r i e f a i r exposure of the extruded melt at >250°C b e f o r e quenching ( l 8 ) . p


POLYMERS

1

1


CARLSSON AND WILES



Figure 5. 1RS ir spectra of air irradiated PPiPA films. Ge re flection element, 45° incidence. Film surface directly irradiated for specified times with Xenon arc Weather-Ometer. Ge curve due to reflection element alone (8).

0.06

-

• \ 0.04

0.02 TRANS. ο TRANS. 0

ι

ι

I

0.2

0.4

0.6

0

I

1

1.0

2.0

3.0*

DEPTH OF PENETRATION (dp,Â)

Figure 6. Surface changes from 1RS spectra during PP photooxidation. Film and irradiation details as in Figure 2, 56 hr irradiation. d measured in from irradiated surface. p


332

The UV t r a n s p a r e n c y o f PP and t h e extremely low a b s o r p t i o n o f t h e chromophores (Table I ) mean t h a t the obverse PP s u r f a c e r e c e i v e s v i r t u a l l y t h e same i n c i d e n t UV i n t e n s i t y as t h e f r o n t s u r f a c e , and so p h o t o - o x i d a t i o n proceeds at t h e same r a t e i n both surfaces.


V.

Backbone S c i s s i o n

The u s e f u l m e c h a n i c a l p r o p e r t i e s o f a l l macrom o l e c u l e s depend on t h e g r e a t l e n g t h o f t h e polymer backbone. T h i s l e n g t h allows e x t e n s i v e entanglement and f o l d i n g l e a d s t o t h e f o r m a t i o n o f c r y s t a l l i t e s i n t e r c o n n e c t e d by amorphous zones. Any backbone c l e a v a g e w i l l r e s u l t i n a weakening o f t h e polymer s t r u c t u r e , p a r t i c u l a r l y s i n c e s c i s s i o n r e a c t i o n s occur l a r g e l y i n t h e amorphous zones, c o n t a i n i n g t i e m o l e c u l e s which connect c r y s t a l l i t e s . For PP, PET and PmPiPA, t h e p r i m a r y photoi n i t i a t i o n steps a l l i n v o l v e backbone s c i s s i o n , y e t none o f t h e s e polymers d e t e r i o r a t e s a p p r e c i a b l y i n m e c h a n i c a l p r o p e r t i e s d u r i n g i r r a d i a t i o n w i t h UV >290 nm i n t h e absence o f 0 . In f a c t b o t h PET (17., 19 ) and PmPiPA (7.) undergo c r o s s l i n k i n g which r e n d e r s the polymers i n s o l u b l e i n t h e i r u s u a l s o l v e n t s ( o c h l o r o p h e n o l f o r PET and d i m e t h y l a c e t a m i d e f o r PmPiPA). C r o s s l i n k i n g i n t h e s e aromatic polymers p r o b a b l y r e s u l t s from r a d i c a l a d d i t i o n t o p h e n y l r i n g s i n a d j a c e n t chains (l£) . F o r PP, even complete p h o t o l y s i s o f t h e v e r y low c o n c e n t r a t i o n s o f chromo phores can have l i t t l e e f f e c t on p h y s i c a l p r o p e r t i e s . In t h e presence o f O2 c r o s s l i n k i n g r e a c t i o n s are reduced due t o O2 i n t e r c e p t i o n o f r a d i c a l i n t e r m e d i a t e s , and a d d i t i o n a l backbone c l e a v a g e r e a c t i o n s become important (Table i ) . F o r PP, t h e p r i m a r y i n i t i a t i o n p r o c e s s e s are v e r y r a r e , but t h e hydro p e r o x i d e which r a p i d l y b u i l d s up ( F i g u r e 3) f o l l o w i n g the i n i t i a l p h o t o - i n i t i a t i o n p r o c e s s e s q u i c k l y becomes t h e dominant source o f f r e e r a d i c a l s . In PP, backbone s c i s s i o n l a r g e l y r e s u l t s from t h e β-scission of macro-alkoxy r a d i c a l s (5.) ( r e a c t i o n I , T a b l e I ) . Roughly 70-80$ o f t h e PP macro-alkoxy r a d i c a l s 3s c i s s i o n , 10-20$ hydrogen a b s t r a c t from PP t o g i v e a l c o h o l groups and t h e remainder are l o s t by combination w i t h other r a d i c a l s (.5*9.) · The β-scission of a t e r t - a l k o x y r a d i c a l may occur by two r o u t e s ( r e a c t i o n s 6 and 7). In w e l l c h a r a c t e r i z e d l i q u i d 2

9

23. CARLSSON AND WDLES


333

phase (3-scissions t h e e l i m i n a t i o n o f a secondary a l k y l r a d i c a l ( r e a c t i o n 6) i s n o r m a l l y f a v o r e d over p r i m a r y CH ~CH -C

+ -CH ~

(6)

^~CH -C-CH ~ + CH '

(7)

2

CH I

3

v

2

3

~CH -C-CH . 2

I ο·

2

2

II

2

3


0 r a d i c a l e l i m i n a t i o n ( r e a c t i o n 7) (20.)· From IR a n a l y s i s o f t h e p h o t o l y s i s p r o d u c t s from PP hydro p e r o x i d e groups, C a r l s s o n and Wiles c o n c l u d e d t h a t "backbone ketone f o r m a t i o n v i a t h e u n u s u a l s c i s s i o n r e a c t i o n 7 o c c u r r e d at r o u g h l y t h e same r a t e as t e r m i n a l methyl ketone f o r m a t i o n (5.) . T h i s might imply the r e v e r s a b i l i t y o f r e a c t i o n s 6 *and 7 i n t h e r i g i d polymer, so t h a t product f o r m a t i o n by t h e e l i m i n a t i o n o f a s m a l l mobile r a d i c a l (CR"3 ) i s f a v o r e d , but o b v i o u s l y needs f u r t h e r c o n f i r m a t i o n . Alkoxy β-scission ( r e a c t i o n IV, T a b l e i ) i s p r o b a b l y a l s o important i n PET p h o t o - o x i d a t i o n (6), as described previously. The presence o f e x t e n s i v e backbone s c i s s i o n d u r i n g p h o t o - o x i d a t i v e d e g r a d a t i o n has been demon s t r a t e d by m o l e c u l a r weight (Mn) d e t e r m i n a t i o n , based on s o l u t i o n v i s c o s i t i e s . PET w i t h an i n i t i a l Mn o f -20,000 decreased t o -1^,000 a f t e r 600 h r xenon a r c i r r a d i a t i o n (21^) . PmPiPA w i t h an i n i t i a l Mn o f ~U0,000 decreased t o -10,000 a f t e r hO h r o f xenon a r c i r r a d i a t i o n (8) . The a c t u a l e x t e n t o f backbone s c i s s i o n i s even more s t a r t l i n g when i t i s remembered t h a t d e g r a d a t i o n has o c c u r r e d o n l y i n t h e i r r a d i a t e d s u r f a c e l a y e r (

5(a)

C0 R

2

2

followed by intermolecular transfer H

H - CH - Ç.

^

-> ^

2

C0 R

C0 R

2

- CH - CH 2

C0 R

2

5(b)

2

C0 R

2

2

The radical remaining can undergo termination by H abstrac tion or recombination leading to further crosslinking. For X = CH and Y = H (an alternating sequence of acrylic and methac rylate monomers). 3

1)

Chain scission: ÇH

H

3

ÇH

ÇH

3

-

Δ EXPERIMENTAL POINT

(0

Ο CALCULATED POINT • EXPERIMENTAL AND CALCULATED POINT EQUAL


0

J_ 38

Η

_J_

76

J_ 114 152 190 228 266 304 342 380 TIME (MINS.)

Figure 5. Comparison of experimental data for C0 with a nonlinear least squares fit of this data to Equation 14 2

417


418

UV LIGHT

INDUCED

REACTIONS

IN

POLYMERS

due to a different kinetic process being responsible for the generation of the molecules producing these fragments. This of course i s not unreasonable because the type of molecules which can generate these fragments are alcohols and esters (e.g. formates) which are probably formed in a two step process. The computer f i t of the CO2 data makes one confident that with the proper kinetic assumptions an analytical expression can be derived to f i t the data, and therefore kinetic parameters arrived at. One complication exists in this process, the M/E = 31 peak, for example, may have contributions from more than one molecule (e.g. CH3OH and CH4CO2). That being the case the curve would be a sum of two or more kinetic processes. This can be overcome by the use of high resolution mass spectrometry. With high resolution capability one can find unique masses which are attributable to each molecule and from that intensity and a library of mass spectral data (9) one can "deconvolute" the mass spectrum into i t s individual components. We currently have the a b i l i t y to acquire such data and are developing the software needed to handle i t . In low resolution finding unique masses for each fragment i s considerably more d i f f i c u l t i f not impossible. However, there i s another approach which may be taken. If one examines the reaction scheme, a prediction of which molecules w i l l be generated can be made. Then u t i l i z i n g a library of mass spectral data a series of simultaneous equations in contribution to intensity of given mass spectral peaks can be written (12). The only assumption necessary for this i s we know a l l the molecules contributing to each peak of interest. Such a procedure was followed with our data. Based on the prepolymer composition i t was decided that Butanol, Butane, Butene, methanol and methyl formate would "be the most l i k e l y degradation products in addition to the CO2 and CO. U t i l i z i n g the M/E = 56, 43, 41 and 31 peaks, a set of simultaneous equations were solved for the composition of the degradation products ob served at the end of the experiment. Table I l i s t s this composition. TABLE I Calculated composition of degradation products from acrylic paint at 25°C found at end of experiment Component Butanol Butane Butene Methanol Methyl formate

Calculated Intensity 3.4 4.4 2.8 2.0 1.3

χ χ χ χ χ

10 10 10 10 10

5 5 5 5 5

27.

KiLLGOAR A N D V A N OENE

Photodegradation of Automotive Paints

419

Using this composition the intensity of several other peaks not used i n the solution of the simultaneous equations and assumed to originate from these degradation products only were calculated. The results are tabulated i n Table II. TABLE II Calculated vs. observed intensity for mass spectral peaks assumed to originate from the degradation products of Table I.


M/E 27 29 39

Calculated Intensity 4.3 χ 10 4.9 χ ΙΟ 2.2 χ 10

5 5

5

Observed

Intensity

4.4 χ 10 β.Ο χ 10 2.5 χ 10

5 5

5

Good agreement with the M/E = 27 and 39 peaks i s obtained, but there i s a f a i r l y large discrepancy with the 29 peak. This indicates that there i s another degradation product we have not considered contributing to the 29 peak. This may be a melamine fragment or some other molecule. In any event i t i s clear the low resolution data can be used to gain insight into the role played by the chemistry of the film in the degradation as well as giving kinetic information. Examination of our data for peaks due to monomer which might be formed from unzipping reactions showed no detectable amount being produced. This i s not unexpected because the prepolymer composition i s approximately 50% aerylate type monomers and, therefore, degradation by unzipping i s unlikely. Conclusions It has been demonstrated that the sensitivity of the mass spectrometer makes i t ideally suited for application to the study of polymer photodegradation. The basic understanding of the system makes i t possible to extract useful kinetic and chemical information about the degradation reactions. Work on the roles of temperature, chemical composition and network structure i s in progress. Acknowled gments The assistance, discussions and suggestions rendered by D. Schuetzel i n the development of this experimental technique, by R. Ullman for his advice i n the theoretical development and M. E. Heyde for her assistance i n a l l phases of this work i s gratefully acknowledged.

420

UV


Literature Cited 1. Hoffman, E. and Saracz, A., J. Oil Col. Chem. Assoc., 55, 1079 (1972). 2. Colling, J . H., Cracker, W. E . , Smith, M. C., and Dunderdale, J., J. Oil Col. Chem. Assoc., 54, 1057 (1971). 3. Perera, D. Υ., and Heertjes, P. M., J . Oil Col. Chem. Assoc., 54, 774 (1971). 4. Tamblyn, J . W., Newland, G. C., and Watson, M. T., Plastics Technol., 4, 427 (1958). 5. Grassie, N. and Farrish, E . , Europ. Poly. J., 3, 627 (1967). 6. Grassie N., Torrence, B. J . D., and Colford, J . B., J . Poly. Sci. Al, 7, 1425 (1969). 7. Grassie, N. and Jenkins, R. H., Europ. Poly. J., 9, 697 (1973). 8. Calvert, J . G. and Pitts, J . Ν., "Photochemistry", John Wiley and Sons, Inc., New York (1966). 9. ASTM Committee E-14 on Mass Spectrometry, "Index of Mass Spectral Data", ASTM, Philadelphia, Pa. (1969). 10. Calvert, J . G and Pitts, J . Ν., "Photochemistry", John Wiley and Sons, Inc., New York, p. 434 (1966). 11. Dye, J . L. and Nicely, V. Α., J . Chem. Ed. 48, 443 (1971). 12. Willard, H. H., Merritt, L. L., J r . , and Dean, J . Α., "Instrumental Methods of Analysis", D. Van Nostrand Co., Inc., Princeton, N. J., p. 449 (1965). 13. Bateman, Η., "Tables of Integral Transforms Vol. I.", McGraw-Hill Book Co., Inc., New York, pp. 245-6 (1954).


;

APPENDIX I SOLUTION OF DIFFERENTIAL EQUATION DESCRIBING TIME DEPENDENCE OF PRODUCT DETECTION I t i s assumed that the f i l m absorbs l i g h t by Beer's law I(x)

= I exp(-ax)

(1)

where the v a r i a b l e s are defined i n the t e x t of t h i s paper. I t i s a l s o assumed that the degradation products a r e formed by a pseudozero th order r e a c t i o n : 3c(x,t) 3t

=

ΦΙ

(2)

The products formed a r e removed from the f i l m by d i f f u s i o n . The r a t e of d i f f u s i o n from the f i l m i s given by

(3)

27.

Photodegradation of Automotive Paints

KiLLGOAR A N D V A N OENE

421

The product distribution within the film i s given by the diffusion equation

91

ο

d X

Solving equation 4 for c(x,t) and then solving equation 3 yields the rate of product into the chamber. The boundary conditions imposed to solve these equations are


c(x,o) = 0 c(~,t) = 0 c(o,t) = 0

(5)

The Laplace transform i s used to solve equation 4 where: pt

c(x,P) =/°°c(x,t) e " 'ο hence / J

o

Γ Jo

. dt

-pt 9c (x, at

e

2

9 c(x,t) 3x^ e

' /

(6)

dt

A e

-

a x e

-

p t

P t

+

dt

(7)

0

Solving

y

r

°°-pt 9c(x,t) dt ce~

Γ /

D

pt

= 0

e

9x

-ptl

,

/"

Since for t = 0

l!|ix^. - P t

0

[

d t

=

*

D

z

Γ

-pt ,. _

c(x,o) = 0

C

.-Pt

=

„

z

3x /

-

0

Λ 9x

z

and

IΙ

A

-αχ Ae

-pt ,^ 1 « -αχ e dt = — Ae

'ο

Ρ

Therefore the transformed d i f f e r e n t i a l equation becomes: A X

~ 1 A ~ - pc = - — Ae 9x^ ρ The complementary solution to equation 8 i s D

7ΓΤ

/ON (8)

1 2 C

=

AJ°)

X

,„ 2 v

+ B e - (

D

)

X

(9)

422

UV L I G H T INDUCED REACTIONS I N

00

since as χ ->

c ->· o, A

1

POLYMERS

must be zero.

The particular solution to equation 8 i s c = Fe"

a x

(10)

substituting into 8 one obtains 1 F = ρ

A ρ -Da

z

and the complete solution i s


Ix F

c-Be"( )

+

A

, , e" p(p -Da ) n

2

a x

(11) '

z

K ± ±

eliminating one of the constants with the boundary condition c(o,t) = 0 we obtain: 1 -pr

-

A

-αχ

Ρ(Ρ-Μ )

C

2

6

A

-V/D

" P(P -D«*)

/

,

C

(

1

2

Λ0 Λ )

Using the tables of transforms (13) we rewrite equation 12 i n terms of χ and t:

_1 A Da t Γ ax [ . - « . ( i ^ 2 Da " 2

2

e

+

2

«

)

*

.

-

«

^

^

.

.

«

î

)

]

28 Triplet States of Photosensitive Aromatic Azides


MINORU TSUDA and SETSUKO OIKAWA Laboratory of Physical Chemistry, Faculty of the Pharmaceutical Sciences, Chiba University, Chiba 280, Japan

Abstract: Although aromatic azides are believed not to emit phospho rescence, 2,4,6-tribromophenylazide was found to phosphoresce in EPA at 77°K, and the peak of the spectrum coincides with the tail of the absorption spectrum of its concentrated solution in ethyl iodide. The measurements of theS ->T absorption spectra in ethyl iodide with v i brational structures were also succeeded in the cases of ethyl p-azidobenzoate and ethyl p-azidocinnamate. The experimantal values coin cided with the (π->π*) transition energies estimated by SCF MO CI calculations(Pariser-Parr-Pople method); the transition energies of T-1 are 20100 cm for the former and 18700 cm for the latter, respec tively, and that for phenylazide is 21700 cm . The results obtained by the calculation of MIM(molecule in molocule) method showed that the contribution of the localized excitation(LE) of azide group to the state function of T of p-azidobenzoate is 61.33%and that of benzoyloxy group is only 18.22%. However, in the case of p-azidocinna mate, the main contribution to T is the LE of cinnamoyloxy group(69%) o

1

3

-1

-1

-1

1

1

Aromatic azides are stable photosensitive compounds and some of them mixed in cyclized rubber are practically used as photosensitive resines.(1) Such photosensitive polymers as poly(vinyl p-azidobenzo ate) (2) and poly(vinyl p-azidocinnamate)(3) are also investigated. These photosensitive resines are known to be spectrally sensitized by triplet sensitizers(1,2,3). But the energies E(T1) of the lowest triplet states T1 of aromatic azides have not been known, be cause these compounds emit no phosphorescence(4) and the trials on the measurement of their S->T1 absorptions are failed(4,5). In order to decide E(T ) of azides, Saunders et al. measured quantum yields for nitrogen evolution in sensitized photolysis of azi des (4) expecting such a relationship that the rate constants kq in the energy transfers from the triplet sensitizers to an azide might con verge to a constant value provided E(T1) of sensitized were larger than Ε(Τ1) of the azide, as Hammond et al. had obtained in the sensi tized cis-isomerization of trans-stilbene(7). In the cases of azides, however, k increases monotonously according to the increase of E(T1) o

1

q

423

424

U V

L I G H T

I N D U C E D

R E A C T I O N S

I N

P O L Y M E R S

of s e n s i t i z e r s . Saunders c o n s i d e r e d t h a t e x t r a p o r a t i o n o f the l i n e s showing the r e l a t i o n l o g k q v e r s u s E(TI) o f s e n s i t i z e r s t o l o g kg=9 suggested t h e E(T1 ) o f a z i d e s , s i n c e k q had converged a t 1 θ 9 - 1 0 * 1 . mol .sec" i n the s e n s i t i z e d c i s - i s o m e r i z a t i o n o f t r a n s - s t i l b e n e . E ( T i ) o f p h e n y l a z i d e s was d e c i d e d t o be 75 K c a l / m o l ( 2 6 3 0 0 c n T ) by the procedure. A l t h o u g h we a c c e p t some o f q u e s t i o n a b l e p o i n t s i n t h e i r d i s c u s s i o n s , the l a r g e s t d i f f i c u l t y i s t h a t the (n->1t*) t r a n s i tion(Fig.1o} o f p h e n y l a z i d e t a i l s t o 75 K c a l / m o l e . T h e r e f o r e , E(Ti ) s h o u l d be l o w e r t h a n t h a t o f t h e l o w e s t s i n g l e t s t a t e . Saunderes a l s o showed t h a t s e n s i t i z e r s as low i n E(TI ) as benzophenone(68 K c a l / m o l ( 23800 c m ) ) gave a p p r e c i a b l e r e a c t i o n i n the p h o t o l y s i s o f p h e n y l a z i de^), w h i c h s u g g e s t s t h a t t h e r e a c t i o n mechanism i s n o t so s i m p l e as in the case o f s e n s i t i z e d c i s - i s o m e r i z a t i o n o f t r a n s - s t i l b e n e . There f o r e , we cannot expect the o b t a i n i n g o f the e x a c t v a l u e o f Ε ( Τ ι ) o f a r o m a t i c a z i d e s by s u c h a p r o c e d u r e o f s i m p l e c h e m i c a l k i n e t i c s as Saunders u s e d . I n t h i s p a p e r , we e f f o r t t o determine E ( T i ) o f a r o m a t i c a z i d e s s p e c t r o s c o p i c a l l y , u t i l i z i n g t h e i n n e r and o u t e r heavy atom e f f e c t s a l o n g w i t h the t h e o r e t i c a l e s t i m a t i o n o f t h e Ε(Τι) by t h e PPP c a l c u l a t i o n s ^ ) , and a l s o d i s c u s s e d t h e e l e c t r o n i c s t r u c t u r e s o f a r o m a t i c a z i d e s comparing t h o s e w i t h t h e d a t a o b t a i n e d by t h e MIM c a l c u l a t i o n s (6). 1

- 1

1

1

1


- 1

Phenyl

Azide.

T r a n s i t i o n E n e r g i e s and Spectrum. The r e s u l t s o f the PPP c a l c u l a t i o n s on t h e t r a n s i t i o n e n e r g i e s and o s c i l l a t o r s t r e n g t h s o f p h e n y l a z i d e a r e shown i n F i g . 1-6The spectrum i n EPA a t 77°K has the same peak a t 40000 c m as t h a t i n n-hexane . s o l u t i o n s a t room temperature w i t h o u t t h e r e m a r k a b l e v i b r a t i o n a l s t r u c t u r e n e a r 35000 cm"" i n the f o r m e r . The v i b r a t i o n a l s t r u c t u r e c o r r e s p o n d s t o t h a t o f B2u s t a t e o f benzene i n t h e number o f peaks and t h e i n t e r v a l s as was shown i n F i g . 1 $ Therefore, i t i s considered t h a t the t r a n s i t i o n a r i s e s from the B 2 t r a n s i t i o n o f benzene. The MIM c a l c u l a t i o n j u s t i f i e d t h e e s t i m a t i o n as was shown i n T a b l e 1, where the c o n t r i b u t i o n o f the LE f u n c t i o n o f B2u i s 84 % i n the S1 s t a t e . The f o r b i d d e n B 2 t r a n s i t i o n becomes a w e a k l y a l l o w e d one by t h e p e r t u r b a t i o n o f a z i d e . The S2 t r a n s i t i o n i n p h e n y l a z i d e shows t h e b l u e s h i f t i n p o l a r s o l v e n t s : t h e peak a t 40000 c m i n n-hexane s h i f t s t o 40400 cm~ i n e t h a n o l and EPA a t room t e m p e r a t u r e ( t h e spectrum i n EPA shown i n F i g . 1 b s h i f t f r o m 40300 cm" to 40000 cm" a g a i n by the f r o z e n c o n d i t i o n s ) . T h i s i s an e x t r a o r d i n a r y phenomenon i n t h e (lT-*1t*) t r a n s i t i o n s . The PPP c a l c u l a t i o n s r e v e a l t h e r e a s o n as i s shown i n F i g . 2 . As a r e f o u n d f r o m t h e m o l e c u l a r diagrams o f So and S2, t h e e l e c t r o n d i s p l a c e m e n t from a z i d e to benzene i n S changes t o t h e o p p o s i t e d i r e c t i o n i n S2 in s p i t e o f t h e same d i r e c t i o n i n S i . I n t h e case o f a n i l i n e , t h e r e d s h i f t i s o b s e r v e d i n p o l a r s o l v e n t s and t h e d i r e c t i o n o f t h e e l e c t r o n d i s p l a c e m e n t i s t h e same i n e a c h case o f SO,S1 and S2* The s i m i l a r i t i e s o f t h e a b s o r p t i o n s p e c t r a o f p h e n y l a z i d e and s t y l e n e a r e a l r e a d y p o i n t e d out by the p r e s e n t a u t h o r ( l O ) . Comparing - 1

1

1

1

U

1

1

U

- 1

1

1

1

1

0

28.

Triplet States of Photosensitive Aromatic Azides 425


TSUDA A N D OIKAWA

Figure lb. The comparison of the calculated values with the experimental ones on the electronic transition energies and oscillator strengths of phenylazide. The S transition has a vibrational structure corresponding to the five peaks of *Β „ of benzene. The absorption spectrum lower than 45,000 cm' was measured at 77°K in EPA. The S absorption is that of neat phenylazide (thin layer). t

£

1

4

426

UV

LIGHT INDUCED REACTIONS IN

POLYMERS

Table 1. The Contributions of Various Electronic Configurations of the MIM model for the State Functions of the PPP model of Phenylazide = Benzene(B) + Azide(A). S1

S2

S3

S

35271

39805

46624

49680

54226

92.30 0.18 0.01 4.62 2.66

0.02 86.48 0.38 3.77 3.16

0.02 39.46 24.55 9.22 22.37

0.02 80.21 8.66 1.71 2.58

0.10 74.58 0.87 6.20 11.98

3.05 41.39 1.93 7.96 37.36

Total

99.78

93.81

95.62

93.18

93.73

91.69

LE+G CT

92.49 7.28

86.88 6.93

64.03 31.59

88.89 4.29

75.55 18.18

46.37 45.32

85.34 ^2u

24.55 A Si 22.37 CT

62.39 *1u

67.14 1B

37.36 CT 35.31 Eiu

T2

T3

Singlet State

So

Transition Energy( cm-1) G LE

* A A

CT Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ch028

0 1

4

B

B->A

Main Contribution

1

Triplet State Transition Energy(cnH)

1U

1

T4

T5

21338

28869

32814

32953

35884

14.66 64.94 4.54 11.51

78.26 14.59 0.94 0.12

87.22 0.25 3.62 2.79

86.76 3.77 2.12 1.24

87.32 0.46 3.22 2.71

Total

95.66

93.91

93.88

93.88

93.71

LE+G CT

79.60 16.05

92.85 1.06

87.47 6.41

90.53 3.36

87.78 5.93

64.94 T1

77.91

80.15

63.02

3E

3E1U

69.17 3B

LE

J

CT 0

Ti

1

S5

4

1

A

B

">

B->A

Main Contribution

A

5

B1U

1 U

2u

Figures are the squares of the coefficients i n the state functions expressed as a linear combination of the electronic configurations of the MIM model. G: Ground configuration, LE: Configuration of the localized excitation, CT: Charge transfer configuration. Bi ,B2u>Elu The electronic states of benzene :

u


28.

TSUDA A N D OIKAWA



428

IN

POLYMERS

the r e s u l t s i n Table 1 and Appendix 1, we can f i n d out the remarkable s i m i l a r i t i e s of the state functions S , S i , S 2 , T i , which are observed, and other states of these two compounds. And we can also see that the S 2 state f u n c t i o n i s s i m i l a r to the S 2 f u n c t i o n of cinnamoyloxy group i n the points of the intramolecular charge t r a n s f e r character and the large c o n t r i b u t i o n of the azide(ethylene i n the l a t t e r ) LE function. 0

T r i p l e t State. A remarkable feature of the T1 state of phenyl azide i s the large contribution of the LE function of azide group. The same tendency was observed i n the cases of stylene,stilbene( 6 ) and cinnamoyloxy group( 6 ) . The calculated value of Ti i s 21700 cm . The S -)Ti absorption spectrum i n E t I gives only t a i l near the calcu l a t e d value. The emission spectrum i n EPA gave such a phosphorescence as was shown i n Fig.3. The i n t e n s i t y increases with the i r r a d i a t i o n time. Therefore, i t i s c l e a r that the emission spectrum i s that of the photodecomposed product, which i s estimated to be a n i l i n e from the s i m i l a r i t y of the phosphorescence of the authentic sample. -1


0

P-Azidobenzoate. T r a n s i t i o n Energies and Spectrum. The UV spectrum of p o l y ( v i n y l p-azidobenzoate) i s the same as that of e t h y l p-azidobenzoate. The spectrum of the l a t t e r i n n-hexane solutions i s shown i n Fig.4 with the calculated t r a n s i t i o n energies and o s i l l a t o r strengths. Hasegawa et a l . divided the spectrum i n t o four peaks assuming the Gaussian d i s t r i b u t i o n s around the t r a n s i t i o n energies lower than 40000 cm (11 ). According to our t h e o r e t i c a l c a l c u l a t i o n s , however, there are only two peaks i n t h i s area. Assuming the Gaussian d i s t r i b u t i o n around the S 2 t r a n s i t i o n energy, we obtained the two t r a n s i t i o n peaks, one of which, the S-j t r a n s i t i o n , has three peaks which has the same i n t e r v a l s as the Si t r a n s i t i o n of phenylazide. Therefore, the Si state i s considered to a r i s e from the B2u state of benzene. The e s t i mation i s supported by the MIM c a l c u l a t i o n s as i s shown i n Table 2 ( also see Appendix 3 and 4 i n the discussions i n t h i s s e c t i o n ) . The S 2 t r a n s i t i o n also the same property as that of phenylazide. When we performed the MIM c a l c u l a t i o n s of p-azidobenzoate = phenylazide + c a r boxyl group, a l l the t r a n s i t i o n s which can be observed by the conven t i o n a l UV spectrometer(S ~ S5,T1) are s u b s t a n t i a l l y those of phenyl azide(purterbed by carboxyl group).(Appendix 2) -1

1

0

The Sn-VTi t r a n s i t i o n energy. The S -*Ti absorption spectrum of e t h y l p-azidobenzoate i n C2H5I was succeeded i n the measurement ( F i g . 5(a)). PPP c a l c u l a t i o n s show that the t r a n s i t i o n energy of T^ i s 20084 cm" and the energy difference between T1 and T2 i s 6300 cm" . Therefore, we can expect the v i b r a t i o n a l structures of Ti i s o l a t e d from T 2 i f i t appears. The spectrum has the progressions of about 2000 cm" which i s i n accordance with the s t r e t c h i n g v i b r a t i o n of a z i de group of IR spectra(2150 cm" ) (2a). The MIM c a l c u l a t i o n reveals that the c o n t r i b u t i o n of the LE function of azido group i s 61.33 and that of benzoyloxy group i s 18.22


430

T a b l e 2 . The C o n t r i b u t i o n s o f V a r i o u s E l e c t r o n i c C o n f i g u r a t i o n s o f the MIM model f o r the S t a t e F u n c t i o n s o f the PPP model o f p - A z i d o b e n z o y l o x y Group = Benzojjpxy G r o u p ( B . A . ) + A z i d e ( A ) Singlet State

S1

S2

S3

S

4

S

5

Energy( cm" )

33048

37142

43338

47413

49562

90.57 1.76 0.05 5.00 2.27

0.01 87.07 0.08 3.60 2.01

0.14 51.95 17.58 11.28 13.11

0.12 73.20 11.56 3.03 4.02

0.89 77.49 3.94 5.62 3.77

0.41 71.02 4.59 1.36 14.37

Total

99.65

92.77

94.06

91.93

91.69

91.75

LE+G CT

92.38 7.27

87.16 5.61

69.67 24.39

84.88 7.05

82.32 9.39

76.02 15.73

30.18 B.A. S 2 34.13 B.A. S4

37.02 B.A. S 3 23.65 B.A. S5

24.18 B.A. S 3 20.73 B.A. S4

T3

T4

T4

Transition

1

G -

γ · A-> Β . A .

B.A.->A

U i


So

Main Contribution

Triplet

83.05 41.41 B . A . S] B.A. S

State

T>|

T

2

2

20084

26388

30169

31157

32711

18.22 61.33 5.21 9.31

72.17 19.39 1.23 0.27

89.61 0.18 0.62 2.60

89.03 0.89 1.79 0.41

84.07 0.94 7.22 0.24

Total

94.07

93.06

93.01

92.12

92.47

LE CT

79.55 14.52

91.56 1.50

89.79 3.22

89-92 2.20

85.01 7.46

Main Contribution

61.24 A Ti

70.06 68.91 B . A . Ti B . A . T 3 12.67 B.A. T2

58.83 B . A . 'Γ 2 20.31 B . A . 'Γ 4

47.75 B.A. 13.22 B.A.

Transition

Energy(cm ) -1

Y"

LE CT

A 4 >

Β

· ·

ι

Α

T4 T2

F i g u r e s a r e the squares o f the c o e f f i c i e n t s i n the s t a t e f u n c t i o n s e x p r e s s e d as a l i n e a r c o m b i n a t i o n o f the e l e c t r o n i c c o n f i g u r a t i o n s o f the MIM m o d e l . G: Ground c o n f i g u r a t i o n , L E : C o n f i g u r a t i o n o f the l o c a l i z e d e x c i t a t i o n , CT: Charge t r a n s f e r c o n f i g u r a t i o n

TSUDA A N D OIKAWA

28.


conclude that the v i b r a t i o n a l structure of the S -)Ti absorption arises from azide group(Table 2). 0

Emission Spectra. With the phosphoroscope, e t h y l p-azidobenzo ate i n EPA at 77°K,gave an emission spectrum which i s shown i n F i g . 6 . The spectrum i s that of the decomposed product, which i s estimated to be e t h y l p-aminobenzoate i n comparison with the phospho rescence of the authentic sample(Fig.6).


P-Azidocinnamate T r a n s i t i o n Energies and Spectrum. P o l y ( v i n y l p-azidocinnamate) was prepared and investigated on the photochemical reactions by the present author(3). Since i t has two photosensitive chromophores, namely azide and cinnamoyloxy group, the characters i n the excited states are i n t e r e s t i n g i n connection with the photochemical reactions. The absorption spectrum i s the same as that of e t h y l p-azidocinnamate. The PPP c a l c u l a t i o n reproduces w e l l the UV spectrum as was shown i n F i g . 7 . The M I M c a l c u l a t i o n reveals that the lower excited states are those of cinnamate(Table 3) and the considerations discussed i n the preceeding paper( 6 ) can be adopted i n the same manner i n t h i s case. Therefore, the Si state a r i s e from the B2u state of benzene and the S2 state has a CT character(or d e l o c a l i z e d character). These e l e c t r o n i c states i l l u s t r a t e w e l l the c h a r a c t e r i s t i c s o f the photochemical reactions of p o l y ( v i n y l p-azidocinnamate)(3), where the c e n t r a l double bonds react i n the same way as cinnamoyloxy group. 1

T r i p l e t States. The measurement of the S ^Ti t r a n s i t i o n energy was succeeded i n the case of e t h y l p-azidocinnamate, t o o ( F i g . 5 ( b ) ) . The lowest t r i p l e t state energy calculated by the PPP method i s 1 8 7 5 5 cm" i n accordance with the observed value. The MIM c a l c u l a t i o n s show that the main c o n t r i b u t i o n to Ti i s the LE of cinnamoyloxy group ( 6 9 io) and the energy difference between Ti and T2 i s only 3200 cm" . We can understand from the small difference that the only one v i b r a t i o n a l structure appears i n Fig.5(b) and can also estimate that the v i b r a t i o n a l structure a r i s e s from the c e n t r a l double bond of cinna moyloxy group and not from azide. The T2 state has the same character as the Ti state of phenylazide where the c o n t r i b u t i o n of the LE func t i o n of azide group i s remarkable(55.02 fo i n t h i s case). The s p e c t r a l s e n s i t i z a t i o n by ervthrosine(Tl=14700 cm ) was suc ceeded i n p o l y ( v i n y l p-azidocinnamate)(3)· The phenomena of the spec t r a l s e n s i t i z a t i o n of azide compounds by the s e n s i t i z e r s having the lower Ti state energy than azides themselves, therefore,has confirmed f o r the f i r s t time i n t h i s paper although Lewis et a l . had speculated the same t h i n g on the u n r e l i a b l e d a t a ( 4 ) . The r e a c t i o n mechanism may contain the exciplex formations, of which the t h e o r e t i c a l i n v e s t i g a tions are now i n progress. The observation i n the erythrosine-sensit i z e d photochemical r e a c t i o n of the c e n t r a l double bond of p-azidocin namate i s i n t e r e s t i n g i n connection with the p o s s i b i l i t y of the e x c i plex formation i n the photodimerization o f poly ( v i n y l cinnamate) spec0

1

1

-1



432

T r a n s i t i o n Energy

( χ 10^

POLYMERS

1

cm* )

Figure 5. S -» T absorption spectra of aromatic azides 50 (vol) % EtI solutions at room temperature. The light path length is 5 cm. a, ethyl p-azidobenzoate; b, ethyl p-azidocinnamate. 0

t

Figure 6. Phosphorescence spectra of ethyl p-aminobenzoate ( ) and the photodecomposed product of ethyl pazidobenzoate (—) at 77°K in EPA

28.


TSUDA A N D OIKAWA

Table 3· The Contributions of Various E l e c t r o n i c Configurations of the MIM model f o r the State Functions of the PPP model of p-Azidocinnamoyloxy Group = Cinnamoyloxy Group(C. A.)+Azide(A) S

T r a n s i t i o n Energy( cm" )

33151

33708

42508

43716

46988

91.94 0.19 0.05 2,47 5.11

0.03 87.73 0.31 2.30 3.08

0.07 73.24 7.57 6.16 6.88

0.00 76.19 4.52 7.10 5.43

0.05 58.00 19.92 11.95 3.55

0.06 85.20 3.14 1.27 3.03

Total

99.76

93.45

93.92

93.24

93.47

92.70

LE+G CT

92.18 7.58

88.07 5.38

80.88 13.04

80.71 12.53

77.97 15.50

88.40 4.30

S

0

1

G LE

°' · A A* C.A. Α

pm C


s

S1

Singlet State

C A M

T

2

s

3

4

s

5

79.20 58.63 23.54 82.23 63.97 C.A. Si C.A. S2 C.A. S 3 C.A. S 3 C.A. S4 19.64 A S1

Main Contribution

T1

T

18755

22201

29102

30270

32136

69.07 16.93 4.23 3.59

31.07 55.02 6.30 2.06

83.76 8.11 0.26 1.04

90.45 0.08 2.71 0.23

85.81 3.46 0.29 3.83

Total

93.82

94.45

93.17

93.47

93.39

LE CT

86.00 7.82

86.09 8.36

91.87 1.30

90.53 2.94

89.27 4.12

T r i p l e t State 1

T r a n s i t i o n Energy(cm~ ) LE

C

:

A

-

A A->C.A. C.A.*A

Main Contribution

2

66.96 54.93 C.A. TiA Ti

T

3

T

4

T

5

61.86 80.26 81.51 C.A. T2 C.A. T3 C.A. T4

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G:Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge transfer configuration.


434

POLYMERS


S2

Transition

Energy

( x lO^cnT ) 1

Figure 7. Comparison of the calculated values with the experimental ones on the electronic transition energies and oscillator strengths of the p-azidocinnamate (ethyl p-azidocinnamate in n-hexane)

Table 4. Parameters Used i n the MO

2

Ν : t r t r t r l T (>N-) N+: d i d i TV T\ (-N=) Ν : d i d i l \ t f (=N ) 2

2

3CC!

Calculations

Ir

(rr| r r )

Z

14.12

12.34

3.9

28.88 14.18

14.30

4.25

2.34 (phenylazide) 2.25 (p-azidobenzoate) 2.31 (p-azidocinnamate)

12,52

r

3.9

(eV unit)

28.


TSUDA A N D OIKAWA

ulated i n the preceeding paper(6 ). 2,4,6-Tribromophenylazide ( TBA) The absorption spectrum of TBA i n Etl(0.426 mol/l) has only t a i l near 22000 cm . I t s phosphorescence spectrum i n EPA(0.028 mol/l) was measured at 77°K. These two spectra are shown i n F i g . 8(a). The agreement of the phosphorescence ( a f t e r corrected) and the S->T<j ab sorption spectrum i s s a t i s f a c t o r y . The i n t e n s i t y of the phosphorescence of TBA at 22200 cm" de creased along with the i r r a d i a t i o n at 34500 cm" l i g h t according to the f i r s t order r e a c t i o n r a t e ( F i g . 9 ) . I t i s w e l l known that phenyl azide photochemically decomposes i n the f i r s t order r e a c t i o n r a t e ( 3 ) . The EPA matrix of TBA colored i n y e l l o w i s h orange by the i r r a d i a t i o n of l i g h t showing the formation of an azo compound and a new phospho rescence peak arose a t 18200 cm" (corrected), the i n t e n s i t y of which increased along the p h o t o i r r a d i a t i o n . I t s e x c i t a t i o n spectrum has a peak at 25600 cm" (corrected). From these f a c t s we can b e l i e v e that TBA emitts the phosphorescence having a peak a t 22200 cm" . Since (7t-*]l*) t r a n s i t i o n s of TBA are at 45400 cm" ( £ (molar e x t i n c t i o n co e f f i c i e n t ) :7.26X 1 0 + ) and 40800 cm" ( £ :1.9Χΐθ4), and (n-yrt*) t r a n s i t i o n i s a t 34500 cm" (£ :1.9Χ1θ3), the phosphorescence(22200 cm- ) i s considered to a r i s e from (lt-»Tt*)3 t r a n s i t i o n . The e x c i t a t i o n spectrum(after corrected) of TBA has a shoulder near 34500 cm-1 which coincides with (n->Tt*) t r a n s i t i o n (Fig.8$. As was shown by a few authors(3>9), the quantum y i e l d s of photocomposi t i o n of aromatic azide by the l i g h t of (n-jKR.*) t r a n s i t i o n were lower than those by that of (ΐΙ-Πΐ.*) t r a n s i t i o n . The f a c t means that (n-yfp-) t r a n s i t i o n should have more emissive property than (lt-»TL*) t r a n s i t i o n provided we assume other energy t r a n s f e r processes are comparable i n e i t h e r case. The spectrum i n F i g . 8 f c j u s t i f i e s the considerations. The phosphorescence newly a r i s e n at 18200 cm" i s estimated to be that of 2,4,6,2',4',6'-hexabromoazobenzene, because the phosphorescence and the e x c i t a t i o n spectrum of authentic 2,4,6-tribromoaniline are i n the higher energy regions than those of the decomposed product(Fig.lO) -1

0

1


1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

1

Experimental Substances: TBA: R e c r y s t a l l i z e d 2,4,6-tribromoaniline(8.5 g) was dissolved i n 50 fo H 2 S O 4 O 0 O ml) at 70-80°C, then cooled to 5°C. D i a z o t i z a t i o n was c a r r i e d out by the a d d i t i o n of NaN02(2.125 g i n 20 ml of water) at 5°C i n the dark room, then converted to TBA by the drop ping of NaN (2.8 g i n 14 ml of water) f o r 30 min. R e c r y s t a l l i z e d from methanol-water. Calc. C:20.25 N:11.81 Br:67.37 Exp. C:20.44 N:12.20 Br:67.48; IR:N3 2140 cm" E t h y l p-azidobenzoate and e t h y l p-azidocinnamate were synthesized from p-amino d e r i v a t i v e s i n a s i m i l a r manner. 3

1

P u r i f i c a t i o n . S i l i c a gel(100 mesh) was p i l e d up to 10 cm i n the glass tube(2 cm diameter) f o r column chromatography and adjusted by


U

!·5

CD Ο Ο

-Ρ

1.0

Χ

W U «5 rH Ο


*

0.5

2^

22

20 1

(XlO^cm" ) T r a n s i t i o n Energy Figure 8a. S -Ti absorption and phosphorescence spec tra of 2,4,6-tnbromophenylazide. 1, S -T absorption of 0.426 mol/l. EtI solution at 20°C. 2, phosphorescence in EPA at 77°K, corrected according to the method of ref. (8). 0

0

Transition

t

Energy

Figure 8b. (η — π*) absorption spectrum and excitation spec trum for the phosphorescence of 2,4,6-tribromophenylazide. 1, (η - π*) (30,000-35,000 cm' ) and tail of (V - π*) absorption spectrum in n-hexane at 20°C. 2, excitation spectrum for the curve 2 in Figure 8a, corrected according to the method of ref. (8). 1

1

1

1


TSUDA AND

Tnplet

OIKAWA

I

States of Photosensitive Aromatic Azides

ι

0.0

ι

0.5

I

1.0

R e a c t i o n Time

(min.)

Figure 9. The decrease (curve 1) of the intensity of phospho rescence of 2,4,6-tribromophenylazide along with the irradiation of 34,500 cm" light following the first order reaction rate (curve 2). C means initial concentration att = 0. 1

0

1 ι—'

25

ι

ι

ι

ι

t

ι

ι

1

1

20 T r a n s i t i o n Energy

1—I

15 1

(XlO^cm" )

Figure 10. Phosphorescence spectra of 2,4,6-tribromoaniline ( the photochemical product of 2,4,6-tribromophenylazide (—) at in EPA

) and 77°K

UV

438

L I G H T INDUCED REACTIONS

IN POLYMERS

flowing 50 ml of n-hexane. Ethyl p-azidobenzoate(6g) i n spectrograde n-hexane(50 ml) and then more 50 ml of n-hexane as the eluent were made to flow through the column. The eluate was held at dry i c e tem perature overnight and the c r y s t a l l i z e d product was c o l l e c t e d . The procedure was repeated three times i n the present case. TBA and ethyl p-azidocinnamate were p u r i f i e d i n a s i m i l a r manner. S -»T1 Absorption. Ethyl iodide was d i s t i l l e d i n dark room and passed through the alumina column immediately before the spectrometry. The most favorable r a t i o of sample and solvent was 1:1(in volume). The quality of the spectrum was s i m i l a r i n either case of 5 and 10 cm light-paths.


n

Spectrometry. The absorption and phosphorescence spectra were measured i n the same manners as i n the preceeding paper( 6 ) . Theoretical The PPP and MIM calculations are performed i n the same manners as i n the preceeding paper( 6 ) . The parameters used are the same ex cept those l i s t e d i n Table ΤΓ The calculations are c a r r i e d out by HITAC 8700/8800 computers of The University of Tokyo.

Literature Cited 1) Kosar,K.Light Sensitive Systems",Wiley,New York (1965) 2) Merrill,S.H. and Unruh,C.C., J.Appl.Polymer Sci.,(1963),7, 273; 2a) Tsuda,M.,Yabe,A. and Takanashi,H., Bull.Soc.Photo.Sci.Tech.Japan, (1974), 23, 12 3) Yabe,A,Tsuda,M.,Honda,K. and Tanaka,H., J.Polymer Sci., (1972), (A-1),10,2379 4) Lewis,F.D. and Saunders,W.Η., J.Am.Chem.Soc., (1968), 90, 7033 5) Kasha,M. and McGlynn,S.P., Ann.Rev.Phys.Chem.,(1956), 7, 403 6) See the preceding paper by Tsuda,M. and Oikawa,S. 7) Herkstroeter,W.G. and Hammond,G.S., J.Am.Chem.Soc.,(1966),88,4769 8) Tsuda,M.,0ikawa,S. and Miyake,R., J.Soc.Phot.Sci.Japan(Nippon Shashingakkai-shi), (1972),35,90 9) Reiser,A. and Marley,R., Trans.Faraday Soc., (1968), 64, 1806 10) Tsuda,M., High polymer,Japan, (1970),19,109 11) Hasegawa,K. and Sano,R., Photo.Sci.Eng., (1971 ),15,309

28.

TSUDA A N D OIKAWA


Appendix 1. The Contributions of Various E l e c t r o n i c Configura tions of the MIM model f o r the State Functions of the PPP model of Styrene = Benzene(B) + Ethylene(E) Singlet State

so

S1

S2

S3

S4

S5

35800 0.00

41000 0.62

46200 0.14

50300 1.09

55900 0.62

90.56 0.01 0.00 4.54 4.54

0.04 83.52 0.00 4.40 4.40

0.00 35.28 21.60 18.97 18.97

0.00 71.35 16.21 2.35 2.35

0.00 71.44 2.81 9.06 9.06

0.00 69.54 16.12 3.60 3.60

Total

99.65

92.36

94.82

92.26

92.37

92.86

LE+G CT

90.57 9.08

83.56 8.80

56.88 37.94

87.56 4.70

74.25 18.12

85.66 7.20

Main Contribution

83.43 B u

21.60

54.53 Blu

62.76

62.39

T r i p l e t State

T1

T2

T

T

22700

31000

33300

34900

35800

30.46 42.71 10.81 10.81

70.16 21.03 0.43 0.43

83.69 0.51 4.16 4.16

76.08 15.17 0.84 0.84

83.55 0.00 4.43 4.43

Total

94.79

92.05

92.52

92.94

92.40

LE CT

73.17 21.62

91.19 0.86

84.20 8.32

91.25 1.69

83.55 8.85

Main Configuration

42.71 Ε

61.78 *1u

82.36 E1

75.30

83.46 B

-1

T r a n s i t i o n Energy( cm ) O s c i l l . Strength G


*

I

CT

B

C T

- »

E

E->B

1

I

«

3

1

2

T r a n s i t i o n Energy (cm"" ) LE

Ε

1

3

3

U

1

E1U

3

3

E

1

E1U

T

4

5

3

I

U

2 u

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G:Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT: Charge t r a n s f e r configuration Blu»B2u>^1u ^he e l e c t r o n i c states of benzene :


440

Appendix 2 . The Contributions of Various E l e c t r o n i c Configura tions of the MIM model f o r the State Functions of the PPP model of p-Azidobenzoyloxy Group = Carboxyl Group(COO)+Phenylazide(PHA) Singlet State

S

92.13 0.67 0.13 5.84 1.02

0.05 88.55 0.02 4.68 0.40

0.84 82.98 1.06 7.29 1.37

0.01 85.60 1.98 5.28 0.18

0.10 84.91 2.45 4.88 1.42

1.03 62.41 13.56 11.34 5.20

Total

99.79

93.70

93.54

93.05

93.76

93.54

LE+G CT

92.93 6.86

88.62 5.08

84.88 8.66

87.59 5.46

87.46 6.30

77.00 16.54

86.56 PHA S1

78.50 PHA S2

78.03 PHA S3

75.92 PHA S

G PHA COO PHA-) COO C00-> PHA

LE p T


s

Si

Main Contribution

T r i p l e t State

S

0

T-j

T

s

2

3

s

4

4

5

34.38 PHA S5

2

91.23 0..19 1.29 0.26

85.36 2.22 5.32 1.15

85.21 0.68 7.29 0.33

88.03 1.88 2.82 0.65

91.70 0.04 1.52 0.59

Total

92.97

94.05

93.51

93.38

93.85

LE CT

91.42 1.55

87.58 6.47

85.89 7.62

89.91 3.47

91.74 2.11

Main Contribution

90.79 PHA Ti

81.36 PHA T

62.98 PHA T3

56.10 PHA T

^ r T

COO PHA-iCOO C00-4FHA

2

4

43.07 PHA T5 26.48 PHA T 4

Figures are the squares of the c o e f f i c i e n t s i n thestate functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G :Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge t r a n s f e r configuration

28.

THplet States of Photosensitive Aromatic Azides 441

TSUDA A N D OIKAWA

Appendix 3. The Contributions of Various E l e c t r o n i c Configura tions of the MIM model f o r the State Functions of the PPP model of p-Azidobenzoyloxy Group = Azide(A)+Benzene(B)+Carboxyl Group(C0(J

Singlet State G Β COO A A4 Β A4 COO B4 COO COO* Β C004 A B4 A

LE


U i

S

s

0

s

1

2

S

84.77

0.00

0.96

0.96

77.63

40.77

0.12 0.04 4.62 0.07

0.02 0.08 3.47 0.00 4.34

0.96 16.36

0.36

5.33 0.94

87.37

86.24

59.05

29.58

75.44 11.26

72.86 14.50

57.42 28.82

24.79 Blu

35-96 B1U

37.23 E1U

19.05 E1

87.69

LE+G CT

85.89

77.73 9.97 75.89 B u

1

76.75 0.62 0.17 0.60 0.01 6.71 0.30 0.01 2.30

79.13

0.17 0.05 8.67

60.86 2.04 18.20 1.06 0.16 4.87 1.06 0.01 0.25

Total

88.52

88.51

LE CT

73.74 14.77

81.10 7.41

Γ Τ

Main Contribution

T

16.11 0.17 57.46 4.79 0.16

1

T

Β COO A A4 Β A-> COO B-> COO C004 Β C004 A B4 A

T1

1

1.33

T3

LE

0.93

57.46 A

0.36

86.69

98.99

T r i p l e t State

12.40 4.42 1.14

88.64

Total

2

5.23

0.01 4.38

2.37 38.23

0.00 3.55

0.00 1.80

1

2.65 0.28 4.57 0.14 0.03

0.31 66.62 2.25 3.68

5

3.59

0.01 2.14

Main Contribution

10.94

S

4

9.50 5.03 0.02 12.77

1.13 0.13 12.73

13.11

0.00 62.67 1.83

9.08 0.80 5.71

S

3

2

5

0.59

0.53

0.01 0.35

0.00 0.21

87.45

86.81

87.86

77.54

81.70 5.11

78.73

9.93

58.29 3

U

T

4

77.82 0.04 0.87 6.97 0.01 1.41

58.20 I

U

1.73 0.84 1.51 0.21 2.44

3

B

1

h u

63.52 3 e

1U

9.13

57.52 B

3

2 u

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G:Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge t r a n s f e r configuration Blu'B >Ei :The e l e c t r o n i c states of benzene 2u

u


442

POLYMERS

Appendix 4 . The Con t r i b u t i o n s of Various E l e c t r o n i c Configura tions of the MIM model f o r the State Functions of the PPP model of Benzoyloxy Group = Benzene(B ) + Carboxyl Group(COO) Singlet State

•S1

S2

S3

S

T r a n s i t i o n Energy( cm" ) O s c i l l . Strength

35400 0.017

43800 0.150

48400 0.565

50500 1.107

92.38 0.56 0.06 5.72 1.09

0.09 87.86 0.08 5.37 0.51

0.69 77.18 0.73 2.04

0.63 66.64 16.19 4.93 5.25

0.12 85.14 1.56 6.06 0.94

Total

99.81

93.91

94.57

93.64

93.82

94.10

LE+G CT

93.00 6.81

88.03 5.88

78.60 15.97

83.46 10.18

86.82 7.00

76.95 17.15

86.04 B u

71.04 Blu

56.26

77.56 Elu

26.17 Elu 24.62 COO

Τι

T

T3

T4

T5

So 1

G L E

Γ Τ


υ

COO B4 COO C00-» Β

Main Contribution

1

2

T r i p l e t State

13.93

1

1

2

S5

4

57100 0.516 0.00 52.33

24.62 3.49 13.66

1

T r a n s i t i o n Energy ( cm"" ^ ) 27100

31700

33000

36200

37400

83.50 3.04 6.61 1.66

85.30 1.46 0.47

88.07 1.39 3.71 0.72

77.02 12.88 1.80 2.71

24.83 62.42 0.65 7.13

Total

94.81

93.55

93.89

94.41

95.03

LE CT

86.54 8.27

86.76 6.79

89.46 4.43

89.90 4.51

87.25 7.78

Main Contribution

79.73 Blu

59.65

84.50 ?E

58.59 B u

62.42 3C00

L E

COO B->C00 C00-4B

3

6.32

3E

1 U

1 u

5

2

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G.Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge t r a n s f e r configuration

98.57

81.31 17.26

L.B4R.B E ->R.B

Total

LE+G CT

L.B4 Ε

Main Contribution

U i

81.29 0.01 0.00 0.01 4.17 0.29 4.17 4.17 0.29 4.17

G R.B LE Ε L.B R.B4 Ε R.B4L.B

1

T r a n s i t i o n Energy ( cm" ) O s c i l l . Strength

20.04 Ε

44.68 42.14

86.82

9.33 2.41 9.33 9.33 2.41 9.33

12.32

20.04

12.32

0.00

73.61 9.60

83.21

0.05 36.78 0.00 36.78 2.14 0.52 2.14 2.14 0.52 2.14

U

68.26 16.52

84.78

4.09 0.08 4.09 4.09 0.08 4.09

34.13

0.00

34.13

0.00

0.00

43600

4

U

78.78 4.52

83-30

0.00 33.56 11.16 33.56 0.87 0.52 0.87 0.87 0.52 0.87

0.13

44400

5

S i n g l e t State s s

U

55.88 27.90

83-78

0.00 27.94 0.00 27.94 5.94 2.07 5.94 5.94 2.07 5.94

0.00

46800

S6 8

10.23 10.23 5.31 5.31

U

58.16 25.22

83.38

C T

17.31 59.32

76.63

9.20 10.23

1.99 5.31

10.23

9.20

13.91 1.70 0.00 1.70

0.00

49700

S

0.00 29.08 0.00 29.08 5.31 1.99

1.35

48400

36.66x2 36.73x2 33.52x2 29.27x2 25.35x2 23.86x2 B2u B2u B1 B1 E1 E1

73.42 9.80

83.22

0.00 36.71 0.00 36.71 2.18 0.54 2.18 2.18 0.54 2.18

0.00

3

0.00

s

1.49

2

34700

S

34600

1

34000

S 9

U

1 0

U

72.03 12.26

84.29

2.66 0.81 2.66 2.66 0.81 2.66

31.29

9.45

31.29

0.00

0.83

55300

s

39.38x2 25.61x2 E1 E1

79.80 3.02

82.82

0.00 39.90 0.00 39.90 0.71 0.09 0.71 0.71 0.09 0.71

0.00

53800

s

9-10 76.22

85.32

0.00 4.55 0.00 4.55 18.83 0.45 18.83 18.83 0.45 18.83

0.00

55900

sn

Appendix 5. The Contributions of Various E l e c t r o n i c Configurations of the MIM model f o r the State Func t i o n s of the MIM model f o r the State Functions of the PPP model of Stilbene = Benzene(L.B) + Ethylene(E) + Benzene (R. Β )


11.10 37.20 11.10 6.54 1.14 6.54 6.54 1.14 6.54 87.84 59.40 28.44 37.20 Ε

Total

LE CT

Main Contribution

82.92 1.30

76.60 7.74

1.92

74.15 9.18

83.33

36.97 0.21 36.97 2.05 0.49 2.05 2.05 0.49 2.05

32100

T5

78.24 4.56

u

82.48 1.86

84.34

33.72 15.04 33.72 0.39 0.15 0.39 0.39 0.15 0.39

39.12 0.00 39.12 1.11 0.06 1.11 1.11 0.06 1.11 82.80

34400

T7

32800

T6

u

83.22 73.58 9.64

73.42 9.80

36.79 0.00 36.79 2.15 0.52 2.15 2.15 0.52 2.15

34700

T9

83.22

36.71 0.00 36.71 2.18 0.54 2.18 2.18 0.54 2.18

34600

T8

U

U

U

2 u

2 u

37.30x2 29.60x2 36.62x2 36.35x2 37.46x2 32.20x2 36.66x2 36.753 Blu B1 E1 E1 E i E i B B

0.32

0.03

83.32 9.42

73.90

81.62

0.32 0.01

1.92 1.92

0.03

1.92

84.34

32100

T4

T r i p l e t State

36.95 0.00 36.95 2.11 0.49 2.11 2.11 0.49 2.11

30200

T3

34.79 12.04 34.79 0.32 0.01 0.32

38.30 0.00 38.30

27200

T2

u

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G : Ground configuration, LE:Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge t r a n s f e r configuration Blu-B2u>Ei :The e l e c t r o n i c state of benzene

LE

R.B Ε L.B R.B4 Ε R.B^L.B Ε *L.B L.B-> Ε L.B^R.B E ->R.B

T r a n s i t i o n Energy ( cm"' )

Ti


28.


TSUDA A N D OIKAWA

Appendix 6. The Contributions of Various E l e c t r o n i c Configura tions of the MIM model f o r the State Functions of the PPP model of A n i l i n e = Benzene(B) + Amino Group(N) S i n g l e t State

S1

S2

S3

S

34300 0.000 0.047

43500 0.081 0.000

50500 0.000 0.925

51000 1.167 0.000

86.65 0.75 12.15

0.00 81.60 7.61

3.46 76.21 8.90

0.00 85.56 5.30

0.56 84.81 2.84

99.54

89.21

88.56

90.86

88.22

So 1

T r a n s i t i o n Energy( cm" ) O s c i l l . Strength X Y G Β N4B


LE CT

Total

78.69

Main Contribution

1B2U

73.07 1

BI

T r i p l e t State

LE CT

Β N4B

Total Main Contribution

E 1

τ

T2

T r a n s i t i o n Energy(cm"^ ) D i r e c t i o n of T r a n s i t i o n

78.02 1

U

U

4

80.18 B

1

E 1

T

3

32200

38100 Y.

78.57 9.84

84.33 4.38

90.66 0.35

91.70

88.41

88.71

91.01

74.35 B1

49.62

79.06

29200 Y

81.94 9.76

3

U

3E

1 U

A

4

X

X

26400

U

53.94 5 b

B

2U

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G : Ground configuration, LE: Configuration of the l o c a l i z e d e x c i t a t i o n , CT:Charge transfer configuration E i a n d E ^ mean the A and Β representation, r e s p e c t i v e l y , i n the C2v point group. J \ Y (short axis) A

u

B

U

X (

l o n g

^ s )

29 Electronic Structures in the Excited States of the Photosensitive Groups of Photocrosslinkable Polymers. (I) Polyvinyl cinnamate


MINORU TSUDA and SETSUKO OIKAWA Laboratory of Physical Chemistry, Faculty of the Pharmaceutical Chiba University, Chiba 280, Japan Abstract : The procedure for SCF MO CI calculation(Pariser-Parr-Pople method) was revised by introducing a new formula for the two center Coulomb repulsion integrals which reproduces well the triplet states of benzene. The application of the revised method to the calculation of the electronic states of cinnamoyloxy group was found to reproduce well the experimental values of its electronic transitions although other authors had failed in the calculations using the usual methods. On the basis of the obtained state functions, which were also rewrit ten in the linear combinations of the electronic configurations of MIM(molecule in molecule) method, the photochemical behaviours of cin namoyloxy group was discussed in details. The theoretical possibili ty was also suggested that the singlet and triplet sensitizations by the low energy sensitizers in the mechanism of complex formation where the atomic configurations are changeable.

Since i t does not suffer dark reactions, poly(vinyl cinnamate) (PVCi) is one of the most excellent and practically important photo sensitive polymers and has been widely investigated(jj. Recently, it was found that a more highly photosensive polymer than PVCi was ob tained by the separation of cinnamoyloxy group through -CH2-CH2-Ofrom the main chain of the vinyl polymer(2). The fact means that by

the modification of the main chain of polymer, a part of cinnamoyloxy groups wastefully used for the intramolecular reaction in PVCi can be converted to those utilizing in the formation of the intermolecular crosslinks which contribute to the photosensitivity; and i t suggests 446

29.

TSUDA

Excited States of Photosensitive Groups

AND OIKAWA

447

t h e p o s s i b i l i t i e s o f the w i d e v a r i e t i e s o f t h e development o f p h o t o s e n s i t i v e polymers u t i l i z i n g cinnamoyloxy group i n f u t u r e . Few r e p o r t s ( 1 , 11 ) have been p u b l i s h e d on the e l e c t r o n i c s t a t e s o f cinnamoyloxy group a l t h o u g h t h e r e a r e many o f t h o s e on t h e p h o t o c h e m i c a l r e a c t i o n s (jl). I n t h i s p a p e r , t h e e l e c t r o n i c s t r u c t u r e s i n t h e e x c i t e d s t a t e s o f cinnamoyloxy group a r e c l a r i f i e d by t h e m o l e c u l a r o r b i t a l c a l c u l a t i o n s and t h e s p e c t r o s c o p i c p r o c e d u r e s , and d i s cussed i n c o n n e c t i o n w i t h t h e i n t e n s i t y o f l i g h t a b s o r p t i o n , p h o t o c h e m i c a l r e a c t i o n p r o c e s s e s and energy t r a n s f e r s i n t h e s e n s i t i z a t i o n w h i c h a r e i m p o r t a n t i n the comprehensive u n d e r s t a n d i n g o f t h e p h o t o s e n s i t i v e polymer.


Theoretical P a r i s e r - P a r r - P o p l e Method(PPP M e t h o d ) . The c a l c u l a t i o n s a r e f o l l o w e d o f t h e o r d i n a r y LCAO SCF MO* CI method w i t h T T - e l e c t r o n a p p r o x i m a t i o n (^^JO^) where m o l e c u l a r wave f u n c t i o n s , φ , o f e l e c t r o n i c c l o u d o n l y a r e w r i t t e n i n g e n e r a l as a sum o f c o n f i g u r a t i o n a l f u n c t i o n s , Y . y a r e e x p r e s s e d as a sum o f S l a t e r d e t e r m i n a n t s c o n s t r u c t e d f r o m MO 7J>1> w h i c h a r e g i v e n a s l i n e a r c o m b i n a t i o n s o f 2pHA0. The c o n s i d e r e d tp are r e s t r i c t e d t o those a r i s i n g from a l l t h e s i n g l y e x c i t e d c o n f i g u r a t i o n s and o t h e r s a r e n e g l e c t e d . I n t h e c a l c u l a t i o n s o f Fock m a t r i x e l e m e n t s , t h e z e r o d i f f e r e n t i a l o v e r l a p a p p r o x i m a t i o n i s a d o p t e d . Then we g e t f o r a c l o s e d s h e l l structure : s

n

n

n

F F

= -I +

r r

iP (rr| rr) + £ ( P

p

r r

r s =Prs

S

S

- Z ) (rrI ss)

(1 )

S

* lP (rr[ss)

(2)

r s

where r , s = 1 , 2 , . . . ,N (N i s t h e number o f AO c o n t r i b u t i n g T T - e l e c t r o n s ) ; P =2^ C' r i C ^ s i (vC r i i s t h e c o e f f i c i e n t o f the r t h AO i n the i t h M O ^ i ) ; I and Z a r e the i o n i z a t i o n p o t e n t i a l o f t h e r t h AO i n t h e v a l e n c e s t a t e and the e f f e c t i v e c o r e charge o f the s t h A O , r e s p e c t i v e ly. One c e n t e r Coulomb r e p u l s i o n i n t e g r a l s ( r r | r r ) = I - E ( E i s t h e e l e c t r o n a f f i n i t y o f t h e r t h AO i n the v a l e n c e s t a t e ) , t h e v a l u e s o f w h i c h adopted i n t h i s paper a r e c o l l e c t e d i n T a b l e 1 w i t h t h e v a l u e s I . Two c e n t e r Coulomb r e p u l s i o n i n t e g r a l a r e e v a l u a t e d by the f o r mula^) . r

u

s

p

i

e

d

r

i

r

s

u

i

r i

g

r

r

r

r

( r r j s s ) = -H(rr|rr)+(ss|ss)) -HÇ

r

+ C

r

^) 3

(3a)

r s

r s

Ρ = KSr+SsKs r

8

ο ( R £ 2 . 8 A, C i s a c o n s t a n t c a l c u l a t e d f r o m the f o r m u l a f o r R = 2 . 8 A ) (3b)

P

r

2

2

C ( r r j s s ) =•—-

ç = Z /n

(Rrs^ -

s)( ·752797P-0.5922504p +0.0809404p ) 1

(3c) ( n i s t h e p r i n c i p a l quantum number o f r t h AO; n =2inthe molecules discussed i n t h i s paper)(3d) r

r

448

UV


POLYMERS

The formula(3) was newly proposed i n t h i s paper and the c o e f f i c i e n t s were so determined as the spectrum of benzene molecules i n n-hexane solutions was exactly predicted i n the c a l c u l a t i o n s considering a l l s i n g l y excited e l e c t r o n i c configurations, The core resonance i n t e grals β are: Γ 3

rs)— Prs= o

r

S

(

0 1

r

911(1

s

axe

neighbors) (4a)

( r and s are non-neighbors)

(4b)

where the subscript c means that the r and s t h AO are 2pTtA0 of car bon. B i s treated as a f r e e parameter depending upon the s i z e of the molecule and the contributions of heteroatoms. S i s the over l a p i n t e g r a l s , the values of which are calculated from Roothaan's formulae based on the S l a t e r orbitals(22). The expression^ T i absorption in ethyl iodide and phosphorescence (in EPA at 77°K) spectra of trans-ethyl cinnamate. The emission intensity was corrected according to ref. 20. The intensity of emission is relative value. The calculated value only shows the transition energy. 0


456


formation readily occurs i f a nodal plane exist at the central double bond in the lowest unoccupied MO(LUMO) and not in the highest occupied MO(HOMO) of the ground state of cinnamoyloxy group within the picture of H u c k e l MO or Extended H u c k e l MO theory. (Pig. 5) In the SCF MO CI picture the bond order at the central double bond reflects the same meaning although the original considerations of the Woodward-Hoffmannfe rule are applicable i n the special case where only one excited elec tronic configuration contributes mainly to the lowest excited state. The state functions in the form of the linear combination of the elec tronic configurations and the MOs required i n the following discus sions are shown in Table 2. The spatial orbital part of the lowest triplet state T^ corresponds to that of the second excited singlet state S2(the main peak) and the bond orders at the central double bond in these two electronic states are smaller than that of the lowest ex cited singlet state Si.(Fig.4) Therefore, the concerted cycloaddition occurs favorably in thesestates. These states are also the special cases in which one singly excited electronic configuration from the HOMO to the LUMO contributes mainly to the state functions(see Table2) and the HOMO and LUMO are satisfy the conditions the Woodward-Hoff mann's rule for the concerted cycloaddition requires. From cyclization occurs more favourably in the triplet state than in the singlet state i f the photochemical reaction takes place in the lowest energy level of each multiplet state as in the usual way. The possibility seems to be supported by the fact that the spectral sensitization of PVCi i s practically very effective, but i s not conclusive because there are no exact data on the quantum yields in both states. Further discussions on this problem w i l l be given later with another possibil ity obtained from the theoretical considerations. Analyses of the Electronic Transitions of Poly(vinyl cinnamate). The concerted cycloaddition of cinnamoyloxy group i s essentially the reaction of the central double bond. Therefore, we can expect to ob tain the clearer image than that hitherto considered in the photochem i c a l reaction i f the contributions of the central double bond for the excited states are calculated quantitatively. It i s known that the state functions of PPP model can be rewritten to the linear combina tions of LE and CT(charge transfer) functions of MIM method(7 ) when we use the same atomic orbitals as the base functions (qj ). Using the technique, the state functions in Table 2 are rewritten and the re sults are collected in Table 3· The relationship between these two methods i s illustrated in Fig.1 (b), (c). The state function of So. The contributions of the ground state functions of phenyl group(equivalent to benzene i n the 71-electron ap proximation), -CH=CH- (equivalent to ethylene) and carboxyl group(equivalent to formic acid) are 83 $ and the CT functions which connect the LE functions i n order to form the molecule, those from the 0M0 of benzene to the UMO of ethylene, from ethylene(OMO) to carboxyl group (UMO), from ethvlene(OMO) to benzene(UMO) and from carboxyl group(OMO) to ethylene (UMOj are totally 16 When examining the contributions of

29.

TSUDA A N D OIKAWA


CO

Ο

OO OO .^(N Ο ΚΩ

W

OJ LfN

Ο


Ο

«1° oui ô ^ Ο

>^ CO CTN O O KMÎ' · v£) Ο

^Λ°^

0 0

458


symmetry plane

/

ROOC^

H/

Journal of the Society of Photographic Science of Japan


Figure 5a.

(top) A pair of cinnamoyloxy groups which has C symmetry 8

A", /

/

~ +

/

\

/\ / / / —

/

r /

I +

A'7

/

V

π

+

^

+ +

θ θ C-C Θ θ

Α'

\

θ θ \ c — c Θ A' \ θ Θ (Β c -— c θ θ

REACTION PATH

_

+_

rneans - ^ - ^

C-C θ θ

Figure 5b. (middle) Correlation diagram of orbitals and their symmetries

Figure 5c. (bottom) Correlation diagram for the cycloaddition reaction of a pair of cinnamoyloxy group. The reaction occurs following the adiabatic potential curve of the A" symmetry. The state ITVTT* is favorable for the concerted cycloadditions (see text) (23). 1

TSUDA

A N D ODKAWA


459

Table 3 . The Contributions of Various E l e c t r o n i c Configurations of the MIM model f o r the State Functions of the PPP model of trans-Cinnamoyloxy Group ($)


Singlet State

Total

98.78

84.76

86.71

85.44

85.35

83.94

LE + G CT

82.93 15.85

71.41 13.35

45.00 41.71

76.10 9.34

53.66 31.69

44.40 39.54

69.04 B u

21.10 Ε

54.92

38.68 Elu

21.03 Elu

*1 18.14 41.72 1.52

*2 66.18 12.95 2.21 0.32 0.06 2.57 0.88 0.02 0.12

3 70.73 0.40 0.14 11.34 1.34 0.10 0.05 0.02 0.62

4 70.81 4.16 4.58 2.16 0.14 1.34 1.12 0.04 0.75

5 78.17 0.00 0.01 1.73 0.22 0.00 0.00 0.12 4.87

Main Contribution

1

2

T r i p l e t State

Γ

B

Ε I coo ' B~>E B-»COO CT ' E-*COO COO->E COO-^B ,E->B

LE

13.31

1.73 5.88 1.25 0.20 5.75

1

Blu T

1

T

1

T

Total

89.50

85.31

84.74

85.10

85.12

LE CT

61.38 28.12

81.34 3.97

71.27 13.47

79.55 5.55

78.18 6.94

41.72 Ε

64.01

55.95 E

70.41 *Mu

59.93 %2u

Main Contribution

3

*1u

i u

3

Figures are the squares of the c o e f f i c i e n t s i n the state func t i o n s expressed as a l i n e a r combination of the e l e c t r o n i c con f i g u r a t i o n s of the MIM model. G : Ground configuration , B:Benzene, E:Ethylene, C00:Carboxyl group. B-| , B 2 and E mean the LE of benzene. u

U

1 u

460

UV


each of the CT configurations i n Table 3, we can f i n d out that e l e c trons favourably d i s t r i b u t e i n carboxyl group even i n the ground state r e f l e c t i n g the e l e c t r o n a g a t i v i t y of heteroatoms. The state f u n c t i o n of S i . The main c o n t r i b u t i o n to S^ i s the LE function of B2u state of benzene r i n g which occupies 69 The bonds order(Fig,4) of the c e n t r a l double bond i s s u b s t a n t i a l l y unchanged compared with that i n t h e ^ t a t e , because the c o n t r i b u t i o n of LE func t i o n of ethylene increases only 0.75 $ and that of the CT function from benzene(OMO) to ethylene(UMO) 4.5 compared with those i n the S state functions. Therefore, we cannot consider t h i s state to be highly photosensitive. 1


0

The state function of Sp. The LE f u n c t i o n of ethylene c o n t r i b utes mainly to S2(about 21 fo). The r e s u l t accords with the f a c t that the experimental data on the photochemical reaction of PVCi are ex plained on the bases of Woodward-Hoffmann's r u l e . CT configurations from the occupied Μθ(θΜθ) of the ground state of benzene to the unoc cupied MO(UMO) of ethylene and the OMO of ethylene to the UMO of ben zene increase 16.6 fo and 4.6 r e s p e c t i v e l y , compared with S .(Fig.6) These contributions decrease the bond order of the c e n t r a l double bond i n S2- I n S^ the contributions of the LE of ethylene and CT are small 0

Therefore, we can expect that the photochemical reaction occurs i n Sp and not m S , because our c a l c u l a t e d energy l e v e l s o f S| and Sâre those i n the Frank-Condon state and may exchange the p o s i t i o n s each other i n the photochemically r e a c t i v e s t a t e . In order to t e s t the p o s s i b i l i t y , t h e o r e t i c a l c a l c u l a t i o n s of the energy l e v e l of transcinnamoyloxy group with lengthening the c e n t r a l double bond are c a r r i e d out. The r e s u l t s are shown i n Fig.7. As was expected, the c o n t r i b u t i o n of the e l e c t r o n i c configuration from HOMO to LUMO i n creases according to the lengthening of the c e n t r a l double bond and the photochemical r e a c t i v i t y increases i n the lowest excited s t a t e . Of cource t h i s c a l c u l a t i o n i s only an approach to the p o s s i b i l i t y , a l though the small bond order i n So compared w i t h that i n S suggests the lengthing of the c e n t r a l double bond. I t i s i n t e r e s t i n g that the lowering of the t r a n s i t i o n energy f o l l o w i n g the change of the atomic configurations suggests the p o s s i b i l i t y of the s i n g l e t and t r i p l e t s e n s i t i zations by the low energy s e n s i t i z e r s inthemechanism ofcomplex formations f

Q

The state function of T i . From the r e s u l t s from PPP c a l c u l a t i o n s i n Table 2, we can f i n d out that the s p a t i a l part of the state func t i o n o f T-| approximately corresponds to that o f S2,and the contribu tion ^ Α ^ 7 and ^fojj shows the mixing of S-j. From the r e s u l t s from MIM methocUTable 3;, the c o n t r i b u t i o n of the LE of ethylene extremely increases i n compared with that i n S2. The f a c t suggests that T^ i s h i g h l y photosensitive on the basis of the concerted c y c l o a d d i t i o n f o l l o w i n g the Woodward-Hoffmann's r u l e . οί>(

Analysis o f the I n t e n s i t y o f the E l e c t r o n i c T r a n s i t i o n of PVCi. The i n t e n s i t i e s of the e l e c t r o n i c t r a n s i t i o n s of photosensitive poly mers are on intimate r e l a t i o n s with the s p e c t r a l s e n s i t i v i t i e s . Most

TSUDA A N D OIKAWA


(a)

-Θ-


-θ-θ- -oe-θ-©--

-θ-θ-

Figure 6. Decrease of the bond order of the central double bond by the contribution of CT functions a, because of the decrease of the electron occupy ing the HOMO of ethylene; b, because of the increase of the electron occupying the LUMO of ethylene.

Bulletin of the Chemical Society of Japan

Figure 8c. Polarized absorption spectrum of transcinnamic acid α-form through the (010) plane. —, the light polarized parallel to the a-axis; , the light polarized perpendicular to the a-axis.

POLYMERS



1.3 Length

l.k of Central

1-5 Double

1.6 Bond

ο (A)

Figure 7. The interchange of S, and S« levels of trans-cinnamoyloxy group with hngthening of the central double bond. The contribution of ψ + 7 is expressed as the filled part in circles. 6

29.

TSUDA A N D OIKAWA

Excited

States of Photosensitive

463

Groups

•s

f

8.

i


I .

•2 |

18 s s

I-3 4s o

&!

e i

e a.

r

a. 8

1£1 s O



POLYMERS

Figure 8b. Atomic configurations in the theoretical calculations of Figure 8a. The Cartesian coordinate was rotated round the Ζ axis.

Ο.Ο81

R o t a t i o n Angle

θ°

29.

TSUDA

AND OIKAWA


465


of the absorption spectra have the shapes s i m i l a r to those of the spectra of the s p e c t r a l s e n s i t i v i t i e s of the photosensitive poly mer s (l5). In t h i s section we s h a l l analyze the r e l a t i o n s h i p between the e l e c t r o n i c structures and the absorption i n t e n s i t i e s i n d e t a i l s . The i n t e n s i t y of the e l e c t r o n i c t r a n s i t i o n i s experimentally obtained as an area of the absorption spectrum displayed with the abscissa of the energy l i n e a r s c a l e ( f o r example cnH , eV,etc.) and the ordinate of the molar e x t i n c t i o n c o e f f i c i e n t s ; and the experimental value corresponds to the t h e o r e t i c a l l y obtained o s c i l l a t o r strength of the e l e c t r o n i c t r a n s i t i o n . The example i s shown i n Fig. 3 . The o s c i l l a t o r strength(f) of a t r a n s i t i o n between two e l e c t r o n i c states of a molecule i s given by f(S *S )= 0

n

(8lfcnc ^(sj/jhe ) G|M(S )| 2

2

(14a)

n

where M i s the t r a n s i t i o n dipole moment expressed as Eq.(l4b), m and e are the mass and charge of an electron, $ i s the frequency of the t r a n s i t i o n , and G i s the number of degenerate wave functions to which absorption can lead. The t r a n s i t i o n moment of an e l e c t r o n i c t r a n s i t i o n i n the dipole approximation, expressed i n terms of the wave func t i o n of the i n i t i a l and f i n a l states i s M = j

i

(14c)

ï

where C±^j i s the c o e f f i c i e n t of the e l e c t r o n i c configuration V^^-j i n Table 2 and the summation extends over a l l the s i n g l y excited e l e c tronic configurations. Therefore, the i n t e n s i t y of the noticed e l e c t r o n i c t r a n s i t i o n i s obtained as the square of the vector sum of the product of the c o e f f i c i e n t of the e l e c t r o n i c configuration which con structs the e l e c t r o n i c state function and the t r a n s i t i o n moment of the configuration. The values of the t r a n s i t i o n moments of the e l e c t r o n i c configurations required i n the f o l l o w i n g discussions are c o l l e c t e d i n Table 4. The absorption i n t e n s i t y of S i . From Table 2, we can f i n d out that the state function of S] contains ^5^7 and Ψβ*8 eqivalence as tne main contributions and the signs of the c o e f f i c i e n t s are oppo s i t e . On the other hand, ζ^ψ^^ΤΕ* e r ( ^ ^ and have i

p

n

G

the same signs i n each of X and Y d i r e c t i o n s and have the s i m i l a r ab solute values as was shown i n Tabled . Therefore, the absorption i n t e n s i t y of Sj may be expected to be very small, because of the cancel l a t i o n by the two moments i n the opposite d i r e c t i o n s . The exact c a l culations considering a l l the s i n g l y excited configurations revealed

466

UV


POLYMERS

that the t r a n s i t i o n moment and the o s c i l l a t o r strength are 0.2452 and 0.0223, r e s p e c t i v e l y : S± i s a very weak t r a n s i t i o n . The p r e d i c t i o n i s true i n the experiment.(exp. f=0.02) Through these discussions we can r e a l i z e that the excited state Si isnot expressedby such a s i n g l e e l e c t r o n i c configuration as was done i n the HMO or the Extended HMO theory. According to the MIM c a l c u l a t i o n s , the Si state i s mainly express ed as the LE f u n c t i o n of B2u t r a n s i t i o n of benzene.(Table 3) The B2u t r a n s i t i o n of benzene(250 nm) i s a forbidden t r a n s i t i o n and i t s o s c i l l a t o r strength vanishes i n the c a l c u l a t i o n s although the experimental value i s f=0.002. The t r a n s i t i o n i s considered to be a v i b r o n i c one having i n t e r v a l s of 925 cm-1 of the a) v i b r a t i o n of benzene r i n g with the s i n g l y e x c i t e d e 2 v i b r a t i o n ( B 2 x E 2 X A i g = E i ; E i i s an allowed t r a n s i t i o n ) . (16) The v i b r a t i o n a l structure i s also observed i n the spectrum of P W i . 1

1

g

1

g

u

g

u

u


#

The absorption i n t e n s i t y of Sp. The state f u n c t i o n of S2 are mainly occupied by^the c o n f i g u r a t i o n Y6j£l> with the minor mixing of and signs i n the Y d i r e c t i o n and the opposite i n the X d i r e c t i o n . The c o e f f i c i e n t s of ψ a n d ^6-*8> however, have the same sign. Therefore, the calculated value of d i r e c t i o n and l a r g e r i n the Y d i r e c t i o n . The d e t a i l i of the o s c i l l a t o r strength as the f u n c t i o n of the d i r e c t i o n s are shown i n Fig.8(a), e

er

h a v e

t h e

s a m e

e

e

i n

t h e

x

The state f u n c t i o n of S2 becomes a complicated one i n the MIM c a l c u l a t i o n s (Table 3), i n contrast with the s i n g l e f u n c t i o n Φβ-fl ^ the PPP method. On the other hand, the s t a t e f u n c t i o n of S-| i s ex pressed by a simple ^B2 f u n c t i o n i n the MIM method although i t be comes very complicated i n the PPP method. These features r e f l e c t the c h a r a c t e r i s t i c of the model adopted i n each of the two methods; MO i s constructed by a l l the AO of the molecule i n the PPP method and by the AO of each chromophore i n the MIM method.(see F i g . l ) From these d i s cussion we can r e a l i z e that S2 i s a e x c i t e d state spread a l l over the molecule( also see MO 6 and 7 i n Table 2), and S^j i s a e x c i t e d state l o c a l i z e d i n benzene r i n g ; and also r e a l i z e the importance of the se l e c t i o n of the t h e o r e t i c a l model i n the comprehensive understanding of the experimental phenomena. n

U

The absorption i n t e n s i t i e s of S^,S4 and S5. From Table 3 i t i s found that the main c o n t r i b u t i o n to S3 i s ^ l u of benzene r i n g and those to S4 and S5 a r e l E i of benzene. The separation of the c o n t r i butions of S3, S4 and S5 from the observed spectrum i s d i f f i c u l t on the absorption i n t e n s i t y , because the i n t e n s i t y of B i of benzene i s about 50 times stronger than 1B2 i n s p i t e of i t s forbidden character. 0 B I has the same progressions 925 cnH of a] g as those of ^B2 > and i s considered to be a v i b r o n i c t r a n s i t i o n with the v i b r a t i o n s e2g and P2gU7).(BluX E2g=Elu, Bl XB2g=A2u) The i n t e n s i t y i s f=0.1 e x p e r i mentally and f=0 t h e o r e t i c a l l y ; Therefore, the calculated i n t e n s i t y of S3 may become smaller than the experimental value. On the other hand, the PPP c a l c u l a t i o n gives the l a r g e r value on the i n t e n s i t y of Ê-j state compared with the experimental one; and the i n t e n s i t i e s of S4 and S5 may become l a r g e r i n the c a l c u l a t i o n s . Recently, i t was u

1

u

U

u

U

u

u

TSUDA AND

29.


OIKAWA

467

found i n our laboratory that the c a l c u l a t e d value of the i n t e n s i t y of 1E>| became close to the experimental one when the contributions of (tf->6*)1 t r a n s i t i o n s were considered i n the c a l c u l a t i o n s .


u

On the Absorption Spectrum of Cinnamic Acid C r y s t a l by the P o l a r i z e d l i g h t . The absorption spectra of cinnamic a c i d c r y s t a l s - f o r m ) by the p o l a r i z e d l i g h t were measured by T a n a k a ( ^ ) , which was shown i n P i g . 8 ( c ) . Since the molecular plane of trans-cinnamic a c i d i s nearly on the (010) plane i n d-form, we can expect to reproduce w e l l the spectra by the PPP c a l c u l a t i o n s . The r e s u l t i s shown i n F i g . ? ( a ) , from which the spectra of Fig.8(c) are considered to be those measured at the 45° r o t a t i o n . At the 45° r o t a t i o n Si has a l i t t l e and S3+S4 has much stronger absorption i n t e n s i t y than S2 i n the Y d i r e c t i o n which coresponds to that perpendicular to the a-axis. In the X d i r e c t i o n , Si and S3+S4 have weaker i n t e n s i t i e s than S2. The i n t e n s i t i e s of S>| and S2 are stronger i n the X d i r e c t i o n than those i n the Y d i r e c t i o n , and i n the case of S3+S4 the i n t e n s i t y i n the Y d i r e c t i o n i s stronger than that i n the Y d i r e c t i o n . These t h e o r e t i c a l r e s u l t s are i n accordance w i t h the experimental observation i n F i g . 8 ( c ) . The forbidden ^B>| and ^B2 t r a n s i t i o n s of benzene become allowed ones by s u b s t i t u t i n g -CH=CH-C00R group f o r hydrogen. Provided the substituent i s considered to be only an o r i g i n of the purterbation, cinnamoyloxy group can be treated as monosubstituted benzenes, f o r ex ample, a n i l i n e , phenol, e t c . . The assumption i s the same one that i s adopted i n the HMO c a l c u l a t i o n , where cinnamoyloxy group belongs to C2v point group. B 1 and B2u representations i n D6 point group r e duce to Ai and B^ i n C2v The t r a n s i t i o n s from the ground state to Ai and B^ states are allowed i n the d i r e c t i o n of the long a x i s of the molecule f o r the former and the short a x i s f o r the l a t t e r . Tanaka ex pected that the d i r e c t i o n of the Si t r a n s i t i o n should be short a x i s ( 4 ). The r e s u l t i n F i g . 8 ( a ) , however, shows that the Si t r a n s i t i o n has both of the components i n the X and Y d i r e c t i o n s at the 6 0 ° r o t a t i o n . The same phenomena i s also found i n long a x i s the S3 t r a n s i t i o n . In the cases of a n i l i n e , phenol, etc., of which It-electron systems exactly belong to C2v point group, the weak t r a n s i t i o n s a r i s i n g from B-j and B2 of ben zene are e x c l u s i v e l y separated i n the d i r e c t i o n of long- and shorta x i s , r e s p e c t i v e l y ( I B ). Therefore, we can consider that the phenomena a r i s e from the f a c t that TC-electron system of cinnamoyloxy group be longs to C and not to C2 symmetry. u

U

U

n

u

s

U

V

On the Spectra of the Sn -> T1 T r a n s i t i o n . The phosphorescence spectrum and S-*T1 absorption spectrum are shown i n F i g 3(b) with the calculated value. 0-0 Band i s about 20000 cm-' i n the both cases and the v i b r a t i o n a l structures also have "the same i n t e r v a l s of about 1500 cm~1. Therefore, the observed phosphorescence i s concluded to be the emission from Ti to S of e t h y l cinnamate and the S->T1 Frank-Condon state of T1 has the same energy l e v e l that 0

0

0

UV

468


POLYMERS

the longest l i v i n g phophorescent state, which may play an important r o l e i n the photochemical r e a c t i o n . Since the calculated value of the t r a n s i t i o n energy i n the PPP method i s that of the Prank-Condon state, these experimental f a c t s support the exactness of the discussions on the photochemical reaction i n the t r i p l e t state based on the t h e o r e t i c a l r e s u l t s by the PPP c a l c u l a t i o n s . As was shown i n Table 3, the c o n t r i b u t i o n of the LE function of ethylene f o r the state f u n c t i o n of T-j i s about 42 Therefore, we can consider that the v i b r a t i o n a l structures i n F i g . 3(b) a r i s e from the s t r e t c h i n g v i b r a t i o n of the c e n t r a l double bond of cinnamoyloxy group. The v i b r a t i o n a l structures of S-^S-| absorption, S^j->S f l u o r e s cence and S ->Ti absorption spectra of s t i l b e n e have the progressions of 1599 cm"', 1635 cm-1 and about 1500 cm~\ r e s p e c t i v e l y . McClure concluded that these progressions arose from the s t r e t c h i n g v i b r a t i o n of the c e n t r a l double bond of stilbene(l1 ). According to our r e s u l t s of the MIM c a l c u l a t i o n s , the LE function of ethylene i s the main con t r i b u t i o n to the state functions of S-j and Ti of s t i l b e n e O ? ). The c o r r e l a t i o n diagram among the state functions of benzene, ethylene, cinnamoyloxy group and s t i l b e n e i s shown i n Fig.l . I t i l l u s t r a t e s w e l l the s i m i l a r i t i e s the e l e c t r o n i c states of cinnamoyloxy group and stilbene 0

0


0

Charge D i s t r i b u t i o n s What i s the Intramolecular Charge Transfer Transition(CT B a n d ) ? . T a n a k a discussed that the strong absorption band at 36000 cm"" (S2) of cinnamoyloxy group i s regarded as an i n t r a molecular charge transfer band because the charge t r a n s f e r from the highest occupied o r b i t a l of the s t y r y l group to the vacant o r b i t a l of the carboxyl group w i l l take place quite e a s i l y . On the other hand, he considered that the 34000 cm-1 band(Si) i s d i r i v e d from t h e B 2 state of benzene i n consistent with our conclusion discussed above. The charge d i s t r i b u t i o n s i n the various e l e c t r o n i c state of c i n namoyloxy group are tabulated i n Table 5, f o l l o w i n g the r e s u l t s of the PPP c a l c u l a t i o n s . Although we can recognize the difference between the charge d i s t r i b u t i o n s of Si and S2, i t i s too small to d i s t i n g u i s h between the l o c a l i z e d excited state and the charge t r a n s f e r state. I f the consideration i s concentrated on the charge d i s t r i b u t i o n s , the electron t r a n s f e r from ethylene to carboxyl group i n S2 compared with Si should be rather recognized than that from s t y r y l to carboxyl group (Table.5). According to the r e s u l t of the MIM c a l c u l a t i o n s where c i n namoyloxy group i s divided i n t o s t y r y l and carboxyl, the contributions of the charge t r a n s f e r configuration from the highest occupied o r b i t a l of s t y r y l group to the vacant o r b i t a l of the carboxyl group(which i s only one i n the ΤΓ-eleetron approximation) are only 11.84 fo and 1.8 fo i n the state function of S2 and S^, r e s p e c t i v e l y . From these discus sions, i t seems that the change of the charge d i s t r i b u t i o n or the r e a l occurance of the electron t r a n s f e r from one chromophore to another f o l l o w i n g the t r a n s i t i o n i s independent of the r e a l meaning of the i n tramolecular charge t r a n s f e r t r a n s i t i o n . What i s the r e a l meaning of the intramolecular charge transfer t r a n s i t i o n , then? We should remember the resemblance between the 1

1

u

U

4

1

1B

1 U

Stilbene

U

B 1

N

Cinnamoyloxy Group

Singlet State

?

« ;

/ /

C00 0.27

COO^B

L.B->R.B R.B*L.B

2.41

4.82

86.82

8.02

E->B

B}E

9.33

37.23

1.30

C0O>E

R.B->E

9.33

37.32

21 .97

41.71

CT

E+L.B

9.33

42.14

5.94

Ε-ΧΣΟΟ

E->R.B

9.33

Figures are the squares of the c o e f f i c i e n t s of the LE and CT functions (the square of the matrix element of M of Eq.(8)), as the l i n e a r combinations of which the state functions are expressed. G: Ground States of the chromophores, E: Ethylene, B: Benzene, L.B: L e f t benzene of s t i l b e n e , R.B: Right benzene of s t i l b e n e , COO: Carboxyl group.

ο

•H

g

g ο

>> ft

ιΗ

Ο

H •H -P CO

rû

CD

Q)

Total

Table 6. The Resemblance of the Si State Function of S t i l b e n e and the S2 State Function of Cinnamoyloxy Group.



472


e l e c t r o n i c s t a t e s o f cinnamoyloxy group and s t i l b e n e . There a r e no d i s p l a c e m e n t s o f e l e c t r o n i n the m o l e c u l e i n any e l e c t r o n i c s t a t e s o f s t i l b e n e . From the c o r r e l a t i o n diagram i n F i g . l , the S2 s t a t e o f c i n namoyloxy group c o r r e s p o n d s t o the Si s t a t e o f s t i l b e n e . The c o m p o s i t i o n s o f the e l e c t r o n i c c o n f i g u r a t i o n s i n the MIM c a l c u l a t i o n s i n t h e s e two s t a t e s a r e c o m p a r a t i v e l y t a b u l a t e d i n T a b l e 6 . From t h i s T a b l e , w e c a n f i n d o u t the remarkable resemblance betweenthe two s t a t e s o f the d i f f e r e n t m o l e c u l e s . T h e c h a r a c t e r i s t i c i s t h a t the c o n t r i b u t i o n o f the charge t r a n s f e r c o n f i g u r a t i o n s i s e x t r a o r d i n a r y l a r g e . ( t o t a l 42$) I n the MIM method, the r o l e o f the CT c o n f i g u r a t i o n i s the c o m b i n a t i o n o f the chromophores, the components o f the m o l e c u l e . T h e r e f o r e , the l a r g e c o n t r i b u t i o n o f the CT c o n f i g u r a t i o n s means t h a t the e l e c t r o n i c s t a t e s p r e a d s a l l o v e r the m o l e c u l e , o r i s d e l o c a l i z e d . T h i s i s the r e a l meaning o f the i n t r a m o l e c u l a r charge t r a n s f e r s t a t e , t o w h i c h the i n t r a m o l e c u l a r CT band appears a t the t r a n s i t i o n o f the m o l e c u l e from t h e ground s t a t e . The d i r e c t i o n and the magnitude o f the t r a n s i t i o n moment a r e independent o f the e x i s t e n c e o f h e t e r o a t o m . The e q u i v a l e n t c o n t r i b u t i o n s o f the f o r w a r d and backward charge t r a n s f e r c o n f i g u r a t i o n i n s t i l b e n e a r e m o d i f i e d i n cinnamoyloxy group because o f the deep i o n i z a t i o n p o t e n t i a l and the l a r g e e l e c t r o n a f f i n i t y o f h e t e r o atoms. The c o n t r i b u t i o n o f t h e CT c o n f i g u t a t i o n f o r w a r d the h e t e r o atom becomes l a r g e r i n the l o w e r energy t r a n s i t i o n s and, on the o t h e r hand, t h a t backward the heteroatoms i n the h i g h e r energy t r a n s i t i o n s ; and t h i s i s t h e cause of the occurence o f t h e d i s p l a c e m e n t o f e l e c t r o n i n the m o l e c u l e . Then, how can we e x p l a i n the l a r g e d i s p l a c e m e n t o f electron i n Sj? From the r e s u l t s o f the MIM c a l c u l a t i o n s i n T a b l e 3 and Appendix 1 , we cannot f i n d out any r e a s o n why the charge d i s t r i b u t i o n i n c a r b o x y l group i s l a r g e i n S i . I t may be c o n c l u d e d t h a t we s h o u l d c o n s i d e r m u l t i - e l e c t r o n e x c i t e d CT c o n f i g u r a t i o n s i n the MIM c a l c u l a t i o n s i n o r d e r t o e x p r e s s the s t a t e o f cinnamoyloxy group i n t h e PPP method. Acknowledgement. The a u t h o r s aknowledge t h e C h e m i c a l S o c i e t y o f Japan and the S o c i e t y o f P h o t o g r a p h i c S c i e n c e and Technology o f Japan f o r the p e r m i s s i o n s o f the r e p r o d u c t i o n s o f F i g . 8 ( c ) and F i g . 5 ( a ) , ( b ) , ( c ) . The c a l c u l a t i o n s a r e c a r r i e d out by HITAC 8700/8800 computers o f The U n i v e r s i t y o f Tokyo.

Literature Cited 1) Minsk,L.M. et al., J.Appl.Polymer Sci.,(1959),2, 302, 309; Tsuda,M., J.Polymer Sci., (1963),B1,215, Makromol.Chem.,(1964),72, 174, 183, ibid, (1973),167, 183, J.Polymer Sci.,(1964),A2,2907, ibid,(T964),B2, 1143, Bull.Chem.Soc.Japan,(1969),42, 905, and many others. 2) Nakane,H.,GazôGizyutsu, (1974),5,(5),39 3) Jaffé,H.H. and Orchin,M.,"Theory and Applications of Ultraviolet Spectroscopy",206,Forth Ed.Wiley.,Ν.Υ.,(1966) 4) Tanaka,J.,Bull.Chem.Soc.Japan,(1963),36, 833 5) Tsuda,M and Oikawa,S.,The 28th Annual Meeting of Chemical Soc. Japan,Tokyo,Preprint I, 1Q30, 1Q31 and 1Q32 (1973)

29.

TSUDA AND OIKAWA



6) Tsuda,M.,J.Scientific Org.Chem.Japan(Yukigôseikagaku),(1972),30, 589, Nippon Shashin Gakkai-shi,(1969),32,82 7) Longnet-Higgins,H.C. and Murrell,J.N.,Proc.Phys.Soc.,(1955),A68,601 8) Parr,R.G.,"Quantum Theory of Molecular Electronic Structure", Benjamin,N.Y.,(1964) 9) Nakanura,Κ.,Bull.Chem.Soc.Japan.,(1967),40,1027 10) Pariser,R. and Parr.,R.G.,J.Chem.Phys.,(1953),21, 767 11) Fueno,T.et al.,Bull.Chem.Soc.Japan.,(1972),45,3294 12) Mataga N.and Nishimoto,K.,Z.Physik.Chem.(Frankfurt),(1957),13, 140 13) Woodward,R.B.and Hoffmann,R,,"The Conservation of Orbital Symme try",Verlay Chemie.GmbH.Weinheim,(1970) 14) Ref.331) in Ref 8) 15) Yabe,A,,Tsuda,M.,Honda,K.and Tanaka,H,,J.Polymer Sci.,(1972),(A-l)

10,2379

16) Herzberg,G.,"Electronic Spectra and Electronic Structure of Poly atomic Molecule",555,D.Van Nostrand,Princeton (1967) 17) Robinson,G.W. et al.,Annual Report Cal.Tech.(Chemistry),(1970),86 18) See the Appendix of the following paper by the authors in this book. 19) Dyck,R.H. and McClure,D.S.,J.Chem.Phys.,(1962),36,2326 20) Tsuda,M.and Oikawa,S. and Miyake,R.,J.Soc.Phot.Sci.Japan(Nippon Shashin Gakkaishi ),(1972),35, 90 21) Baba,Η.,Suzuki,S. and Takemura,T.,J.Chem.Phys.,(1969),50,2078 22) Roothaan,C.C.J.,J.Chem.Phys.,(1951),19,1445 23) Tsuda,M.,J.Soc.Phot.Sci.Japan(Nippon Shashin Gakkai-shi),(1969), 32,82

473

UV

474


POLYMERS

Appendix The Contributions of Various E l e c t r o n i c Configurations of the MIM model f o r the State Functions of the PPP model of Cinnamoyloxy Group = Styrene(s) + Carboxyl Group(COO) Singlet State

So

S1

S2

S3

S

91.82 0.31 0.06 6.36

0.03 90.36 0.02 1.92

1.14 77.74 0.77

1.23

0.12

1.68

0.04 88.72 0.07 3.54 0.50

0.15 85.30 0.30 6.62 0.46

0.12 57.80 17.30 2.91 15.44

Total

99.78

92.45

93.86

92.67

92.83

93.57

LE+G CT

92.19 7.59

90.41 2.04

79.65 14.21

88.83 3.84

85.75 7.08

75.22 18.35

T1

T2

T3

T

T5

83.30 1.72 8.62 1.65

86.63 2.43 2.73 0.95

90.01

Total

95.29

92.74

LE CT

85.02

89.06 3.68

G L E


υ ι

COO S -> COO COO S

T r i p l e t State

M

c'oo S -* COO COO s

10.27

12.53

4

4

S5

84.66 5.05 1.62

92.07

1.21

0.21

92.33

92.54

92.59

90.17 2.16

89.71 2.83

92.08 0.51

0.16 2.08 0.08

0.01 0.30

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G: Ground configuration, LE: Configuration of the l o c a l i z e d e x c i t a t i o n , CT: Charge t r a n s f e r configuration

29.


TSUDA A N D OIKAWA

475

Appendix The Contributions of Various E l e c t r o n i c Configurations of the MIM model f o r the State Functions of the PPP model of Cinnamoyloxy Group = Benzene(B) + A c r y l y l o x y Group(AC) So

S1

s

89.42 0.14 0.05 6.36 3.60

0.16 76.27 1.02 12.12' 2.02

1.31 22.90

Total

99.57

LE+G CT

89.61 9.96

S3

S

0.04 56.36 3.97

29.29 8.43

0.09 69.27 15.79 5.20 1.21

91.59

93.32

77.45 14.14

55.60 37.72

Ti

T2

19.69 52.32 16.01 6.41

Total LE+G CT

Singlet State G

Ic

L E

B+ AC AC-> Β


Γ Τ U i

T r i p l e t State LE u i

* AC AC->B

2

4

S5

5.30

2.89 25.24 30.60 17.85 13.51

91.56

91.96

90.09

85.15 6.41

60.37 31.59

58.75 31.36

T3

T

T5

71.48 19.34 0.41 0.15

76.67 0.71 13.61 0.67

76.51 11.67 2.46 0.83

84.4? 0.01 2.09 5.22

94.43

91.38

91.66

91.47

91.74

72.01 22.42

90.82 0.56

77.38 14.28

88.18

84.43 7.31

31.39

26.29

4

3.29

Figures are the squares of the c o e f f i c i e n t s i n the state functions expressed as a l i n e a r combination of the e l e c t r o n i c configurations of the MIM model. G: Ground configuration, LE: Configuration of the l o c a l i z e d e x c i t a t i o n , CT: Charge t r a n s f e r configuration

INDEX

Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0025.ix001

A Absorbed photon 247 Absorbers, ultraviolet 216, 341 Absorption apparent 87 characteristics of the blends 79 coefficient 60, 415 of the copolymers, carbonyl 279 distribution, U V 328 in ethyl iodide 455 intensities 465, 466 light 174 photosensitizer 138 spectra of aromatic azides, S -»Ti 432 changes 252 of cinnamoyloxy compond 453,463 of cinnamic acid crystal 467 of phenyl azide 425 of trans-cinnamic acid α-form ... 461 of 2,4,6-tribromophenylazide 436 0

So-^T!

438

Abstraction, hydrogen 212, 396 A B P (4-acryloxybenzophenone) 77 Acceptor(s) complexes, comonomer donor9 concentration, effect of 32 effect of different 25 in the photocationic crosslinking .... 26 Acenaphthylene homopolymer 194 Acetyl acetonates, transition metal .343,346 Acetyl radical 279 Acid-crosslinkable prepolymers 168 Acrylate(s) benzoin ether photoinitiated polymerization of 12 ester derivatives of conventional ink vehicles 171 ester derivatives of epoxy resins ... 172 Acrylic enamel, melamine crosslinked 408 monomers 93 paint 418 polymers 408 Acrylonitrile complexation of 3 photoinitiated copolymerization of 2,7 terpolymerization of 5,6 4-Acryloxybenzophenone(ABP) 77 Actinic deterioration of common plastics 318

Activation for photoinitiation, energy of 386 Activities, specific 16 Addition of hydrogen atoms, direct 396 to double bond, direct 396 vinylidene 351 Additive(s) on C==C loss yield of photosensitive polymers, effects of 49 effect of miscellaneous 30 flexibilizing 159 on photocrosslinking, effects of .28,47,48 system for photodegrading polymers 290 Adhesive, pressure sensitive 159, 160 Adhesives based on Uvimer 160 A I B N (α,α'-azobisisobutyronitrile) ...13,29 Air-irradiated PPiPA films 331 α-Alkoxy-a-phenyl acetophenones 12 Alkyl peroxides, primary photodisso ciation of 266 Alkylbenzenes 64,70 Alkyl radical 333 Allylic hydrogen 267 Allylic hydroperoxides 399 monosubstituted197 Amino-azidobenzoic acid derivatives, Amplification by spatial polymeriza tion, quantum 128 Anatase titanium dioxide 136 Anhydride, maleic 8 Aniline Black type dye 203 phosphorescence spectra of 429 PPP model of 445 Anionic polymers 226 Anionic polystyrene, oxidized 232, 233 Anthraquinone 266 Antioxidants 340 in singlet oxygen quenching 403 stabilizers 217 A O , coefficient of 451 Apparent absorption 87 Aromatic azides 423, 432 hydrocarbons, polycyclic 266 side chains 193 Arrhenius plots of PP degradation 374 Aryl peroxides 266 A T R spectra 403 Autoexcitation 8

479


480 Automotive paints Autoxidation Azide(s) compounds, photosensitive 197, group, localized excitation ( L E ) of light-initiated decomposition of S —>Ti absorption spectra of aromatic triplet states of photosensitive aromatic p-Azidobenzoate 428, p-Azidobenzoyloxy group 430, 440, p-Azidocinnamate 431, p-Azidocinnamoyloxy group Azido photopolymers Azobenzene residues 189, Azobisisobutyronitrile

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