Solid State NMR of Polymers, Volume 84 (Studies in Physical and Theoretical Chemistry)

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studies in physical and theoretical chemistry 84

SOLID STATE NMR OF POLYMERS

studies in physical and theoretical chemistry Recent titles in this series

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studies in physical and theoretical chemistry 84

SOLID STATE NMR OF POLYMERS Edited by ISAO ANDO

Department of Polymer Chemistry Tokyo Institute of Technology Ookayama, Meguro-ku, Tokyo 152 Japan and TETSUO ASAKURA

Department of Biotechnology Tokyo University of Agriculture and Technology Naka-cho, Koganei, Tokyo Japan

1998 ELSEVIER Amsterdam

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Lausanne

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New

York

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Oxford

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Shannon

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Singapore

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Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

Llbrary oF Congress Cata]ogtng-tn-Pub]tcatton

Data

So]ld state NMR oF po]ymers/ edited by Isao Ando and Tetsuo Asakura. p. cm. - - (Studles In physical and theoretical chemistry ; 84) Inc]udes blb]lographtca| references and index. ISBN 0-444-82924-5 1. Po]ymers--Ana]ysls. 2. Nuc|ear magnetic resonance spectroscopy. I. Ando. I. (Isao). 1941. I I . Asakura, Tetsuo. I I I . Series. OD139.P6S65 1998 547'.7--dc21 97-53184 CIP

ISBN' 0-444-82924-5 9 1998 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U . S . A . - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands.

Preface

As materials polymers are almost always used as "solids". A structural and dynamic characterization of the polymers in question is necessary in order to understand the relations between properties and structure and, on the basis of these relations, to design new polymer materials. As is well known, the X-ray diffraction method has contributed to the structural determination of polymers with high crystallinity. However, most polymers have low crystallinity and so structural information about the noncrystalline region, which is the major component, cannot be obtained by X-ray studies. Therefore, the X-ray diffraction method has a limitation for the structural analysis of such systems. Further, it can be said that chain segments in the noncrystalline region are sometimes in a mobile state, so that the X-ray diffraction method provides no structural or dynamical information. On the other hand, the solid state NMR method provides information about the structure and dynamics of a sample irrespective of whether the region studied is crystalline or noncrystalline. Recently, high resolution NMR studies of solids have been realized by using advanced pulse and mechanical techniques, and so have provided a variety of structural and dynamic information about polymer systems. Further, it can be said that solid state NMR has provided characteristic information that cannot be obtained by other spectroscopic methods, and that it has become a very powerful means for elucidating the structure and dynamics of polymer systems. In polymer science and technology, the advanced development of various polymer materials with ideal properties and functions is desired. To achieve this, the close relationship between physical properties and molecular structure and dynamics must be clarified precisely. Therefore, powerful techniques are required for the elucidation of this relationship. One of these is solid state NMR spectroscopy. This book is divided into two parts: the basic principles of solid state NMR and its application to polymer systems in the solid state. In the former part, the principles of NMR, important NMR parameters such as chemical shifts, relaxation times, dipolar interactions, quadrupolar interactions, pulse techniques and new NMR methods are covered. In the latter part, applications of NMR

vi

PREFACE

to a variety of polymer systems in the solid state are discussed. The book is intended for graduate students and researchers in academic environments. It provides information relevant to beginners as well as those who are experts in solid state NMR applied to polymer science and technology, materials science, chemistry, biochemistry, physics, and so on. We are delighted that so many active authors, who are leaders in the field of NMR spectroscopy and polymer characterization, have contributed to this work. We hope this book will be welcomed by the widespread NMR community and that all readers, from beginner to expert, everywhere will find the details of the various techniques and applications helpful. ISAO ANDO TETSUO ASAKURA

July, 1997

Contents

Preface Introduction I. A n d o and T. Asakura

XV

I. Basic Principles

1. 2. 3. 4. 5. 6. 6.1. 6.2. 6.3.

6.4. 6.5. 6.6.

NMR Chemical Shift and Electronic Structure I. A n d o , N. Asakawa and G.A. Webb Distance Information and Dipolar Interaction H. Saito, S. Tuzi and A. Naito NMR Relaxations and Dynamics F. Horii Spin Diffusion in Solids M. Ernst and B.H. Meier NMR Imaging and Spatial Information B. Bliimich, P. Bliimler and K. Saito Multi-nuclear NMR 1H NMR B. C. Gerstein 2H NMR A.S. Ulrich and S.L. Grage 3H NMR J.P. Bloxsidge, J.R. Jones, J.C. Russell, A.P. Sharratt, T.A. Vick and D. Zhong 15N NMR T.A. Cross 170 NMR S. Kuroki 19F NMR R.K. Harris, G.A. Monti and P. Holstein

23 51 83 123 165 166 190

212 218 236 253

vii

viii

CONTENTS

II. Applications of Solid State NMR ~

Structure and Dynamics of Crystalline and Noncrystalline Phases in Polymers T. Yamanobe

~

Oriented Fibers and Polymers T. Asakura and M. Demura

,

267 307

Polyethylene and Paraffins T. Yamanobe and H. Kurosu

327

10. Polymer Blends and Miscibility A. Asano and K. Takegoshi

351

11. Polyolefins A. Aoki and T. Asakura

415

12. Polyamides I. Ando and T. Asakura

445

13. Thermoplastic Polymers and Polyimides A . K . Whittaker

469

14. Poly(ethylene terephthalate) T. Asakura and T. Ito

491

15. Crosslinked Polymers R.V. Law and D.C. Sherrington

509

16. Electrically-Conducting Polymers H. Kurosu

589

17. Inorganic Polymers T. Takayama

613

18. Fluoropolymers R.K. Harris, G.A. Monti, and P. Holstein

667

19. Hydrogen-bonded Polymers F. Horii and K. Masuda

20.

Polymer Gel Systems H. Yasunaga, M. Kobayashi and S. Matsukawa

21.

771

Polypeptides I. Ando, T. Kameda, and N. Asakawa

23.

737

Biodegradable Polymers Y. Inoue

22.

713

819

Proteins T. Asakura, M. Demura, N. Nishikawa and H. Yoshimizu

853

CONTENTS 24. 25.

Polysaccharides and Biological Systems H. Saito, S. Tuzi, and A. Naito NMR Characterization of Functionalized Polysiloxanes G.E. Maciel

ix

891 923

III. Conclusions I. A n d o and T. Asakura

985

Subject Index

987

This Page Intentionally Left Blank

Contributors

Professor Isao Ando Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Akira Aoki Plastic Laboratory, Tokuyama Co., Harimicho, Tokushima, Japan Professor Tetsuo Asakura Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan Dr. Tsushi Asano Department of Chemistry, The National Defense Academy, Hashirimizu, Yokoshuka, Japan Dr. J.P. Bloxsidge Department of Chemistry, University of Surrey, Guildford, Surrey, UK Professor Bernhard Blfimich Institut for Makromolekulare Chemie, RWTII, Worringer Weg 1, D-52056 Aachen, Germany Dr. P. Blfimer Institut ftir Makromo|ekulare Chemie, RWTII, Worringer Weg 1, D-52056 Aachen, Germany Professor Timothy A. Cross Department of Chemistry, Florida State University, Tallahassee, Florida, USA Professor Makoto Demura Department of Biotechnology, Tokyo University of Agriculture and Technology, Naka-cho, Koganei, Tokyo, Japan xi

xii

CONTRIBUTORS

Dr. Matthias Ernst NSR-Center for Molecular Structure, Design and Synthesis, Laboratory of Physical Chemistry, University of Nijmegen, Toernooiveld 1, NL-6525 ED Nijmegen, The Netherlands Professor Bernard C. Gerstein Department of Chemistry, Iowa State University, Ames, Iowa, USA Dr. S.L. Grage Institut ftir Molekularbiologie, Friedrich-Schiller-Universitat Jena, Winzerlaerstrasse 10, 07745 Jena, Germany Professor Robin K. Harris Department of Chemistry, University of Durham, Durham, UK Dr. Peter Holstein Institut for Experimentelle Physik I, Universit~it Leipzig, Leipzig, Germany Professor Fumitaka Horii Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan Professor Yoshio Inoue Department of Biomolecular Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Takuro Ito Co. R&D Tokyo Seikan Group, Yokohama, Kanagawa, Japan Professor John R. Jones Department of Chemistry, University of Surrey, Guildford, Surrey, UK Dr. Tsunenori Kameda Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Dr. Shigeki Kuroki Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan Professor hiromichi kurosu Department of Textile and Apparel Science, Nara Women University, Kitauoyahigashi-cho, Nara, Japan

CONTRIBUTORS

xiii

Dr. Robert V. Law Department of Chemistry, Imperial College of Science, Technology and Medicine, London, UK Dr. Kenji Masuda Institute for Chemical Research, Kyoto University, Uji, Kyoto, Japan Professor Gary E. Maciel Department of Chemistry, Colorado State University, Fort Collins, Colorado, USA Dr. Shingo Matsukawa Department of Food Science and Technology, Tokyo University of Fisheries, Konan, Minato-ku, Tokyo, Japan Professor Beat H. Meier Laboratory for Physical Chemistry, University of Nijmegen, Toernooiveld, NL-6525 ED Nijmegen, The Netherlands Dr. Gustavo A. Monti Department of Chemistry, University of Durham, Durham, UK Professor Akira Naito Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan Dr. Naoki Nishikawa Department of Biotechnology, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan Dr. Jeremy C. Russell Biocompatibles Ltd., Brunel Science Park, Kingston Lane, Uxbridge, Middlesex, UK Professor Hajime Saito Department of Life Science, Himeji Institute of Technology, Kamigori, Japan Dr. A.P. Sharratt ICI Chemicals and Polymers, The Heath, Runcorn, Cheshire, UK Professor David C. Sherrington Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow, UK

xiv

CONTRIBUTORS

Dr. Toshio Takayama Department of Applied Chemistry, Kanagawa University, Rokkakubashi, Kanagawa-ku, Kanagawa, Japan Professor Kiyonori Takegoshi Department of Chemistry, Kyoto University, Kitasirakawa, Sakyo-ku, Kyoto, Japan Dr. Satoru Tuzi Department of Life Science, Himeji Institute of Technology, Kamigori, Hyogo, Japan Professor Anne S. Ulrich Institut ftir Molekularbiologie, Friedrich-Schiller-Universitat Jena, Winzerlarstrasse 10, 07708 Jena, Germany Dr. T.A. Vick Biocompatibles Ltd., Frensham House, Farnham Business Park, Farnham GU9 8QL, UK Professor Takeshi Yamanobe Department of Materials Engineering, Gunma University, Kiryu, Gunma, Japan Dr. Hidekazu Yasunaga Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Gosyokaido, Matugasaki, Sakyo-ku, Kyoto, Japan Dr. Hiroaki Yoshimizu Department of Materials Engineering, Nagoya Institute of Technology, Gokiso, Shouwa-ku, Nagoya, Japan Professor Graham A. Webb Department of Chemistry, University of Surrey, Guilford, Surrey, UK Dr. Andrew K. Whittaker Centre for Magnetic Resonance, University of Queensland, Queensland 4072, Australia Dr. Desong Zhong Department of Chemistry, University of Surrey, Guildford, Surrey, UK

Introduction

Polymers generally form a variety of primary, secondary and higher-order structures in the solid state. This comes from the characteristic fact that a polymer chain is formed from an extremely large number of bonds and has sometimes irregular configurational structure and regiostructure. Due to such structural features some regions are found to be in the crystalline state and some in the noncrystalline state. In the former region, polymer chains are aligned like crystals and, on the other hand, in the latter region, they are randomly irregular in structure with and without molecular motion. The existence of these polymer structures is closely associated with their properties. For this reason, it becomes important to carry out precisely both structural and dynamic characterizations. It has been demonstrated that solid state N M R spectroscopy provides useful information about the structure and dynamics of polymers in the bulk. At present, in polymer science, solid state N M R is recognized as one of the most powerful means for elucidating the structure and the dynamics of solid polymers in addition to X-ray diffraction. The history of solid state NMR, which has been used in polymer science, is very old. The appearance of new techniques in solid state N M R has certainly contributed to the development of polymer science and technology. From such a background, the principles of solid state N M R and its applications to structural and dynamic characterization of polymers will be described. Previously, many excellent books and periodical monographs on fundamental N M R and advanced N M R spectroscopies, have appeared. Also some excellent books of solid state N M R of polymers have appeared. Some of these books are mentioned for the convenience of readers below [1, 2].

References 1. Basic NMR books: For example (a) E.R. Andrew, Nuclear Magnetic Resonance, Oxford University Press, Oxford, 1954; (b) A. Abragham, Principles of Nuclear Magnetism, Oxford University Press, Oxford, 1961; (c) R.K. Harris, Nuclear Magnetic Resonance, Pitman, XV

xvi

INTRODUCTION

London, 1983; (d) M. Mehring, High Resolution NMR in Solids, Springer-Verlag, Berlin, 1985; (e) B.C. Gerstein and C.R. Dybowski, Transient Techniques in NMR of Solids, Academic Press, New York, 1985; (f) C.P. Slichter, Principles of Magnetic Resonance, Springer-Verlag, Berlin, 1990; (g) E.O. Stejskal and J.D. Memory, High Resolution NMR in the Solid State, Oxford University Press, Oxford, 1994. 2. Solid state NMR for Polymers: For example (a) C.A. Fyfe, Solid State NMR for Chemists, C.R.C. Press, 1983; (b) R.A. Komoroski (Ed), High Resolution NMR of Synthetic Polymers in Bulk, VCH Publishers, 1986; (c) V.J. McBriety and K.J. Packer, Nuclear Magnetic Resonance in Solid Polymers, Cambridge University Press, Cambridge, 1993; (d) K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, London, 1994; (e) G.A. Webb and I.Ando (Eds), Ann. Repts. NMR Spectroscopy (Special Issue: NMR in Polymer Science), Vol. 34, Academic Press, London, 1997.

Chapter 1

Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved

N M R Chemical Shift and Electronic Structure Isao Ando ~, Naoki Asakawa 2 and Graham A. Webb 3 ~Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 2Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuda, Yokohama, Japan, and 3Department of Chemistry, University of Surrey, Guildford, Surrey, UK

1.1

Introduction

High-resolution solid-state NMR spectroscopy, combined with quantum chemistry, provides detailed information on the structure and electronic structures of solid polymers through the observation of the NMR chemical shift [1]. In the liquid and solution states, NMR chemical shifts of polymers are often the averaged values for all of the possible conformations because of rapid interconversion by rotations about bonds. However, in the solid state, chemical shifts are often characteristic of specific conformations because of strongly restricted rotation about the bonds. The NMR chemical shift is affected by a change of the electronic structure arising from structural changes. NMR chemical shifts in the solid state provide, therefore, useful information about the electronic structure of a polymer or polymers with a fixed structure. Furthermore, in the solid state, the components of the full chemical shift tensor can often be determined. The complete chemical shift tensor provides information on the local symmetry of the electron cloud around the nucleus and so provides much more detailed knowledge of the electronic structure of the polymer compared with the average chemical shift associated with the structure. Such a situation applies to many polymers and, in order to establish the relationship between the NMR chemical shift and the electronic structure of polymers, it is necessary to use a sophisticated theoretical method which takes account of the characteristics of polymers. Some methodologies for obtaining structures and the electronic structures of polymers, both in the solution and solid state, involve a combination of the observation and calculation of NMR chemical shifts. This approach has been applied to various polymer systems. Theoretical calculations of NMR chemical shifts for polymer systems have been achieved using two main approaches. One approach is that model compounds, such as the dimer,

2

ISAO ANDO

ET AL.

trimer, etc., as a local structure of polymer chains, are used in the calculation by combining quantum chemistry and statistical mechanics. In particular, this approach has been applied to polymer systems in solution [2]. However, in solid polymer systems it should be recognized that the results of quantum chemical calculations on model compounds are not readily transferable to polymers because of differences in the electronic structure, including longrange interactions such as intrachain and interchain interactions. Electrons are constrained to a finite region of space in small molecules but this is not necessarily the case for polymers and, hence, some additional approaches are required. Another approach is to employ the tight-binding molecular orbital (TB MO) theory, which is well known in the field of solid-state physics, to describe the electronic structure of linear polymers with periodic structure within the framework of the linear combination of atomic orbitals (LCAO) approximation for the electronic eigenfunctions [3-11]. These approaches lead to the determination of the spatial structure and/or electronic structure of polymer systems including polypeptides in the solution and solid state. The essence of these two approaches are described below.

1.2 Approach using model compounds 1.2.1

The origin of NMR chemical shift

The chemical shift of an atom depends on its electronic and molecular enviroments [12]. Note that the chemical shift relative to a standard reference is expressed by 6 and the chemical shielding by o-. The chemical shielding or for atom A can be estimated by the sum of the following terms: (1.1)

OrA = O-d .qt_ o-P _+_ O " ,

where erd is the diamagnetic term, o-p is the paramagnetic term and or' is another term which comes from the magnetic anisotropy effect, polar effect and ring-current effect. For nuclei with 2p electrons, such as 13C, lSN, etc., the relative chemical shift is predominantly governed by the paramagnetic term, and for the 1H nucleus by the first and third terms in Equation (1.1). The paramagnetic term is expressed as a function of excitation energy, bond order, and electron density according to the sum-over-states (SOS) method in the simple form as follows: (rP -- - C ~ ( r - 3 ) 2 p ( t m _ E n )

-1Q ,

(1.2)

N M R C H E M I C A L SHIFT A N D E L E C T R O N I C S T R U C T U R E

3

Table 1.1. Calculated ~3C chemical shieldings of hydrocarbons by FPT I N D O method

Sample

Calculated ~ (ppm)

Experimental c (ppm)

crd

crp

CH4

57.7

-129.3

-68.0

0

0

Ethane C2H6

57.4

-136.2

-75.7

7.7

8.0

Ethylene

57.9

-230.3

- 169.3

101.3

124.9

Methane

C2H4

O'A

6 (cal)b

(~(exp)

The negative sign means deshielding. b Relative to CH4. c Relative to CH4.

where E m - E n is the singlet-singlet excitation energy from the nth occupied to the mth unoccupied orbitals, and Q is a factor including the bond order and electron density. The quantity (r-3)Zp is the spatial dimensions for a 2p electron while C is the coefficient incorporating universal constants. This term is calculated by semi-empirical MO or ab initio MO methods. The former has some features which give the substantial aspects of the chemical shift behavior associated with the spatial structure and/or the electronic structure. The diamagnetic term is estimated from the calculated electron density. Using these procedures, the chemical shielding o-; of the model compound with any specified conformation is calculated. For example, the contributions of the paramagnetic term and diamagnetic term to the relative 13C chemical shifts of small hydrocarbon molecules, such as methane, ethane and ethylene using the FPT (finite perturbation theory) with the I N D O (semiempirical MO) method, are calculated as shown in Table 1.1 together with the experimental data [13]. Note that the negative sign of the shielding constant o- indicates deshielding and, therefore, shielding variations can be compared with the observed chemical shift 6 where a positive sign denotes deshielding. This table indicates that the paramagnetic term predominantly contributes to the relative ~3C chemical shift, and the contribution of the diamagnetic term is very small. These results show that it is very important to estimate exactly the paramagnetic term for the chemical shift calculations of nuclei with 2p electrons. 1.2.2

Medium effects on N M R chemical shifts

Most MO calculations of nuclear shielding relate to the case of a molecule in a vacuum. For nuclei forming the molecular skeleton, such as 13C, and

4

ISAO ANDO ET AL.

nuclei with small shielding ranges, such as ~H, this may not be an unreasonable approximation. This is particularly true if comparison of the theoretical results is to be made with experimental data taken on a molecule dissolved in an inert solvent. More reactive atoms, especially those with lone pair electrons such as 14N, ~SN, 170 and 19F, are very likely to have their nuclear shieldings influenced by interactions with solvent molecules. Such interactions may be specific, e.g., hydrogen bonding, or nonspecific, e.g., polarizability/polarity, or perhaps a combination of both specific and nonspecific solute-solvent interactions. An empirical procedure has been developed for quantitatively unravelling the contributions made to the shielding of solute nuclei by specific and nonspecific interactions. Nonspecific solute-solvent effects on nuclear shielding may be described by MO calculations which include the influence of solvent polarity/polarizability by means of the solvent dielectric constant, e. This approach is epitomized by the use of the solvaton model together with a semi-empirical MO basis set [14-19]. This method has successfully accounted for the observed variations of 13C and 15N shielding for a number of solute molecules in solvents with various values of e. The solvaton model has been demonstrated to predict correctly both the sign and magnitide of the solute shielding variation as the value of E of the medium is changed. This approach provides a good understanding of the various solute-solvent interactions on the 13C chemical shifts of polymers such as poly(vinyl chloride) associated with the stereochemical configurations [15]. Ab initio MO calculations of the effect of solvent on the 29Si shielding of some solvated molecules have been reported by Arshadi et al. [20]. The extended basis set calculations employed the IGLO nuclear-shielding procedure coupled with a continuum solvent model in which the solute is placed inside an appropriately dimensioned cavity within a polarizable continuum with a given value of E [21]. The calculated variation in the silicon nuclear shielding, as a function of E, agrees satisfactorily with experiment. In general, the variation in the silicon shieldings of the solvated molecules is small but not negligible. Specific solute-solvent interactions, such as hydrogen bonding or protonation, may be included in the calculation of the shielding of solute nuclei by a supermolecule approach. The appropriate structure of the solute-solvent supermolecule can be obtained using molecular mechanics simulations. At the semi-empirical MO level this approach has been used successfully to describe the effects of hydrogen bonding on the nuclear shielding of small molecules. Ab initio MO calculations, using the gauge independent atomic orbital (GIAO) orocedure, has been aoolied in a studv of the effects of

NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE

Fig. 1.1. The planar conformation of any specified configuration of a vinyl polymer. polysaccharide [22]. Calculations using a 6-31G** basis set show that hydrogen bonding can result in a change in 170 nuclear shielding by up to about 70 ppm, whereas a few ppm change at most is predicted for the XH and 13C shielding on hydrogen-bond formation. 1.2.3

Applications to polymers

A polymer chain can assume an enormous number of conformations because of the various possibilities of rotation around the chain bonds, due to molecular motion [23]. Thus, the factors governing the appearance of the NMR spectra include the structures, the relative energies of the rotational isomers, the chemical shifts and spin couplings. If molecular motion in the polymer chain is extremely slow on the NMR timescale, the spectrum represents the superposition of the spectra for the various conformations. However, if the rotation around the chain bonds is very fast on the NMR timescale, the experimentally observable chemical shift for nucleus A is given as [2, 24-281 n

<SA) = 2 PiSg.

(1.3)

/=1

The numerical indices refer to the preferred conformations, and Pg and 6/ are the probability of occurrence and the chemical shift of the preferred conformation i, respectively. This indicates that the chemical shift of a given nucleus can be obtained by a combination of a quantum chemical and a statistical mechanical method as described below. The probability, Pi, of the preferred conformations in various configurations in vinyl polymers as a function of the racemic units necessary for the calculation of the averaged chemical shift (6A) is estimated by statistical mechanics for a polymer chain. For convenience, the planar conformation in any specified configuration of a vinyl polymer is shown in Fig. 1.1. The statistical weight factors are r/, 1 and r for trans (T), positive gauche (G § and negative gauche ( G - ) conformations, respectively. The statistical weight factor for G * G - is ~o (the so-called pentane effect) [23]. The statistical weight matrices for the characterization of the array of chain conformations of vinyl polymers can

6

I S A O A N D O E T AL.

be used. The purpose of calculating the probability that the pair of skeletal bonds within the kth dyad, the kth triad, etc., in a chain are in particular rotational states is that such statistical weights are used to construct statistical matrices U = U'U". In such matrices, rows are associated with rotional states about bond i--1 and columns about bond i. The statistical weight matrices in the case of pairs of bonds adjoining C H R groups are designated U". These matrices are expressed as

;t

?

U =

B

(.o

rt 1

too

for the first skeletal bond of a dyad and

7qw 1

=

=

1

too t

TI

O)

7"09

rio

oo

too 2

r/

w

~Tw

1

r

rtw

w

rw /

for the second; the former matrix, Equation (1.5), is for a meso dyad and the latter, Equation (1.6), for a racemic one. The configuration partition function Z for the entire chain can be expressed as the sum of the statistical weights for all molecular conformations of the chain consisting of n bonds or x = n/2 repeating units 9n / 2

--

1

Z = J

--.,i--,,i

(1.7)

J

i/2 = 1

where J* = (100) and J is the transpose of (111). Let/3 and y denote indices from the set T, G + and G - . The probability Pt3~k" that the pair of skeletal bonds within the kth dyad are in the rotational states/3 and 3', respectively, is the ratio of the sum of the statistical weights for all conformations. It is given by

)

P/3v:k" = z - l J *

UhUh'+l h=l

,(xl

t

tt

(UkU(/3,y)k

U'TT" t

i ~"i+ 1 J ,

i=

1

(1.8)

NMR CHEMICAL SHIFT AND ELECTRONIC STRUCTURE

7

in which U" is the matrix representing the second bond of the kth dyad with all the elements, except for U", which is replaced by zero. P~g~,, is estimated by generating Monte-Carlo chains. An averaged chemical shift is then obtained using the values obtained for P~v~,, and 6i (2). Using this developed methodology, the structural behaviors of polyethylene and paraffins, vinyl polymers such as poly (vinyl alcohol) and polypropylene, etc., in the solution have been successfully elucidated on the basis of their observed spectra [2, 24-28]. Also, this can be applied to noncrystalline and crystalline phases in polymers. In the crystalline state polymer chains assume a fixed conformation. In this case, the structural information obtained from the chemical shift corresponds to the fixed conformation. The calculation of I3C chemical shifts for a dipeptide fragment (N-acetyl-N'-methyl-L-alanineamide) [Ac-L-AlaNHMe] of poly(L-alanine) and L-alanine residue containing proteins has been attempted using the FPT INDO method in order to understand and predict the13C chemical shift behavior of polypeptides associated with a secondary structure such as an a-helix,/g-sheet, etc., and the determination of secondary structure through the observation of 13C chemical shifts [29]. The 13C chemical shifts of the C~ carbon of an alanine residue in various peptides and polypeptides vary significantly depending on the conformation, which may be a right-handed a-helix, /3-sheet, or other conformation. Such sizeable displacements of the 13C chemical shifts can be characterized by variations in the electronic structures of the local conformation as defined by the dihedral angles (d~, ~). The calculated contour map for the C~ carbon is shown in Fig. 1.2. From this map, we can estimate the 13C shielding for any specified conformation. It has been demonstrated, from comparison of the experimental data and the predicted values given by this chemical shift map, that the map successfully predicts the 13C chemical shifts of alanine residues in polypeptides and proteins [30, 31]. For example, as shown in the map, the 13C chemical shift of the right-handed a-helix form appears at high frequency by 2.5 ppm compared to the /3-sheet form. This reasonably explains the experimental result. For the chemical shift calculation of the 15N nucleus, which is also popular in polymers, the same method can be used. Most recently, ab initio calculations for the NMR chemical shifts have become available for medium-size molecules because of the remarkable power capabilities of modernworkstations, personal computers and supercomputers [32]. This leads to a quantitative discussion on the chemical shift behaviors. For example, the ab initio MO calculation with the 4-31G basis set using the GIAO-CHF (gauge independent atomic orbital-coupled Hartree-Fock) method on N-acetyl-N'-methyl-L-alanineamide which is the same model molecule as the case of the above FPT INDO calculation will be

8

ISAO A N D O ET AL.

6

-180

t.

-120

-60

0

60

~. degree

120

180

Fig. 1.2. The calculated 13C chemical shift map of the C~ carbon in N-acetyl-N'-methyl-Lalanine amide obtained using the FPT INDO method. The chemical shifts were calculated at 15~ intervals for the dihedral angles (~b, ~).

introduced [33]. All of the geometrical parameters are energy-optimized. Figure 1.3(a) shows the calculated isotropic 13C chemical shift map of the C~ carbon as a function of the dihedral angles, where the positive sign means shielding. The whole trend for this map is similar to that obtained by the FPT INDO method as shown in Fig. 1.2. The isotropic shielding constants (or) for the C~ carbon are 186.4 ppm for the dihedral angles (4~, g0, which correspond to the antiparallel/3(/3A)-sheet conformation, 189.4 ppm for the right-handed a(aR)-helix, 189.6 ppm for the left-handed a(aL)-helix; on the other hand, the observed isotropic chemical shifts (~) are 21.0 ppm for the /3a-sheet, 15.5 ppm for the aR-helix and 15.9 ppm for the aL-helix [34]. Such

-p,,

0 co

v

o

CO

,~

0 t~l

t~l

o

0 ~

~

0 ~D

~J

o

o -9

CO

o.

o.

0 C')

,,..

,.

,-

o o . o .

0

0

,,-

,.. ,. _ ~ , . . , _ , ,

,,-

e9

&

o

~

o.

o

0 t~9

o.

0

~

o.

o

o CO

o

,

o.

o

o

o

o O)

":"

o

0

8

0

c~

~.~

A

NMR C H E M I C A L SHIFT A N D E L E C T R O N I C S T R U C T U R E

~

0 to

o

I~

d

,d

9

10

A

,,-

)

0 CO 1.-

.-

I.-

1.-

0 C~l

0 ,e--

0 IZ)

.-

0

0 O~

o

o

,,-

0

o_ ~ o - ~ _ . . _ ~ /

.-

0 e')

0

..

'

i

0 COD

,--

o

'

o

0 e')

ISAO A N D O ET AL.

9

C) CO

(-Sap)

iI o e

0

o

o

s

o

8

o (o

"T

'7,

~

"o

ID

J

~2

180

180

(e)

150

"~. o) (1) 9-

150

120

120

90

90

6O 159.5

6O

J

140

o

158.4 ~

30

142 144 146 t48 150 52

9-

157.3

~38

30

-30

.60

-60

-9O

.90 -70

-40

-10

20

~

142 144 146 k~6

-100

136

0

-30

-130

~

1 3 8 ~

134 ~

156.2

-160

140 "

50

-160

80

150 152 --154

.... -130

-100

-70

-40

-10

(deg.)

(deg.) Fig. 1.3. e, f.

20

50

80

180

180

150

157

155

9"

6O

~

159

165165

163

161

30

0f

-60t / -90[ -160

(1)

c~ 60

\

165

9"

\ =

153

\ 1s3 155

~

~

_ ~ ~

~

) / I !

1e3--130

-100

( ) 161f

-70

\157

]

\

!159

/ -40

-10

/

167 ! 63 161 159 lS7 155 "

-30

9

90

163

~161

(h)"

120

157

~

90 ~ Q)

150

153

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186 184 182 180

20

50

80

t

30

'/ ~

166

~~7o

/

9

" ~

174

" ~

176

~

)

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172/-

178

~

174

> 0 >

176 178

178

.I

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J

-9oL / / -160

~) (deg.)

0 nl

176

-30

174 172 170 / -130

\ -100

~ I -70

I

>

| -40

-10

20

50

80

r (deg.) Fig. 1.3. g,h.

Fig. 1.3 9The dependences on the dihedral angles (4), q,), of the chemical shielding constants for the L-alanine residue Ca and Ct3 carbons in peptides. Chemical shielding calculations were carried out using the GIAO-CHF method with 4-31G ab initio basis set. The 4-31G optimized geometries for the model molecules, N-acetyl-N'-methyl-L-alanineamide, were employed: (a) the isotropic; (b) o"11; (c) o'22; (d) o'33 for the Ct3 carbon (unit in ppm); (e) the isotropic, (f) o'11; (g) o'22; and (h) o'33 for the Ca carbon (unit in ppm).

NMR C H E M I C A L SHIFT AND E L E C T R O N I C S T R U C T U R E

13

experimental chemical shift behavior is well explained by the calculated behavior. It is found that the change of the dihedral angles dominates the isotropic chemical shift behavior of the L-alanine residue C~ carbon. In order to understand the isotropic chemical shift changes, the calculations of the principal components for chemical shielding tensor have been made. The (qS, ~) dependences of 0-11, 0"22, and 0"33 (these are defined from the least shielded to the most shielded, respectively) are shown in Fig. 3(b-d). From these figures, the fact that the isotropic C~ chemical shift for the aRhelix appears at a lower frequency (15.5 ppm) than that for the /3A-sheet (21.0 ppm) is understood by the explicit differences in the value of 0"11(A0"11 ~ 9 ppm). Careful investigation of the chemical shielding tensor shows that the paramagnetic term for 0"11 dominates the total o" value. The 2p orbital for the C,, carbon, which contributes to the C ~ C ~ or-bond, is the most effective contribution to the paramagnetic term of o"11, since the magnetic dipole-coupling integral of electrons, (qS~I L~/r3 1~) (where, 13 = 1 in this case), is dominantly estimated by the 2p-orbital perpendicular to the direction of o'al. However, in the 4-31G optimized geometry, the distance between the C~ and Ct3 carbons is 1.512 A for the /3A-sheet and 1.516 A for the aR-helix. Although this bond-length difference seems to be too small to explain the behavior of the o'11 value, the fact that the typical C ~ C t 3 bond length determined by X-ray diffraction is estimated as 1.51 A for the/3A-sheet and 1.53 ]k for the aR-helix, respectively, might indicate that the C,,~Ct3 bondlength differences, which are driven by conformational changes, do control the o'11 value. Figure 1.3(e) shows the dependence of isotropic chemical shielding for the C~ carbon against the main-chain dihedral angles. From this figure, the isotropic chemical shieldings for the C~ carbon are 160.4 ppm for the/3A-sheet, 159.6 ppm for the aR-helix 159.2 ppm for the aL-helix, 161.4 ppm for the 3x-helix and 157.9 ppm for silk I and (Ala-Gly)n form II. For these calculated shielding, the observed isotropic chemical shifts are 48.7 ppm for the/3A-sheet, 53.0 ppm for the ag-helix, 50.1 ppm for the aL-helix, 49.7 ppm for the 31-heli~ and 51.5 ppm for (Ala-Gly)n form II, respectively. Although it is obvious that there exists the main-chain dihedral-angle dependence on chemical shift for the C~ carbon, it seems more complicated than that for the C~ carbon, because the orientation of the chemical shift tensor for the C~ carbon, with respect to the molecular fixed flame, is different from one (~b, qJ) to another. Additionally, because, in a case in which the L-alanine residue carbonyl- or amide-group would form the hydrogen-bond, the hydrogen-bonding structure can also affect the behavior of the chemical shift for the C~ carbon. The structure of (Ala-Gly)~ form II and silk I, needs chemical shift calculation of the hydrogen-bonding taken into

14

I S A O A N D O ET AL.

consideration and the investigation of chemical shift carbonyl-carbon will be needed. To give a further insight into the C~ chemical-shift behavior, principal values of the chemical shielding tensor were also calculated [33]. From Fig. 1.3(f-h), it is obvious that all the principal values are quite sensitive to the (q~, 0) differences. In particular, the 0"33 is the most sensitive to the (q~, 0) differences. For almost all (q~, 0)or, the principal axis of the o33 is aligned to the C ~ C ' bond but with ca. 30 ~ deviation and is also nearly perpendicular to the C ~ C ~ bond. Since it is generally known that bond extension makes shieldings decreased [32], it is predicted that the C ~ C ' bond-length differences associated with dihedral-angles variations would contribute dominantly to the 0"33 differences. The investigation of the shielding calculation procedures provides information as to where a chemical shift change comes from. Regarding the o33 for the C~ carbon, it was found that the diamagnetic contribution for the o33 dominates the changes in the total 0"33. Therefore, the most crucial factor for this behavior is changes in geometric parameters of the C, carbon moiety along the 0"33 axis for model compounds with several main-chain dihedral angles. It should be noted that the other principal values, o11 and o22, change their orientation of principal axes for one (q~, 0) to another. One of the complexities--the orientation of the chemical shift tensor--will be discussed later, and the other complexity--the hydrogen-bonding effect--seems particularly intricate because of the chemical shift for the C~ carbon, especially the principal values of the chemical shift tensor, which would be greatly affected by differences in not only the hydrogen-bond length ( R N ~ O and R H ~ O ) , but also on the hydrogen-bond angles (e.g., < N ~ O - - - C , < H ~ O = C , etc.). The hydrogen-bonding effect on the C~ chemical shift will be estimated by the solid-state NMR measurements combined with GIAOCHF chemical-shielding calculations. As mentioned above, the principal values of the chemical shift tensor give information about the three-dimensional electronic state of a molecule. However, in order to understand the behavior of the principal values, one should obtain information about the orientation of the principal axis system of a chemical shift tensor with respect to the molecular fixed frame. Figure 1.4(a-d) shows the calculated orientations of the principal axis systems of the chemical shift tensors of the L-alanine Ct3 carbons in some peptides whose L-alanine moieties have different main-chain dihedral angles, (~b, 0) = (-57.4 ~ -47.5~ (--138.8 ~ 134.7 ~ [/3A-sheet], (-66.3 ~ -24.1 ~ [31~-helix] and (-84.3 ~ 159.0 ~ [31-helix]. Figure 1.4(a-d) shows that the 0"33 component nearly lies along the C ~ C t 3 bond for all peptides considered here, and also show that O'11 is nearly perpendicular to the plane which is

r

..o v

r

9. - -

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11) v

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"...

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I

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9

N M R CHEMICAL SHIFT A N D ELECTRONIC S T R U C T U R E

C)

"'.. .--

Uz-":

..9 ~ ( ....... ..,,,, 6~ ~ u.--"::--'~--L.

o

Z ~

I

i I

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0

15

0 I

011 (a) ctn-helix

(b)

l

[3A-sheet

13.2 . ~ _ 033

""

.-T . . . .

~'-- :-I-

I 1-. or

,

~

Ab3isoI ' " .... ~ .....

113a>

A B laa>

A

B

A A

Fig. 2.7. Energy level diagram in the homonuclear two-spin system under near rotational resonance condition. 2

mR(t) =

O0(B m) exp(imoort).

~

(2.25)

m = --2

The Fourier components in this equation can be further expanded by the chemical shift anisotropy as follows" 2

O.)(n) =

~

tUB'(m)"(m--n)uA

,

(2.26)

m = --2

where, CO(B '~ is the consequence of dipolar CO(B m) and chemical shift anisotropy a(A"). When the rotational resonance condition A~Oiso = nOOr is fulfilled, ~O(B ") is the Fourier component which is equal to the energy gap between two-spin states (Fig. 2.7) and, therefore, the spin-exchange, or mixing of the wavefunctions, becomes efficient. At the rotational condition, the exchange rate R can be expressed as R 2 - r2 -

41 ~o~) 12,

(2.27)

40

A K I R A NAITO ET AL.

where r = 1/T z ~ represents the zero quantum transverse magnetization rate and ] ~o(BmI implies the rotationally driven exchange rate. When R2> 0, the difference of the longitudinal two-spins is given by ?.

_

( I z - Sz>(t)= e rt/2[cosh(Rt/2) +--sinh(Rt/2)],

R

(2.28)

w h e n R 2 < 0, ?-

( I z - Sz>(t)= e-rt/2[cos(iRt/2) + - sin(iRt/2)]. iR

(2.29)

As expected, T z ~ chemical shift anisotropy and dipolar interaction are involved in the exchange rate of the longitudinal magnetization under the MAS condition. Therefore, the separation of the dipolar interaction is quite complicated compared with the R E D O R experiment.

2.8

Practical aspect of RR experiment [26]

Experimentally, homonuclear dipolar interactions can be determined by measuring the extent of the exchange rate of the longitudinal magnetization as a function of mixing times 'I" m using the pulse sequence shown in Fig. 2.8. In the pulse sequence, a selective inversion pulse is used to invert one of the two resonances followed by the mixing time. One of the advantages of the R R experiment is that it can be performed by an ordinary double-resonance spectrometer as long as the spinner speed can be controlled using a spinner frequency controller. It is advised to use n = 1 as the rotational resonance ~2

H ~ CP

Decoupling

n/2

m:2

13c CP

INV ~

'Cm--I~

Fig. 2.8. Pulse sequence of RR experiment. INV is the selective pulse to invert one resonance.

DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES

41

condition because the chemical shift anisotropy is strongly affected when the n value is greater than three. When a high-field spectrometer is used, chemical shift anisotropy increases proportional to the frequency used. In that case, one has to include the chemical shift anisotropy tensor for both nuclei in the analysis of the longitudinal magnetization. In the analysis of the R R experiment, one has to use the T z ~ as discussed in the previous section. This value is difficult to determine experimentally. Practically, the T z ~ value can be approximated from the single quantum relaxation time (Y2) of the two spins by the expression [29] 1 TZO

=

1 TI1

+

1

(2.30)

TI2 '

or

T2z ~ =

1

1r(vi, + vi~)

.

(2.31)

When the chemical shift difference is very small, it is difficult to perform the R R experiment, because of the overlap of the resonance lines of the dipolar coupled nuclei, leading to difficulty in the analysis of the R R data. In this case, a rotating resonance experiment in the tilted rotating flame [30] can be used because a much higher spinning speed can be adopted for the case of a smaller chemical shift difference in the system. The natural abundance background signal can also affect the apparent amount of R R magnetization exchange. Consequently, the observed magnetization exchange rate yields a smaller magnetization exchange rate than the observed one. Therefore, a rate smaller than the real one is obtained resulting in an overestimation of the interatomic distance. Incomplete proton decoupling prevents magnetization exchange between the coupled spins because of the H1 field inhomogeneity. It is advised to irradiate with a strong proton decoupling field (>80 kHz) to a small size of sample in the rf coil to prevent H1 inhomogeneity. Overall, much care has to be taken for the R R experiment to obtain accurate interatomic distances.

42

AKIRA NAITO ET AL.

2.9 Illustrative examples for the determination of three-dimensional structure based on accurately determined interatomic distances

2.9.1

Peptides and proteins

Marshall et al. [31] synthesized an emerimicine fragment (Ac-Phe-[113C]MeAZ-MeA-MeA-Val-[15N]Gly6-Leu-MeA-MeA-OBz). The 13C-15N interatomic distance of 4 residues apart was determined to be 4.07 A by the R E D O R method. It was concluded that the structure is a-helix, because the expected distances are 4.13 and 5.87 A in the cases of the a-helix and the 3~o-helix, respectively. In a similar manner, the 19F-13C interatomic distance was measured for the 19F, 13C and 15N triply-labelled fragment (19FCHzCOPhe-MeA-MeA-[1-13C]MeA-[15N]Val-Gly-Leu-MeA-MeA-OBzl) [32] and found to be 7.8 ~ by the T E D O R method [8] after transferring the magnetization from 15N to 13C. Because the T E D O R method makes it possible to eliminate background signals due to naturally abundant nuclei, quite remote interatomic distances can be determined. Hing and Schaefer [33] also tried to determine the C - N interatomic distances of an ion channel peptide Val 1[1-13C]GlyZ-[15N]Ala3-gramicidin A in a DMPC bilayer. The dipolar interaction of the peptide in the lipid bilayer showed much smaller values compared with that in the powder state, because the helix motions significantly averaged the dipolar interactions. The extent of the scaling of the dipolar interaction shows that gramicidin A consists of the dimer with a single helix. A magainin analogue in the membrane was investigated by 13C, 31p R E D O R [34]. The result indicates that the a-helical Ala19-magainin 2 amide is bound to the head group of the lipid bilayers. It is mentioned here that the data analysis described above seems to be worth considering in order to improve the accuracy of the R E D O R data. A complete three-dimensional structure can be determined by combining a variety of interatomic distances [18, 22, 35, 36]. Garbow and coworkers have synthesized three labelled peptides which were labelled at different positions. These interatomic distances were converted to the torsion angles to yield the/3-turn II structure [22, 35]. Naito et al. [18] systematically applied this technique to elucidate the three-dimensional structure of N-Ac-Pro-GlyPhe. They proposed that the carbonyl carbon of the i-1 residue, and the amino nitrogen of the i + lth residue, should be labelled with 13C and 15N, respectively. Namely, [1-13C]N-Ac-Pro-[15N]Gly-Phe (I), N-Ac-[1-13C]Pro Gly-[15N]Phe (II) and [1-13C]N-Ac-Pro-Gly-[15N]Phe (III) were synthesized and the resulting distances determined to be 3.24, 3.43 and 4.07 ~ , respectively, utilizing the R E D O R factor obtained for the infinitely diluted state to prevent errors from the contributions of the neighbouring labelled nuclei. No


43

correction from the contribution of the naturally abundant nuclei turned out to be necessary. Surprisingly, these distances do not agree well with the values obtained from an X-ray diffraction study [37] available at that time, showing the maximum discrepancy between them to be 0.5 A. This value seems to be much larger than the expected error in the R E D O R experiment (___0.05 A). The reason why the distances are so different is explained by the fact that the crystal (orthorhombic) used for the R E D O R experiments is different from that used in the X-ray diffraction study (monoclinic). To check the accuracy of the R E D O R experiment, an X-ray diffraction study was performed on the same crystals used for the R E D O R experiment. It is found that the distances from the new crystalline polymorph (orthorhombic) agree well within an accuracy of 0.05 A as shown in Table 2.1. Conformational maps based on the possible combinations of the torsion angles of the Pro and Gly residues are calculated as shown in Fig. 2.9. Furthermore, the difference of the chemical shifts between the C~ and C v carbons of the Pro residues A ~ was used as a constraint to determine the 0 value ( - 1 3 ~ [38]. Since the 4~ angle of the Pro residue is restricted in many instances to - 7 5 ~ which shows the minimum energy in the residue. Therefore, the torsion angles of the Pro residue are uniquely determined to be ( - 7 5 ~, -28~ Using these torsion angles, conformational maps were calculated as shown in Fig. 2.9. Finally, two pairs of torsion angles were selected as ( - 1 1 2 ~ 48 ~ and ( - 1 1 2 ~, -48~ Energy minimization by molecular mechanics yielded the structure of the/g-turn I structure, as shown in Fig. 2.10. It was found that the three-dimensional structure of this peptide was well reproduced by a molecular dynamics simulation by taking into account all of the intermolecular interactions in the crystals [18, 39]. Elucidation of the three-dimensional structure of an opioid peptide Leuenkephalin crystal, Tyr-Gly-Gly-Phe-Leu grown from M e O H / H 2 0 mixed solvent, was performed by the R E D O R method [27] alone. This seems to be an additional challenge for this technique to reveal the three-dimensional structure of more complicated systems. Six differently-labelled Leu-enkephaline molecules were synthesized following the strategy described above and the resulting interatomic distances accurately determined. It turns out, however, that the crystalline polymorph under consideration was very easily converted to another form. Therefore, it is necessary to check whether or not the six differently-labelled samples are all in the same crystalline polymorph by means of the ~3C chemical shifts. Otherwise, meaningless data can be obtained without this precaution. When the distance data are converted to yield the necessary numbers of torsion angles, a unique combination of torsion angles in the corresponding conformational map are determined by using the chemical shift data as additional constraints. A three-dimensional

4~ 4~

Table 2.1. C - N I n t e r a t o m i c distances (A) determined from R E D O R experiments as compared with those by X-ray diffraction and M D [18]. Experimental

peptides

REDOR

X-ray

Orthorhombic

Orthorhombic

Monoclinicb

3.24 + 0.05 (3.43 _+ 0.05) c 3.43 +- 0.05 (3.66 -+ 0.05) 4.07 + 0.05 (4.45 + 0.05)

3.19 3.35 3.99

3.76 3.21 3.91

I II III

>

Calculated

Labelled

Energy-minimized a

a Energy-minimized structure based on R E D O R data. b Ref. [36]. c D a t a from Ref. [19] based on fully packed 7.5 m m rotor system.

MD

>

Orthorhombic

Monoclinic

3.22 -+ 0.10 3.32 + 0.10 3.92 _+ 0.12

3.63 - 0.10 3.33 +_ 0.10 3 83 + 0.10

> 3.17 3.57 4.17

9 t'rl

DIPOLAR INTERACTIONS AND ITERATOMIC DISTANCES 180

180

.

.

.

.

45

.

b

"x ~JPro 0

-180 -180

B .

.

i .

.

0

.

~Pro

180

-180 -180

.

.

.

.

0

,

180

(~Gly

Fig. 2.9. Conformation maps for the torsion angles in Pro (a) and Gly residues (b), respectively. The A and B regions were obtained from the intersections of the constraint of ~b angles of the Pro residue. The C and D regions were then obtained by a cross-section of the two types of conformation maps [18].

"!:"

'

A

"4.07

,

Fig. 2.10. Optimized conformation of N-acetyl-Pro-Gly-Phe as obtained by the minimization of energy from the initial form as deduced from the R E D O R experiment [18].

structure was thus determined as shown in Fig. 2.11. However, this structure is not the same as that previously determined by X-ray diffraction because one is dealing with a crystalline polymorph which is not fully explored. The RR method has been used to characterize the structures of fragments of amyloid [40, 41]. Griffin et al. [40] have synthesized the/3-amyloid fragment (HzN-Leu-Met-Val-GIy-Gly-Val-Val-Ile-Ala-COzH) which is the C-terminus of the /3-amyloid protein. The structure of this molecule was

46

AKIRA NAITO ET AL.

Fig. 2.11. Three-dimensional structure of Leu-enkephalin crystal determined by the R E D O R experiment [27].

determined by the 13Cm13Cinteratomic distances and the 13C chemical shift values. The a-carbon of the ith residue, and the carbonyl carbon of the i + lth residue, were doubly labelled and the interatomic distance A[ai, i + 1] observed by using a rotational resonance method. Similarly, the interatomic distances of B[i, a(i + 2)], C[i, a(i + 3)] were also determined. Since the rotational resonance signal of A does not show a dilution effect, an intermolecular contribution does not exist. On the other hand, B and C show strong intermolecular contributions from B* and C*. Therefore, it turns out that the fragment forms an antiparallel /3-sheet. Furthermore, the intermolecular contribution indicates that/3-strands consist of antiparallel/3-sheets forming hydrogen bonds with the position which is slipped from the Nterminus position. It is interesting that the information on the intermolecular contribution made it possible to reveal the assembly of the amyloid molecules. Another important application of the R E D O R and RR methods is to determine protein structure. However, it is still difficult to determine the three-dimensional structure of the whole protein molecule using these methods. Instead, Schaefer and coworkers [42-45] have determined the structure of the ligand and the binding site of the ligand protein complex. It is possible to determine the interatomic distance by the R E D O R method in


47

the case of the enzyme-analogue complex, because the reaction will not forward. However, it is difficult to measure the interatomic distances of the enzyme-substrate complex since they will react in a short time. Evans and coworkers [46, 47] freeze the reaction instantaneously and the structure of the intermediate state of the complex could be observed by the R E D O R method. Griffin and coworkers [48-50] have used the RR method to determine retinal configurations in various states of photointermediates in the membrane protein bacteriorhodopsin.

2.9.2

Syntheticpolymers

In contrast to biological systems, the accuracy of the measured distance is less stringent for synthetic polymers, because they do not form high quality crystals and it is not easy to perform specifically isotope label. Local packing in a glassy polycarbonate has been examined in a homogeneous mixture of 5% [Carbonyl-a3C]polycarbonate and 95% [methyl-d6]polycarbonate using R E D O R NMR. The 13C~2D distance from the carbonyl carbon of one polycarbonate chain to the average deuterium position of the closest methyl group of the nearest-neighbour polycarbonate chain is 3.8/k. These results indicate that one of the methyl groups is closer to the carbonate group than the other. It is also demonstrated that the distribution of intermolecular carbonate-isopropylidene distances in the glass is quite narrow [51]. In addition, 13C~2D distances of the average nearest-neighbour interchain ringcarbon to ring-deuterium, and ring-carbon to methyl-deuterium, were determined to be 2.6 and 3.2 A, respectively. The short ring-ring distance indicates that the phenyl groups are tightly packed. This fact is consistent with the fact that cooperative intermolecular motions are required for ring flips [52]. The existence of locally parallel chain segments were examined by means of DRAMA, C E D R A and DANTE-selective R E D O R experiments. In this system, the 13C--13C distance in pure [carbonyl-~3C]polycarbonate, and in a homogeneous blend of [carbonyl-X3C]polycarbonate-dx4 and [methyl13C]polycarbonate, were determined [53]. R E D O R was also applied to examine the structure and dynamics of interfaces of heterogeneous polymer blends. A heterogeneous blend was prepared from [carbonyl-13C]polycarbonate and poly(p-fluorostyren-co-styrene) copolymer of p-fluorostylene. The blend was formed by coprecipitation from chloroform into methanol. A fluorine dephased 13C R E D O R signal indicates that the 1 polycarbonate chain in 20 exists at the interface, suggesting that the polycarbonate phase is embedded in a continuous polystyrene matrix which is 200 A thick or 400/k in diameter [54].

48

AKIRA NAITO ET AL.

2.10

Concluding remarks

It is emphasized here that the accurate determination of the interatomic distances are a prerequisite to achieve the three-dimensional structure of peptides, proteins and macromolecules. A protocol for R E D O R and R R experiments for this purpose is described in depth from both the theoretical and practical points of view. In addition, a systematic procedure to construct the three-dimensional structure from these distance constraints is described together with a brief review of some related subjects so far reported. We believe that the measurement of accurate interatomic distances can be the most promising and valuable means to reveal the three-dimensional structure of macromolecules such as membrane proteins in the future.

References

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7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

T. Gullion and J. Schaefer, Adv. Magn. Reson. 13 (1989) 57. E. Bennett, R.G. Griffin and S. Vega, NMR 33 (1994) 1. T. Gullion and J. Schaefer, J. Magn. Reson. 81 (1989) 196. D.P. Raleigh, M.H. Levitt and R.G. Griffin, Chem. Phys. Lett. 146 (1988) 71. K.T. Mueller, T.P. Javie, D.J. Aurentz and B.W. Roberts, Chem. Phys. Lett. 242 (1995) 535. T.P. Javie, G.T. Went and K.T. Mueller, J. Am. Chem. Soc. 118 (1996) 5330. M.H. Levitt, D.P. Raleigh, F. Creuzet and R.G. Griffin, J. Chem. Phys. 92 (1990) 6347. A.W. Hing, S. Vega and J. Schaefer, J. Magn. Reson. 96 (1992) 205. R. Tycko and G. Dabbagh, Chem. Phys. Lett. 173 (1990) 461. B-Q. Sun, P.R. Costa, D. Kocisko, P.T. Lansbury, Jr. and R.G. Griffin, J. Chem. Phys. 102 (1995) 702. T. Gullion and S. Vega, Chem. Phys. Lett. 194 (1992) 423. A.E. Bennett, J.H. Ok, R.G. Griffin and S. Vega, J. Chem. Phys. 96 (1992) 8624. G.J. Boender, J. Raap, S. Prytulla, H. Oschkinat and H.J.M. de Groot, Chem. Phys. Lett. 237 (1995) 502. T. Fujiwara, K. Sugase, M. Kainosho, A. Ono and H. Akutsu, J. Am. Chem. Soc. 117 (1995) 11351. K. Schmidt-Rohr,Macromolecules 29 (1996) 3975. M. Baldus, R.J. Iuliucci and B.M. Meier, J. Am. Chem. Soc. 119 (1997) 1121. D.P. Weliky and R. Tyco, J. Am. Chem. Soc. 118 (1996) 8487. A. Naito, K. Nishimura, S. Kimura, S. Tuzi, M. Aida, N. Yasuoka and H. Sait6, J. Phys. Chem. 100 (1996) 14995. A. Naito, K. Nishimura, S. Tuzi and H. Sait6, Chem. Phys. Lett. 229 (1994) 506. T. Gullion and J. Schaefer, J. Magn. Reson. 92 (1991) 439. Y. Pan, T. Gullion and J. Schaefer. J. Magn. Reson. 90 (1990) 330. J.R. Garbow and C.A. McWherter, J. Am. Chem. Soc. 115 (1993) 238. D. Suwelack, W.P. Rothwell and J.S. Waugh, J. Chem. Phys. 73 (1980) 2559. W.P. Rothwell and J.S. Waugh, J. Chem. Phys. 74 (1981) 2721.


49

25. A. Naito, A. Fukutani, M. Uitdehaag, S. Tuzi and H. Sait6, J. Mol. Struc. 441 (1998) 231. 26. M. Kamihira, A. Naito, K. Nishimura, S. Tuzi and H. Sait6, J. Phys. Chem. (in press). 27. K. Nishimura, A. Naito, C. Hashimoto, S. Tuzi and H. Sait6 (manuscript in preparation). 28. O.B. Peersen, M. Groesbeek, S. Aimoto and S.O. Smith, J. Am. Chem. Soc. 117 (1995) 7728. 29. A. Kubo and C.A. McDowell, J. Chem. Soc. (Faraday Trans 1) 84 (1988) 3713. 30. K. Takegoshi, K. Nomura and T. Terao, Chem. Phys. Lett. 232 (1995) 424. 31. G.R. Marshall, D.P. Beusen, K. Kociolek, A.S. Redlinski, M.T. Leplawy and J. Schaefer, J. Am. Chem. Soc. 112 (1990) 4963. 32. S.M. Holl, G.R. Marshall, D.P. Beusen, K. Kociolek, A.S. Redlinski, M.T. Leplway, R. Makey, S. Vega and J. Schaefer, J. Am. Chem. Soc. 114 (1992) 4830. 33. A.W. Hing and J. Schaefer, Biochemistry 32 (1993) 7593. 34. D.J. Hirsh, J. Hammer, W.L. Maloy, J. Blazyk and J. Schaefer, Biochemistry 35 (1996) 12733. 35. J.R. Garbow, M. Breslav, O. Antohi and F. Naider, Biochemistry 33 (1994) 10094. 36. R.C. Anderson, T. Gullion, J.M. Joers, M. Shepiro, E.B. Villhauer and H.P. Weber J. Am. Chem. Soc. 117 (1995) 10546. 37. S.K. Brahmachari, T.N. Bhat, V. Sudhakar, M. Vijayan, R.S. Rapaka, R.S. Bhatnagar and VS. Aranthanarayanan, J. Am. Chem. Soc. 103 (1981) 1703. 38. I.Z. von Siemion, T. Wieland bs K-H. Pook, Angew. Chem. 87 (1975) 712. 39. M. Aida, A. Naito and H. Sait6, J. Mol. Struc. Theochem 388 (1996) 187. 40. P.T. Lansbury, Jr., P.R. Costa, J.M. Griffiths, E.J. Simon, M. Auger, K.J. Halverson, D.A. Kocisko, Z.S. Hendsch, T.T. Ashburn, R.G.S. Spencer, B. Tidor and RG. Griffin, Nature Structural Biology 2 (1995) 990. 41. J.M. Griffiths, T.T. Ashburn, M. Auger, P.R. Costa, R.G. Griffin and PT. Lansbury, Jr., J. Am. Chem. Soc. 117 (1995) 3539. 42. A.W. Hing, N. Tjandra, P.F. Cottam, J. Schaefer and C. Ho, Biochemistry 33 (1994) 8651. 43. A.M. Christensen and J. Schaefer, Biochemistry 32 (1993) 2868. 44. L.M. McDowell, A. Schmidt, E.R. Cohen, D.R. Studelska and J. Schaefer, J. Mol. Biol. 256 (1996) 160. 45. L.M. McDowell, C.K. Klug, D.D. Beusen and J. Schaefer, Biochemistry 35 (1996) 5396. 46. Y. Li, R.J. Appleyard, W.A. Shuttleworth and J.N.S. Evans, J. Am. Chem. Soc. 116 (1994) 10799. 47. Y. Li, F. Krekel, C.A. Ramilo, N. Amrhein and J.N.S. Evans, FEBS Lett. 377 (1995) 208. 48. L.K. Thompsom, A.E. McDermott, J. Raap, C.M. van der Wielen, J. Lugtenburg, J. Herzfeld and R.G. Griffin, Biochemistry 31 (1992) 7931. 49. K.V. Lakshmi, M. Auger, J. Raap, J. Lugtenburg, R.G. Griffin and J. Hertzfeld J. Am. Chem. Soc. 115 (1993) 8515. 50. K.V. Lakshmi, M.R. Farrar, J. Raap, J. Lugtenburg, R.G. Griffin and J. Herzfeld, Biochemistry 33 (1994) 8854. 51. A. Schmidt, T. Kowalewski and J. Schaefer, Macromolecules 25 (1993) 1729. 52. P.L. Lee and J. Schaefer, Macromolecules 28 (1995) 1921. 53. C.A. Klug, W. Zhn, K. Tasaki and J. Schaefer, Macromolecules 30 (1997) 1734. 54. G. Tong, Y. Pan, M. Afeworki, M.D. Poliks and J. Schaefer, Macromolecules 28 (1995) 1719.


Chapter 3

Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved

NMR Relaxations and Dynamics F. Horii Institute for Chemical Research, Kyoto University, Uji, Kyoto 611, Japan

3.1

Introduction

NMR observations basically contain spin relaxation processes which are associated with molecular motions with different specific frequencies in a given system. For quantitative measurements to determine the compositions of the system or selective measurements of particular components with different relaxation parameters, it is essential, therefore, to understand the principle of the relaxation mechanism. When our interest is focused on molecular motions, spin relaxation parameters such as spin-lattice relaxation time T~, spin-spin relaxation time T2, and the nuclear Overhauser enhancement (NOE), are directly measured as a function of temperature or field frequency by using appropriate pulse sequences. Such temperature or frequency dependencies of the spin relaxation parameters are analyzed in terms of appropriate models to obtain detailed information of molecular motions with frequencies of 106-1012 Hz in the system. In this chapter, the basic theories and analyses for the spin relaxation parameters are described somewhat in detail. As shown in Fig. 3.1, NMR can also probe a wide range of frequencies for molecular motions which are reflected differently on various NMR parameters. In particular, spectra reflecting the chemical shift anisotropy (CSA), the C ~ H dipolar interaction, and the 2H quadrupolar interaction are sensitive to the mid-range of frequencies, which are closely associated with important properties for glassy polymers such as impact strength and gas permeability. The basic equations and analyses for the 2H and 13C CSA spectra are also described in this chapter.

3.2

Spin relaxation parameters

The total Hamiltonian of the system containing spins I is generally given by the following equation [1]:

52

F. HORII

Fig. 3.1. Frequencies of the molecular motion detectable by NMR spectroscopy.

Ht-- Hz + Ho + Ho + Hs + Hj.

(3.1)

Here, Hz is the Zeeman term, Ho is the quadrupolar interaction term for nuclei with 1 i> 1, HD is the dipolar interaction term for nuclei with I = 1/2, Hs is the electron shielding term and Hj is the J-coupling term. Spin relaxations will be induced by the time fluctuations of these interaction terms. For example, 2H spin-lattice relaxation behaviour is dominated by Ho, whereas Ho mainly determines the relaxation process of the 1H or 13C magnetization in organic materials. In some cases without significant contributions from H o and HD, the time fluctuations of Hs and Hj also induce spin relaxation; for example, the magnetization of a carbonyl carbon with a large chemical shift anisotropy relaxes due to the contribution from the time fluctuation of Hs. Nevertheless, since the main interest of polymer scientists is 13C NMR, we focus on the description of the 13C relaxation process in this chapter. 3.2.1

Basic theory

In the case of conventional organic polymers, 13C T1, T2 and NOE values are determined mainly by the time fluctuation of the magnetic dipole-dipole

NMR RELAXATIONS AND DYNAMICS

53

interaction between the ~3C and 1H nuclei. Therefore, the spin relaxation theory for the 13Cm1H two-spin system is described here in some detail [13]. 3.2.1.1 Transitions among eigenstates and spin relaxation parameters According to traditional conventions, a two-spin system consists of spins, I and S, with spin I of 1/2 and these spins are coupled with each other through the magnetic dipole-dipole interaction (hereafter, I and S correspond to 1H and 13C nuclei, respectively). When two eigenstates of the spins defined with respect to the direction of the static magnetic field (the z direction), which correspond to their energy eigenstates, are expressed as [+ ) and i - ), the corresponding eigenstates in the z direction for the two-spin system composed of I and S are described as

I+ + > , 1 + - > , 1 - +>,1

(3.2)

>,

Assuming that there are enough numbers of these two spins in the system, and that rapid transitions occur among these eigenstates as shown in Fig. 3.2, then T1 and N O E can be described in terms of transition probabilities, w0, w~ and w2, among these eigenstates as follows: 1 T 1 --

(3.3)

,

Wo + 2w~ + w2

w

I+->

--

Wo

w,

!-+>

I+ + ~ Fig. 3.2. Four eigenstates defined for the two-spin system with respect to the direction of the static magnetic field.

54

F. HORII

)/

U 1'

u

[+ -)x ~

= I-

Ho

l+

Fig. 3.3. Four states defined for the two-spin system with respect to the direction perpendicular to the static magnetic field.

NOE = 1 +

1422- 1420

TH

9 , Wo + 2w~ + w 2 ')/c

(3.4)

where yi-i and Yc are the gyromagnetic ratios of the XH and ~3C nuclei, respectively. On the other hand, similar four states can also be defined in the direction perpendicular to the static magnetic field, for example, in the x direction, as shown in Fig. 3.3. However, these states are not eigenstates but general states which can be described by linear combinations of the four eigenstates in the z direction shown in Equation (3.2). In this case, T2 is expressed by the following equation using transition probabilities, Uo, u~ and u2, among these four states as shown in Fig. 3.3.

=

1

.

(3.5)

Uo + 2u~ + H2

3.2.1.2 Transition probabilities The total Hamiltonian H t of the system is assumed to be composed of the unperturbed Hamiltonian Ho and the perturbed Hamiltonian H' as follows"

Ht = Ho + H ' .

(3.6)

NMR R E L A X A T I O N S AND DYNAMICS

55

Here, the contribution from H' will be fairly small compared to that from Ho, and the time fluctuation of H' is assumed to induce the transitions among the eigenstates determined by Ho. According to perturbation theory, the respective transition probabilities among the eigenstates, which appear in Equations (3.3)-(3.5), are given by

1 f~

Wij = --~

(n~ I H' *(t + ~) [nj)(nj l H'(t) l n~) exp(-iwq~-) dr

(3.7)

where wq = (Ej - E~)/h, and E; and Ej are the energies for the states In)) and ]nj), respectively. These states correspond to the states shown in Figs. 3.2 and 3.3. The upper bar in Equation (3.7) indicates the ensemble average for spins and the asterisk denotes the complex conjugate. In conventional polymer systems, H' corresponds to the magnetic dipoledipole interaction HD between 13C and 1H nuclei. As is well known, HD is expressed in terms of the spin operator functions Oq'S and the orientation functions Fq's of the 13C~IH internuclear vector against the static magnetic field Bo as follows: 2

HD

=

2 2 2 -3 YcYHh r

E

F_qOq,

(3.8)

q=--2

with Fo = 1 -

3n 2

F+_1 -- (l +- im)n F+_2 = (l + i m ) 2

(3.9)

~0 = IxSx- (1/4)(I+S - + I - S +) ~+1 = (-3/2)(I+S~ + IxS ) ~+2 = (-3/4)1+S + 9

(3.10)

Here 1, m and n are the direction cosines of the 13C~IH internUclear vector in the laboratory frame, as shown in Fig. 3.4, and r is the internuclear distance. The substitution of these equations into Equation (3.7) gives the following equation:

56

F. HORII

/' l

B0

H

r

xJ Fig. 3.4.

Jc r = li + mj + nk

A schematic representation of the C m H internuclear vector in the

xyz

coordinate.

wi] - ]/2T2h2r-6 s (ni l ~ ' I nj)(gt][~qlni) q ,q' x ;~~ F*q,(t + r)F_q(t) exp(-iw~ff) d r .

(3.11)

Here, it is assumed that the distance r is a constant independent of time. After the time-consuming calculations for Equations (3.3)-(3.5) and (3.11), T1, T2, and N O E can finally be expressed by the auto-correlation functions Gq('r) of the orientation functions Fq, which describe the random time fluctuation of the C ~ H vector, or by the spectral densities Jq(o)) that are the Fourier transforms of Gq(7) with frequency o) as follows: 1

NT1 1 .....

NT2

2 2 ,-2 YHYCn

~

16r 6 2

{Jo((.OH- a)c) + 18Jl(Coc) + 9J2(O)H + WE)},

2 3,2

THTC n

36r 6

{4Jo(0) + JO(WH -- we) + 18J1(o9c) + 36J1(wi-i)

+ 9J2(O)H -t- O)c)}, NOE = 1 +

(3.12)

9J2(09H + OgC) -- Jo~on + ~Oc)

(3.13) "}/H

9 . Jo(OgH- Wc) + 18Jl(WC) + 9J2(OgH + Wc) Yc

(3.14)

Here, N is the number of protons chemically bonded to a given carbon and O)H and o)c are the Larmor frequencies of the 1H and 13Cnuclei, respectively. Moreover, the correlation functions Gq(7") and the spectral densities Jq(o)) are described as follows"


G q ( r ) = F~(t + r)Fq(t)

57 (3.15)

q = 0, 1, 2

(3.16)

Jq(o)) = I-~7 G(r) e x p ( - i w r ) d r . d-o~

3.2.1.3 Molecular motion models A large number of structural models describing the random motions of the C ~ H internuclear vector have already been proposed for the calculations of Gq(7") or Jq(o)). In this section, the equations for Jq(m) are shown for some representative structural models as well as for different levels of model-free treatments. (a) Single correlation-time model. When the C ~ H internuclear vector undergoes isotropic random motion, the following well-known equations for Jq((.o) a r e obtained:

Jq(~O) = Kq

2Tc

2 2

l+wrc

K0 = 4/5, K1 = 2/15, K2 = 8/15.

(3.17)

Here, rc is the correlation time for the molecular motion of the C ~ H vector, meaning the time (or the life time) in which the C ~ H vector stays in the same direction without any motion. Figure 3.5 shows T1, T2 and NOE as functions of rc, which are obtained using Equation (3.17). (b) Distributions of correlation times rc. When the system in question is an ensemble of spin systems with different rc values, the distribution functions P(rc) for re should be introduced. In this c a s e , Jq(r are given by

Jq(~O) = K q

2P(rc)rc drc 1 q- 022Y 2

P(rc) drc = 1.

(3.18)

For example, in the box-type distribution P(rc) is expressed as In P(rc) - (In ,c){ln(ln_~e)-

Tcs ~ T c ~ ETcs

otherwise

,

(3.19)

and then Jq(oJ) are described as a function of the average value -? of rc and the parameter 9 for the width of the distribution shown in Equation (3.19) as follows:

58

F. HORII

I00~

I

"

I

'

i

"'

10

I

"

I

'

~-!

6.3T

~

4 . 7 T ~

=]

o.11

1

I0-12 i0-~'

0 . 01 ...

101

i.

i0-7 I0-6

. i . . . . 10-:o 10-9 lO-S !

I

I'

! '

'

':

L

]"~ _-

/9.4

T

~ . 3 T

I0 -I st

_:

4.7 T " ~ " ~

I0-2 K 2

10-3

.

I .

i0-12 lO-,l

3.0

....

,

10-1o

~ .....

I

.

!

..IX.

10-9 lO'S 10-7 I0-6 j

i

~

" '

2.3T

u~

o Z

2.0-

4.7.7'

-

6.3 9.4

-

1.0 ,,, J .... ~ .... ~.. 10-12 lO-ll lO-lO 10-9

,

!

lO-S

,

-

!'i

10-7

,

" 10-6

rr Fig. 3.5 13C T1, T2 and NOE vs. ~c under different magnetic fields (Ref. [3]).

NMR RELAXATIONS AND DYNAMICS 2Kq

- 1

Jq(w) = ~ tan ~o In e

59

(.OT In(e)

1 + e ( e - 1)-2(ln e

)2 o2=2.

(3.20)

In t h e log-x 2 distribution [4], which is frequently used for polymer systems, the distribution function is expressed as P(s) ds = ~

1

r(p)

( p s ) p - l e - p s ds ,

(3.21)

where p is the parameter used to determine the width of the distribution, s is given by (3.22)

s = logb[1 + ( b - 1)Zc/'~],

where b is an adjustable parameter between 10-1000. In this case Jq(og) is 2Kq

Jq(cO) = ~ t a n co In e

- 1

O)T In(e)

2-2" 1 + e(e - 1)-2(ln E)zw z

(3.23)

(c) C o n f o r m a t i o n j u m p m o d e l s . Some models called conformation jump models were proposed to describe the segmental motion of polymers in more realistic ways. One of the representative conformation jump models is a model proposed by Hall and Helfand [5] and Weber and Helfand [6] (hereafter referred to as the H W H model). In this model, two kinds of conformations, + and - , are assumed along a polymer chain and each structural unit is assumed to undergo a single + ~ - jump motion and a co-operative +- ~-+ jump motion. If the correlation times of the single and cooperative jump motions are Zo and z~, the auto-correlation function Gq('r) is given by G q ( g ) - Kq

exp(-z/zo)exp(-z/Zl)Io(z/'r 1).

(3.24)

Here, I o ( r / r l ) is a modified Bessel function. This equation contains two kinds of correlation time and, therefore, it will correspond to the 2z model described in the following section. Dejean et al. [7], proposed another conformation jump model (referred to as the DLM model), in which a librational motion of the jump axis was introduced as the third motion. This model will correspond to the 3z model described below but the derivation of Gq('r) w a s empirically made in this case,

60

F. HORII Gq(T) = (1 -

A)exp(-z/'ro)exp(-'r/'rl)Io(z/'rl) + A exp(--'r/'rL). (3.25)

(d) Multiple correlation-time models. There are many kinds of models which describe the complicated segmental motion of polymeric chains in the solution and solid states. In generalized structural models for such motions, which are referred to as multiple-correlation-time models [8-10], the thermal fluctuation of the C ~ H internuclear vector should be described in terms of the superposition of several independent random motions. Let O1, 02, 9 9", Op_l be the rectangular coordinate systems defined in the respective motional units. Here, O I indicates the coordinate system used to describe the most local motion of the C ~ H vector and Op denotes the laboratory frame. The direction cosines in the laboratory frame, which appear in Equation (3.9), are then related to the direction cosines ll, m i, and F/1 of the C ~ H vector in the O1 coordinate system by the following equation"

t'mt t'1t =Tp...T3T2

ml

(3.26)

nl

where Tj are the 3 x 3 matrices of the orthogonal transformation from coordinate system Oj-1 to coordinate system Oj:

T~

t cos Sj cos 0j cos ~bj - sin ~. sin ~bj sin Sj cos 0j cos ~bj + cos Sj sin 4~j - s i n 0j cos ~bj - c o s Sj cos 0j sin ~bj - sin Sj sin ~bj

c~ 0J sinai 1 - s i n Sj cos 0j sin ~bj + cos Sj cos ~bj sin 0j sin 9 sin 0j sin ~bj COS Oj

(3.27)

Here, q~j, 0j and ~bj are the Euler angles that describe the Oj_ 1 coordinate system in the Oj coordinate system. The thermal fluctuation of the C ~ H internuclear vector should be expressed, therefore, by the time fluctuation of the Euler angles in each coordinate system in the multiple correlationtime models. Jq(tO) can finally be derived when the modes of molecular motions are defined in the respective coordinate systems. Woessner [11] proposed such a molecular motion model whereby p = 2, which is hereafter referred to as 2z model. In this model the C ~ H vector


61

undergoes the diffusional rotation about the Z l axis in the O~ system, while the Zl axis independently changes in orientation by the isotropic random motion in the laboratory frame. Then Jq(tO) are given by

Jq(tO)

= Kq[A

k

2ri 1 + (.02T 2

-'[" B

2rl 27"2 ] 1 + w2r 2 + C 1 + w2r2J ,

(3.28)

with --1 1 --1 r x = r~- + rR , r2-1 = ri- 1 + 4rR 1 ,

(3.29)

and A = (3

COS 2 OR -

1)2/4,

B = 3 sin 2 0a cos 2 0 a , (3.30)

C = (3/4)sin 2 OR.

Here, rR and T I are the correlation times for the diffusional rotation and the isotropic random motion, respectively. OR is the angle between the C ~ H internuclear vector and the Zl axis. In contrast, Howarth [12] derived Jq(tO) for the 3r model corresponding to p = 3, where three independent motions are assumed to be superposed for the overall motion of the C ~ H vector as shown in Fig. 3.6. Namely, the C m H vector undergoes diffusional rotation about the Zl axis in the O1 frame, whereas the Zl axis librates within a cone whose axis is parallel to the z2 axis in the 02 frame. Moreover, the z2 axis undergoes the isotropic random reorientation in the laboratory frame. Although an empirical approximation was made in the previous calculation, we obtained the following equations by the exact mathematical derivation [8-10]:

2rl + AR(1 -- AL) 2rl 2r2 Jq(o)) = Kq ARAL 1 + o)2T2 1 + c02r2 + BRBL 1 + w2r22 +BR(1 -- BL)

273 2r4 + CRCL 1 + w2r 2 1 + w2r 2

+ CR(1

2r5 ] 1 + ~2r2

-

eL)

(3.31)

62

F. HORII Z2 ..-.

\x"-XN~_..

----~

r162 -'7/.4

,, /x

Z1

I Y2

X2 Fig. 3.6. A schematic representation of the 3r model describing the motion of the C - - H internuclear vector.

with --1 "/'1 : --1

r2

--1

r3

= ri = r{

--1 T4 =

--1

r5

TI 1

ri

= rI

1 1 1

1

+

--1 TL --1

+ rR

+ r{~

1

--1

+ rR

+ 4rR 1 + rE

1

+4rR

1

(3.32)

and ~ ( 1 + c o s 0L)2/4

AL

=

BL

- sin 2 0L(1 + c o s 0L)2/6

COS 2

CI~ = (2 + c o s 0L)2(1 -- COS 0 L ) 2 / 2 4 , AR

=

BR =

(3

COS 2 OR -

1)2/4

3 sin 2 OR cos 2 OR

(3.33)

NMR RELAXATIONS AND DYNAMICS CR = (3 sin 4 0R)/4 9

63 (3.34)

Here, 7R, 7L and '7"I are the correlation times for the diffusional rotation, libration and isotropic motion, respectively. OR and OL are halves of the vertical angles of the cones associated with the corresponding motions. The librational motion is defined here as time-dependent reorientation in which the Zl axis randomly changes in direction within the larger cone with the z2 axis as shown in Fig. 3.6. Equation (3.31) apparently differs from the Jq(0)) derived empirically but both Jq(0)) equally reduce to the following equation when ~'R ~ ~'L ~ ~'I: I 27~ J q ( w ) - Kq ARAL 1 + 092"/-2 nt- A R ( 1 -- AL) +BR

27"L 1 + 0)27 "2

2~'R +CR 2(~'R/4) -] 1 + 0)2T2R 1 + W2(7"R/4)2_]

(3.35)

This equation is expressed as a linear combination of Lorentzian contributions from the respective random motions, if the fourth term is assumed to be negligibly small. This indicates that Equation (3.35) is a model-free equation for three types of independently superposed random motions, which is in good accord with the results of the model-free treatments described in the following section, even though a specific structural model, shown in Fig. 3.6, was used for the derivation of Equation (3.35).

(e) Model-free treatments. In the polymer system, the single-correlation-time model is not valid and many kinds of models were proposed to explain the temperature or frequency dependencies of the spin relaxation parameters. Through these historical processes, it seems reasonable to introduce plural independent motions to describe the random motion of the C ~ H internuclear vector. Nevertheless, there may exist some limitation in using detailed structural models for the respective motions. In other words, it would be very important to examine the possibility of model-free treatments in which the number of independent motions and their correlation times are first considered without introducing detailed structural models for the respective motions. Here, the first- and second-order model-free treatments are described. (i) The first-order model-free treatment. The time fluctuation of the C - - H internuclear vector should be described in terms of the superposition of several independent random motions, and the autocorrelation function Gq(7)

64

F. HORII

is assumed to be a linear combination of the exponential decay contributions from the respective motions when their correlation times differ greatly from each other as follows: P

Oq('r) = Kq E Aj exp(-'r/~'cj).

(3.36)

j=l

Here, p is the number of random motions and ~'~/are the correlation times of the respective motions. Aj seem to be thc weights of the respective motions with EAj = 1, but are simply assumed to be adjustable parameters without giving any explicit physical meaning in the first-order model-free treatment [9, 13]. King and coworkers [14, 15] also derived Equation (3.36) in a more general fashion.

(ii) The second-order model-free treatment. The second-order model-free treatment will correspond to the treatment in which explicit physical meaning is given to the coefficients of the respective terms in Equation (3.36). Lipari and Szabo [16] proposed that the motion of the C ~ H vector can be expressed as the superposition of the overall isotropic motion and the anisotropic inner local motion, and that the total Gq(~') should be given by the product of the respective contributions as follows: Gq(,/') = G I ( T ) G A ( T ) , Gi(7" ) = Kq e x p ( - ~ ' / ~ - i ) , G A ( T ) = 8 2 + (1 - S 2) e x p ( - - ~ ' / ~ ' A )

9

(3.37)

S 2 is called the generalized order parameter that indicates the extent of the spatial limitation of the anisotropic motion. When T I T~2a. Recovery of magnetization of the crystalline domains, R(t), due to transfer of magnetization from noncrystalline to crystalline domains is monitored at variable times t.

The resulting recovery of magnetization of the crystalline domains, ~(t), is then monitored by a 90x "interrogation pulse" which again places the magnetization in the transverse plane for observation. The basic scheme is shown in Fig. 6.1.7. The recovery of magnetization of the crystalline domains, dO(t) = 1 - [ M x a ( t ) - Mxa(t ~ oo)]/[Mxa(t- O) - Mxa(t ~ ~)],

(6.1.20)

is fit to possible solutions of Equation (6.1.19) with appropriate boundary conditions. As an example of such a fit, Fig. 6.1.8 shows a comparison between the experimental recovery curve from Goldmen-Shen experiments on a semicrystalline polypropylene film (black points), and two different models [48], one which invokes two different phases (crystalline and noncrystalline), with different sizes for the crystalline and noncrystaline domains. The second is again a two-phase model, but allows a distribution of sizes among the do-

184

B.C. GERSTEIN 1.0

.a

1

I

!

!

!

\\

0.8 - ~~ O\ \\

O.6 -

~\\

e 0.4 -

~~---~~ Xke

2P-MD

eq.(33)

\x,x,~,.r /b = 61A o.2 -

2p-2D

eq.( 13)7. b = 89A 0.0 0.00

A/ = 0..~5

\\",,.o /

\" " . ~ _ ~. 9 9

9

,

i

',

,

,

0.05

0.10

0.15

0.20

0.25

4t

0.30

( s e c 1/2)

Fig. 6.1.8. Fit of recovery curves ~(t), on semicrystalline polypropylene film (black dots) to two different models of phases and domains [48]" two-phase, two-domain, and two-phase multidomain [48].

mains. The model with a distribution of domain sizes is found to best fit the data. A variant of the Goldman-Shen sequence to generate a magnetization gradient in a polymer system containing regions that do not vastly differ in dipolar broadening, is that of using a dipolar echo sequence with cycle time, t~, arranged to produce an average dipolar Hamilton which is zero for one portion of a polymer blend, but not for another. VanderHart [46] used this technique to study chain proximity in a blend of a dominantly aromatic polymer, e.g., a rigid-rod polymer, poly(benzo-[a,d]-dithiazol-2,6-diyl-l,4phenylene), PBZT, and a dominantly aliphatic polymer, e.g., nylon 6,6. The dipolar-broadened linewidth of the proton NMR spectrum for the pure nylon 6,6 is ca. 45 kHz. That of the PBZT is ca. 24 kHz. The pulse spacing of the dipolar echo sequence could be arranged, therefore, to average to zero the dipolar Hamiltonian for the proton-poor aromatic portion of the blend--that

1H NMR

0 ii

185

0,,

-NH(CH2)6 NHC(CH2)4Cnylon 6,6

1H Multiple Pulse

1H PBZT

S

-50

O kHz

50

10 O PPM

Fig. 6.1.9. 200 MHz spectra of aH pertaining to spin diffusion experiments on a nylon 6,6 PBZT blend (VanderHart [46]). Left, magnetization gradient created by dipolar echo sequence with spacing 30 Ixs, thus, initially favoring the PBZT portion of the blend. Right, results of spin diffusion observed using CRAMPS to obtain high resolution proton NMR of the blend, and observation of magnetization transfer between the phenyl protons of the PBZT, and the methylene protons of the nylon 6,6.

containing the PBZT--while allowing the magnetization of the proton-rich portion, the nylon 6,6, to dephase. Storage of the magnetization along the direction of the static field for a variable diffusion time with interrogation after this time could then monitor the diffusion of magnetization from the PBZT to the nylon 6,6. The left side of Fig. 6.1.9 illustrates the type of results obtained. On the left are the proton NMR at 200 MHZ, resulting from generating a polarization gradient using a single dipolar echo-pulse sequence [49], [90x, z, 90y] (echo) with pulse spacing, z, of 30 txs. During diffusion times, ranging from 50 IXS to 50 ms, magnetization diffusing from the PBZT to the nylon is reflected in the increasing linewidth of the spectrum of the blend with increasing diffusion

186

B.C. G E R S T E I N

time, thus, indicating close proximity of the two polymers in the blend. A more fine-grained view of the diffusion process in this system is provided by observing spin diffusion between chemically shifted species, e.g., the phenyl protons in the PBZT, and the methylene protons in the nylon. In order to observe this effect, a technique pioneered by Ernst and coworkers [50] was used, in which CRAMPS [26] was used to provide high-resolution NMR of chemically shifted protons, and storage of the magnetization for a variable diffusion time allowed for transmission of magnetization between aromatic protons in the PBZT, and methylene protons in the nylon 6,6. The results, as a function of diffusion times between 40 ~s and 100 ms, are shown on the right side of Fig. 6.1.9. Here, Mo refers to a scaled equilibrium spectrum of the blend, and "nylon" refers to the CRAMPS spectrum of pure nylon 6,6. Again, a gradient in magnetization was created by opening a window in the symmetrized dipolar echo sequences used to average the dipolar Hamiltonians, such that the magnetization of the nylon 6,6 could decay while retaining magnetization in the PBZT component. Storage along the static field for diffusion times varying between 40 ~s to 100 ms, followed by an interrogation period during which, again CRAMPS is used to provide the high-resolution proton NMR allowed observation of the increase in magnetization of the methylenes in the nylon 6,6 component at the expense of the aromatic protons in the PBZT. For variations on these ideas, the reader is referred to VanderHart [46]. As a final example of the use of proton NMR invoking spin diffusion to study miscibility of polymer blends, the use of CRAMPS to remove proton dipolar coupling in a blend of an aromatic poly(ether-imid) (PEI), and a poly(aryl-ether-ketone) (PEEK), with detection of the magnetization of the 13C in the blend under high resolution conditions is cited [51]. Here, detailed information on the chemical composition of the phases present, as inferred from high resolution NMR of 13C, is linked to typical sizes of domains as reflected in spin diffusion of proton magnetization. The basic idea in producing a gradient in the proton magnetization of chemically shifted protons is that under the multiple-pulse sequence used to average the proton dipolar coupling Hamiltonian to zero, the effective magnetic field about which chemically shifted spins precess is in the (1, 0, 1) direction in the rotating frame. This is to say that the spin portion of the average Hamiltonian associated with the chemical shift becomes (I~ + Iz) [52]. Two chemically shifted spins, initially polarized and aligned with each other along z, but precessing about the (1, 0, 1) direction of the effective field under the homonuclear decoupling sequence, can be "caught" after a suitable time period, such that one is aligned along z, the other along x. If

1H NMR

1H CRAMZPS

187

13C CP MAS

U.H

H

. UH

/,

_ tJ .---x

.

,

I

10

.....

!

. . . .

5

ppm

!

0

. . . . . .

H! !. . . . .

200

,H

Vl t

.

,U.H! ~

UU

! . . . .

150

!.

100

,_:._

f

50

.....

!

0

ppm

Fig. 6.1.10. Comparison of 1H CRAMPS and 13C high-resolution spectra of a blend of PEI (U) and PEEK (H), under cross-polarization from 1H to 13C, before and after, a chemical shift filter (see text) selectively destroys the magnetization of protons in the aromatic region between 5 and 10 ppm [51].

the homonuclear decoupling is turned off at this time, the effective magnetic field then becomes the static field along z. The chemically shifted spin aligned along z will then be stored, while the spin initially aligned along x will dephase in the x,y plane of the rotating frame under homonuclear dipolar coupling, thus creating a magnetization gradient between the two chemically shifted species. Then cross-polarization from IH to X3C will reflect enhanced magnetization for those carbons attached to the proton spins which have been stored along z; which have not dephased under dipolar coupling. Thus, a "chemical shift filter" is applied to the cross-polarization process. With appropriate times taken to allow spin diffusion to take place between the chemically shifted proton spins which are, and are not magnetized, and subsequent crosspolarization, the intensities of the observed high-resolution carbon signals can be utilized to monitor the diffusion process. This effect is illustrated in Fig. 6.1.10.

188

B.C. GERSTEIN

Acknowledgements I am grateful to Dr Peter Cheung, to Professors Karen Gleason, Klaus Schmidt-Rohr, Hans Spiess and to Dr Dave VanderHart for supplying me with timely examples of their work on proton NMR in polymers.

References

.

.

5.

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

B.C. Gerstein, Chapter 9 in R. Komorski (ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, VCH Publishers, Inc., 1986. C.R. Dybowski, Multipulse 1H and 19F techniques, in Solid State NMR of Polymers. Plenum, New York, 1991. NMR Spectroscopy of Polymers, R.N. Ibbett (ed.), Blackie Academic and Professional, New York, 1993. F.A. Bovey, NMR of Polymers. Academic Press, San Diego, 1996. V.J. McBrierty, Nuclear Magnetic Resonance in Solid Polymers. Cambridge University Press, Cambridge, U.K., 1993. K. Schmidt-Rohr and H.W. Spiess, Multidimensional Solid State NMR and Polymers, Academic Press, San Diego, 1994. A.N. Garroway, Polymer MRI, in The Encyclopedia of Nuclear Magnetic Resonance. Wiley, 1996. B.C. Gerstein and C.R. Dybowski, Transient Techniques in NMR of Solids; An Introduction to the Theory and Practice. Academic Press, Orlando, 1985. M. Goldman, Quantum Description of High Resolution NMR in Liquids. Oxford Press, 1988, Section 2.5. D.C. Champeney, Fourier Transforms and their Physical Applications. Academic Press, London, 1973. U. Haeberlen, High Resolution NMR in Solids, Selective Averaging, Supplement 1, Advances in Magnetic Resonance. New York: Academic Press, 1976. D.L. VanderHart, Magnetic susceptibility and high resolution NMR of liquids and Solids, in The Encyclopedia of Nuclear Magnetic Resonance. Wiley, 1996. U. Haeberlen and J.S. Waugh, Phys. Rev. 175 (1968) 453. J.S. Waugh, Average Hamiltonian Theory, in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996, and references therein. See Chapter 4 in Ref. [8]. R.M. Wilcox, Exponential operators and parameter differentiation in quantum physics, J. Math. Phys. 8 (1967) 962. B.C. Gerstein, Echos in Solids, in The Encyclopedia of NMR, Wiley, Chichester, 1996. R. Andrew, Magic angle spinning, in The Encyclopedia of NMR, Wiley, Chichester, 1996. Jian Zhi, Hu and R. Pugmire, Magic angle turning and hopping, in The Encyclopedia of NMR, Wiley, Chichester, 1996. Ajoy K. Roy and Karen K. Gleason, J. Polymer Science B32 (1994) 2235. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess J. Magn. Res., Series A 114 (1995) 264. Equation 2.161 in Ref. [8].

1H NMR 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52.

189

Son Jong Hwang and B.C. Gerstein, Bulletin of Magnetic Resonance 15 (1994) 213. Ref. [8], Equation (4.19). L.M. Ryan, R.E. Taylor, A.J. Paff and B.C. Gerstein, J. Chem. Phys. 72 (1980) 508. B.C. Gerstein, R.G. Pembelton, R.C. Wilson and L.M. Ryan, J. Chem. Phys. 66 (1977) 361. R. Prigl and U. Haeberlen, Advances in Magnetic and Optical Resonance, 19 (1996) 1. D.P. Burum and A. Bielecki, J. Magn. Res. 94 (1991) 645. D.P. Burum, HETCOR in Organic Solids, in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996. P.W. Anderson and P.R. Weiss, Rev. Mod. Phys. 25 (1953) 269. W.Z. Cai, K. Schmidt-Rohr, N. Egger, B. Gerharz and H. W. Spiess, Polymer 34 (1993) 267. A.K. Roy and K.K. Gleason, J. Polymer Science B32 (1994) 2235. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess, Chem. Phys. Lett. 227 (1994) 79. H. Green, J.J. Titman, J. Gottwald and H.W. Spiess, J. Magn. Res. All4 (1995) 264. J. Gottwald, D.E. Demco, R. Graf and H.W. Spiess, Chem. Phys. Lett. 243 (1995) 314. R. Graf, D.E. Demco, J. Gottwald, S Hafner and H.W. Spiess, J. Chem. Phys. 106 (1996) 885. D.W. McCall and D.C. Douglass, Polymer 4 (1963) 433. D.C. Douglass and G.P. Jones, J. Chem. Phys. 45 (1966) 956. B. Christ and A. Peterlin, J. Polym. Sci. Part A-2 7 (1969) 1165. G.E. Wardell, V.J. McBrierty and D.C. Douglass, J. Appl. Phys. 45 (1974) 3441. A.C. Lind J. Chem. Phys. 66 (1977) 3482. R.A. Assink, Macromolecules 11 (1978) 1233. V.J. McBrierty, D.C. Douglass and T.K. Kwei, Macromolecules 11 (1978) 1265. V.J. McBrierty, Faraday Discuss. Chem. Soc. 68 (1979) 78. T.T.P. Cheung and B.C. Gerstein, J. Appl. Phys. 52 (1981) 5517. D.L. VanderHart, Macromol. Chem., Macromol. Symp. 34 (1990) 125. M. Goldman and L. Shen, Phys. Rev. 144 (1966) 321. T.T. Peter Cheung, Polymer Prep. (Am. Chem. Soc., Div. Polym. Chem.) 38 (1997) 892. B.C. Gerstein, "Echos in Solids", in The Encyclopedia of Nuclear Magnetic Resonance, Wiley, 1996. P. Caravatti, P. Neuenschwander and R. Ernst, Macromolecules 18 (1985) 119. K. Schmidt-Rohr, J. Clauss, B. Bltimich and H.W. Spiess, Magnetic Resonance in Chemistry 28 (1990) $3. See pp 185-187 in Ref. [8] for the physical picture, and Table 5.7 therein for the mathematical result.

Chapter 6.2

Solid State NMR of Polymers, edited by I. Ando and T. Asakura Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All rights reserved

2H NMR A.S. Ulrich* and S.L. Grage Institut fiir Molekularbiologie, Friedrich-Schiller-Universitiit Jena, Winzerlaerstrasse 10, 07745 Jena, Germany

6.2.1

Introduction

This chapter provides an introduction to the characteristic N M R properties of the deuterium nucleus and presents an overview of various applications to investigate polymer structure and dynamics. Deuterium has a vanishingly low natural abundance and possesses a spin of 1, and these particular features can be turned to advantage. Selective labelling is employed to obtain local information about a specific site on the molecule. The quadrupolar interaction dominates the wideline 2 H N M R spectrum and, to a good approximation, the symmetry axis of this interaction is aligned along the C m Z H bond. Therefore, the quadrupolar lineshape and relaxation behaviour reveal the geometry and motion of the deuterated segment in a direct manner. In this chapter, we will first summarize some general 2 H N M R theory as a basis to understand the orientation-dependent spin interactions in powders as well as oriented samples. (Magic-angle spinning techniques are not discussed here.) Various cases of motionally averaged lineshapes will then be illustrated for the fast, intermediate and slow motional regimes. Different nuclear relaxation mechanisms are also described with a view to the respective motional timescales that can be addressed. Finally, we have compiled an overview of some important pulse sequences for one- and two-dimensional wideline experiments. The manipulation of the spin-1 system is visualized here in terms of a vector model based on a simplified density operator framework. For further information, the interested reader is referred to the more detailed, excellent reviews that have been published over the past two decades o n 2 H N M R of synthetic polymers and liquid crystals [1-13], as well as on biopolymers and lipid membranes [14-22]. Deuterium labels can be introduced synthetically into the polymer backbone or side-chains, or deuterated solutes can be infused into the matrix. Thus, the labelled molecular segment is analyzed in terms of its structural and dynamic properties, and the influence of environment and experimental *Corresponding author: [email protected]

2H NMR

191

conditions can be examined. 2H NMR covers an extremely wide range of motional timescales, such as side-chain flips, backbone dynamics or diffusion processes. The degree of motional averaging may be interpreted in terms of molecular order, e.g., to discriminate between crystalline or amorphous regions within the polymer. When a uniaxially oriented sample is available, such as a drawn fibre or a liquid crystal, it is possible to measure the bond angle of the deuterated segment with respect to the macroscopic axis. Thus, 2H NMR is ideally suited to address many types of questions. Some recent examples from the past couple of years, representing by no means a comprehensive list, have been concerned with polymer phase transitions [23-28], molecular miscibility and heterogeneity [29-32], backbone and side-chain flexibility [33-40], molecular structure of oriented samples and quality of alignment [41-50], self-diffusion and diffusion of solutes [51-54], or specific aspects about molecular weight or chemical structure of cross-links and end-groups [55-59]. It ought to be emphasized that biological materials such as lipid membranes and polypeptides have been studied by similar approaches as synthetic polymers, and many 2H NMR applications have benefitted from the mutual stimulus of these complementary areas. With the exception of its intrinsically low sensitivity (less than 1% compared to 1H), deuterium offers distinct advantages due to the ease of data collection, spectral analysis and the detailed information provided [1-5]. 9 Deuterium is an essentially nonperturbing probe when substituted for a proton. It can be synthetically incorporated into virtually any site of interest on a polymer, and at relatively low cost compared to ~3C. 9 In view of the dominant quadrupolar interaction of deuterium, its weak dipolar and chemical shift interactions can be neglected, which simplifies the theoretical analysis of the quadrupolar lineshapes and relaxation data (see Sections 6.2.2 and 6.2.3). 9 The quadrupole splitting of a suitably oriented sample represents the angle of the deuterated C~2H segment relative to the axis of alignment. Thus, quasi-atomic coordinates within the molecule can be determined, and the angular distribution of the sample estimated (see Section 6.2.4). 9 The 2H NMR lineshape readily reveals molecular anisotropy and motional averaging at a segmental level. It provides information about fast and intermediate dynamics, and about molecular order (see Section 6.2.5). 9 Relaxation experiments and 2D exchange spectra cover an extremely wide range of motional regimes (101~ -2 Hz), from fast methyl rotation to molecular self-diffusion. Concise information can be obtained about the type of motion, its geometry and its frequency (see Sections 6.2.5 and 6.2.6).

192

A.S. ULRICH AND S.L. GRAGE

9 In terms of spectrometer hardware, short high-power pulses are required to cover a spectral width of up to a few hundred kHz, and fast digitizing is essential for echo experiments [13, 19]. Since T~ relaxation proceeds efficiently by a quadrupolar mechanism, rapid data acquisition is possible. 1H decoupling is not necessary, unlike the case with many other nuclei. 9 To obtain quantitative information, computer lineshape simulations may be essential. Routines and algorithms are available for most evaluations, and in all figures shown below, we are using computed spectra to illustrate the characteristic features of the 2H NMR experiments.

6.2.2

General 2H NMR theory

In the following section, the transition energies between the three quantized energy levels of deuterium (spin I = 1) are derived, in order to explain the orientation-dependence of the quadrupole splitting [1-5]. The total energy of a deuterium nucleus in a static magnetic field contains contributions from the Zeeman interaction (Hz), dipolar interactions (HD), scalar coupling (Hj), the chemical shift interaction (Hcs), and the electric quadrupolar interaction (Ho). Therefore, the complete spin Hamiltonian is given by [59] H = Hz + HD + H j + Hcs + H Q .

(6.2.1)

The Zeeman Hamiltonian Hz describes the interaction between the nuclear magnetic moment, /XN, and the external magnetic field Bo. It determines the Larmor frequency COoof deuterium, which is, for example, 76.8 MHz at 11.7 T (corresponding to a 500-MHz spectrometer). The chemical shift range for deuterium, as well as its dipolar and scalar interactions, are of the order of only a few ( V2

1

Fig. 6.2.1. Top: 2H N M R powder lineshape, which arises from the spherical distribution of

C--2H bonds over all directions in space. Selected frequencies are marked by arrows for some of the bond orientations that contribute to the hatched ~'+ transition. For example, the characteristic peak corresponds to 0 - 90 ~ Bottom: In an oriented sample, the C - - 2 H bonds form a cone with an opening angle 7. When the sample axis is aligned parallel to the external field Bo (c~ = 0~ 7 can be calculated from the quadrupole splitting according to Equation (6.2.8). Positive and negative splittings cannot be distinguished, however, as shown here for the case of 7 - 40~ (left), and 7 - 73~ (right), unless the sample is tilted.

196

A.S. U L R I C H AND S.L. G R A G E

dN=(N) 47rr 2 27rr sin 0 r dO = I N sin 0 dO.

(6.2.9)

Thus, the probability density corresponds to p(O)= (sin 0)/2. Defining the reduced resonance frequency ~'___in units of Co (see Equation (6.2.8)), it is

+(3cos20 1)2

(6.2.10)

The frequencies, ~'+ and ~'_, for the two transitions (Am = _ 1) are centred about the spectral origin, andl are defined within the respective regions of 1 >i ~+ >I - ~ and - 1 ~< ~'_ ~< ~. Thus, the lineshape of the powder pattern is given by the probability function p ( ~ ' ) = p + ( s r) +p_(~'). Here, p(g)dg describes the fraction of signal between ~" and sr + ds~, which contributes an intensity p_ from the transitions ~'+_. The two probability densities p(O) and p_+(sr_+) are related to each other according to dO 1 dO p+_(~+_) = p(O) ~ = -sin 0~ = dsr___ 2 dsr___

I d cos 0

2 d~'___

.

(6.2.11)

It follows from Equations (6.2.10) and (6.2.11) that [21]

1 P---(sr+-) ~ v~/_+"-+ zg + 1"

(6.2.12)

The powder lineshape p(sr) has two characteristic singularities at frequencies sr = _+1, which correspond to bonds inclined at 0 = 90 ~ with respect to Bo. Note that the splitting of a completely rigid powder sample is equal to Co ~ 125 kHz for aliphatic bonds, and it is reduced by molecular motions (see Section 6.2.5).

6.2.4

Lineshape analysis of oriented samples

When an oriented sample is available, such as a drawn fibre, a stretched polymer film, or an anisotropic liquid crystal [1-5, 12, 41-50], it is possible to measure the orientation of the labelled C ~ 2 H bond vector with respect to the sample axis. When local orientational constraints are collected for several connected groups in a polymer, they can be combined to calculate

2H NMR

197

its three-dimensional molecular structure. This procedure is equivalent to determining not only the relative atomic coordinates of the molecule but also placing them into the reference frame given by the sample axis. Furthermore, it is straightforward to estimate the quality of alignment in a fibre, and thus to discriminate between crystalline and amorphous regions. In a uniaxially oriented sample, the C~2H bond vectors form a cone with an opening angle y around the axis of ordering. The lower part of Fig. 6.2.1 illustrates the case of two fibres, which are measured at a series of different inclinations c~ between the sample axis and the spectrometer field direction Bo. When the fibre axis is aligned parallel to Bo (a = 0~ all C~2H bonds within the sample give rise to the same quadrupole splitting (0 = y). Since this situation may be regarded as a "single crystal", the angle y can be calculated directly from the quadrupole splitting A vo. However, for the range of angles between 35 ~ and 90 ~ there are two possible solutions to Equation (6.2.8), since positive and negative values of Avo cannot be distinguished experimentally. This ambiguity is illustrated in Fig. 6.2.1, where two different intramolecular geometries (y = 40 ~ and 73 ~ give rise to the same splitting at = 0 ~ The two different bond angles can nevertheless be discriminated by measuring a tilt series at different sample inclinations, and by analyzing the more complex lineshapes. In a tilted fibre (where a # 0 ~ the orientation of the C~2H bond vector assumes a range of values for 0, which all contribute to the resulting lineshape. In analogy to the powder lineshape derived in Section 6.2.3, the equation for an oriented lineshape is given as a function of the reduced resonance frequency ~'_+ [43] p __(g'__)

X/+-2~'+-+l 3

/X/+-2(3+l_cos(c~+y)~/cos(]

X/---2•'+_ + 1

,

.

3

(6.2.13) This lineshape function possesses up to three singularities for each of the two transitions ~'+ and ~'_. The positions of these peaks depend on the value of y and vary with the sample inclination c~. Representative 2H NMR lineshapes are shown in Fig. 6.2.1 for the two formerly indistinguishable cases of y = 40 ~ and 73 ~ Differences in the lineshapes become apparent when tilting the sample away from a = 0 ~ so the two possible solutions of y can now be clearly discriminated [41-44].

198


Experimental 2H NMR spectra are broadened significantly compared to the theoretical lineshapes shown in Fig. 6.2.1, as a result of various effects. In addition to the intrinsic linewidth, an oriented spectrum is broadened further when the molecules in the sample are not perfectly aligned. The degree of microscopic disorder in the sample can be estimated from the linewidth of the oriented spectrum at a = 0~ [12, 42, 43]. When the intrinsic linewidth is known from the powder pattern, a comparison provides the orientational distribution of the aligned sample.

6.2.5 Motionally averaged lineshapes 2H NMR is ideally suited to explore molecular motions in the polymer. Different types of motion can be discriminated on behalf of their timescale and geometry of exchange. One-dimensional quadrupole echo lineshapes (see Section 6.2.7.1) are particularly sensitive to segmental dynamics [1-6, 912], when there is either fast exchange between discrete geometries (with ~-c~ 1/Avo) or when the motion occurs on the intermediate timescale (~c ~ 1/Avo). Dynamic processes in the intermediate to slow motional limit (~-c>> 1/Az,o) are addressed by 2D exchange spectroscopy (see Section 6.2.7.4). This approach offers a detailed and model-free description of the distribution of jump-angles, and can cover an enormous range of timescales only limited by relaxation [1, 4, 61-64]. In the following section, both 1D and 2D 2H NMR lineshape analysis will be described. Any motion on the fast or intermediate timescale changes the appearance of the static powder spectrum, as shown in Fig. 6.2.2 on the left. When the orientation of the C ~ 2 H bond gets rapidly averaged around a symmetry axis (with ~-c~ 1/Az,o), the spectrum retains its axially symmetric powder lineshape but its width is narrowed by a geometric factor. Given that 6 is the new effective angle between the motional symmetry axis and B0, Equation (6.2.8) can be deconvoluted into a product of several factors, where a time average is denoted by angular brackets [19, 21]

(ApQ) =

CQ(3 COS20 -

1)= CoO cos2 (~ - 1)/(3 \

COS2 t.,t~ --

2

1).\ /

(6.2.14)

For the most simple case of fast continuous rotation around a well-defined axis,/3 represents the unique angle between the C ~ 2 H bond and the motional symmetry axis. The narrowing factor is (3 cos 2 / 3 - 1)/2, because a tensor rather than a vector is being averaged. For example, rotation of a tetrahedral

2H NMR

static ~ powder

0

199

pr

_.../----

I I ~ ] rJ~~

rotating methyl

1"1'- 0 T- 70.5~

~

rotating phenyl

rl'-0 y - 60 ~

rotating water

TI'= 0

2-site flip of phenyl

1"1'=0.6 [3= 120~

2-site flip of water

TI'---- 1

~/= 54.7 ~

13= 109.5~

[ I

-1

I

t I

0

I

I

1

Fig. 6.2.2. Left: Simulated 2H NMR lineshapes that are averaged by various characteristic segmental motions. In the case of fast rotation, y represents the angle between the rotation axis and the C--2H bond. For a two-site jump, /3 denotes the angle between the C--2H bond in the two configurations, and the effective asymmetry parameter becomes ~'~= 0. Right: Calculated 2D exchange spectra for a two-site jump with /3 = 120~ (top), and for continuous diffusion (bottom). The distribution functions P(/3) of the reorientation angle are shown, together with the contour maps of the corresponding spectra. All data are displayed on the reduced frequency scale in units of Co, and mixing times '7" m are set equal to the motional correlation time rc.

200


methyl group around its intramolecular angle of 109.5 ~ causes a narrowing by a factor of - 1 / 3 , as seen in Fig. 6.2.2. Rotation around the magic angle, as shown for the case of a water molecule, even leads to a virtual collapse of the quadrupole splitting, but this is only due to the geometric term rather than free tumbling. Fast anisotropic fluctuations without any well-defined geometry, such as librational or diffusive motions around a symmetry axis, also lead to a narrowing of the powder pattern. However, in this case the variation of/3 over a range of values gives a more complicated time-average of (3 cos 2 / 3 - 1) that cannot be calculated analytically. Thus, it is convenient to describe the extent of averaging in terms of a segmental or local order parameter, which is defined as Sz~ = (3 COS2 / ~ - 1)/2 [19-21]. The molecular order parameter of a structural entity describes the librational disorder in a selected reference frame [65]. Any known contribution from a well-defined rotational axis needs to be factorized out according to Equation (6.2.14). Used this way, deuterium order parameters can provide much qualitative information, for example about phase transitions or phase separations in a polymer, or about the mobility profile along a chain. One-dimensional quadrupole echo 2H NMR lineshape analysis of powder samples is particularly informative when fast, discrete jumps occur between sites of well-defined geometry as, for example, in a phenyl group undergoing two-site exchange. In this case, the characteristic Pake-pattern is transformed into an axially asymmetric lineshape with an apparent asymmetry parameter r l ' # 0 (see Equation (6.2.3)) [1-8]. The asymmetric lineshapes, shown on the left in Fig. 6.2.2, can be derived by considering how the individual components of the principal EFG tensor become averaged by the discrete jumps. Within the molecular frame, and in units of V~ as defined by Equation (6.2.2), the static axially symmetric tensor consists of the components Vzz = 1, V~ = - 1/2, and Vyy = - 1/2. This traceless tensor satisfies the condition V~ + Vyy + V~ = 0 and 77 = 0. Note that the unique V~z component lies along the C m Z H bond. During the two-site jump of a 2,3,5,6deuterated phenyl ring, for example, the Vz~ component is averaged around an angle of /3= 120 ~ which causes a reduction by a factor of (3 cos2(120 ~ - 1)/2, giving - 1 / 8 . The principal component that corresponds to the plane perpendicular to the jump axis, on the other hand, remains unaffected at - 1 / 2 . Since the trace of the EFG must remain zero, the third principal component becomes 5/8. Hence, the effective asymmetry parameter of the motionally averaged tensor is calculated to be rt' = 0.6 for the phenyl ring undergoing fast two-site exchange, and the resulting spectrum is shown in Fig. 6.2.2. The lineshape calculation used here is based on Equation (6.2.7), where the quadrupolar contributions are summed up over the two rotation angles 0 and 4>. Note that for any symmetry higher than C2 (e.g. a methyl

2H N M R

201

group), even the discrete jump model leads to an axially symmetric spectrum [6]. Motions on the intermediate timescale are somewhat harder to analyze using simple quadrupole echo spectra, since the lineshape will be determined not only by the geometry and the rate of motion but also by relaxation effects. In the slow (%>> 1/Avo) or fast (rc ~ 1/Avo) limit, the echo is virtually completely refocussed, since T2 relaxation is comparatively slow. In the intermediate regime, on the other hand, much of the signal decays irreversibly during the echo delay time, and different orientations may decay with different T2 rates. Thus, the total intensity of the spectrum tends to be reduced and the lineshape can be significantly distorted [3-6, 8-11]. Both these effects depend on the length of the echo delay time, which can be varied deliberately to discriminate among different motions. Motionally averaged spectra in the intermediate regime should be interpreted with care, but with an appropriate model, experimental lineshapes have been well reproduced over a range of correlation times and temperatures [3, 6-12]. While there may exist ambiguities in the interpretation of quadrupole echo lineshapes, 2D 2H NMR exchange spectroscopy provides an essentially model-free approach to describe dynamic processes in the polymer [1-5, 6164]. An appropriate mixing time is included in the experiment, during which reorientations of the deuterated segment can occur. Such motions reveal themselves by the appearance of off-diagonal intensity, and the jump-angles are projected directly into the 2D exchange spectrum. An example is shown on the right in Fig. 6.2.2, where the discrete two-site jump of a phenyl-ring changes the relative C~2H bond orientation by/3 = 120~ Using the projection map of the elliptical pattern, the jump-angle/3 can be directly evaluated as tan(/3) = a/b from the ratio of the principal axes. For the case of continuous rotational diffusion, on the other hand, there exists a distribution of jumpangles. This leads to a rather different appearance of the 2D exchange spectrum [1-5, 61-64], characterized by parallel ridges, as illustrated in Fig. 6.2.2. In both simulated examples, the mixing time T m of the 2D exchange experiment was set equal to the motional correlation time r~, which defines the jump rate or diffusion rate. In practice, it is possible to probe a wide range of motional timescales by choosing appropriate mixing times between about 500 p~s to 20 s.

6.2.6

Nuclear spin relaxation

The previous section has been concerned with a 2H NMR lineshape analysis of motional effects, which is most informative on the intermediate timescale. Any dynamics in the very fast or ultraslow correlation time limit, however,

202


are more suitably investigated by relaxation time measurements [1-5, 14, 18, 19, 66]. Since deuterium relaxation is dominated by an intramolecular quadrupolar mechanism, relaxation times provide information about the rate of motion of each individually labelled group. In systems as complex as polymers there invariably exists a wide range of dynamic processes, which contribute differentially to the various relaxation pathways [2, 15]. For deuterium there are five different relaxation times, involving three different spectral densities Jo(0), Jl(~oo), and J2(26Oo). After the absorption of electromagnetic radiation, the nuclear spins will relax back to their equilibrium state corresponding to a thermal Boltzmann distribution. Coherent precession of the spins generates a net component of transverse magnetization. This decays with an exponential time constant T2 as a result of the gradual dephasing of the spins, giving rise to the transverse or "spin-spin" relaxation. Strictly, the conventional T2 consists of the transverse Zeeman Tzz and the quadrupolar transverse Tzq relaxations [14, 19], the meaning of which will be explained in Section 6.2.7.

T2 = - 2

z + Y2q

= 8 --

2 Jo(0) + 2 Ja(wo) + J2(Zwo)

9

(6.2.14)

Transverse relaxation is used to detect slow motions near Jo(0) with correlation times longer than T~ > 10 -6 S. It can be measured either with a quadrupole echo (T2~ or a Carr-Purcell-Meiboom-Gill pulse train (T q-CPMG 2 ). By comparing the data from both methods, it is possible to discriminate very slow motions down to ~-r 10 -3 s, as described in Section 6.2.7. The recovery of the Zeeman polarization to its equilibrium value is characterized by the longitudinal, or "spin-lattice" relaxation time constant Txz. Spin-lattice relaxation occurs through dissipation of the excess energy of the spins to the surrounding lattice, brought about by fluctuating fields of the appropriate frequencies, i.e., close to the Larmor frequency COoand to 26Oo.

Tlz- 8

[Jl(WO) + 4J2(2Wo)] 9

The decay of double quantum coherence is described by

(6.2.15)

2H N M R

1 1(7) 8

TDo

[Jl(WO) + 2J2(2Wo)].

203

(6.2.16)

The decay of the quadrupolar polarization is given by

Tlq

--8

[3Jl(O)o)] 9

(6.2.17)

Spin-lattice relaxation is most sensitive to fast segmental fluctuations such as side-chain rotation and backbone oscillations with correlation times of around 10 -11 S < "t"c < 10 - 6 S. A combination of different relaxation measurements makes it possible to discriminate among the different spectral densities [2, 15]. For a complete description of the frequency dependence, however, a series of measurements needs to be carried out at several different field strengths.

6.2.7

Summary of 2H NMR pulse sequences

The following paragraphs provide an overview of some important 2H NMR experiments. In analogy to the intuitive picture of a spin I = 1/2 precessing in space, the behaviour of the deuterium nucleus with a spin-1 will be visualized here in a higher-dimensional coordinate system. Some excellent reviews are recommended for further reference, which cover the special properties of spin-1 in more depth [3, 14]. In order to calculate the time evolution of a spin system and its manipulation by radio frequency (rf) pulses, it is conveniently described by a density operator o-(t). This operator can be written in a basis of (21 + 1) 2 operators Pi, which are related to the angular momentum operators Ix, Iy, I z [14] (21 + 1)2-- 1

~r(t) = coPo +

~

ci(t)pi .

(6.2.18)

i=1

A convenient basis set has been suggested in the literature and will be used here [16, 68]. For a constant number of spins, Co is not a function of time and may be set equal to 1, and Po is the unity operator. Thus, for spin I = 1/2, the system is described by the three coefficients Cl(t), C2(t) and c3(t). They reflect the three components of the magnetization vector which can be readily visualized in the rotating frame as the x, y and z coordinates. For I = 1, eight instead of three coefficients are now required.

204


Therefore, the usual picture of a vector in real space breaks down and has to be substituted by a vector in eight-dimensional space. Nevertheless, with the above choice of the basis operator set p~, the first three components c~(t), c2(t) and c3(t) can still be thought of corresponding to the respective components in real space. They are referred to as Zeeman orders. The three components C4(t), Cs(t) and c6(t) are called quadrupolar orders, and the remaining c7(t) and c8(t) are double quantum orders. Only Cl and c2 are accessible by experiment, since their changes correspond to the precessing magnetization that induces a voltage in the rf coil. In spin operator mechanism the time evolution of the density operator is given by the Liouville-von-Neumann equation, do"

dt

= - i [ H , o-].

(6.2.19)

This important equation governs, like an equation of motion, the time development of the system under a Hamiltonian H. It yields eight coupled differential equations for the coefficients ci(t). The resulting solutions for various Hamiltonians resemble rotations in an eight-dimensional space spanned by the eight nontrivial basis operators. With the convenient basis set of Pi [14, 16, 68], the time evolution under Zeeman interaction can be visualized as a precession in the Pl-P2, P5-P6 and P7-Ps planes. The axially symmetric quadrupolar interaction, on the other hand, mixes between Pl and P6 and between P2 and P5. Therefore, evolution under quadrupolar interaction does not lead to precession of "magnetization" within the x - y plane, but rotates it out of this plane into a not directly accessible order and back again. Irradiation with rf, where B1 lies along the x- or y-axis, can be represented by the Hamiltonian H1 - yBlIx or H1 = yBxIy, respectively. For a spin-l/2 system, application of a pulse of length tw rotates the magnetization around the x- or y-axis, i.e., in the P2-P3 or P3-Pl plane, by an angle of q~= yBltw. In spin-1 systems, on the other hand, the Hamiltonian H1 with B~ along the x-axis leads to precession in three planes. The first two are defined by P2-P3 and Ps-P5, and the third one is spanned by P6 and an axis along p4,7- V3/2p4 + 1/2/)7. With B1 pointing along the y-axis, the corresponding rotations takes place in the P3-Pl, Ph-P8, and P4, -7-P5 planes (p4,-7 = X/-3]2p4-1/2p7). As can be seen, the time course of a 2H NMR experiment can be determined, without too much theory, by using the picture of a precessing vector in the corresponding planes. Thus, pulse sequences can be designed or rewritten for spin-l, e.g., to obtain echoes yielding undistorted spectra,

2H NMR

205

to reveal relaxation times, or to correlate the states of motion at two different times. Some frequently used experiments are discussed below and summarized in Fig. 6.2.3. 6.2.7.1 Quadrupole echo In solid state NMR, it is impractical to obtain a spectrum with a single pulse because the FID rapidly dephases during probe ring-down over the first 10 ~s following the pulse [13]. To avoid this problem in spin-l/2 experiments, the two-pulse Hahn echo sequence may be used [69]. For spin-1 systems, the conventional Hahn echo is replaced by the quadrupole echo sequence 90y - z - 90x - t [67]. Refocussing is achieved by a pulse with half the length of what is used for spin-i/2, since the precession of spin-1 in the corresponding plane occurs at twice the frequency. A 90~ difference between the two pulses is essential for refocussing along a detectable order, and phase cycling is applied to cancel unwanted coherences. Pulse lengths should be of the order of 5 ~s or less, when large spectral widths are to be covered, and echo delay times ~-are typically a few tens of ~s. Note that if there is motion in the intermediate regime, the echo lineshape and intensity depend critically on the delay time, as discussed in Section 6.2.5. 6.2.7.2 Relaxation time measurements In the case of spin 1/2, the transverse relaxation time T2 corresponds to dephasing in the Pz-P3 plane, and the longitudinal relaxation time T1 describes magnetization recovery along the Pl axis. For spin-1 systems, on the other hand, there exist additional relaxation pathways, as described in Section 6.2.6. Transverse dephasing of magnetization is possible during several different modes of precession, and the approach towards thermal equilibrium occurs along several of the eight components with distinct timescales T1. Considering the evolution under Zeeman interaction, only P3 and P4 are invariant orders, while the components of Pl and P2, of P5 and P6, and of P7 and P8 are intermixing. Therefore, the longitudinal relaxation times describe the approach of thermal equilibrium of the P3 and P4 orders, which are referred to as Tlz (longitudinal Zeeman) and Tlq (quadrupolar), respectively [19, 70]. Dephasing of P l-P2 coherence is denoted by Tzz (transverse Zeeman), of Ps-P6 coherence by Tzq (transverse quadrupolar) and of Pv-P8 coherence by TDo (double quantum) relaxation. The longitudinal relaxation time Tlz can be measured in an analogous way as for spin-l/2 systems by an inversion recovery experiment as depicted in Fig. 6.2.3. The recovery of signal intensity is monitored as a function of incrementing the delay time ~'1 from a few p~s up to around 5 • T1. Other relaxation times corresponding to the higher orders can be revealed by trans-

206

A.S. ULRICH AND S.L. G R A G E

Fig. 6.2.3. Pulse sequences of some important 2H NMR experiments. The response of the spin-1 nucleus to pulses and precession periods is explained in the text in terms of rotations within an eight-dimensional space.

2H N M R

207

ferring the system into the appropriate coherence state, where dephasing occurs with the desired rate. Since several orders are coupled and interchange among each other during the evolution, the measured rate usually consists of contributions from several relaxation times. For example, in analogy to spin-i/2, the quadrupolar echo can be used to measure spin-spin relaxation. However, the relaxation time T2~ determined by a variation of the pulse spacing ~- is composed of Yzz and Tzq, as given by Equation (6.2.14). Two other prominent relaxation experiments are based on the JeenerBroekaert [9, 71] and the stimulated echo sequence [69, 72], which are outlined in Fig. 6.2.3. The Jeener-Broekaert experiment explores the Tlq (quadrupolar) and TDQ (double quantum) relaxation rates by transferring the initial state into a mixture of quadrupolar and double quantum orders (corresponding to P4 and P7) via a 9 0 y - ' / 1 - 45x pulse combination. The length and phase of the second pulse are optimized to transfer the magnetization into the P4 and P7 orders while, for example, the 90x pulse of the quadrupolar echo prevents such orders altogether. After an incremented evolution time z2, another 45x pulse is applied to bring the system back into an experimentally observable order. The signal after this pulse still carries the echo information from propagation during 71, which gives rise to an echo at a time ~'1 after the third pulse. Furthermore, a second, virtual echo builds up at a time z l before the third pulse. Only the former echo is directly detectable, but an added quadrupolar echo refocusses both the real and the virtual echo. The FID then carries a positive echo at z 3 - Zl and a negative echo at 73 + ~'1 following the last 90~ pulse. Since their amplitudes are functions of +[3 exp(-~'z/Taq)+ exp(--~'z/TDQ)], a variation of T 2 reveals both relaxation times as independent values [14]. In the stimulated echo experiment, also shown in Fig. 6.2.3, the second pulse transfers the system into a mixture of Zeeman and double quantum order (along P3 and Ps). Here, the relevant relaxation times are Tlz (longitudinal Zeeman) and TDO (double quantum), for which the 45x pulses of the Jeener-Broekaert sequence are replaced by 90y pulses. Again, two echos evolve at _+71 around the third pulse, and are refocussed by the fourth pulse. The two negative echo amplitudes vary as function of ~'2, with -[exp(-~'z/Tlz) w exp(--~'z/TDo)], and both Tlz and TDO can be determined as separate values [14]. 6.2.7.3 Correlation time filtering Beyond the standard quadrupole echo experiment, multipulse sequences provide an alternative and versatile approach to measure transverse relaxation. The relaxation time constant T2~ is obtained from a series of experiments with different pulse spacings ~-. Extending this sequence by n further 90~

208

A.S. U L R I C H A N D S.L. G R A G E

pulses, separated by 2~-, yields a train of echoes at times k2~', with k = 1, 2 , . . . , n. The corresponding sequence is the spin-1 version of the CarrPurcell-Meiboom-Gill experiment (q-CPMG), also referred to as MW-4 (Mansfield-Ware 4) [16, 73-75]. Each echo is refocussed again by the following pulse in the sequence, however with reduced amplitude due to transverse relaxation. Hence, within a single experiment, the relaxation time constant Tq-CPMG 2 is determined from the amplitudes as A ( k 2 ~ ' ) - A(0) exp(-k2~-/ q-CPMG T2 ). The q-CPMG relaxation time differs from T2~ Assuming that the quadrupolar interaction is relaxed through random fluctuations with a single correlation time ~c, the Tq-CPMG relaxation rate is a function of the echo delay --2 time ~- [76] l

Tq_CPMG ~ - AM2~'c 1 - - - tanh --2

.

(6.2.20)

T

Here, AM2 is the second moment of the spectrum after the effect of motional averaging has been taken into account. By appropriate selection of ~', it is possible to explore two limiting cases. Using a long z > ~c, the value of Tq-CPMG is independent of z and equal to T2~ With z < Zc, on the other 2 hand, the relaxation rate is proportional to ~.2, giving 1/Tq-CPMG('r) ~ AM2"r2/3"rr Thus, it is possible to use the q-CPMG experiment as a correlation time filter, since decreasing z suppresses more and more contributions from slow motions to the relaxation. When a motional process is dominated by a single correlation time, a series of experiments with different z reveals the value of ~'c from the slope of 1/T q-cPMG versus ~.2. 6.2.7.4 2D exchange spectroscopy Among two-dimensional experiments, wideline exchange spectroscopy plays a prominent role in 2H NMR [1, 4, 61-64]. By correlating the frequency distributions at two different times, any changes in the resonance frequency due to reorientation can be detected in the off-diagonal intensity pattern. With the aid of lineshape simulation and by comparing different mixing times, detailed conclusions about the type and rate of motion can be drawn, as illustrated in Section 6.2.5 and Fig. 6.2.2. A typical 2D exchange experiment consists of three parts, as seen in Fig. 6.2.3. First, in an evolution period tl the system evolves with its initial frequency distribution, which corresponds to the simple Pake-pattern. Subsequently, the spin state is stored throughout a mixing time tm, during which

2H NMR

209

molecular reorientations can occur, which lead to a change in the orientationdependent spectral frequencies. In the final detection period, the evolution of the spin system is switched on again. The magnetization of any spin that is frequency-encoded from before the mixing time, will now propagate with a potentially different frequency after tm. The FID is acquired in t2, and the second dimension is obtained by incrementing tl. In order to obtain a fully complex data set in the indirectly detected dimension (equivalent to quadrature detection), two separate experiments need to be carried out with differing phases of the second and third pulse. The component kept during the mixing time will then be either the sin or cos of the phase acquired during the evolution period, and the two data sets are added up later. After preparation by a 90y pulse, the quadrupolar interaction causes a precession in the Pl-P6 plane (rather than in the Pl-P2 plane as for spin-i/2). The Pl and P6 orders are then transferred into an invariant order (P3, P4, P7 or Ps) to store the system's evolutionary state during the mixing time. When acquiring the first set of data, the Pl order is transferred into P3 by a/3_y pulse, while P6 in the second experiment is transferred into P4 and P7 by a /3-x pulse. After the mixing time, the reverse transfer is achieved by a fly or fix pulse, respectively, followed by a quadrupole echo. Any undesired contributions, which add to the FID while precession occurs between the diverse orders, can be cancelled by phase cycling [64]. The choice of the pulse length/3 is not in fact critical to the experiment, but for/3 = 54.7 ~ both data sets theoretically contribute equally to the complex 2D spectrum and give the best S/N ratio. Since different relaxation pathways may lead to different contributions, in practice the two data sets are weighted empirically such that the cross-diagonal has the least intensity.

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2H NMR

211

48. P. Fischer, C. Schmidt and H. Finkelmann, Macromol. Rapid Commun. 16 (1995) 435. 49. D.J. Schaefer, R.J. Schadt, K.H. Gardner, V. Gabara, S.R. Allen and A.D. English, Macromolecules 28 (1995) 1152. 50. S.M. Fan and G.R. Luckhurst, J. Chem. Phys. 101 (1994) 3255. 51. S. Luesse and K. Arnold, Macromol. 29 (1996) 4251. 52. J. Rault, C. Mace, P. Judeinstein and J. Courtieu, J. Macromol. Sci. - Physics B35 (1996) 115. 53. D. Radloff, C. Boeffel and H.W. Spiess, Macromol. 29 (1996) 1528. 54. E.A. Schmitt, D.R. Flanagan and R.J. Linhardt, Macromol. 27 (1994) 743. 55. A. Abe, H. Furuya, S.Y. Nam and S. Okamoto, Acta Polym. 46 (1995) 437. 56. W.S. Price, N. Kikuchi, H. Matsuda, K. Hayamizu, S. Okada and H. Nakanishi, Macromol. 28 (1995) 5363. 57. D. Pressner, C. Goeltner, H.-W. Spiess and K. Muellen, Acta Polym. 45 (1994) 188. 58. T.W.N. Bieze, J.R.C. van der Maarel, C.D. Eisenbach and J.C. Leyte, Macromol. 27 (1994) 1355. 59. D. Bucca and B. Gordon, Macromol. 27 (1994) 862. 60. C.P. Slichter, Principles of Magnetic Resonance. Harper and Row, New York, 1963, 160176. 61. H.W. Spiess, and H. Sillescu, J. Magn. Res. 42 (1981) 381. 62. S. Kaufmann, S. Wefing and H.W. Spiess, J. Chem. Phys. 93 (1990) 197. 63. S. Wefing, S. Kaufmann and H.W. Spiess, J. Chem. Phys. 89 (1988) 1234. 64. S. Wefing and H.W. Spiess, J. Chem. Phys. 89 (1988) 1219. 65. C. Schmidt, B. Bli~mich and H.W. Spiess, J. Mag. Res. 79 (1988) 269. 66. N.O. Petersen and S.I. Chan, Biochem. 16 (1977) 2657. 67. M. Bloom and E. Sternin, Biochem. 26 (1987) 2101. 68. J.H. Davis, K.R. Jeffrey, M. Bloom, M.I. Valic and T.P. Higgs, Chem. Phys. Lett. 42 (1976) 390. 69. A.J. Vega and Z. Luz, J. Chem. Phys. 86 (1987) 1803. 70. E.L. Hahn, Phys. Rev. 80 (1950) 580. 71. K.R. Jeffrey, Bull. Magn. Reson. 3 (1981) 69. 72. J. Jeener and P. Broekaert, Phys. Rev. 157 (1967) 232. 73. N.S. Sullivan, D. Esteve and M. Devoret, J. Phys. C. Solid State Phys 15 (1982) 4895. 74. H.Y. Carr and E.M. Purcell, Phys. Rev. 94 (1954) 630. 75. S. Meiboom and D. Gill, Rev. Sci. Instrum. 29 (1958) 688. 76. P. Mansfield and D. Ware, Phys. Rev. 168 (1968) 318. 77. J.S. Blicharski, Can. J. Phys. 64 (1986) 733.

Solid State NMR of Polymers, edited by I. Ando and T. Asakura

Chapter 6.3

Studies in Physical and Theoretical Chemistry, Vol. 84 9 Elsevier Science B.V. All fights reserved

3H NMR J.P. Bloxsidge 1, J.R. Jones 1, J.C. Russell 2, A.P. Sharratt 3, T.A. Vick 2 and D. Zhong a 1Department of Chemistry, University of Surrey, Guildford, Surrey, UK: 2Biocompatibles Ltd., Frensham House, Farnham Business Park, Farnham GU 9 8QL, UK; 31CI Chemical and Polymers, The Heath, Runcorn, Cheshire, UK

A key factor in the development of high resolution 3H NMR spectroscopy [1] of tritiated compounds in solution was the use of a sealed sample assembly within the conventional NMR tube making it very unlikely that any radioactivity would be released even in the event of a tube breakage. Since then improvements in magnet design and higher fields [2, 3] have made it possible to produce satisfactory 3H NMR spectra at much lower levels of radioactivity (> i). Above 30~ T1 and Tlo increase as the temperature is increased. The molecular motion is nearly in the extreme narrowing region (o~, ~ 1) above the first transition temperature. No appreciable change is observed at the melting temperature. This means that, at the first transition, a large jump of correlation time for molecular motion takes place and motional narrowing already occurs and spin diffusion occurs effectively between the mobile and immobile regions. Figure 9.7 shows the powder pattern spectrum of c-C24H48 at ambient temperature [11]. As the temperature in the CPMAS probe is thought to be more than 30~ this is the spectrum above the first transition. The powder

c-C2taH~ 8

9 t"

.._.._.___

~

J

~

~_30

Z: t~ Z

. . . .

I

,

J

> Z

10 KHz 50 ~

> >

s

~ ra,l

Ii

9 ,

oc

10 KHz 2 KHz

Fig. 9.6. Temperature-dependence of 1H NMR spectra for c-C24H48 [15].

+

_

336

TAKESHI YAMANOBE AND HIROMICHI KUROSU

Fig. 9. 7. CP powder pattern spectra of c-C24H48at ambient temperature [11].

pattern spectrum for c-C24H48 shows an axially symmetric pattern instead of the usual tent-like one. An axially symmetric pattern means that the molecular motion takes place around a certain axis and is not perfectly random. On the basis of these results the structure of c-C24H48 can be described. Below the first transition temperature, 29~ the molecular motion of cC24H48 is frozen, i.e., it assumes a compact conformation with the shape of two parallel trans-zigzag straight chains bridged at two G G T G G loops. Further in the range from 29~ to the melting temperature, c-C24H48 has two components: these are the mobile and immobile regions that comprise the folded chain structure region and the trans-zigzag structure region, respectively. The molecular motion in the mobile region comes from the fast transition between the trans- and gauche-conformers. On the other hand, the molecLdar motion in the immobile region comes from the gauche migration in the trans-zigzag chains as follows:

.... TTTTT G . . . . . . . . TTTT G T . . . . . . . TTT GTT . . . . . . TT G TTT... T GTTTT..

The motional modes allow the transition between the trans- and gaucheconformers to occur in a cyclic paraffin without a large scale change in the overall shape of the cyclic paraffins.

337

POLYETHYLENE AND PARAFFINS

9.5

Structural and dynamic studies of polyethylene

The amorphous region of 13C-labeled polyethylenes crystallized under different conditions were studied by VT 13C CP/MAS NMR [18]. The dynamics of the amorphous region were discussed by measuring the 13C spin-lattice relaxation times (T1) and dipolar-dephasing relaxation times (TDD) over a wide temperature range, from -120 to 44~ Two types of 13C-labeled polyethylene samples were prepared. One is single 13C-labeled (polymerized using 90% single 13C-enriched ethylene) polyethylene and the other is double labeled (polymerized using 90% double 13C-enriched ethylene) polyethylene. Typical 13C CP/MAS NMR spectra of the single labeled solution-crystallized polyethylene (PESL; the crystallinity is 95%) and single labeled meltquenched polyethylene (MQPESL; the crystallinity is 66%) are shown in Fig. 9.8. Each of these spectra consists of three peaks corresponding to an orthorhombic crystalline peak, O, at 33.0 ppm, a monoclinic crystalline peak, M, at 34.4 ppm (a small shoulder on the left side of the orthorhombic peak), and an amorphous peak, N, which appears at 30.8-31.3 ppm. The 13C T1 values of PESL and MQPESL over a wide range of temperatures are obtained using the inversion-recovery method with the PST (pulse saturation transfer)

PES, I

45

'

'

' '

"1

4O

'

'

'

'

I

35

'

'"'

'

I"

3O

'

'

"

'

I'"

25

'

'

'7

2O

Fig. 9.8. Typical 13C CP/MAS NMR spectra of samples of PESL and MQPESL at room temperature [18].

338

TAKESHI YAMANOBE AND HIROMICHI KUROSU 0.6

0.5

0.4 A

I---

0.3

0.2

o.1

.

,.

,i

0.0O4

.

1/T (K"1)

,.

.

,

0.005

Fig. 9.9. Plots of amorphous 13C spin-lattice relaxation times: TIS for PESL (0) and MQPESL (O) versus the reciprocal absolute temperature [18].

pulse sequence. The PST pulse sequence enhances the intensity of the signals of mobile methylene carbons. The resulting T1 values for PESL and MQPESL are plotted against the inverse of the absolute temperature (l/T) in Fig. 9.9. It had been suggested previously [19, 20] that local molecular motion in the amorphous region of polyethylene is independent of the degree of crystallinity, high-order structures or morphologies. However, these suggestions are not supported by the experimental results because: (1) the TlS of the amorphous region of PESL and MQPESL at room temperature are 570 and 440 ms, respectively, and the difference of 130ms between them is beyond experimental error; and (2) the T1 minimum values for the two samples (Fig. 9.9) are at different temperatures, the T1 minimum of PESL appearing at -10.5~ and that of MQPESL at -32~ These facts show that the local molecular motions in the amorphous regions of PESL are more constrained than those of MQPESL. In order to study whether the dynamic behavior of the two kinds of amorphous region is also different on the T2 timescale, TDD values of PESL and MQPESL were measured over a wide range of temperatures. The relative intensity of the amorphous peak obtained from the computer simulation was plotted against the delay time ~" in Fig. 9.10. It can be clearly seen that the amorphous peak of PESL relaxes more quickly than that of MQPESL. The dipolar-dephasing time, TDD, usually depends on molecular motion, carbon-proton dipolar interactions, MAS rate and spin diffusion [21]. Fundamentally, it can be said that the dipolar-dephasing time in the

339

POLYETHYLENE AND PARAFFINS 0

"

'

II

9

'

II

9

'

II

"

II

"" '

>., omm

u)

r

0

0

r

..,=,.,.

e

.01

' " 100

"

' 200

"

" 300

"

' 400

" 500

(ps) Fig. 9.10. Intensity of amorphous peaks of samples of PESL and MQPESL versus delay time r [18]. The peak intensity was obtained from computer simulation of the 13C partially relaxed dipolar-dephasing NMR spectra.

amorphous region becomes a measure of molecular motion. Therefore, the longer TDD value of MQPESL, compared with that of PESL, obviously suggests that the carbon-proton dipolar interaction is partially averaged by molecular motion on the Y2 timescale. Furthermore, Chen et al. [22] have measured the ~3C CP/MAS spectra of a ~3C-labeled solution of crystallized polyethylene at temperatures from -120 to 144~ The measurements (Fig. 9.11) were taken in order to study changes of structure and molecular motion of the polymer with temperature variation for the crystalline and amorphous regions. As the crystalline and amorphous signals are incompletely resolved in the ~3C CP/MAS spectrum of the polyethylene sample, computer-fitting of the spectra is performed, and the determined ~3C chemical shifts of the crystalline and amorphous signals are shown in Fig. 9.12. It is shown that the ~3C chemical shift of the amorphous signal decreases with an increase in temperature, whereas the ~3C chemical shift of the crystalline signal does not change greatly with temperature before the melting point.

340

TAKESHI YAMANOBE AND HIROMICHI KUROSU (b) -120"c -100"c -80"c

144"c

-60"c

132.8"c

-50'C 121.6"c

40"c -30"c

110.4~

-20"c

99.2"c

O'c

88"c

11.5"c RT I''''1''''1''''1"""~i'"''1 37 36 ~15 34

33

32

....

I ....

31

I .... 30

I''''l

29

28

65.6"c 3~ '~ "2~i' ' '314' ' '3~?.' ' '3~' ' 'Z~8' ' ' ~

Fig. 9.11. -120~

1 3 C CP/MAS NMR spectra of the PESL sample as a function of temperature: (a) to room temperature; (b) 65.6 to 144~ [22].

34

A

E (3. cL v

33

9 9e e

..ww.,C r,/)

rj E to 0

32

0

0

0

eee

9

ee

9

9 eeeee

0000 0 0 0

31

0

0000

,,,,,.

30 .140

9 ' ,.100

9 | .60

9

!

-20

9

I

20

=

I

60

=

I

0

9

100

C I

140

T(~

Fig. 9.12. Plots of the

1 3 C chemical shifts of the crystalline ( e ) and amorphous ( 9 signals of the PESL sample against temperature [22].

341


O

A

E

(:3. Q.

O O

v

J~

0

0

0

O

O

"13

O

0 r !

o~ ,o

'r"

1

9

9

9 Q0000

0 0

9

D

(3 0

-140

9

I

-100

I

l

-60

9

I

-20

9

!

20

9

I

60

9

1

100

,,

I

140

T(~ Fig. 9.13. Plots of the width at half the maximum peak height of the crystalline (O) and amorphous ( 9 signals of the PESL sample against temperature [22].

This shows that an increase of the fractional population of the t r a n s conformer leads to the observed high frequency shift of the amorphous signal with a decrease of temperature. At temperatures below -80~ the 13C chemical shift of the amorphous signal does not change with temperature. Such a result indicates that the molecular motion in the amorphous region of PESL is completely frozen below -80~ on the NMR timescale. The halfheight width (halfwidth) of the crystalline and amorphous 13C signals are plotted against temperature in Fig. 9.13. The halfwidth of the amorphous region becomes a maximum at -30~ This means that molecular motion of the crystalline region occurs at a frequency corresponding to the amplitude of the proton decoupling field (about 55 kHz in this case). Contrary to the results of the amorphous peak, the halfwidth of the crystalline peak is almost constant at temperatures from 0 to -120~ The halfwidth decreases from 0.9 to 0.7 ppm as the temperature is increased from 0~ to room temperature. Further elevation of temperature from 65.6~ causes line broadening and the halfwidth becomes a maximum at 110.4~ This result may suggest that there is an a-transition, T~, in the crystalline region of polyethylene and thus the temperature (110.4~ at which the maximum of the halfwidth is observed may be correlated with T~. The structure and dynamics of polymer chains adsorbed on the surface of solids have been studied using spectroscopic methods such as IR, ESR and

342


II

II

i""

40

9

9

I

38

'

9

'

I'"

36

9

'

I'"

34

"~"

I

32

9

9

9

I

30

9

9

9

I

28

9

'

' I

" ~

26 ppm

Fig. 9.14. 13C CP/MAS spectra of 13C-labeled crystallized polyethylene [23].

so on. The NMR sensitivity is very low compared with IR and ESR. This is a problem for NMR experiments on polymers adsorbed on the surface of solids. Therefore, to obtain a high-resolution solid-state 13C NMR spectrum with a reasonable signal-to-noise (S/N) ratio, isotope-labeled polymers must be prepared [18]. Inoue et al. [23] have studied the structure and dynamics of the 13C-labeled polyethylene adsorbed on the surface of silica gel as a function of temperature by high-resolution solid-state ~3C NMR spectroscopy. A 13C CP/MAS spectrum of 13C-labeled polyethylene in bulk at room temperature is shown in Fig. 9.14. As reported previously [4, 24-35], this spectrum consists of three peaks corresponding to an orthorhombic crystalline transzigzag methylene peak II at 33.0 ppm, a monoclinic crystalline trans-zigzag methylene peak I at 34.3 ppm and a noncrystalline methylene peak III at 30.8-31.3 ppm. The methylene carbons are undergoing a fast transition between the trans- and gauche-conformers in the amorphous region, and so the observed amorphous chemical shift is an average value for the trans- and gauche-conformers. Therefore, the 13C chemical shift of the amorphous peak reflects the fraction of the gauche-conformer. 13C CP/MAS and PST/MAS NMR spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel are shown in Fig. 9.15. The ~3C chemical shift values and peak intensities of the 13C CP/MAS spectrum obtained by computer-fitting are listed in Table 9.2. In the 13C CP/MAS spectrum (Fig. 9.15(a)), the three peaks appear at

343


1

a)

2

3

b)

__

40

38

J

36

34

32

30

28

26

ppm Fig. 9.15. (a) 13CCP/MAS and (b) 13C PST/MAS spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel [23]. Table 9.2. 13C chemical shift values and peak intensities of the 13C CP/MAS spectrum of 13Clabeled polyethylene adsorbed on the surface of silica gel. Peak number

13C Chemical shift/ppm

Peak intensity

1 2 3

32.9 30.6 27.5

16 8 1

32.9, 30.6 and 27.5 ppm, and are designated by 1, 2 and 3 from high frequency, respectively, in the ~3C CP/MAS spectrum of an NMR rotor without polyethylene and with silica gel there are no peaks in the region of 20-40 ppm. In the ~3C CP/MAS experiments, it is known that the CP efficiency strongly depends on molecular motion, and when the reorientation rate is close to the 1H decoupling frequency (~60 kHz), the CP efficiency is greatly reduced and so the signal sometimes disappears. On the other hand, in the

344


PST/MAS experiments the intensity of the peak is governed by the NOE enhancement. In the 13C CP/MAS spectrum, the intensity of peak 1 is more intense than that of peak 2, but in the 13C PST/MAS spectrum (Fig. 9.15(b)) the intensity of peak 2 is relatively increased compared with the a3C CP/MAS spectrum. This means that the molecular motion of the methylene carbons for peak 2 is faster than that for peaks 1 and 3. The 13C CP/MAS spectral pattern of adsorbed polyethylene is very different from that of bulk polyethylene. The fractions of the mobile components for these polyethylene samples are different from each other. The fraction of the mobile component in adsorbed polyethylene is larger than that of bulk polyethylene. The chemical shift value for peak 1 is close to that for trans-zigzag methylene carbons and the difference in X3C chemical shift between peaks 1 and 3 is 4.4 ppm which corresponds to the 1 y-gauche effect value. From these results, it can be said that peaks 1 and 3 come from the trans and gauche parts in which the molecular motion is frozen on the NMR timescale. This is caused by adsorption on the surface of silica gel (this part is designated as the region A). Peak 2 comes from the methylene carbons which are undergoing fast transitions between the trans- and gauche-conformations (this part is designated as the region B). The ratio of intensities for peaks 1, 2 and 3 obtained by computer-fitting is 16:8:1 in the CP/MAS spectrum of PEDA (doubly 13C-labeled polyethylene adsorbed on the surface of silca gel). As described above, because the CP efficiency depends on molecular motion, the peak intensities do not correspond exactly to the ratio of each component. However, it serves as a rough standard for the ratio of each component, especially the trans- and gauche-conformers. In order to study the dynamic behavior in region B, 13C spin-lattice relaxation times T1 are measured by varying the temperature. As an example, the partially-relaxed 13C spectra of polyethylene at room temperature obtained by the inversion recovery method are shown in Fig. 9.16. Using these results, 13C T1 values were determined as listed in Table 9.3. The 13C T1 values were plotted against inverse absolute-temperature in Fig. 9.17 together with those of the amorphous region for MQPESL (meltquenched polyethylene (single 13C-labeled)) which was melted at 150~ and quenched to -70~ and PESL (polyethylene single 13C-labeled) which was dissolved at 130~ in xylene at a concentration of 0.03% and crystallized. The T1 curves for these three samples have the minimum at different temperatures. The T1 minimum for polyethylene adsorbed on the surface of silica gel appears at the highest temperature compared with other samples. According to the BPP theory, the correlation time for molecular motion at the T1 minimum corresponds to the resonance frequency. All of the T1 values for the three samples are measured using the same


345

Delay Time 1; ( x l 0 m s )

400

200

150

80

~

f

40 20

10

I

34

'

'

'

I

32

'

'

'

I

30

'

"

'

I

28 ppm

Fig. 9.16. Partially relaxed 13C NMR spectra of 13C-labeled polyethylene adsorbed on the surface of silica gel at room temperature obtained by the inversion recovery method [23].

Table 9.3. 13C T1 values of methylene carbons in the region B of 13C-labeled polyethylene adsorbed on the surface of silica gel. Temperature/K

TI/S

313 284 273

0.65 0.49 0.50

346


0.7

0.6

m

,..,

-.e-

0.5

PEDA PESL MQPESL

_

' ~ 0.4 0.3

m

% 0.2

J

-

0.1 0.003

0.004

0.005

1 / T ( K "1) Fig. 9.17. Plots of 13C spin-lattice relaxation times T1 against inverse absolute-temperature for laC-labeled polyethylene adsorbed on the surface of silica gel [23].

spectrometer operating at 67.8 MHz. For this, it is apparent that the molecular motion of the methylene carbons in the region B is more restrained, compared with that of bulk polyethylene samples. From the single correlation-time model based on the BPP theory, the correlation time, rc, for molecular motion was calculated using the determined 13C T1 values [23]. The plot of rc against inverse absolute temperature 1/T for P E D A is shown in Fig. 9.18. The temperature dependence of the correlation time usually obeys the Arrhenius form [38] as rc = roexp(AE/T), where AE is the activation energy which is corresponding to the potential barrier for the transgauche transition and ro is the pre-exponential factor. The AE value can be estimated from the slope of ln(rc) against 1/T. By this procedure, ro is found to be 1.37 x 10 -14 S and the AE value is found to be 6.60 kcal/mol for the molecular motion of the methylene carbons in the region B of PEDA. As reported previously, the AE values are 5.19 and 3.72 kcal/mol for the molecular motion of the methylene carbons in amorphous region for PESL and MQPESL samples, respectively. This means that the potential barrier for the trans-gauche transition for the methylene carbons in the region B for P E D A is larger than those in the amorphous region of PESL and MQPESL samples. It is reported that the rotational barrier for the central C-C bond

347

POLYETHYLENE AND PARAFFINS 1 0 .8

r.~

1 0 "9

L---'

10 -10 0.003

i

0.0035

0.004

I / T ( K -1) Fig. 9.18. Plots of correlation times zc in molecular motion for the methylene carbons in the region B for 13C-labeled polyethylene adsorbed on the surface of silica gel against the inverse absolute temperature [23]. in n-butane is about 3.3 kcal/mol as determined from thermodynamic data [36, 37]. Since this value is very close to that for M Q P E S L , it has been suggested that the trans-gauche transition rate in the noncrystalline region of melt-quenched polyethylene is very fast. On the other hand, in the amorphous region of PESL, the AE value is somewhat larger than that for n-butane, and so the trans-gauche transition rate is slower than that for M Q P E S L . In the region B of P E D A , the AE value is larger than those for M Q P E S L and PESL. This means that the trans-gauche transition rate is much slower than those for M Q P E S L and PESL. It is shown that the contributions of 13C--13C dipolar interaction to the 13C T1 value can be neglected, compared with the 1 3 C - - 1 H dipolar interaction as discussed below. The ratio of the contributions of the 13Cm13C and 1 3 C - - 1 H dipolar interactions to relaxation rate R = l/T1 can be expressed by R-

(1/T1)cc = (1/T1)cH

4

6

yc/rcc

2 6 ' T~TH/rCH

(9.1)

where 7c and ")/H are the magnetogyric ratios of the carbon and hydrogen nuclei, respectively ( 7 c = 0.6726 x 104 and YH = 2.675 X 104 rad s -1 gauss--i), and rcc and rcH are C ~ C and C ~ H bond lengths, respectively (rcc = 1.54 A and rCH = 1.1 A). Therefore, R can be estimated as R = (0.6726/2.675)2 x (1.1/1.54) 6 - 0.0043. It is apparent that the contribution of the 13C~13C dipolar interaction is negligibly small compared to that of

348


the 13C--1H dipolar interaction. According to the above discussion, it is considered that the morphology of polyethylene adsorbed on the surface of silica gel is as follows. Methylene chains roughly contain two components with different molecular motions. The first is directly the adsorbed part on the surface of silica gel and its molecular motion is strongly restricted. In this part, methylene carbons are frozen in t r a n s - or gauche-conformations in an NMR timescale. As described above, the t r a n s conformation is the major one and the ratio of t r a n s - and gauche-conformers is approximately 16: 1. The other part is assigned to a polyethylene which is not directly adsorbed on the surface of silica gel, i.e., away from the surface and there is a fast transition between the t r a n s - and gauche-conformations which is similar to the amorphous region of polyethylene in bulk. The mobility of this part is more restricted compared with the noncrystalline region of polyethylene in bulk. The chain length of this part may be shorter than that of the amorphous region of bulk polyethylene.

References

.

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

T. Yamanobe, R. Chujo and I. Ando, Mol. Phys. 50 (1983) 1231. D.M. Grant and E.G. Paul, J. Am. Chem. Soc. 86 (1964) 2984; W.R. Woolfenden and D.M. Grant, J. Am. Chem. Soc. 89 (1966) 1496; A.E. Tonelli and F.C. Schilling, Acc. Chem. Res. 14 (1981) 233. D.L. VanderHart, J. Magn. Reson. 44 (1981) 117. D.L. VanderHart and F. Khoury, Polymer 25 (1984) 1589. T. Yamanobe, T. Sorita, T. Komoto, I. Ando and H. Sato, J. Mol. Struct. 131 (1985) 267. A. Keller, Phil. Mag. 2 (1957) 1171. P.J. Flory, J. Am. Chem. Soc. 84 (1962) 2857. E.W. Fischer and R. Lorenz, Kolloid. Z. 189 (1963) 97. B.A. Newman and H.K. Kay, J. Appl. Phys. 38 (1967) 4105. P. Groth, Acta Chem. Scand. A 33 (1979) 199. I. Ando, T. Sorita, T. Yamanobe, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Polymer 26 (1985) 1864. F.A.L. Anet, A.K. Cheng and J.J. Wagner, J. Am. Chem. Soc. 94 (1972) 9250; F.A. Anet and A.K. Cheng, J. Am. Chem. Soc. 97 (1975) 2420. M. M611er, W. Gronski, H.-J. Cantow and H. H6cker, J. Am. Chem. Soc. 106 (1984) 5093. I. Ando, T. Yamanobe, T. Sorita, T. Komoto, H. Sato, K. Deguchi and M. Imanari, Macromolecules 17 (1984) 1955. M. Takenaka, T. Yamanobe, T. Komoto, I. Ando and H. Sato, Solid State Commun. 61 (1987) 563. M. M611er, H.-J. Cantow, H. Dortloff, D. Emeis, K.-S. Lee and G. Wegner, Makromol. Chem. 187 (1986) 1237. S. Ishikawa, H. Kurosu and I. Ando, J. Mol. Struct. 248 (1991) 361.


349

18. Q. Chen, T. Yamada, H. Kurosu, I. Ando, T. Shiono and Y. Doi, J. Polym. Sci.: Part B: Polym. Phys. 30 (1992) 591. 19. J.J. Dechter, R.A. Komoroski, D.E. Axelson and L. Mandelkern, J. Polym. Sci. Polym. Phys. 19 (1981) 631. 20. B. Schroter and A. Posern, Makromol. Chem. 182 (1981) 675. 21. D.E. Axelson, in: R.A. Komoroski (ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk, Chapter 5, p. 197, VCH Publishers, Florida, 1986. 22. Q. Chen, T. Yamada, H. Kurosu, I. Ando, T. Shiono and Y. Doi, J. Mol. Struct. 263 (1991) 319. 23. D. Inoue, H. Kurosu, Q. Chen and I. Ando, Acta Polymer. 46 (1995) 420. 24. C.A. Fyfe, J.R. Lyerla, W. Volksen and C.S. Yannoni, Macromolecules 12 (1979) 757. 25. J.J. Dechter, R.A. Komoroski, D.E. Axelson and L. Mandelkern, J. Polym. Phys. Ed. 19 (1981) 631. 26. B. Schroter and A. Posern, Makromol. Chem. 182 (1981) 675. 27. D.E. Axelson, J. Polym. Phys. Ed. 20 (1982) 1427. 28. R. Kitamaru, F. Horii and K. Murayama, Polym. Bull. 7 (1982) 583. 29. D.E. Axelson, L. Mandelkern, R. Popli and P. Mathieu, J. Polym. Sci. Polym. Phys. Ed. 21 (1983) 2319. 30. R. Kitamaru, F. Horri and K. Murayama, Macromolecules 19 (1986) 636. 31. I. Ando, T. Yamanobe, S. Akiyama, T. Komoto, H. Sato, T. Fujito, K. Deguchi and M. Imanari, Solid State Commun. 62 (1987) 785. 32. A.L. Cholli, W.M. Ritchey and J.L. Koenig, Spect. Lett. 21 (1988) 519. 33. S. Akiyama, T. Komoto, I. Ando and H. Sato, J. Polym. Sci. Polym. Phys. Ed. 28 (1990) 587. 34. D. Doddrell, V. Glushko and A. Allerhand, J. Chem. Phys. 56 (1972) 3683. 35. K.J. Packer, I.J.F. Poplett, M.J. Taylor, M.E. Vickers, A.K. Whittaker and K.P. Williams, Makromol. Chem. Macromol. Symp. 34 (1990) 161. 36. K.S. Pitzer, Ind. Eng. Chem. 36 (1944) 829. 37. W.B. Person and G.C. Pimentel, J. Am. Chem. Soc. 75 (1953) 539.


Chapter 10


Polymer Blends and Miscibility Atsushi Asano I and K. Takegoshi 2 ~Department of Chemistry, National Defense Academy, Hashirimizu, Yokosuka, Japan; 2Department of Chemistry, Graduate School of Science, Kyoto University, Kitasirakawa, Sakyo-ku, Kyoto, Japan

10.1

Introduction

Here, the main discussion concerns solid-state NMR techniques as applied to a mixture of two different polymers (a polymer blend). Due to its inherent heterogeneity, even one polymer is a difficult target to characterize. Then, why be concerned by the mixture? It is because the blending of polymers is a simple and economic method to produce new materials. At least, the blending is much easier than finding new monomers and polymerization techniques. Various pairs of polymers have been examined [1] with the hope of realizing a unique set of and/or some designed properties. Macroscopic properties of a blend depend on its microscopic phase (domain) structure. One may naively expect that a homogeneous one-phase blend shows the average properties of component polymers, while a heterogeneous two-phase blend keeps the original properties of component polymers. Therefore, microscopic characterization of a blend is important and we will see that solid-state NMR is particularly useful for that. In fact, examination of heterogeneity in a polymer blend is, in a sense, easier than that in a homopolymer, because each component polymer can be discriminated by its distinct 13C peaks. The resolved 13C peaks can be used as clues to unravel the heterogeneity in a blend. Numerous NMR works have been published and some of them were conveniently reviewed recently [2, 3]. In this review, the references are not intended to provide a complete compilation of the literature, because it would be outdated in a month. We would like to take this opportunity to apologize to all authors and readers affected by this.

10.2

Miscibility

Macroscopic properties of polymer blends are influenced by the degree of mixing between component polymers [4, 5]. Miscibility is a term based on thermodynamics, and a miscible state means a homogeneous single phase on

352

A T S U S H I A S A N O A N D K. T A K E G O S H I

a molecular level. In practice, the miscibility depends on how closely we look at the blend; if the domain size is smaller than the characteristic space scale of a particular observation, the blend appears to be miscible/homogeneous. A blend which is homogeneous for a certain observation is often found to be heterogeneous by another observation with a smaller scale of observation. For instance, poly(2-methylstyrene)/poly(2,6-dimethylphenylene oxide) (P2MS/PPO) shows a single glass transition by differential scanning calorimetry (DSC). This shows that P2MS/PPO is miscible from the DSC point of view. However, this blend is observed to be heterogeneous by using solidstate NMR [6]. This discrepancy comes from the smallest domain size that can be detected by Tg (DSC) is 10-20 nm [7], while it is 2-5 nm for NMR (see below). It is true that the Tg measurement is a very useful and good method to judge heterogeneity of a blend on a 10-20 nm scale, but care must be taken if one's problem originates from the heterogeneity of much smaller scales. In this section 10.2, we review the various solid-state NMR methods used to investigate interpolymer interactions, molecular motion and the spatial structure of a polymer blend. An interaction between component polymers affects the chemical shifts and lineshapes (see Section 10.2.2.1) and the molecular motions of the component polymers (see Section 10.2.2.2). In Section 10.2.3.1, microheterogeneity from 2 to 50 nm is studied by measuring XH spin diffusion indirectly from its effects on XH spin-lattice relaxation. The ~H spin-diffusion processes can also be monitored by several methods based on the Goldman-Shen experiment [8] (see Section 10.2.3.2). Homonuclear and heteronuclear two-dimensional correlation experiments reveal how and to what extent component polymers interact with each other (see Section 10.2.3.3). Section 10.2.3.4 deals with cross-relaxation experiments. Before discussing NMR experiments, the thermodynamics of a blend is briefly outlined. 10.2.1

Thermodynamicbackground

In general, the miscibility of a pair of polymers depends on temperature and composition. Figure 10.1 schematically shows three typical phase diagrams. The ordinate and the abscissa axes represent temperature and composition, respectively. The solid line in Fig. 10.1(a), below which the blend becomes immiscible (two-phase), is referred to as an upper critical solution temperature (UCST). However, Fig. 10.1(b) shows a lower critical solution temperature (LCST) behavior. Some polymer pairs display both UCST and LCST as shown in Fig. 10.1(c). As will be shown in the following, UCST is rarely observed for a polymer blend.

353

POLYMER BLENDS AND MISCIBILITY

a)

b)

c) 9

,

,t

.

,

-

,,

Single Phase

Single Phase Single Phase

Fig. 10.1. Schematic illustration of a phase diagram for a polymer blend, showing: (a) upper critical solution temperature (UCST)" (b) lower critical solution temperature (LCST)" and (c) UCST + LCST. Ordinate and abscissa show temperature and composition, respectively.

According to the second law of thermodynamics, polymer pairs will be miscible if the Gibbs free energy of mixing AGm is negative. The AGm may be described by the following three contributions [9-11]" (1) combinatorial entropy of mixing; (2) free volume difference between the component polymers; (3) exchange interaction energy, which can be represented as

AGm ~A (liB = ~ In (/)A -~ ~ In 4~ + XAB~AIJ)B, RTV mAVrA mBVrB

(10.1)

where R is the ideal gas constant, T, q~g, Vri and m~ represent the absolute temperature, volume fraction, molar volume and polymerization degree of component-i, respectively. Thus, the first two terms represent the combinatorial entropy of mixing. /I{'AB includes both the entropic free volume contribution and the exchange interaction energy between polymers A and B. Generally, the value of XAB is positive because polymers interact with the dispersion forces, which do not favor the mixing. For polymer pairs with small m~, the first two terms are negative and larger than the positive third

354


term. Therefore, AGm becomes negative. Hence, a pair of short polymers would be miscible and show the UCST phase diagram (Fig. 10.1(a)). For larger mi values, the combinatorial entropy of mixing becomes too small to overcome the disadvantage of free volume differences between the component polymers. The A Gm value becomes positive if there are no special exchange interactions between A and B. Therefore, most of the polymer pairs are immiscible. More advanced treatments have been published elsewhere [12]. For two polymers with higher molecular weight to be miscible, a certain exothermic exchange interaction is required for the third term (3) to be negative. If such exothermic interactions exist, a polymer blend shows the LCST phase diagram. Because the free volume contribution in XAB becomes large with increasing temperature, and at a certain temperature (LCST), it overcomes a negative exchange interaction. Several specific interpolymer interactions, such as charge transfer [13, 14], ion-amide [15], ion-dipole [16] and hydrogen bonding [3, 17, 18], have been found to work in miscible blends. There are a few polymer pairs which require no specific interpolymer interactions [19-21]. For example, poly(methyl acrylate) and poly(vinyl acetate) (PMA/PVAc) mix by entropic advantage [20, 21].

10.2.2

Polymer-polymer interactions

In the above, we have seen that a certain interpolymer interaction is required for different polymers to be miscible. Here, we will see that high resolution NMR enables us to locate interacting regions in component polymers. One of the most useful methods is the nuclear Overhauser effect (NOE) between ~ H ~ H and ~H~13C. NOE can be observed between spins whose distances are less than about 0.5 nm. The one- (1D) and two-dimensional (2D) NOE experiments have been used to reveal the spatial structure of biomolecules in solutions. Of course, these can be applied to locate interacting regions in a blend in solution and in solids [3]. For example, Crowther et al. [22] and Mirau et al. [23] applied NOE experiments to polystyrene/poly(vinyl methyl ether) (PS/PVME) in a toluene solution, and show that the interpolymer NOE signals between the aromatic protons of PS and the methoxy protons of PVME can be observed at polymer concentrations higher than 25 wt%. In the solid state, Heffner and Mirau [24] measured 2D ~ H ~ H NOESY (NOESY: nuclear Overhauser effect spectroscopy) spectra of 1,2-polybutadiene and polyisoprene (1,2-PB/PI) and observed NOE cross-peaks between these component polymers. White and Mirau observed interpolymer NOE interactions between the ~H spins of PVME and the ~3C spins of deuterated


355

PS in solids [25, 26]. These homo- and heteronuclear correlations in solids can also be used to study miscibility (see Sections 10.2.3.3 and 10.2.3.4). In a miscible blend, a specific interaction would influence chemical shifts and/or lineshapes of component polymers (see Section 10.2.2.1). Molecular motion is also affected by the interpolymer interaction, and is investigated by spin-lattice relaxation, 13C and 2H lineshapes and 2D exchange 2H NMR (see Section 10.2.2.2). 10.2.2.1 Chemical shifts and lineshapes By comparing the NMR spectrum of each component polymer with that of a blend, we can often see that some changes occur in a chemical shift and/or a lineshape. Such apparent changes can be attributed to modifications of both chemical structure and polymer conformation upon blending, reflecting a specific interpolymer interaction. In solution, even a small chemical shift change upon blending can be detected due to its high resolution [27]. Unfortunately in most solid polymer blends, changes in a chemical shift and/or a lineshape are obscured by broadening of resonances. For instance, Natansohn and Simmons [13] studied poly([N-ethylcarbazol-3-yl]methyl methacrylate) and poly(2-[(3,5-dinitrobenzoyl)oxy] ethyl acetate) (PECMMA/PNBOEAc), which are mixed via a charge-transfer (CT) interaction. The aromatic 13C resonances of the blend of their monomeric units (ECMMA/NBOEAc) show low frequency shifts of 1-5 ppm due to the CT interaction. However, such shifts are not observed for the polymeric PECMMA/PNBOEAc blend. Grobelny et al. [28] observed changes in the lineshapes of imide carbonyl carbons of polyimide (PIm) in the blends with poly(ether sulphone) (PES). They attributed the lineshape changes to a polar interaction between the imide carbonyl group of PIm and ether oxygen or sulphone of PES. Among several interactions, the hydrogen-bonding interaction causes appreciable changes in a 13C spectrum. Several blends show a high frequency shift and/or a lineshape change due to hydrogen bonding between component polymers. For example, Yang et al. [29] found a broadening and high frequency shift at the carbonyl carbon resonance of poly([1-hydroxy-2,6-phenylene]methylene) in the blend with poly(N,N-dimethyl acryl amide). Several groups investigated blends of poly(4-vinylphenol) (PVPh) [30-36]. They found that the resonance of the aromatic region of PVPh is sensitive to the interactions occurring in its surroundings, particularly the hydrogen-bonding interaction. In the blends of PVPh, the hydroxy group of PVPh acts as a proton donor, and the oxygen of the carbonyl group of another component polymer acts as a proton acceptor. Belfiore et al. [30] examined the chemical shifts and lineshapes of carbonyl carbon resonances of several polymers in

356

ATSUSHI ASANO AND K. TAKEGOSHI

PEA, wt.% 10 20

30 40 50 70 80

90 100 ]I~5

18~0

17S

170

PPM

'~C Solid State Chemical Shift Fig. 10.2. 13C CP/MAS NMR spectra of the carbonyl chemical shift region of PEA in PVPh/PEA. The weight percent of PEA in the blend is indicated on the left of each spectrum. (Reprinted with permission from Ref. [30]. 9 1993, John Wiley, New York).

the blends with PVPh. For instance, Fig. 10.2 shows 13C CP/MAS spectra of PVPh/poly(ethylene adipate) (PEA) blends with several weight percent of PEA. Although a single Tg was observed for whole compositions, the coexistence of two morphologically inequivalent microenvironments are appreciable as shown by the two peaks at 174 and 177 ppm. These two peaks are attributed to carbonyl carbons in a rigid amorphous phase and those hydrogen-bonded with the hydroxyl proton of PVPh.


357

Zhang et al. [37, 38] and Feng et al. [39] adopted poly(vinyl alcohol) (PVA) as a proton donor. The signal of the methine carbon (CH) of pure PVA shows the characteristic triplet lineshape due to hydrogen bonding [40]. On blending with a proton acceptor such as poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMAA), rearrangement of hydrogen bonding occurs and the characteristic lineshape disappears. Contrary to the widely accepted statements that hydrogen bonding produces a shielding decrease, Zhang et al. [37] observed an increase of ---3 ppm for the carbonyl carbon of PVA/ PMAA. The intrapolymer hydrogen bonding among PVA is destroyed on blending and interpolymer hydrogen bonding is formed. The former brings a low and the latter a high frequency shift. They concluded that the intrapolymer hydrogen bonding in PVA has a larger deshielding effect than the interpolymer one. Other interesting studies are found in Refs. [17, 18, 41431. Recently, it was demonstrated that the 129Xe nucleus is very sensitive to the free volume changes, and its NMR resonances provide a quick examination of miscibility. For miscible PB/PI [44] and PS/PVME [45] blends, a single 129Xe NMR signal is observed at an intermediate position of the two resonances for two component polymers. From the chemical shift differences and the 129Xe diffusion constant D (typically 10-11"-'10 -13 m 2 S-1) [46, 47], the domain size, below which only one 129Xe signal is expected, is estimated to be 90 nm [45] to 600 nm [48]. Mansfeld et al. [49] observed 129XeNMR in a rubbery copolymer of polyethylene (PE) and polypropylene (PP) embedded in a rigid PP. They find an approximate linear relation between the shift and the PE/PP ratio of the copolymer. For a blend, Miyoshi et al. [45] observed a nonlinear relation between the 129XeNMR chemical shift and the composition ratio in PS/PVME (Fig. 10.3). They showed that the observed 129Xe chemical shifts in the blend is explained by the total volume of the blend. Therefore, when an interpolymer interaction exists, the linear relation does not hold, because the total volume in a blend is smaller than that weighted average sum of the volumes of component polymers. Furthermore, they observed that the 129Xe NMR linewidth of PS/PVME = 50/50 (250 Hz) is narrower than that of PS (450 Hz), and broader than that of PVME (100 Hz). They suggested that PS in the blend becomes mobile by blending with mobile PVME. In Section 10.3.2.2, it is demonstrated that the 1D/2D 129Xe NMR is a powerful tool to determine the domain size of phase-separated blends. Line broadening of 13C CP/MAS signals is often observed at the temperature range of Te, + 30 to Tg + 60 K. For PS/PVME = 50/50, line broadening of the 13C resonance for the CH carbon of PVME on blending with PS was

358

ATSUSHI ASANO AND K. T A K E G O S H I

230

225

I

220

(_1

o,~

215

33 u 210

205 0

20

40

60

80

100

PVME c o n t e n t / w t % Fig. 10.3. Component-ratio dependence of the 129XeNMR chemical shifts in PS/PVME blends. Circles express the measured shifts. A solid line represents the calculated chemical shifts using a simple weighted sum of the volumes of pure PS and PVME. Crosses are the calculated chemical shift using the observed total volumes of the blends measured by Shiomi et al. [50] (Reprinted with permission from Ref. [45]. 9 1997 Elsevier, Amsterdam.)

observed at 311 K (Fig. 10.4) [51, 52]. This broadening is also due to the interpolymer interaction in the blend. Since the broadening effect is a motional effect, it is discussed in the next section. 10.2.2.2 Effects of blending on motion The effects of blending on motion may be caused by the different free volume of the blend compared with that of homopolymers or specific interpolymer interactions between component polymers. For example, it is well known for a miscible blend that a composition-dependent glass transition occurs at a temperature between two Tg values of the respective component polymers. The change of the original Tg value indicates that large-scale main-chain motions of a few 10 kHz are affected by blending. Macroscopic properties of a polymer, such as impact strength and ductility, are largely influenced by such molecular motions [53]. Thus, it is important to study the effects of blending on motion. In solid-state NMR, the lineshapes and relaxation rates


Ps

a)

@/

-cH2cH-

OCH3

PVME ,,-.-..,. ",~

CH

240 220 200 180 160 140 120 100 80

b) PS

PVME

-CH2CH-

CH OCH3

CH2 ~.

5/5

•••i1••`•j•••l••••••••t••••••••1••••••.••`•••••

359

5/5

60

40

20

0

-20 -40

220 2s

t80 160 140 120 100 80

60

40

20

0

-20

Fig. 10.4. 13C CP/MAS spectra of pure PS, pure PVME and PS/PVME = 50/50 at (a) 311 K and (b) 228 K. The peak marked with asterisk (*) in (a) is from a silicon-rubber cap to prevent leaking of the blend at higher temperatures. The peaks marked with SSB denote spinning side bands. (Reprinted with permission from Ref. [52]. 9 1994 Butterworth-Heinemann, UK.) would be influenced by such large-scale motion of a few 10 kHz. For example, Kwei et al. [54] observed a transition of the 1H 7'2 values of P S / P V M E = 50/50 from 15 to 3 ms at ---323 K. This is ascribed to the onset of large-scale motion at the glass transition. The free induction decay (FID) is composed of two T2 components which are different from those observed for pure PS and PVME. Each T2 component shows a different transition temperature, indicating different "glass transition" temperatures for the component polymers. For poly(ethylene oxide)/poly(methyl methacrylate) ( P E O / P M M A ) , Brosseau et al. [55] found that the temperature dependence of T2 of P E O obeys a Williams-Landel-Ferry (WLF) equation [56] with the reference temperature shifted 50 K higher than Tg. The motional narrowing of a ~H resonance occurs when the frequency of motion exceeds the 1H static linewidth and an averaging of the 1 H ~ H dipole interaction becomes appreciable. Similar motional narrowing occurs for a dilute spin, such as t3C and 29Si. The linewidth of a dilute spin is governed

360


by an anisotropic chemical shift interaction and a heteronuclear dipolar interaction between 1H. The linewidth in general is a few 10 kHz for a rigid solid, therefore, the linewidth of a dilute spin is also sensitive to motion of a few 10 kHz. Newmark and Copley [57] measured e9si NMR spectra of poly(dimethylsiloxane) (PDMS)/silicone at various temperatures. The linewidth decreases gradually with increasing temperature beyond Tg from --~1200 Hz at 173 K to --~100Hz at 295 K. For high-resolution solid-state 13CNMR using 1H dipolar decoupling (DD) and magic-angle spinning (MAS), the effects of motion on linewidth are rather complicated. This is because the anisotropic chemical shift interaction and the 1H--13C heteronuclear dipolar interaction are already averaged to be zero by MAS and DD. Since random molecular motion interferes with the artificial coherent averaging of DD and MAS, motional broadening is observed, instead of motional narrowing [58]. When the motional frequency is close to the MAS frequency, the chemical shift anisotropy interaction is reintroduced. Practically, however, this broadening is negligibly small when the magnetic field is less than 10 T, so that the observed broadening of the 13C resonances upon changing temperature has been attributed to the interference between motion and DD. When the motional frequency is close to the strength of DD in Hz (---50 kHz), the heteronuclear dipolar interaction is reintroduced. For most glassy polymers below Tg, the 13C linewidth is ---4 ppm and temperature independent. With increasing temperature beyond Tg, the line broadens and reaches a maximum width at --~T~ + 50 K. Further increases of temperature narrow the line to a few 10 Hz. Miller et al. [59] examined the temperature dependence of the 13C linewidths in miscible PI/poly(vinylethylene) (PVE). They found that the 13C linewidths of the two component polymers show different temperature dependencies. This shows that even though a blend is thermodynamically homogeneous, carbons on the respective components exhibit a distinct "glass transition". Takegoshi and Hikichi [51] gave an analytical equation for a temperature-dependent 13C linewidth in a glassy polymer. They explained the linewidth of 13C under DD and MAS as follows: Below Tg, the linewidth reflects a distribution of the isotropic chemical shift arising from a variety of local conformations of polymer in the glassy state. Above T~, molecular motion averages out the distribution, hence this term decreases gradually to zero (motional narrowing). With increasing temperature above Tg, the interference between the random (incoherent) modulation due to molecular motion and the coherent modulation by DD occurs. As a result, motional broadening instead of narrowing occurs. At temperatures far above T~, motional averagings of the anisotropic chemical shift and the 13C~IH dipole interaction become effective, and the line becomes


361

narrower even without DD and MAS. The remaining linewidth is ascribed to the intrinsic one accounting for various static line broadening mechanisms such as an inhomogeneous static field. Thus, the 13C linewidth is represented as [51] 2 AM2 8 = 8111 + - arctan{a(To- T)}] + ~ 7"1"

7rO) 1

7"O)1

1+

r2 w 2

+ 60.

(10.2)

Here, the first term empirically describes the motional averaging of the distribution of local conformations, where ~1 is half the linewidth due to the distribution, a describes steepness of the transition, and To is a characteristic transition temperature. The second term describes the interference between motion and DD [58], where A is the reduction factor of the second moment M2, r is the correlation time of motion and (.O1 is the strength of DD. This shows that the interference is effective when the motional frequency comes close to the frequency of DD (ro)l "~ 1). The third term 8o is the intrinsic linewidth. Figure 10.5 shows the observed temperature dependence of the CH carbon of PVME in PS/PVME [51]. 13C CP/MAS spectra of PS/PVME = 50/50 have been already shown in Fig. 10.4. The motional broadening with a linewidth of 1000 Hz occurs at 311 K for PS/PVME = 50/50, which is about Tg + 52 K. It is clearly shown that the dispersion of the isotropic chemical shift in the glassy state is very similar for each blend at below Tg, which is about 270 Hz (---4 ppm). Figures 10.4 and 10.5 show the molecular motion of PVME is affected by the blending with PS and the curve shifts to the higher temperature side with increasing the content of PS. The solid lines in Fig. 10.5 are the "best-fit" ones to Equation (10.2). It is concluded that the onset of anisotropic short-range motion of the PVME chain is not related to the macroscopic glass transition. Menestrel et al. [60] also investigated the 13C r e s o n a n c e s of PS in PS/PVME. Both groups conclude that the PS and PVME components do not share the same local chain dynamics, even though it is thermodynamically homogeneous. This conclusion is also supported by the aforementioned result of 1H T2 [54] and the 2H NMR study [61] for PS/PVME. Similar 13C linewidth behavior showing dynamic heterogeneity has been found for PIP/PVE [59], PVPh/PEO [34, 35], PVPh/PMA [36] and PMA/PVAc [62]. Landry and Henrichs [63] applied dynamic mechanical spectroscopy and 2H NMR to investigate sub-Tg motion in polycarbonate(PC)/PMMA and PC/poly(cyclohexylene dimethylene terephthalate)(PCHDMT). Examination of 2H NMR spectra and relaxation times led them to conclude that local

362


7.0

6.0 ~-

^ Q

4

3.o

A v

l,j

0

2.5

.!

I 3.0

I

I 3.5

I

!. 4.0

I

l 4.5

IO00K/T Fig. 10.5. Temperature dependence of the linewidth of the CH carbon of pure PVME (O), P5/PVME - 20/80 (x), PS/PVME - 50/50 (A) and PS/PVME = 80/20 (n). The solid curves through the data points are "best fits" calculated using Equation (10.2). (Reprinted with permission from Ref. [51]. 9 1991 American Institute of Physics, New York.)

motions in the PC backbone are slower in the miscible blends than that in pure PC, while local motions of PMMA are relatively unaffected by the blending with PC. It is true that blending affects the frequency of the main-chain motion of component polymers, but does it affect the mode (e.g., jump angles) of the motion? Among many NMR experiments, the 2D exchange 2 H NMR experiment is particularly useful for investigating the motional mode of component polymers [64]. The pulse sequence for the 2D 2 H exchange NMR experiment is shown in Fig. 10.6(a) [64]. Figure 10.6(a) shows two pulse sequences for the cosine and sine magnetization components in the t~ period. The initial pulse produces transverse magnetization. After the evolution time t~, the second pulse induces either longitudinal magnetization (cosine) or quadrupole order (sine). Segmental reorientation occurring during a mixing time tm alters the quadrupole coupling for a spin. After the mixing time, the stored Zeeman magnetization or quadrupole order are again transformed

363

P O L Y M E R B L E N D S A N D MISCIBILITY

a) 90, 54 . 7.,

I I .

54.7,

Zeemanorder .

.

.

.

.

_AlfX

.

_

~VV v ) < I

II

tl 90~ 54.7.,

)

t2 <s i n ( t o ~ t ~ ) s i n ( u 2 t . 2) >

~VV ~

b)

~.t /"x

_

_A

~ttttttttt~

-150-1OO-50

O 50 1OO 150 kHz

-150-1OO-50

O 50 IOO 150 kHz

Fig. 10.6. (a) Pulse sequence for the 2D 2H exchange N M R experiment. (b) The 2D 2H exchange NMR spectrum of pure d3-PS at 373 K [65]. The contour plot is shown in the righthand side. The 7r/2 pulse length was 2.2 ms and the mixing time (tm) was 10 ms. The pulse interval for the quadrupole echo (A) is 20 ms.

364


into transverse magnetization and detected by using the quadrupole echo with the echo period of A. Figure 10.6(b) shows the 2D 2H exchange NMR spectrum of main-chain deuterated PS (d3-PS) at 353 K, which is below the Tg of PS (---375 K), with the mixing time of 10 ms [65]. The 2D spectrum shows a typical rigid powder pattern along the diagonal axis without appreciable off-diagonal signals. This indicates that the motion of d3-PS is much slower than the order of Hz below Tg. However, the spectrum at 393 K shows appreciable cross-peaks (Fig. 10.7(a)), and the off-diagonal pattern shows no particular ridges [65]. Similar spectra have been also observed by Wefing et al. [66] and they conclude that the main-chain motion of PS can be ascribed to an isotropic rotational diffusion. For d3-PS/PVME, a similar pattern was also observed at 333 K (Fig. 10.7(b)). This shows that the mode of molecular motion of PS is not affected by the blending of PVME. Note, however, the cross-peak pattern in the blend observed at 333 K, which is much below the corresponding temperature (393 K) for homopolymer. This indicates that the main-chain motion of PS in PS/PVME = 50/50 of 100-1000 Hz commenced at temperature much below the Tg of the PS homopolymer [61]. Similar conclusions by Chin et al. [67] show that component polymers retain their distinct motional characteristics in a miscible blend, even though their dynamics in blends are very different from those in homopolymers. They studied glass transition dynamics of miscible PPO/PS = 25/75 by 2D exchange NMR experiments of ~3C and 2H. From those exchange spectra, they found that the chain motion of PPO and PS of a few kilohertz in the blends commenced at temperature of 10-15 K below the Tg (401 K). In contrast, such motion of homopolymers appeared at a temperature above their Tg's. The motion exhibited the characteristics of rotational Brownian diffusion with an associated broad distribution of correlation times. Both distributions of PPO and PS are a bimodal distribution and considerably broader than those typical for pure PPO and pure PS, respectively. They employed a statistical lattice model to evaluate local concentration fluctuations and explained the observed relative ratio of the mode. The dynamic heterogeneity in nominally miscible polymer blends has been studied also by Chung et. al. [68] by observing 2D exchange 2H NMR. They observed that the two components of PI/PVE displayed distinctly different segmental mobilities, even though this blend is characterized by a single glass transition. They explained the dynamic heterogeneity depending on PVE content using the effective glass transition temperature obtained by fitting the correlation time and WLF equation. Kumar et al. [69] explained the dynamic heterogeneity in several miscible blends by using a model based on concentration fluctuations. They assumed

365

P O L Y M E R BLENDS A N D MISCIBILITY

a)

i Y

i I

I'""!'"'"

-150-100-50

I

"'" I ' " ' I'""' I ' " ' I 0 kHz

50

100 150

'"'i'l"l""l"l'l""l

-150-100-50

0 kHz

50

''l~

100 150

b) /,

I f'''

-150--100-50

'"" I"'"I"" 0 kHz

50

I"" I

100 150

''''lll''llli'l'"'l''''

-150--100-50

0 kHz

50

l'''V'

100

150

Fig. 10.7. 2D 2H exchange N M R spectra and contour plots of (a) pure d3-PS at 393 K and (b) d3-PS in d3-PS/PVME = 50/50 at 333 K [65].

366


that, although the probability of occurrence of concentration fluctuations is symmetric about the mean value in a given volume, the cooperative volume over which a fluctuation must occur for it to be detected by a dynamic probe is not a constant. They conclude that the dynamic heterogeneity is a consequence of the coupling of concentration fluctuations, which occur symmetrically about the mean composition in any fixed volume, with a cooperative volume that changes monotonically with composition for systems with significant Tg contrast. The blending effects on motion have also been studied by measuring the ~3C spin-lattice relaxation time T~. Feng et al. [70] measured ~3C T~ of PPO and PS in PPO/PS. They showed that the respective ~3C T~ values for the aromatic carbons of PPO (---10 s) and PS (---39 s) homopolymers become the same value of 17-18 s when PPO and PS are blended at PPO/PS = 60/40. They concluded that the aromatic rings of PPO drive those of PS to move cooperatively, which indicates a strong rr-rr electron conjugation interaction between the aromatic rings. This approach was also applied to investigate poly(styrene-co-acrylonitrile)/poly(ethyl methacrylate) (SAN/PEMA) and SAN/PMMA [71]. Interpolymer interactions between the phenyl groups of SAN and the carbonyl groups in PEMA or PMMA were examined. Schantz and Ljungqvist [72] measured ~3C Tx of poly(3-octylthiophene) (POT) and PPO in partially miscible POT/PPO. They found that the ~3C T~ values of alkyl side chain carbons of POT increase considerably when POT is blended with PPO at POT concentrations less than 30%. This reflects an increasing of flexibility of POT on the blending with PPO. Recently, self-diffusion constants of polymer blends and copolymers have been observed by using pulsed-field gradient NMR (PFG-NMR) techniques in solution and solids. Meier et al. [73] examined interpolymer diffusion in PDMS/poly(ethylmethylsiloxane) (PEMS) at temperatures far above its Tg by the ~H correlation spectroscopy and PFG-NMR. Miyashita and Nose [74] examined the dynamic critical behavior of PS/PVME in deuterated benzene solution at the concentration where polymer chains are weakly entangled. They applied PFG-NMR, quasielastic light scattering and shear viscosity measurements to conclude that the self-diffusion of constitutional polymers is not affected by critical fluctuations. Uemura and Macdonald [75] investigated the binding of a hydrophobic ethoxylated urethane (HEUR) associating polymer (AP) to PS latex (diameter 168 nm). They observed the enhancement of the self-diffusion of HEUR-AP on addition of PS latex due to the breakup of the associated network. Pinder [76] calculated the X parameter from selfdiffusion and slow mode diffusion constants obtained by PFG-NMR and dynamic light scattering for PS/PMMA in deuterated solvents.


10.2.3

367

Spin diffusion and domain size

Since miscibility (degree of mixing) influences macroscopic properties of a blend significantly, it is important to know the size and morphological information of domains in a blend. In Section 10.2.3.1, the effects of spin diffusion on IH Tx and Tip are discussed, which can be used to deduce the domain size on a scale of 2-50 nm. Sections 10.2.3.2, 10.2.3.3 and 10.2.3.4 discuss several experiments to monitor spin diffusion. To monitor spin diffusion, the following three periods, which are formally analogous to cross-relaxation and chemical exchange NMR experiments in liquids, may be required: (1) the preparation of nonequilibrium magnetization (polarization gradient) among the spins of component polymers or between different domains; (2) the variable spin-diffusion time, where spin diffusion takes place; (3) the observation of the resulting magnetization. Various methods have been proposed for (1) and (3). In Section 10.2.3.2, methods for (1) based on the Goldman-Shen experiment [8] are reviewed. In Section 10.2.3.3, 2D NMR experiments, which enable us to detect the region of specific interpolymer interaction and the domain size on a scale of 2 nm, are discussed. In Section 10.2.3.4, heteronuclear cross-relaxation experiments between 1H~13C,1H~ZH,19F~13C,electron to 1H, etc., which can be applied to study the intimacy between component polymers, are reviewed. 10.2.3.1 Spin-lattice relaxation experiments In solids, different 1H relaxation rates of respective spins tend to be averaged by a mechanism called spin diffusion. Spin diffusion is the equilibration process of nonequilibrium polarizations of spins at different local sites through mutual exchange of magnetization. Since the efficiency of spin diffusion is governed by a strength of a dipole-dipole interaction, which is a function of the internuclear distance, we can obtain information about the domain size of a blend by analyzing the spin-diffusion rate among component polymers. In this section, effects of 1H spin diffusion on a 1H spin-lattice relaxation rate are discussed. Figure 10.8 shows a schematic illustration of how the T1 relaxation process for ~H spins in a blend of polymers A and B proceeds with spin diffusion (SD). Here, we assume that (1) 1H spins are divided into two species: species A for polymer A and species B for polymer B, and (2) both A and B are characterized by their common relaxation times TIA and T1B, respectively.

368


SD

Tt

\ /

A

B

ltttt iiii Ill |

m

i

i

i

4,

Ill T1A >

j

. . . .

P~

C ~o > > Z

i

> C ~o 0 0 ---"

300

200

100

200

100

ppm

.....

' _ - - - A , - - - . A , ,

200

100

=---"

. . . .

J ....

200

'

. . . . . .

100

Fig. 14.11. 13C CP N M R spectra of carbonyl carbon-labeled uniaxially draw ( x 5 at 80~ P E T film as a function of ilL, the angle between the draw direction and the magnetic field. Full and dotted curves show observed and calculated spectra, respectively. The fractions of the three components, amorphous (55%) and two oriented ones (33 and 12%), was determined by simulation. The structural parameters were fiE = 90o, ~ F = 24 -+ 10 ~ and p = 24 ~ (low oriented component) and GF = 9 0 ~ ~ F = 16 -+ 6 ~ and p = 5 ~ (high oriented component), respectively.

i

!

!

(high)

._t

300

200

t

.

Unoriented component (powder pattern)

Oriented component (low)

Oriented component

4

.

200

I00

I00

200

2O0

I00

I00

9 trn

ppm

rn

o-

i 7~

rn

> t>

, 300

,

'~

v-~ ~ ~.,_~, 200

100

l

.

.

.

.

.

.

.

.

.

. 9 ....

200

_

l

.

.

.

.

|

.

.

.

200

IO0

.

i

.

.

.

.

t

100

__

o,o

200

100

ppm

Fig. 14.12. 13C CP N M R spectra of carbonyl carbon-labeled uniaxially draw ( x 5 at 80~

P E T film after heat t r e a t m e n t at 170~ /3L is set as 0 and 90 ~ Full and dotted curves show observed and calculated spectra, respectively. The fractions of the three components, amorphous (30%) and two oriented ones (35 and 35%), was determined by simulation. The structural parameters were aF -- 90 ~ ~F --" 20 + 10 ~ and p = 8 ~ (low oriented c o m p o n e n t ) and O~ F = 90 ~ ~ F = 11 - 5 ~ and p = 2 ~ (high oriented c o m p o n e n t ) , respectively.

t.n ~t~

506

TETSUO A S A K U R A AND T A K U R O ITO

Table 14.1. Fraction (%) of disordered and ordered components for uniaxially oriented PET films determined from 13C CP NMR

Drawn at 45~

Drawn at 80~

Heat treatment

Draw ratio

x3

x2

x3

x4

x5

x5.66

x5

Disordered Ordered

35 65

100 0

80 20

70 30

55 33 (1) 12 (h)

47 38 (1) 15(h)

30 35 (1) 35 (h)

(1) = low ordered component. (h) = high ordered component. Table 14.2. Structural parameters (degrees) of the ordered components for uniaxially ordered PET samples determined from 13C CP NMR (az was assumed to be 90 ~

Draw ratio

p /3F 0

Drawn at 45~

Drawn at 80~

Heat treatment

x3

x3 & x4

x5 & x5.66 low high

x5 low

high

24 24 --+ 10 16 -----10

8 20 --+ 10 20 -----10

2 11 --+ 5 29 -----5

22 18 -----6 22 -----6

24 20 -----10 20 -----10

5 16 -- 6 24 -----6

p: = the orientational distribution around the fiber axis (chain axis or MD). 0 = the angle between the phenylene para-C--C and chain axes (or MD) of PET film.

is determined as 29 m 5o. The structural parameters obtained for uniaxially oriented PET films are summarized in Tables 14.1 and 14.2. The three components observed in the spectra of well-oriented PET samples might correspond to the three-region model composed of an NMR crystalline, highly oriented component, a rigid, NMR amorphous (low oriented) component, and a mobile NMR amorphous region (unoriented component) proposed by Havens and VanderHart [12], and Gabrielse et al. [2] on the basis of 13C CP/MAS NMR relaxation experiments.

References 1. J.-M. Besnoin and K.Y. Choi, J. Macromol. Sci., Rev. Macromol. Chem. Phys. C29 (1989) 55. 2. W. Gabrielse, H. Angad Gaur, F.C. Feyen and W.S. Veeman, Macromolecules 27 (1994) 5811. 3. M.D. Sefcik, J. Schaefer, E.O. Stejskal and R.A. McKay, Macromolecules 13 (1980) 1132. 4. A.D. English, Macromolecules 17 (1985) 2182. 5. M. Wilhelm and H.W. Spiess, Macromolecules 29 (1996) 1088. 6. S. Rober and H.G. Zachmann, Polymer 33 (1992) 2061.

POLY(ETHYLENE TEREPHTHALATE)

507

7. B.F Chmelka, K. Schmidt-Rohr and H.W. Spiess, Macromolecules 26 (1993) 2282. 8. T. Asakura,T. Konakazawa, M. Demura, T. Ito and Y. Maruhashi, Polymer 37 (1996) 1965. 9. D. Walls, J. Appl. Spectrosc. 45 (1991) 1193. 10. S. Arnott and A.J. Wonacott, Polymer 7 (1966) 157. 11. G.S. Harbison, V.-D. Vogt and H.W. Spiess, J. Chem. Phys. 86 (1987) 1206. 12. J.R. Havens and D.L. VanderHart, Macromolecules 18 (1985) 1663.

Chapter 15


Crosslinked Polymers R.V. Law ~ and D.C. Sherrington 2 1Department of Chemistry, Imperial College of Science, Technology and Medicine, London, SW 7 2AZ, UK; 2D.C. Sherrington, Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK

15.1

Introduction

Crosslinked polymers are key materials in a wide range of technological applications ranging from the "high-tech" aerospace area to the "low-tech" use of wood-derived products. Inevitably, the bulk properties and materials performance are controlled in the final analysis by the polymer molecular structure, although to relate the backbone structure to performance in applications is a complex exercise. This often involves the requirement for numerous intermediate correlations e.g., molecular structure with micromorphology, micromorphology with phase separation, etc. Evaluating the molecular structure of linear polymers has become relatively straightforward in recent years and solution phase ~H and ~3C NMR techniques have played an important role in this context. When linear polymers are crosslinked to form an infinite network, either during the polymerisation process, or as a post-polymerisation chemical treatment, then analysis by routine solution NMR spectroscopy rapidly becomes impossible as a result of signal broadening. When the level of crosslinking remains low, solution phase methodology applied to highly solvent swollen polymer can still be useful (see Sections 4.3 and 4.4). In general though, most crosslinked polymers are relatively highly crosslinked, and become amenable to analysis by NMR spectroscopy only by using solid phase techniques.

15.2 Solid-state ~3C and ~SN cross-polarisation/magic-angle spinning (CP/MAS) NMR 15.2.1

Background

Despite the obvious benefits of quantitative solid-state NMR via single pulse excitation methodology (SPE) (see Section 3), cross-polarisation combined with magic-angle spinning (CP/MAS) [1] has many intrinsic advantages which

510

R.V. LAW AND D.C. SHERRINGTON

have proved very valuable in evaluating the structure of crosslinked polymers. The principle benefit of CP is that it enables a spectrum to be acquired very quickly, as more scans can be obtained in any given time, giving a better signal-to-noise (S/N) ratio. The increase in S/N ratio is due to the fact that in many organic materials the proton spin-lattice relaxation mechanism, by which the system can relax back to equilibrium, is approximately an order of magnitude shorter that for 13-carbon. Though it is desirable to use SPE it is often impractical to carry out a fully quantitative analysis because experimental conditions e.g., MAS at high temperatures, means that there is limited time available and therefore a compromise, using only CP, has often to be made. If time permits an ideal approach is to use a combination of both SPE and CP as these techniques complement each other well (see Section 3). The other advantage that CP brings is important information about the molecular dynamics within the polymer from CP variable contact studies. The parameters typically obtained include the cross relaxation time between 1H and 13C, TcH, and the relaxation time in the rotating frame, ~H Tip. Furthermore, there is also a large (--~ factor 4 for 13C) sensitivity enhancement for ~3C when carrying out CP as polarisation is transferred from the isotopically abundant 1H (100%) to the rare 13C (1.1%). This section will deal principally with crosslinked polymers which have been characterised with CP combined with MAS. Earlier monographs and reviews have dealt, in part, with the application of CP/MAS to crosslinked polymer systems. These include reviews by Yu and Guo [2], Andreis and Koenig [3], books by Komorski [4], Mathias [5], McBrierty and Packer [6], Ibbett [7], Bovey and Mirau [8] and the annual reports by Webb [9]. This section will focus on more recent papers dealing with the application of solid-state CP/MAS NMR spectroscopy to the analysis crosslinked polymer systems.

15.2.2

Phenol-formaldehyde resins

Phenol-formaldehyde (PF) resins have been used as model compounds for the study of pyrolysis and combustion reactions that occur in solid fuels [10]. Utilising these resins it is possible to incorporate a wide range of heteroatomic and hydrocarbon moieties to simulate compounds that arise naturally in the solid fuels. A series of phenol resins crosslinked with thiophene, dibenzothiophene, diphenylsulfide, benzyl phenyl sulfide, thioanisole, 8-hydroxyquinoline and 2-hydroxycarbazole were synthesised. These samples were then cured at 200~ (Fig. 15.2.1) and the resulting resins examined by solid-state NMR spectroscopy. The ~3C CP/MAS spectra of a standard PF resin is shown

CROSSLINKED POLYMERS

511

OH '---'- S ~

OH

Excess CH20 "OH

/

CH20H

S

CH2

Curing

/

CH2

S

CH, -

Fig. 15.2.1. Incorporation of diphenyl sulfide into the structure of the co-resite and the resultant

resite after curing.

in Fig. 15.2.2. This was compared to the partially and fully cured resins. (Figs. 15.2.3 and 15.2.4). By curing the standard resins at increasing temperatures (up to 200~ it was possible to show that peaks at 35 and 70 ppm, attributable to methylene and alkyl ether bridges respectively, were converted from the ether to methylene bridges at the final cure temperature. The aryl ether peak at 160 ppm possibly arose from the condensation of two phenolic moieties. These resins were studied in order to obtain the optimum cure conditions. The sulphurcontaining resins show a wide range of ether, ethylene and methylol constituents. The peaks at 38, 55-80 and 90 ppm arise from methylene and methylol carbons attached to the ortho and para positions of the phenol ring and hemiacetal carbons, respectively. Fully cured resins contain only aliphatic peaks at 18 and 38 ppm with no remaining alkyl ether linkages. The aryl ether peak at 160 ppm also increases slightly in intensity. A major side reaction that was

512

R.V. L A W A N D D.C. S H E R R I N G T O N

't

j .......

I

200

. . . .

90~ (48 hours) ~._

I. . . . .

100 PPM

-

--I

0

. . . . . .

Fig. 15.2.2. Solid state 13C C P / M A S N M R spectra of a normal PF resin after various cure periods. SB = spinning side band.

identified for both the normal and sulfur containing resins was the formation of an arylmethyl moiety which gave a peak at 18 ppm. Understanding the curing process in PF resins and the analogues employed in this work gave a useful insight into the mechanisms that occur in solid fuels. Though useful as model compounds, PF resins have many commercial uses in their own rights, these include thermal insulation, mouldings, and use as wood resins. Therefore the systematic understanding of their molecular structure gives an insight into their physical properties. One important physical property is the resistance of the resin towards acid and bases; understanding this enables a better approach to the correct formulation of the resins.

513


(a)

(c)

200

1SO

leo PPH

50

0

Fig. 15.2.3. Solid state 13C CP/MAS NMR spectra of the partially cured (130~ containing (a) dibenzothiophene (b) thioanisole (c) phenyl benzyl sulfide.

resites

This has been studied by 13C CP/MAS NMR where the degradation of PF resins in the presence of acid (oxidising and nonoxidising), base and formalin [11] has been monitored (Fig. 15.2.5). The main structural components believed to be present in the cured resin are shown in Fig. 15.2.6. The proportions of each species depends upon the initial P : F ratio, p H value, catalyst and temperature. At relatively low concentrations (---1 M) exposure to acid and base simply leads to neutralisation or formation of the phenoxide salt of the resin. This is shown by the change in intensity of the peak at ca. 152 ppm. Also in the presence of alkali, peaks at 73 ppm disappeared giving a proportional rise in intensity at 65 ppm. This was explained by the cleavage of dimethylene ether linkages to produce the corresponding methylol groups. Treatment with formalin also produced methylol groups at positions ortho or para with respect to the phenolic linkage, producing peaks

514

R.V. LAW A N D D.C. S H E R R I N G T O N

(b)

(c)

(d)

200

150

I00 PPM

50

0

Fig. 15.2.4. Solid state 13C CP/MAS NMR spectra of the fully cured (200~ resites containing

(a) dibenzothiophene (b) phenyl benzyl sulfide (c) diphenyl sulfide (d) thioanisole.

at 65 ppm. There was also evidence, from the presence of peaks at 40 ppm, of the formylation of p,p'-methylene linkages formed. Under stronger nonoxidising acidic conditions (sulphuric acid 36 N) (Fig. 15.2.7) all of the methylol and dimethylene ether linkages are cleaved. Some of this cleavage gives rise to CHO (194ppm) and CH3 (18ppm) moieties. There is also evidence that there is sulphonation of the aromatic ring in the ortho and para position to the phenolic hydroxide group (ca. 152 ppm). The use of strong oxidising acid (15 N nitric acid) brought about more major structural changes

CROSSLINKED P O L Y M E R S

515

Phenol i c ring carbons

herl•rbons /

~

~C_H2OAr'

ArCHO

ArCH2Ar'

b

C 9

i"~'~'T

200

,"'i

1

'

150

'

'

i 1 "" 100

'

'

I

50

""'~ ' I 0

'

PPM

Fig. 15.2.5. 15.1 MHz 13C CP/MAS N M R spectra of (a) resole type resin (b) cured resin after treatment with 1.0 N sodium hydroxide under N2(g) for 65~ for 3 days, and (c) cured PF resin after treatment with 36.8% formal under N2(g) at 25~ for 1 day.

(Fig. 15.2.8). These included the conversion of phenolic rings into cyclic ketones and nitration of the rings. Methylol and methyl groups are also similarly nitrated. 15.2.3

Melamine-formaldehyde resins

These are common materials that have found many applications e.g., as hard, durable and chemically resistant surfaces. A series of uncured and cured

516

R.V. LAW AND D.C. SHERRINGTON OH

OH

OH

0

~

OH

Fig. 15.2.6. Main moieties present in a PF cured resin.

melamine-formaldehyde (M-F) resins have been examined by solution state (1H, 13C) and solid-state (13C, 15N) NMR respectively [12]. The resins were either laboratory synthesised or obtained from an industrial manufacturing process. The solid-state NMR studies involved four resins, two laboratory synthesised and two obtained from industry, all of which had different M : F ratios. The 13C solid-state NMR spectra of the synthetic resins are shown in Fig. 15.2.9. The peak at 166 ppm in readily assigned to azine ring carbons. The more interesting region, however, is the methylene region (40-60 ppm) in which there are five components at 48, 52, 58, 66, and 72 ppm. These have been assigned to the following structures,

--NHCHzNH m,

=NCH2NH--,

--NCH2N=,

mNHCH2OCH2~,

This indicated that resins synthesised with M ' F ratios of 1.0" 1.5 contain principally methylene linkages where as higher ratios of M ' F (1.0" 3.0) contain equal amounts of methylene and methylene ether linkages. The spectra, however, showed no evidence for methylol groups which should give peaks at 64 and 69 ppm. The was also confirmed by examining the 15N solid-state NMR spectra (Fig. 15.2.10). The 15N solid-state NMR spectra of model compounds containing methylolmelamines showed peaks at 87 and 107 ppm (indicative of ~ N H C H 2 O H and ~N(CH2OH)2 which were absent in the spectra of the resins. What the 15N spectra did indicate, however, was a peak at 77 ppm and a shoulder (for the M ' F 1.0" 3.0 resin) at 93 ppm indicative of ~ N H C H 2 ~ and - - N C H 2 ~ linkages. The industrially synthesised resins showed very similar spectra indicating the presence of high concentrations of branched and linear methylene and methylene ether linkages.

517

CROSSLINKED P O L Y M E R S

Phenolic ri ng carbons ot--~hercarbons

I

COCH2Ar'

J!

ArCH2OAr' ~ AI.CH2OCH2Ar'

C_OH ArCHO

ArCH2OH

\

- ArCH2Ar

a

b

C 200

!50

100

50

0

PPI4

Fig. 15.2.7. 15.1 MHz 13C CP/MAS NMR spectra of three residues of cured PF resin after treatment with sulphuric acid under N2(g) and three different conditions (a) 1.0 N sulphuric acid solution at 65~ for 3 days (b) 36 N sulphuric acid at 25~ for 1 day (c) 36 N acid at 65~ for 1 day.

15.2.4

Urea-formaldehyde resins

Currently, one of the most important commercially available materials today are the urea-formaldehyde (U-F) resins. Their applications include coatings, adhesives, castings, moulding compounds and textiles. Maciel et al. have produced a series of extensive papers in this area concentrating on both 13C and 15N CP/MAS [13-16].

518


a

b 0

d

-

'"

1.-

' 1"

2S0

'

' ' I '''

200

' I '''''''~

150

'

'1"

100

'""

I '

50

''

~1

~

0

'

1 '

-50

PP~I

Fig. 15.2.8. 13C CP/MAS NMR spectra of the residue of cured PF 50 resin after treatment with 15 N nitric acid in air at 25~ for 1 day (a) 50.3 MHz (b) 22.6 MHz (c) 15.1 Mhz and (d) 15.1 MHz 50-1~s dipolar dephasing. Spinning sidebands are marked with an asterisk.

A systematic 13C CP/MAS study was undertaken of a large array of different U-F resins synthesised under a wide range of conditions including pH, concentration, and U : F ratio [13]. Catalyzed by both acid and base a great variety of reactions can occur leading to a large and complex range of moieties depending upon the synthetic conditions. At high pH the principal linkages present are methylol urea (65, 72 ppm) and dimethylene ether (69, 70 ppm), also present under the basic conditions is a small amount of methylene methyl ethers (55 ppm). Under acidic conditions other reactions predominate. In addition to the formation of linear methylene linkages (47, 54,

519


Ca)

L

1

300

J- .

200

.

.

.

100

.

1

. . . . . . .

0

ppm

I

-100

(b)

L

300

~

:

200

..........

l

100

.

.

.

.

ppm

__1

0

.j

-100

Fig,. 15.2.9. Solid state 13C CP/MAS NMR spectrum of M-F resin.

60ppm) resins contained a substantial amount of crosslinking methylene linkages (69, 76ppm) which increase as the F : U ratio increases. At very high F : U ratios methylene dimethylene ethers, methylol and hemi-formals occurred (69-72 ppm). Also present in large quantities were disubstituted urons (75, 79 ppm) which increased as pH increased. In an attempt to formulate alternative [14] resins N,N'-dimethylolurea or paraformaldehyde was used as a different source of formaldehyde to crosslink the urea molecules. The resins produced by these methods generally exhibited similarities to those previous synthesised using formaldehyde [13]. Small

520


(a)

r

(b)

i

I

I

400

300

,

J_

_

200

1

t

100 ppm 0

,

.1

-100

Fig. 15.2.10. Solid state 15N CP/MAS N M R spectrum of M-F resin.

differences between the resins produced by the different synthetic route appeared to be due principally to the problems of solubility of the N,N'dimethylolurea or paraformaldehyde. A large volume rotor MAS system was used to examine the natural abundance 15N present in urea-formaldehyde resins [15]. Increasing the amount of material which is examined has enabled the investigation of the isotopically low 15N present (0.37%) in the resins without having to resort to synthesising 15N enriched materials. There are four possible interaction sites between urea and formaldehyde (Fig. 15.2.11).

CROSSLINKED POLYMERS O II

0 II

~--N--C--NCH2OH + H20

~-~N--C--NH + HOCH2OH

I

I

O II ,~,N--C--NCH2(OCH2)nOH

I

I

0 II ,,,,,,~N--C--NCH20H 9

I

I

I

OH

,,~N--C--NCH2(OCH2)n+IOH + H20

0 II + ,,---N--C--NH

I

~

I

O O II II ,~-N--C--NCH2N--C--NH.'~ + H20

I

I

I

I

O O II II ,~,N--C--NCH2OCH2N--C--NH,w~ + H20

~-~N--C--NCH2OH

--N--C--N--N [ ~H2 CH2

0 Ii

+HOCH2OH

0 I!

o II

521

I

~

",,

O

N

~ jN

I

I

I

/ + H20

O

OH

Fig. 15.2.11. Structural units present in UF resins.

For resins synthesised under acidic conditions tertiary amides initially seen by 13C CP/MAS were confirmed by 15N CP/MAS. Under neutral or basic conditions the main constituents of the resin are N,N'-dimethylolurea (102 ppm), monomethylolurea (102 and 78 ppm) and dimethylene ether linkages (90 ppm). Using dipolar dephasing and cross-polarisation times it was possible to distinguish primary, secondary and tertiary substituted nitrogens. These results confirmed the existence of many moieties postulated by 13C CP/MAS. The widespread application of U-F resins has meant understanding the mechanism of the degradation process is important if an improvement in resin stability is to be obtained [16]. Therefore the way in which U-F resins change when they undergo hydrolysis was examined. The resins have been described previously [13]. There are a number of possible mechanisms which involved the hydrolysis of the moieties in the U-F resins. A typical degraded resin is shown in Fig. 15.2.12.

522

R.V. LAW AND D.C. S H E R R I N G T O N

A

b)---J -I-

i ....

I"

i

"!

I

i ....

200 180 160 140 120 100 80

I'

60

I

40

I"

20

I --

0 PPM

Fig. 15.2.12. (a) 50.3 MHz 13C CP/MAS NMR spectrum of a UF resin sample prepared from formalin (F) and urea (U) with an equivalent F/U/water ratio of 2.00/1.00/1.07 at pH 3 and (b) its solid residue after hydrolytic treatment at pH 4 and 86~ for 20 h. Spinning side bands are marked with asterisks.

It was demonstrated that resins prepared with an equivalent molar ratio of F : U gave the highest stability towards hydrolytic treatment. Resins which contained higher F : U ratio (2.0: 1.0) contained a wide range of moieties which were more readily susceptible to hydrolysis, the products formed include dimethylene ether linkages, poly(oxymethylene glycols) and methylols attached to tertiary amine groups. These moieties are the sources of formaldehyde when the resin degrades. Resins of different composition showed similar degradation patterns. 15.2.5

Isocyanurate based resins

In a series of papers cured resins based upon ~SN enriched 4,4'-methylenebis(phenyl isocyanate) (MDI) have been examined by utilising ~3C and 15N

CROSSLINKED POLYMERS RNCO + H20 R'NCO + RNH2 ~ ~ - ' P "

RNCO + R'NHCONHR"

523

~-" RNH2 + CO 2

RNHCONHR' ~" R"NHCON(R")CONHR

Fig. 15.2.13. Reactions of isocyanate units that occur in MDI-polyisocyanurate resins.

CP/MAS NMR. In the first of these papers the structures within the resins were examined as a function of cure temperature [17]. The chemistry of the resins is very complex but one of the principle reactions is the formation of stable isocyanurate structures from three isocyanate units. Other species are also present e.g., amine, urea and biuret. These are formed by the reactions shown in Fig. 15.2.13. 13C CP/MAS NMR spectra of the resins indicated that the optimum cure temperature was 120~ at which most of the isocyanate groups were converted to isocyanurate (Fig. 15.2.14). The peak at 150ppm is due to the isocyanurate carbonyl carbon, the benzylic substituted aromatic carbons para- to an isocyanurate moiety are shown by a peak at 145 ppm, the 130ppm peak is due to the protonated aromatic carbons, the shoulder at 125 ppm is due to both isocyanate and ortho aromatic carbons. These signals were confirmed by using dipolar dephasing spectra. However, because of the complexity of the spectra ~SN CP/MAS NMR was used to clarify the structures present at the different cure temperatures. A summary of the moieties found in the resin is given in Fig. 15.2.15. In addition Duff et al. also undertook a quantitative analysis by utilising the large difference in cross-polarisation time for protonated and nonprotonated nitrogen. In a related study a series of resins were examined in which biuret linkages predominated [18]. These were formed by the reaction of formic acid and 4,4'-methylenebis(phenyl isocyanate) (MDI). The principal reactions that occur in the resins are shown in Fig. 15.2.16. The 13C and 15N CP/MAS spectra showed that when the formic acid" MDI ratio increased the biuret linkage predominates. The pathway for this was initially the formation of MDI-based urea and formic anhydride moieties which further reacted with isocyanate groups to form the biuret linkages and possibly diformyl imide groups. In a further study the same resins were analysed straight after curing and again after a 7 months exposure to air [19]. Three different cure temperatures were used, 100~ 120~ and 160~ A typical example of the 15N CP/MAS spectra is given in Fig. 15.2.17. The predominant structure in these resins is the isocyanurate linkage. These are relatively stable and it is the chemistry of the residual isocyanate groups that dominate the formation of new bonds in the system during the

524


160 *C

140 ~

120 ~

100 ~

80 ~ 160

140

120

100 PPM

Fig. 15.2.14. 50.3 MHz 13C CP/MAS spectra of MDI-polyisocyanurate resins prepared at different temperatures.

exposure to the air. Using ~SN CP/MAS it was possible to identify clearly the products of isocyanate hydrolysis which involved principally the formation of amines and the urea-linkage condensation products, there was no significant formation of biuret linkages. Structural assignment for these resins was further substantiated by 13C CP/MAS. These results aided the identification of the decrease and increase in concentrations of isocyanate groups and urea linkages, respectively. Duff et al. also attempted a quantification experiment using the ~SN CP/MAS results to determine the relative concentrations of the


525 15N Chemical Shift

Isocyanate

ArNCO

46

Amine

ARNH 2

53

Urea

ArNHC(O)NHAr

104

114 ArNHC(O)N(Ar)C(O)NHAr

Biuret

141

.0

L Uretidione

Ar--N

\]l/

(NH) (N)

145

N--At

O

Isocyanurate

O

'~N

N/Ar 149

I

Ar Fig. 15.2.15. Structures and 15N chemical shift data pertinent to MDI-based resins.

RNCO + R'COOH

~"

RNCO + 2R'COOH RNCO + R'NHC(O)R" RNCO + R'C(O)OC(O)R'

RNHC(O)R'

+ C02

RNHC(O)NHR + R'C(O)OC(O)R' ~--

RNHC(O)NR'C(O)R"

~

R'C(O)NRC(O)R' + C02

Fig. 15.2.16. Reactions that occur between formic acid and 4,4'-methylenebis(phenyl isocyanurate) (MDI).

526

R.V. LAW AND D.C. S H E R R I N G T O N

B

A l

200.00

J_

1

| ~ , ~0

J .....

!

i013.=

I

_

l

50,00

,,.J

!

E, O0 PPM

Fig. 15.2.17. (a) 20.3 MHz 15N CP/MAS spectra of MDI-polyisocyanurate cured at 100~ (b) Same resins after 7-month exposure to air.

different nitrogen containing moieties before and after prolonged exposure to air [20]. The differing extents to which hydrolysis occurred in the samples was interpreted in terms of the structural effect that the cure conditions had. The possibility that the different morphologies present could be responsible for the amount of hydrolysis was further investigated by study of the 1HT~p of the samples. Finally the thermal degradation of the samples was examined. In this study it was possible to show that the degradation of all biuret and uretidione linkages occurred at 230~ a decrease of residual isocyanate took place on heating to temperatures up to 240~ an increase in urea linkages occurred in samples heated up to 240~ followed by a decease in these from 250-260~ A steady increase in amine groups and a decrease in isocyanurate groups was also observed. The peaks in both the ~3C and ~SN CP/MAS NMR


527

spectra broadened with increasing temperature. This was attributed to the formation of free radicals and substantiated by ESR spectroscopy. The thermal stabilities of the relevant groups were in the order biuret, uretidione < urea < isocyanurate < urea', where urea' is a urea-type more stable than isocyanurate. 15.2.6

Synthetically crosslinked natural polymers

Natural polymers crosslinked by synthetic molecules represent many resins used for industrial applications. These are now being more closely examined by solid-state N M R spectroscopy to try to understand more fully what occurs in these systems and how it is possible to improve them. Of industrial importance are the polyphenolic tannin resins crosslinked by hexamethylenetetramine. These principally contain flavan-3-ols (Fig. 15.2.18) in the tannin [21] and have been examined by ~3C CP/MAS solid-state N M R spectroscopy. Hexamethylenetetramine was used in preference to formaldehyde as it has showed a much faster rate of reaction. The intermediates in this reaction are tribenzyl-, dibenzyl-4~, and monobenzylamines some of which rearrange to give the dihydroxydiphenylmethane crosslinking bridges in the resin. The exact nature of the crosslinking process, however, is still in debate and the study was undertaken to try and clarify the issue. To examine this process fully, a comparison was made between pine tannin (high in flavan-3-ol) (Fig. 15.2.19) pine tannin hardened with paraformaldehyde (Fig. 15.2.20) and pine tannin hardened with hexamethylenetetramine (Fig. 15.2.21). For the paraformaldehyde cured species three new peaks were observed that were representative of the C4 unsubstituted flavonoid site (38 ppm) and (OH) OH H

OH

H

(OH)

Fig. 15.2.18. A typical flavonoid structure. The parentheses indicate variations in flavonoid structure with some hydroxyl groups absent.

528


..[

......

, ....

~00

Fig. 15.2.19. 13C C P / M A S N M R

t. . . . . . . . . .

150

l . . . . . . . . .

I00

ppm

I

. . . . . . . . .

50

! . . . . . . . . . .

I

0

spectrum of pine tannin.

the formation of methylene bridges between two phenolic rings (36.8-37 and 33 ppm). Unsurprisingly methylene bridges were formed exclusively. For the hexamethylenetetramine cured species peaks assigned to the formation of methylene bridges (identical to the first reaction) together with peaks assigned to the formation of tribenzyl- (57.5 ppm), dibenzyl- (51.0 ppm) and monobenzylamines (45 ppm) linkages were observed. In the latter case it appeared that 40-50% of the crosslinks were the benzylamine type (with tri- and monobenzylamine predominating) the remaining being methylene bridges between phenolic type structures. Further evidence for this was shown from the peak at 98 and 105-110 ppm representative of the free and reacted C6/C8 sites respectively, the former decreasing and the latter increasing in both cases with reaction with the formaldehyde and the hexamethylenetetramine. Pecan nut tannin with another predominant flavonoid form of differing reactivity was also reacted with hexamethylenetetramine. In this case the dibenzylamine and tribenzylamine units were the dominate moieties present in the


.!

.........

200

f .........

150

! .........

I00

ppm

f .....

50

....

529

1 .........

f

0

Fig. 15.2.20. 13C CP/MAS NMR spectrum of pine tannin extract hardened with paraformal-

dehyde.

system, also the level of methylene bridging was much lower representing only 20% of the total crosslinks. In two closely related studies Wendler and Frazier examined, by ~SN NMR, the interaction between both model cellulose compounds [22] and wood with 15N enriched polymeric diphenylmethane diisocyanate (pMDI) [23]. The resin formed is used commercially as a wood adhesive. Previous work [17] had shown that this reaction is sensitive to moisture, the formation of different products depending upon the degree of moisture present (Fig. 15.2.22). Urea and biuret type linkages were all characterised. Biuret structures were predominant when the moisture content was low, gradually being replaced by urea linkages when there was higher moisture content, the formation of urethane and amine moieties also occurred at intermediate moisture contents. This is in contrast to the previous study [17] where only a small amount of biuret linkages were detected. This may be due to the presence

530


! .....

200

, ....

! ....

150

~ ....

1 .....

, ....

100

I .....

50 ppm

, ....

t ..........

1

0

Fig. 15.2.21. 13C CP/MAS NMR spectrum of pine nut tannin extract hardened with hexamethylenetetramine.

of the large amounts of hydroxyl groups available from the cellulose to react further with the urea linkages. 15.2.7

Polyacenicpolymers

The potential applications for conducting polymers are enormous and this has stimulated a large amount of research into this area. Not surprisingly, solid-state NMR spectroscopy has been applied to study these amorphous, insoluble and in many cases crosslinked materials [24]. Looking at the 13C CP/MAS spectra of a series conducing polyacenic polymers, some of which were doped with iodine, it was possible to see the effect of the halogen upon conductivity. These resins were prepared by a conventional procedure for the preparation a Novolak-type phenol-formaldehyde resin. After synthesis, the phenol-formaldehyde resin were dissolved and solutions were cast as a film and heat treated to between 590-670~ in a Ne atmosphere to form the polyacenic film. The electrical conductivity of the films was shown to increase

531


1

t

250

1

200

! ....

150

, ....

! . . . . . . . . .

100

;0 . . . . . . . . .

I

PPM

Fig. 15.2.22. ~SN CP/MAS spectrum of wood/15N-pMDI composite as a function of precure moisture content.

with higher temperature. The addition of iodine to one of the films gave rise to substantial increase in electrical conductivity. The postulated structures are shown in Fig. 15.2.23. Typical 13C CP/MAS spectra are shown in Fig. 15.2.24. By using the reference spectrum of a phenol-formaldehyde resin the peaks for the polyacenic films were assigned. The main peak at 127-130 ppm moves upfield and broadens with increasing temperature indicating an increased amount of polyacenic-type structures. The peak at 150 ppm assigned to quaternary aromatic carbon substituted by hydroxy groups and the peak at 40 ppm assigned to methylene carbons decrease in intensity with increasing temperature but are never completely removed. Using dipolar dephasing spectra the aromatic region also revealed further signals and it was possible

532


a

a

Fig. 15.2.23. Structures of polyacenic films. Initial phenol-formaldehyde film (top), partially cured (middle). fully cured (bottom).

CROSSLINKED

533

POLYMERS

2,3 5,6

i

~-

i

~)o

~

~

i

9

15o

, .

100

.

.

.

.

50

Fig. 15.2.24. 13C C P / M A S spectra of a polyacenic film.

to assign them using chemical shifts from model compounds. The peaks at 129.5 and 151ppm were due to protonated/nonprotonated and hydroxy substituted carbons respectively in the residual phenol-formaldehyde resin structure. The peaks at ca. 125 and 138 ppm are the methine and quaternary carbon in the polyacene. The ratios of two of the aromatic peaks, obtained by deconvolution, were related to the degree of electrical conductivity. For the iodine doped sample it was shown that the iodine interacts significantly with the polyacene part and not with the phenol formaldehyde part. This was shown by peak broadening of the polyacene type peaks. 15.2.8

Polyethers

A semi-crystalline poly(1,3-dioxolane) was examined by solid-state NMR spectroscopy looking at both linear and crosslinked polymers [25]. The

534


OCH2CH20

OCH20

(b)

,'l",~']"'"'i

le5

100

95

....

I ....

$0

! .... "T"'"I

85

80

....

75

I' '"" i '''~:i . . . .

7e

65

6a ~p~

i

Fig. 15.2.25. (a) 13C single pulse excitation and (b) 13CCP/MAS spectra of poiy(1,3-dioxolane).

crosslinks were formed by introduction and reaction of acrylate groups allowing the formation of a network and control of the molecular weight. The systems were examined by 13C MAS and typical spectra appear in Fig. 15.2.25 where two characteristics peaks at 67.5 ppm ( O C H 2 0 ) and 96.0 ppm ( O C H 2 0 ) are diagnostic. For the crosslinked polymer the CP spectrum, being more sensitive to static molecular motion, revealed a further peak at 93.4 ppm (the O C H 2 0 region) which was assigned to a less mobile phase. To clarify these results variable contact CP and cross-depolarisation experiments were carried out and by deconvolution of the peaks in the NMR spectrum three regions, a crystalline, an interfacial and elastomeric one, were indicated. Further study carried out using 1HTlp which is indicative of kHz motion in


535

the polymer, also suggested that there were three phases present in the polymers. 15.2.9

Epoxide based resins

In a high temperature study [26] of two epoxide resins, the samples were heated to above Tg whilst still carrying out magic-angle spinning to remove residual line broadening interactions. At these temperatures (ca. 260-290~ the molecular motion of the system had increased to such an extent that it was possible to use conventional one (~H, DEPT) and two dimensional (HECTOR) solution state NMR experiments on the samples. The networks looked at were the oligomer of diglycidyl ether of bisphenol A (DGEBA) cured with 100 and 66% of diaminodiphenyl sulphone (DDS) (Fig. 15.2.26). The spectra of the 100% cured resin at ambient and high temperatures are shown in Fig. 15.2.27. The principal reactions are between the epoxy and the primary and secondary amines, further reactions between hydroxyl groups are also possible. The principal reaction moiety for the 100% cured polymer is indicated in Fig. 15.2.28. For the 66% cured polymer the situation was more complex the spectrum is shown in Fig. 15.2.29 and the major structural moiety present is indicated in Fig. 15.2.30. The increase in resolution at higher temperature is clearly evident and the possibility of using conventional solution state editing techniques is advantageous as they greatly aid peak assignment. Unfortunately this technique may be applicable only where there is a substantial increase in motion above Tg. Many highly crosslinked polymers do not show a discrete Tg and therefore would not be expected to show any substantial decrease in line broadening if they were heated to high temperatures. There is also the question as to whether further post-curing reactions may occur when polymers are heated to such temperatures. Epoxide resins made from 2,2-[4-(2,3-epoxypropyl)phenyl]propane (DGEBA) polycondensed with 4,4'-sulphonyl-dianiline (DSS) produce a three-dimensional insoluble network which was examined by CP/MAS NMR spectroscopy [27]. In this study the chemical structures and the cure kinetics were determined. A cured epoxy synthesised from a mixture of the diglycidyl ether of bisphenol A (DGEBA) and 1,3-phenylenediamine was studied by ~H NMR spectroscopy including multiple pulse techniques and spin-lattice relaxation in the rotating frame, T~o. The study [28] focused on the water distribution based upon possible variation in the cross-link density measured by spin diffusion. From the analysis involving a combination of T~p and multiple

t~ ta~

/o~

c.,---c.c.,o

Y" /~

I

o.

~---~xk ..../f--oc~c.c.,o

]/----x

I /--x.

/o~

X~
, Z

DGEBA

h :z:

./

N

sch -

o u

It

N

Z

\ H

DDS Fig. 15.2.26. Structure of diglycidyl ether of bisphenol A and 4,4'-diaminodiphenylsulphone.

0 Z

II

537


_ ~L_L_=_

I

180

I

I

lz~O

i

I

100

~

i

I

60

I

I

20

~c/ppm

Fig. 15.2.27. 13C MAS spectra of DGEBA-DDS with 100% stiochiometry (a) 23~ with crosspolarisation and (b) 290~ without CP (c) expanded.

HO

0

\

I

'

I

0

\

/OH

I

o

Fig. 15.2.28. Major structure present in the 100% cured DGEBA-DDS resin.

pulse it was possible to postulate that the water was molecularly dispersed in the epoxy rather than aggregated in the voids. Also there seemed to be an absence of two distinct sites for water affinity. The presence of accelerators, either magnesium perchloroate or N,Ndimethylbenzylamine (DMBA) [29], on the curing of bisphenol A diglycidyl ether with butane-l,4-diol (BADGE-BD) were studied by CP/MAS (Fig. 15.2.31). Magnesium perchlorate was shown to induce the consumption of all the diol whereas the DMBA showed only approximately 50% consumption

538


q I

--

I

180

I

,

,

,

'I

1/.,0

,

I

L

.....

I

100

,

I

9

9

9

I

60

LJ ,

I

c/ppm

20

Fig. 15.2.29. 13C MAS spectra of DGEBA-DDS with 66% stoichiometry (a) 23~ with crosspolarisation; (b) 260~ without CP and (c) expanded.

as indicated by the residual primary alcohols from the butanediol (Fig. 15.2.32). 15.2.10

Methacrylate-based resins

Polymer composites are increasingly used for dental applications [30], the durability and aesthetic appeal has made them ideal substitutes for the more traditional amalgam fillings. The dental polymer composites are principally composed of an organic matrix and a powdered ceramic phase. The organic matrix is composed of an aromatic or urethane dimethacrylate such as 2,2bis[4-(2-hydroxy-3-methacryloyl propoxy) phenyl]propane (bis-GMA) with

539


O

o

Fig. 15.2.30. Major structure present in the 66% cured DGEBA-DDS resin.

I _

s

150

,,,

!

.....

!

110

I

,

r

70

1,

!

....

30 ppm

Fig. 15.2.31. 13C CP/MAS spectrum of BADGE-BD system with accelerator Mg(C104)2.

another monomer such as triethylene glycol dimethacrylate (TEGDMA) to alter viscosity. This system has the further advantage that it can be photopolymerised. ~3C solid-state NMR spectroscopy has been used to study the extent of the crosslinking reaction. A series of commercial and laboratory synthesised resins were examined by CP/MAS and SPE to determine more accurately the relative amounts of unreacted resin present (Fig. 15.2.33). In the synthesis of a methacrylate-based metal chelating resin, ~3C CP/MAS spectroscopy has been used to confirmed that the target resin had been made [31]. Here the imidazole ligand bis(imidazo-2-yl)methylaminomethane (bimam) was attached to a glycidyl methacrylate-co-trimethylolpropane trimethylacrylate (pGMT) resin. The peaks in the ~3C NMR spectrum were

540


_

t

150

!. . . . .

!

110

....

s,

t. . . .

70

! ....

t

30 ppm

Fig. 15.2.32. 13C MAS spectrum of BADGE-BD system with accelerator DMBA.

assigned as follows: 7.3 ppm, hindered methyl in trimethylolpropane residue; 24.3 ppm, backbone methyl; 41.4 ppm, ~ C H N in ligand; 46.0 and 56.1 ppm, polymer backbone; 67.5 ppm, ~ O C H 2 ~ in epoxy linkage; 127.5 and 145.6, imidazole carbons; and 176.3, carbonyl carbon. In an attempt to provide alternative supports to styrene-divinylbenzene resins [32] for use in reactive chemistry, poly(hydroxyethylmethylacrylate) (poly(HEMA)) has been employed. In this study poly(HEMA) was crosslinked with ethyleneglycol dimethacrylate and the CP/MAS recorded. The peaks were assigned as follows: 18.6 ppm is due to the methyl attached to the aliphatic backbone; 56.0 and 45.4 ppm are due to the methylene and quaternary of the backbone; the ~ O C H 2 ~ forming the crosslink are at 63.0ppm; the conjugated and unconjugated carbonyls are at 167.2 and 177.6 ppm and finally the methylene and quaternary carbons of the unreacted vinyl group are at 126.3 and 137.0 ppm. Another alternative is the use of crosslinked ethylene dimethacrylate [33]. Here, the unreacted double bonds in resins were used as a graft point for further reaction. The level of unreacted double bonds was determined by the relative areas of the carbonyl peaks at 176.3 and 166.3 ppm (Fig. 15.2.34) determined by CP/MAS before and after reaction with glycidyl methacrylate. These results were in good agreement with data from Raman spectroscopy. Spin diffusion is a valuable method by which it is possible to examine the heterogeneity of a polymer [34]. Spin-lattice relaxation times in the rotating

541


260

P-20

180

140

100

60

20

0

ppm

Fig. 15.2.33. 13C CP/MAS spectra of commericially available dental acrylate resins. (a) Tetric; (b) Zl00; (c) Duo Bond; (d) Coltene bonding agent; (e) TEGMA the labels r and u indicate carbonyl peaks from reacted and unreacted methacrylate groups.

flame have been used to determine the rate of spin diffusion. Tip data from three solid polyacrylate networks made by photopolymerisation of poly(ethylene glycol) diacrylate (PEGA), trimethylolpropane triacrylate (TMPTA), and dipentaerythritol pentaacrylate (DPHPA) have been used in this way. The photopolymerisation was carried out by a laser and this was related to the degree of crosslinking that occurred which was quantified in terms of the signals from the residual double bonds in the CP/MAS NMR spectra. The level of heterogeneity in these resins was measured by the 1HTlp and was related to the degree of crosslinking.

542


1

190

I

180

I

170

I

160

PPM

Fig. 15.2.34. Part of the 13C CP/MAS n.m.r, spectrum of poly(ethylene dimethacrylate) showing the peaks used for the determination of the double bond content.

In an attempt to obtain high surface area glycidyl methacrylate-co-trimethylolpropane trimethacrylate resins were synthesised [35] with a variety of porogens. The degree of unreacted double bonds was determined by CP/MAS NMR spectroscopy. Oligomers containing ether-ester groups were synthesised [36] in order to obtain a crosslinking agent that gavegood cure kinetics and was uniformly distributed in the network structure. The crosslinking agents were modified so that vinylidene groups were incorporated to enable them to be polymerised free radically with styrene or methyl acrylate. The oligomer was incorporated (5-50%) in the polymer to give clear hard resins and these were characterised by CP/MAS NMR (Fig. 15.2.35). The peak at 30 ppm is due to the tert-butyl group, the broad peaks for


543

/

V

200

160

ASO

t40

t20

100 ~X

60

40

2O

0

Fig. 15.2.35. Solid state 13C CP/MAS spectra of three samples of poly(methyl methacrylate)

crosslinked with 5 (lower), 20 (mid) and 50 (top) % of t-butyl acrylate end-capped oligomer containing ca. 2-3 HDDA repeat units.

the ethyl and ester carbons bonded to the oxygen are at 65-80 ppm, and the carbonyl carbon on the upfield side of the P M M A carbonyl at 178 ppm. Polyacrylamides can be synthesised by two methods, either polymerisation of an acrylamido monomer or chemical modification of another polymer e.g., poly(methylmethacrylate). In the latter case 13C CP/MAS N M R spectra show clearly the loss o f - - O C H 3 groups as these are replaced b y - - N H C H z - groups (Fig. 15.2.36). The latter approach has the advantage that polymethacrylates like P M M A can be obtained easily in a bead form of uniform size and are physically convenient for further exploitation. CP/MAS was used in a study [37] of the kinetics of reaction between P M M A and a series of amines. This was carried out by taking aliquots of the reaction mixture at certain times and recording the solid-state spectrum after the reaction had been quenched. The degree

544


_JA

~"-'~

720 rain.

360 min.

240 rain. 120 rain.

,

J 200.00

~

I 150.00

,

1 100.00

PpM

, 50.00

PMA

-0.00

Fig. 15.2.36. 13C CP/MAS spectra of PMMA reacted after various times with 1,6-diaminohexane.

of reaction was determined by 13C CP/MAS and further structural evidence was provided by 15N~ CP/MAS. 15.2.11

Styrene-basedpolymers

The degree of unsaturation in styrene cured polyesters was investigated [38] by using dipolar dephased 13C CP/MAS NMR data. The commercial resins contained fumarate, isophthalate and propylene glycol structural units and were cured with styrene. The use of dipolar dephasing (see Fig. 15.2.37) suppressed the strong phenyl peak at 129 ppm and therefore allowed the determination of the degree of unsaturation in the resin. The signal at 131 ppm was attributed to the isophthalate units, and the peak at 144 ppm to the quaternary substituted carbon from the styrene. This peak also showed

545


180

160

lt, O

Chemico[ shift

(ppm}

12O

Fig. 15.2.37. Part of a 13C CP/MAS spectrum of a solid polyester obtained (a) without dipolar dephasing and (b) with dipolar dephasing. Chemical shift are shown by the numbers.

partial resolution into two peaks at ---142 and --~146ppm which may have been due to styrene units in sequences of differing lengths. The two carbonyl peaks at 165 and 172 ppm were ascribed to unreacted fumarate/isophthalate and reacted fumarate carbonyls, respectively. The degree of residual unsaturation was calculated by determination of the relative ratios to these peaks. From this it was possible to determine the optimum level of styrene (47%) needed to give the lowest degree of unsaturation in the resin. Polyesters derived from maleic anhydride and 2,2-di(4-hydroxyphenyl)propane were copolymerised with styrene and then studied by CP/MAS NMR [39] spectroscopy. The three dimensional-crosslinked network formed by the polymerisation was examined using spin-lattice relaxation times in the rotating frame. A correlation between reaction conditions and the structure of the resulting material was found. The degree of residual unsaturation was determined by subtraction of two relaxation times from a linear additivity model used for crosslinked polymer systems. In two closely related papers [40, 41] CP/MAS was used to examine a series of styrene-divinylbenzene (St-DVB) and chloromethylated resins. In the first part of this study the authors were concerned with trying to determine the residual amount of unreacted vinyl groups present in St-DVB resins (see Section 3). In order to increase the sensitivity of the method the authors used 13C-labelled divinylbenzene (labelled in the methine position) and combined this with unlabelled styrene (1-20% by weight). The final resins were examined by CP/MAS and it was found that even for a very lightly crosslinked

546


species (1% DVB-St) at 70~ residual unreacted vinyl groups were present. Post-curing reactions, carried out by swelling the resin in a solvent and then heating the resin to 155~ in the presence of initiator, showed it was possible to remove the residual unreacted vinyl groups. For resins with higher amounts of DVB (10 and 20%) it was impossible to react all the residual vinyl groups by using this method. This showed that the unreacted vinyl groups are trapped in inaccessible locations within the polymer network. The second part of the study was concerned with studying resins in the unswollen glassy state and the solvent swollen state using CDC13. St-DVB and a chloromethylated resin were examined. For samples with low levels of crosslinking ( 200 ms. From the RAD at tm = 2 and 20 ms, the calculated correlation time of the two-site jump motion ~-~ was 20 +_ 5 ms at 97~ The timescale of these motions agreed well with the so-called a-relaxation detected by dielectric and mechanical spectroscopy. Comparision with X-ray data showed that the NMR results were consistent only with the conformation of Takahashi et al. [68]. They proposed the existence of disorder between four possible chain orientations in the unit cell. Transitions between these polarisation states require molecular reorientation.

682

R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN

Four different models of molecular motion were in agreement with the jump angle determined by NMR. However, of these possible motions only one was in agreement with the dielectric relaxation results of Miyamoto et al. [69]. This motion is defined by a dipole-moment transition and a conformational change (tg+tg-~--~ g-tg*t) yielding an effective dipole-moment reversal only along the chain axis and a reorientation angle of 113 ~ for the C ~ 2 H bond directions. As discussed in Section 6.6, proton high-power decoupling during 19F observation is not an easy technique to implement, because of the proximity of the relevant frequencies. An alternative to continuous wave 1H decoupling, involving proton 180 ~ pulses during windows of MREV8 19F pulse sequences, has been shown [70] to be effective for PVDF, giving an improved shielding powder-pattern bandshape. Until recently, surprisingly little attention has been paid to high-resolution (i.e., using MAS) 19F spectra of PVDF or other fluoropolymers which also contain protons. However, Harris and coworkers have now conducted a number of studies [71-75], utilising high-power proton decoupling. Without such decoupling, PVDF powder obtained from the melt shows, at ambient probe temperature, a major signal at ~ ~ -91 ppm, together with hints of shoulders (on both high and low frequency sides) and a weak doublet at lower frequency ( ~ - -116 and -120 ppm). Proton decoupling has a significant, but not dramatic, effect by improving the resolution of the shoulders (see Section 6.6, Fig. 6.6.2). Spectral quality also relies on MAS rates being high (> 10 kHz). Solution-state 19F-{1H} NMR [76-78] indicates that the main peak arises from the regular alternating CF2CH2 structure expected for the polymer, whereas the weak doublet can be assigned to chain imperfections of the type H T H T T H H T (where H and T refer to "head" CH2 and "tail" CF2 groups, respectively). It is noteworthy that a TT link is followed by a HH link to resume the original monomer sequence (thus giving t w o 19F signals for the imperfections). The solid-state shifts for the most intense signal and for the tail-to-tail doublet in the solid are very similar to those for solutions (6F = --91.6, --113.6 and -116.0 ppm for N,N-dimethylacetamide as the solvent [76]), indicating that chain conformations are similar in the two phases. The shoulders in the solid-state spectra can be shown to arise from crystalline domains in the sample. For material produced from the melt, these domains are of the a form, and NMR techniques of discrimination proved to be very effective [73,74]. Cross polarisation is more efficient for crystalline regions than for amorphous parts, since the chains in the latter are more mobile. Discrimination in favour of the crystalline domains can be enhanced

FLUOROPOLYMERS

683

a.

'

I

-60

~

I

-80

J

I

-100

~

I

-120

Fig. 18.9. Fluorine-19 CPMAS spectra of PVDF (biaxially stretched film), showing the discriminatory effect of a precontact delay (with 1H spin-locking) following a 90 ~ proton pulse, combined with the use of a short (100/xs) contact time. (a) Precontact delay zero. (b) Precontact delay 40 ms. The signal for amorphous domains does not appear in (b). The sample contains both a and/3 crystallites. The signals from head-to-head units are also lost in (b) and, therefore, reside principally in amorphous regions. [Figure reproduced with permission from Ref. 72.]

by spin-locking the protons prior to CP contact (since T1H is substantially shorter for amorphous chains) (see Fig. 18.9). Such an experiment (known as a proton Tip filter) allows the investigation of the effect of processing on the polymeric material. Drawing in one or two dimensions converts a crystallites into the /3 form. Figure 18.10 shows the spectra of a and/3 forms (the sample of the latter contains ca. 20% of the former). It is clear that the /3 form gives a single signal at 6F = - - 9 8 ppm, whereas, the a crystallites give two signals at 6F = --82 and - 9 8 ppm. These results can be understood in terms of the known conformations. The/3 form has an all-trans conformation, whereas the a polymorph is tg§ Figure 18.7 shows clearly that the two CF2 fluorines are equivalent for the former, but nonequivalent for the latter. Moreover, the/3-fluorines have two 7-gauche interactions with carbon, as has one of the a-fluorines, thus explaining the near coincidence of the signals (see Fig. 18.11). The second a-fluorine has one 7-gauche and one 7-trans carbon, and its chemical shift reflects this fact. Thus, the difference in shifts of the a fluorines represents one 7-gauche (C,F) effect, which is therefore ca. - 1 6 ppm, in good agreement with the value estimated [77] from solution-state NMR. Discrimination in favour of the amorphous domains may be achieved by

684

R . K . HARRIS, G. A. MONTI AND P. HOLSTEIN

I'"'I""I""I

-60

-80

.... I'"'I""I'"'I

-100

%/pp m

-120

Fig. 18.10. The effect of crystallite phase on PVDF 19F CPMAS spectra. Top. sample crystallised from the melt (a form). Bottom: 9-1xm thick biaxially drawn film (/3 form with a little a). Spectra were obtained using the T~o filter (precontact ~H spin lock 40 ms, contact time 50 Ixs).

,,.C

A F"

] C

tg+tg -

C all-trans

- FORM

/~ - FORM

Fig. 18.11. Chain conformations for PVDF to show T-gauche interactions of carbon to fluorine. Left: a-form, where FA has two T-gauche interactions with carbon, but FB has one T-gauche and one T-trans. Right: /3 form, with two y-gauche interactions for each fluorine.

the dipolar-dephasing pulse sequence [73,74], since static spectra are relatively narrow because of the extensive chain motion. This reveals that the tail-to-tail imperfections are largely in the amorphous domains and spinning sidebands are not as prominent for amorphous signals as for those from the crystalline regions. Both effects are as expected. Proton and fluorine relax-

685

FLUOROPOLYMERS

'

9

I

,'

.

.

.

. 1 .

.

.

40

'

'

I

. . . .

40

.

.

.;

.

.

.

.

.

.i

i

0

i

. . . .

i

0

..v..

40

-40

. . . .

i

. . . .

i

,

-40

,

'

'

!

0

. . . .

40

I

-40

'

. . . .

0

" +

'

,

. . . .

i

'

'

-40

Fig. 18.12. Slices in the 1H dimension for a 19F/1H WISE spectrum of PVDF. The two lefthand spectra are for crystalline domains. The right-hand spectra are for the amorphous region (upper right) and the head-to-head defects (bottom right). The fluorine nuclei were decoupled during the time tl. The frequency scale in kHz. [Figure reproduced with permission from Ref. 75.]

ation times (T~ and Tao) have been measured by several different methods [72,73] (see Section 6.6). The influences of temperature and poling on the spectra have been studied [74]. Cross polarisation from 19F to 1H has also been carried out [71] for solid PVDF, though the potential of this experiment is probably not great. The two-dimensional (1H,19F) wideline separation (WISE) sequence has been applied [75] to PVDF, both with and without 19F decoupling during the 1H evolution time tl. The resulting spectra yield values of proton second moments, discriminated in terms of the 19F chemical shifts by the second dimension, thus quantifying the mobility differences between the amorphous and a-crystalline chains (Fig. 18.12). Moreover, a suitable preparation period in the pulse sequence, namely application of a 1H Tlo filter, allows signals of the crystalline region to be obtained selectively in the two-dimensional plot [75]. A period for spin diffusion facilitates the study of phase separation. Full restoration of amorphous magnetisation occurs in 16 ms. A 13C spectrum of solid PVDF, obtained with cross polarisation from 1H plus simultaneous 1H and 19F decoupling appears to have been first reported by Fleming et al. [33], on a static sample. "Reasonably well-defined" powder patterns for separate CF2 and C H 2 signals were obtained and shielding tensor principal components reported. On the other hand, T6k61y et al. [79] showed

686


j A

I

200 $C

I

I

I"

100 ppm 0 200 $C

I

100 ppm

I

0

Fig. 18.13. Carbon-13 CPMAS spectra of PVDF. Left-hand: 19F'-~13C CP. Right-hand"

1H----~13C CP. Top: 1H-decoupled. Middle: 19F decoupled. Bottom: Double-decoupled, {19F,1H}, triple-resonance spectra. [Figure reproduced from Ref. 83.]

the rather poor-quality CPMAS spectra that are obtained with only proton decoupling. Veeman and coworkers [2,80,81] have combined the triple-channel 13C-{1H,19F} and MAS experiments, using them as the basis for studying blends containing PVDF. Peaks at 6 c - - 4 3 ppm (CH2) and 120 ppm (CF2) were seen for PVDF itself (see Fig.18.13), but no additional signals such as have been observed [78,82] for triple-channel 13C-{1H,19F} experiments on PVDF solutions. However, Holstein and Harris [83], using 13C-{1H,19F} triple-channel experiments, have shown that the c~ and /3 polymorphs give rise to slightly different spectra (peak separations ca. 4 ppm), but such spectra are inferior to those of 19F for distinguishing domains. The CP dynamics have been explored [83]. Relatively little solid-state NMR work appears to have been carried out on homopolymers in this category other than PVDF. However, some work has been reported on poly(trifluoroethylene) (PTrFE). McBrierty et al. [84] measured proton T1, Tlo and T2 as a function of temperature over the range -1 - 1 0 0 to + 140~ Above ca. +60 ~ two-component T2 (measured as the e point of the FID) was observed as motion in the amorphous region became significantly faster than in the crystalline region. A crystallinity of ca. 55% was indicated. The authors suggested that there are three minima in Tip, which were assigned to c~,/3 and 3' relaxation processes. The c~and y processes were attributed to the amorphous region, whereas the /3 process was tent-

FLUOROPOLYMERS

687

atively associated with the crystalline region. Measurements of T1 provided corroboration for these conclusions. Katoh and Ando [85] have reported I3C MAS and pulse-saturation transfer experiments (the latter giving enhanced signals by the NOE) for two poly(Lglutamate) samples with n-fluoroalkyl side chains over a temperature range from 30 to 100~ It was deduced from the measured chemical shifts that the sample with short (~,-trifluoroethyl) side chains takes the helix form whereas the one with longer (~/-n-2-perfluorodecylethyl) side chains is in the/J-sheet form. Zumbulyadis et al. [86] selected a fluorinated polyphosphazene, containing CF3CF2CF2CF2CH20 side chains (attached to phosphorus atoms in the polymer backbone) to explore the potential of 19F MAS/NOESY experiments. MAS at 3.82 kHz sufficed to resolve all four 19F sites, though effects of isotropic indirect coupling were obscured by the linewidths. Cross peaks are seen in the two-dimensional NOESY spectrum. The authors argue that these arise from NOE processes and not from spin diffusion. Wooley et al. [3] have used 13C, 19F R E D O R experiments to study the solid-state shape, size and intermolecular packing of a series of benzyl ether dendrimers (generations 1-5) based on 3,5-dihydroxybenzyl alcohol. A fluorine label was placed at the core, and samples contained site-specific 13C isotopic enrichment near the chain ends. Dipolar coupling constants were measured, giving average intramolecular (C,F) distances of ca. 12 A for generations 3-5, indicating inward folding of chain ends. Intermolecular measurements were consistent with decreased interpenetration for larger dendrimers. Some polystyrene-diluted materials were also examined.

18.5

Copolymers and blends involving only one fluorinated component

In this section, copolymers of the above type are dealt with first because, in principle, discriminating experiments originating from the 19F magnetisation in one component are possible, giving a more clear-cut situation than when both components contain fluorine. Within this category, even further simplifications become possible if the fluorinated component does not contain hydrogen. This is the situation for PE/PTFE blends, which have been examined by at least three groups of authors. Nagumanova et al. [87] studied low-density PE/PTFE mixtures with a range of compositions. Proton second moments were used to obtain mobility information and it was concluded that there were conformational restrictions at the LDPE/PTFE interface. Nagarajan and Stachurski [88] expanded on the scope of this work, using broadline 1H and 19F resonances to determine second moments as a function

688


of composition and of sintering times in the preparation. They proposed a model for the interaction of PE and PTFE units involving interdiffusion at the interface. Sugiura et al. [89], on the other hand, studied blend films of ultra-high molecular weight polyethylene with PTFE using 13C MAS NMR. Carbon-13 relaxation times T1 and T2 were measured by the Torchia method and by direct polarisation saturation recovery for the former, and direct polarisation spin-echoes for the latter. The signals from the Torchia method for the orthorhombic crystal form of PE were monitored in this process. The T1 decay curves were decomposed into three components. The direct polarisation results were assigned to amorphous PE units. Whereas T1 values were almost independent of composition, T2 (for the amorphous region) increased with PTFE content. It would appear that no direct measurements relating to the PTFE were made in this study. None of the above three investigations have capitalised, therefore, on the presence of fluorine in the systems examined. This is also the case for a 15N NMR investigation [90] of curing, post-curing and hydrolytic degradation of a polyimide resin produced with hexafluoroisopropylidene bis-phthalic hemi-methylester as one component. Douglass and McBrierty [91] reported a detailed study of 19F relaxation in PVDF/PMMA and PVDF/PEMA blends. In addition to measuring T1F, T~ and Tl~p, they determined the parameters affecting cross relaxation between the 1H and 19F spin systems using the transient Overhauser experiment. Thus they monitored the 19F magnetisation at a variable time after a 180~ proton pulse. All measurements were carried out over the temperature range -200 to +160~ and were compared to the analogous data for the relevant homopolymers. The authors concluded that a substantial fraction of the PVDF units are at nearest-neighbour distance from PMMA or PEMA units and that there is evidence of extensive premelting of PVDF crystallites in the blends. Blends of PVDF and PMMA were studied by T6k61y et al. [79] using 13C MAS NMR, with cross polarisation from protons. They obtained data on PMMA 13C magnetisation as a function of 1 H ~ 13C contact time for the homopolymer and a series of blends (both quenched and annealed). For high contents of PVDF, the plots showed a double-stage character, which the authors interpreted in terms of partial crystallisation of PVDF. The conclusions were reinforced by the experiments on annealed samples. However, the lack of fluorine decoupling meant that no direct study of PVDF signals was feasible. A major series of articles by Veeman and coworkers [2,80,81,92,93] has remedied that situation. Using a commercial probe modified in-house, they obtained [2] 13C spectra of blends of PVDF with PMMA using {1H,19F} double decoupling and 19F ~ 1H ~ 13C double cross

FLUOROPOLYMERS

t9 F

I,

H

689

1

I

'a C Fig. 18.14. Pulse sequence used by Veeman and coworkers [80,81,92] to determine the intimacy of mixing of PVDF and PMMA (see text).

polarisation. The PMMA carbonyl and methoxy resonances, being well removed from PVDF signals, were monitored as a function of contact time in 19F----~13C{1H,19F} experiments. Fluorine and carbon Tlo values were determined separately. Note that a "physical" mixture of PMMA and PVDF yielded no PMMA signals under the procedure used, whereas the data on the blends allowed an average distance between a fluorine atom and carbons on PMMA to be calculated, the result ranging from 2.6 to 3.1 ~ . Further work [80] used, (i) 19F----~IH----~13C double cross polarisation, and (ii) 1H ~ 19F cross depolarisation followed by 1H ~ 13C cross polarisation (see Fig. 18.14). Both techniques are affected by proton spin diffusion. This renders technique (i) only valuable in a qualitative sense, showing intimate mixing (Fig. 18.15), but the ability of proton spin diffusion to transfer magnetisation from PMMA protons to PVDF protons under experiment (ii) was successfully used to quantitatively distinguish various phases in the PVDF/PMMA blends. The authors used a model with four phases: "isolated" PVDF, "isolated" PMMA, "intimately mixed" PMMA/PVDF (giving 1H ~ 19F depolarisation); and PMMA sufficiently close to the mixed phase that spin diffusion from it is important. Radial spin diffusion was assumed, with a sphere of intimately mixed phase of radius A and a shell of "close" PMMA of thickness L - A (up to a point of abutment with a neighbouring sphere). Figure 18.16 shows experimental data for the 60:40 PMMA/PVDF blend, which is fitted to A = 6 A, L = 12 ~ and 30% "isolated" PMMA. The same experiment was then employed [81], together with 19F ~ 13C{1H,X9F} CP, to study the influence of PMMA tacticity on PMMA/PVDF miscibility, yielding evidence for a specific interaction between segments of the two polymers. The mixed PMMA/PVDF phase was determined to have a mean radius of 6-8 A, with some dependence on PMMA tacticity. Large differences were found in the

690

R. K. HARRIS, G. A. MONTI AND P. HOLSTEIN

(bl

200 ........

"'

1 0 0

....

0 ....

*c/ppm

Fig. 18.15. CPMAS spectra of a P M M A / P V D F 60:40 blend: (a) 19F---->13C direct CP (contact time 2 ms); (b) 19F ----->1H ----->13C double CP (19F ----->1H contact time 0.4 ms, 1H ~ 13C contact time 2 ms). Experiment (b) clearly shows the intimate contact between PMMA and PVDF chains. [Figure reproduced with permission from Ref. 80.]

amounts of isolated PMMA between melt-mixed and coprecipitated blends. This theme was continued by a study [92] of the crystallisation behaviour of PMMA/PVDF 60:40 blend, and an amorphous PVDF interphase was found to be present. The fraction of isolated PVDF increased rapidly as a function of annealing time (see Fig. 18.17). Polymer interdiffusion above Tg was then monitored [93] by using technique (ii) at ambient probe temperature following various times during which the polymer system was held at 190~ Initially, the sample consisted of stacks of discs cut from PMMA and PVDF sheet (average thicknesses 92.5 and 88.2 txm, respectively) alternately. Evaluation of the diffusion equation gave intrinsic diffusion coefficients (7 _ 5) • 10 -11 and (15 ___5) • 10 -11 c m 2 s -1 (at 190~ for PMMA and PVDF, respectively. Amorphous polymers or regions of polymers can be regarded as microporous materials, and can be studied, therefore, by monitoring the 129Xe. chemical shifts of adsorbed xenon gas. Mansfeld et al. [94] used this method to distinguish between incompatible blends (of polypropylene with a polypropylene/polyethylene copolymer) and compatible blends (PMMA and PVDF). In the former case, two 129Xe signals were observed, whereas only

FLUOROPOLYMERS

691

1.0 -I

_

:~o.5

0.0

-

0.0

I

0.5

cross

I

1.0

I

1.5

I

2.0

depolarization

i

2.5

time

I

3.0

I

3.5

(ms)

Fig,. 18.16. Experimental data for a PMMA/PVDF 60" 40 blend, obtained for the 13C carboxyl

resonance of PMMA. The solid line is a calculated depolarisation curve. The dashed line represents the offset from 30% of "isolated" PMMA. [Figure reproduced with permission from Ref. 80.] one was seen in the latter. However, for P M M A / P V D F blends, the 129Xe linewidth shows variations with composition that are not yet explained.

18.6

Other copolymers and blends containing fluorine

A number of synthetic, noncrystalline fluorocarbon copolymers exhibit elastomeric properties when vulcanised. Such elastomers are of commercial interest because they have unusual combinations of properties; e.g., high melting point, high thermal stability, insolubility, low coefficient of friction and flexibility at low temperatures. They are designed for demanding service applications in hostile environments characterised by broad temperature ranges and contact with chemicals, oils or fuels. Copolymers of vinylidene fluoride (VDF) (CF2=CH2) and hexafluoropropylene (HFP) (CF2=CFCF3) were reported 40 years ago by Dixon, et al. [95]. Ferguson [96] carried out a 19F NMR solution-state analysis of the samples. The gross structure was deduced from the spectral assignments on the assumption that it was a random linear copolymer, except that no adjacent

692

R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN 0

.0

,

,

,.

o

I

I

I

I

I

'l

I

O 30-

0

0

O [30 O

I

V

I

==

"0 -~,,W

I

20-

EX)O 10-o ID

0-

0

I

I

200

'

O 120

~

o

140

"C

I

'

4o0

I

600

'

I

8o0

'

I

lOOO

'

I

2oo

(rain.) Fig. 18.17. Fraction of isolated PVDF in a 60"40 PMMA/PVDF blend as a function of

annealing time at 120 and 140~ [Figure reproduced with permission from Ref. 92.] hexafluoropropylene-hexafluoropropylene units were present. The assumption was justified by the lack of spectral evidence for branched and/or unsaturated structures and on chemical grounds. Randomness in the incorporation of the two monomers was expected from the method of polymerisation. Significant homopolymerisation of hexafluoropropylene under similar conditions has not been reported, and it appeared to be impossible to incorporate more than 50 mol% hexafluoropropylene into the copolymer. In the late 1950s, such copolymers were developed on a commercial scale by 3M (Fluorel) and by DuPont (Viton). Also, copolymers of vinylidene fluoride and chlorotrifluoroethylene (CTFE), CF2zCFC1, became available from Kellog in 1955 under the trademark Kel-F. In the 1960s, terpolymers of vinylidene fluoride, hexafluoropropylene and tetrafluoroethylene (TFE) (CF2~CF2), were developed and commercialised by DuPont as Viton B. These kinds of elastomers from various companies typically contain a range of impurities or additives such as from initiators (organic or inorganic peroxy compounds, e.g., ammonium persulfate), emulsifying agents (usually fluorinated acid soaps) and chain-transfer reagents such as carbon tetrachloride,

FLUOROPOLYMERS

693

Table 18.1. Typical fluorocarbon elastomer polymerisation recipe

Component

Grams

Vinylidene fluoride Hexafluoropropylene Carbon tetrachloride Potassium persulfate Perfluoro-octanoic acid Potassium phosphate, dibasic Water

61 39 0.12 1.2 0.90 3.6 340

methanol, acetone, diethyl malonate and dodecylmercaptans [97]. Most fluorocarbon elastomer gums also contain a cure system, consisting of an organic onium cure accelerator, such as triphenylbenzylphosphonium chloride, and a bisphenol cross-linking agent, e.g., hexafluoroisopropylidenediphenol. For complete formulation, reinforcing fillers and metallic oxides are added, the latter as acid acceptors. Raw Viton gums contain no curatives. A typical polymerisation recipe for a vinylidene fluoride and hexafluoropropylene copolymer is shown in Table 18.1. The relationship between the molecular motion and properties of copolymers of VDF and CTFE was investigated by Katoh et al. [98] by means of wideline ~H pulse NMR over a wide range of temperatures (20-120~ For the proton spin-spin relaxation time (T2) measurements, the solid-echo pulse sequence was used. Copolymers of VDF, CTFE and unsaturated peroxide (CHzzCH--CHzmO--CO--OmOmtmBu), referred to as "base polymers", were prepared at low temperature with different number-average molecular weights: 5.6 x 10 4 and 2.6 x 10 4, designated H and L, respectively. Following side-chain cleavage at elevated temperature, further polymerisation with VDF occurred, producing "graft polymers" H and L, respectively. Two blend copolymers were obtained by mixing base polymer H with PVDF and base polymer L with PVDF at 180~ The Y2 curves of the base copolymers consist of two decays. For the H base copolymer the T2 constants are 65 and 29 ms, assigned to amorphous and "interfacial" phases, respectively. The T2 curves for the graft copolymers consist of three decays, with decayconstants of 108, 33 and 15 ms. By comparison with neat PVDF (Y2 = 16 ms) the shortest Y2 is assigned to the crystalline component of VDF units. The remaining two components correspond to those observed for the base copolymer. The nature of the "interfacial" phase in the base polymer is unclear, but its value of Y2 is close to the intermediate value in the graft system. It was shown that the molecular motion of the base copolymer is restrained with increase of molecular weight, but that the molecular motion of the graft

694


polymer is independent of the molecular weight of its base polymer. The base polymers showed different mechanical properties and different fractions of the immobile component. However, graft polymers H and L have the same mechanical properties and the same 1H T2 behaviour. The authors concluded that the molecular motions are clearly associated with mechanical properties. The temperature-dependence of T2 for the blends lies between those for the base and graft systems. Several wideline NMR experiments were carried out on copolymers of vinylidene fluoride and trifluoroethylene (TrFE) [84,99-112]. Such copolymers are of special interest since TrFE in proportions greater than 10% induces the VDF component to preferentially crystallise in the /3 form. In contrast with neat PVDF, the copolymers exhibit a "ferroelectric" phase transition at a temperature well below melting. McBrierty et al. [84,99] studied proton relaxation behaviour of unpoled [84] and poled [99] 52/48 mol% vinylidene fluoride and trifluoroethylene copolymer. They reported TI, Tip and T2 data as a function of temperature in the range -120 to 120~ (Fig.18.18). The unpoled sample showed a discontinuity in the relaxation parameters at 70~ more pronounced in the T2 data, which, below 70~ were represented by two components. The discontinuity in T2 occurs in both long and short components. The magnitude of the short T2 above the transition is comparable to that for pure crystalline TrFE. The absence of a T2 characteristic of the crystalline phase of neat PVDF was also noted. The behaviour of T2 around the transition temperature indicates that both amorphous and crystalline regions are affected at the 70~ transition. The transition may be interpreted in terms of either motional or order-disorder fluctuations such as the proposed ferroelectric transition. In the poled sample (Fig. 18.3), the "ferroelectric" transition is shifted to higher temperatures by 10 to 15 K. Additionally, the transition becomes sharper. The poled material has improved crystallographic packing in the ordered regions. The activation of chain rotation in the ordered region as the temperature is increased is associated with the onset of the "ferroelectric" transition. Legrand et al. [100] reported 19F wideline NMR measurements on a 70/30 mol% VDF-TrFE random copolymer, made in order to study molecular motion both below and above the ferroelectric transition temperature, Tc. The samples consisted of semicrystalline copolymer films of 0.51 mm thickness, with biaxial orientation of the crystalline axis. The samples were rolled (without poling) at 70~ with a draw ratio of 300%. The 19F resonance was chosen, rather than the proton resonance, because the abundance ratio of 19F to 1H nuclei is 1.4. In addition, the 19F free-induction decay (FID) lasts longer than that of the proton, which decreases the influence of spectrometer deadtime. FID analyses were made assuming a simple superposition of two

695

FLUOROPOLYMERS t

I

~ I-

i

I

'

I

'

I

'

I

'

I

'

TI

1o0

1p (n

uJ

-2

9

10

9

FZ o_ F 80~ but not at lower temperatures, and especially around room temperature as the glass transition temperature is approached (Tg is about 0~ The FID analyses showed that the relative intensity of the crystalline signal starts to increase at about the temperature of the rolling process (70~ reaching 65% just before the transition occurs. Another increase of the crystallinity up to 85% occurred on cooling throughout the transition temperature Tc. The transverse relaxation rate of the amorphous phase exhibits the same behaviour upon heating and cooling, while the linewidth for the crystalline phase narrows by a factor of two around 100~ upon heating and returns to its initial value, with a hysteresis of 40~

696


upon cooling. These changes are associated with the anomalies of the specific heat at the ferroelectric transition. The results show that the disorder of the high-temperature paraelectric phase (T < Tc) is of dynamical origin. Fluorine19 spin-lattice relaxation was also investigated. For measurements at 9.14 MHz the observed T1 appears to be dominated by the dynamics of the amorphous phase and exhibits no anomaly through the phase transition. However, from measurements at 20 MHz, well-defined minima in T1 were observed, and associated with the ferroelectric transition. Hirschinger et al. [101] have studied the structure and morphology of VDF-TrFE copolymers containing 30 to 100 mol% VDF. They used ~H and 19F wideline NMR to obtain information on the crystalline fraction and crystalline morphology of the copolymers, and on the relative fraction of VDF and TrFE units in the rigid and mobile phases. Films of copolymers have been cast at 180~ quenched at room temperature and annealed at 120~ A variable-temperature study of the 70/30 copolymer reveals that amorphous and two crystalline phases must be considered to describe the ferroelectric transition. Proton FID analysis of the 70/30 P(VDF-TrFE) copolymer reveals that the total second moment undergoes a substantial change above 60~ with a large hysteresis loop reflecting the first-order crystalline transition, in agreement with the 19F r e s o n a n c e results of Legrand et al. [100]. At low temperature, the structural change between the crystalline forms with increasing TrFE content is detected clearly. At room temperature, the morphology of VDF-rich copolymers is readily analysed, with two components having the same 1H to 19F ratios. On the other hand, below 70 mol% VDF, the two components have different 1H to 19F ratios, which implies segregation between TrFE-rich and VDF-rich sequences. In other articles [102,103] the authors reported studies of the 70/30 P(VDF-TrFE) copolymer. They examined the dynamics and morphology of the copolymer in the ferroelectric [102] and in the paraelectric [103] phase. The spin dynamics of protons and fluorines were investigated by measurements of T1 and Tip (the spin-lattice relaxation time in the rotating frame as a function of the spinlock angle 0), the transient Overhauser effect (TOE) and the FID. The 0 results obtained from Tip in the ferroelectric phase demonstrated the presence of local motions within the crystallites, implying the existence of crystalline "defects". These defects act as sinks for relaxation via spin diffusion and probably have an important role in the initiation of the Curie transition. The authors found that a simple model treating cross relaxation and spin diffusion was appropriate to describe all the relaxation data in the laboratory frame. The model uses the Hunt and Powles [113] correlation function associated with a one-dimensional diffusion process. In particular, when compared with the results in the paraelectric phase [103], the shape of the correlation

FLUOROPOLYMERS

697

function in the amorphous phase confirms the existence of a similar highfrequency mode both below and above Tc. For the paraelectric phase, proton and fluorine relaxation times T1, from 6 to 300 MHz, and Tip from 3 to 100 kHz, were measured at different temperatures, and analyses of the FID signals on both oriented and nonoriented samples were carried out. No transient Overhauser effect was observed over the whole temperature range. Two relaxation modes were found: (i) a fast anisotropic motion showing the characteristic "one-dimensional" 0) - 1 / 2 dispersion of l/T1 and 1/Tip, and (ii) a slow motion considered to be isotropic. The two modes are present in the amorphous phase, while the fast motion alone is involved in the dynamics of the crystalline phase. The fast process was described by means of three-bond motions. Their important effect is to cause diffusion of the existing C ~ C orientations along the chain, giving 1D fluctuations. In a series of articles, Ishii et al. [104-108] reported proton spin-lattice relaxation time and linewidth measurements (obtained by field-sweep methods and reported in field units, AH), over a wide range of temperatures, in drawn film samples of copolymers of VDF and TrFE (with VDF contents of 72, 65 and 52 mol%). Three kinds of T1 relaxation processes, referred to a s j~, of t and a~, were observed, respectively, below the Curie Temperature (Tc), around Tc, and in a certain temperature region above Tc. The thermal hysteresis of T~ and AH vs. temperature for each relaxation process became smaller as the VDF content of the copolymer decreased. The motional processes in the /3 region are associated with a flip-flop of TRFE-rich groups correlated to "free" rotation of the VDF segments. In the O~t and a~ regions, the motional modes are associated with the conformational change from trans to gauche bonds. The minimum value of T1, and the discontinuous change of AH in the O~t region observed for each copolymer, are attributed to a ferroelectric phase transition and their thermal hystereses are related to their first-order nature. The behaviours of both Wl and AH in the paraelectric phase for each polymer are described in terms of a one-dimensional diffusional motion of conformational defects along the chain. The activation energy of the motion, obtained from the dependence of Tx on temperature, is about 36 kJ/mol and does not depend on the PVDF content. The anomalous behaviour of T1 and AH, observed around Tc, was investigated [107-109] by applying an effective Ising spin model to the configurational fluctuation of the trans segment. Tanaka et al. [110] studied the ferroelectric phase transition of a 52/48 random copolymer of PVDF and TrFE by pulsed proton NMR. Attention was focused on the dynamic properties of the phase transition near the Curie point. Two samples were studied; one was isothermally crystallised at 125~ and the other was rapidly quenched in liquid nitrogen from the melt. The

698


FID curves were well fitted by a single Weibull function for each sample. The authors analysed the FID, fitting the Kubo-Tomita [114] relation to the transverse magnetisation and, hence, obtained the correlation time (~c) of the local field and the second moment of the rigid lattice separately. The correlation time showed a cusp at the Curie point, which reflect the critical slowing down of the order parameter fluctuation in that region. The second moment decreases rapidly up to Tc and levels off at Tc. There is almost no change above Tc. The temperature behaviour of the second moment was explained by a structural change (accompanied by an intramolecular conformational, trans to gauche, process) and the abrupt lattice expansion around the Curie point. The relaxation rate, l/T1, showed a logarithmic divergence at the Curie point, explained by long-range dipolar interactions or co-operative conformational changes. The authors also concluded that the amorphous phase is not mobile enough to cause the motional narrowing, and that there is no significant difference in motion between crystalline and amorphous phases. Clements et al. [111] carried out proton broadline NMR measurements on an oriented 70:30 copolymer of VDF and TrFE. The spectra show two components, one broad and one narrow, identified with the crystalline and amorphous regions, respectively, as discussed above. A procedure was devised for modeling the rigid-lattice NMR lineshape of the copolymer and used to decompose the signal into the two components. The rigid mass fraction was determined by calculating the ratio of its integrated signal intensity to the total integrated intensity. By following the evolution of the rigid mass fraction with the temperature, the authors found an appreciable reversible change in crystallinity with temperature. Calculations showed that this change could be a significant contribution to the pyroelectricity response of the material. In addition, they observed the increasing libration of the chains prior to the Curie transition, which could also contribute to the pyroelectric response. Stock-Schweyer et al. [112] reported a high-pressure effect on molecular motions in the paraelectric phase of a (70/30) VDF and TrFE copolymer. Fluorine-19 NMR relaxation times (T1 and Tip) w e r e studied over a range of pressures from 0.1 to 200 MPa. Correlation times of the molecular motions, as functions of pressure and temperature, were obtained and the activation parameters determined. The experimental data confirmed the presence of a slow motion in the amorphous phase in addition to the fast anisotropic motion. The results indicated that the relaxation times of the copolymer are controlled by the effects of both temperature and volume. The authors concluded that ---40-50% of the mobility increase of segments with increasing temperature under constant pressure results from volume expansion.

FLUOROPOLYMERS

699

Kochervinskii and Murasheva [115] studied the microstructure of copolymers of vinylidene fluoride and tetrafluoroethylene of 71:29 composition using 19F NMR. They showed that there is 5 mol% of diads in the tetrafluoroethylene blocks and 2.5 mol% of head-to-head defects in the VDF blocks. A pioneering work on the WAHUHA multiple-pulse sequence applied to 19F NMR of a fluorinated polymer was reported by Ellett et al. [116]. They obtained resolved chemical shifts for the OCF3 and CF2 peaks (separation 73 ppm) of a copolymer of 60/40 TFE and perfluoromethylvinyl ether. Good agreement between the area of the peaks and the known composition of the copolymer was obtained. The anisotropy of the chemical shift of the CF2 groups was approximately determined. Vega and English [13] obtained 19F spectra and relaxation times for static samples of a copolymer of tetrafluoroethylene and hexafluoropropylene (TFE-co-HFP; 85 mol% HFP) by the multiple-pulse technique (MREV8) at various temperatures (see Fig. 18.19). They observed CF2 group lineshapes in crystalline and amorphous regions, and also CF3 and CF lineshapes. After subtracting the crystalline contribution to obtain the amorphous lineshape, the latter contribution was analysed as a function of temperature to obtain information about the type of molecular motion present. The/3 and 3i relaxCFz ~ 2 9 1 " 215" 174"

143" 82 ~

31" 5* -:58* -I05~

Fig. 18.19. Fluorine-19 multiple-pulse (REV8) spectra of a TFE/HFP (8.5 mol% HFP) copoly-

mer as a function of temperature. [Reproduced with permission from Ref. 13].

700


ations were observed as in the case of TFE homopolymer [13], but the amorphous-region local chain-axis reorientation is inhibited by the sterically larger CF3 groups below the melting point. An indication that one-third of the CF2 groups are in the crystalline environment was also obtained. As mentioned above, most commercial cured fluorocarbon elastomers are solvent resistant. Hence, it is important to achieve 19F high-resolution solidstate spectra of these polymers in order to understand their microstructure directly in the state in which they have practical applications. Dec et al. [117] studied copolymers of VDF and HFP, copolymers of VDF and CTFE, and terpolymers of VDF, TFE and HFP. They have demonstrated the feasibility of obtaining 19F high-resolution solid-state spectra, at room temperature, of these samples using direct polarisation and MAS speeds of about 18 kHz. They were able to make assignments of chemical shifts in terms of five-carbon sequences (pentads). Most of the pentad assignments for each chemical shift were made by reference to the solution-state work of Ferguson [118] and of Murasheva et al. [119]. All the major features apparent in the 19F NMR spectra of the solubilised polymers [118,119] are readily measured with highspeed MAS techniques. Some indication of a very small amount of polymerisation of HFP was found. Sufficient resolution was obtained to determine the relative concentration of each carbon pentad and also to determine monomer compositions from the 19F NMR data. Good agreement with known compositions was obtained. We have studied a commercial sample of Viton provided by Goodfellow (Cambridge, UK). The sample is a black sheet 2-3 mm thick, density 2.0 g/cm 3, lower working temperature - 10 to -50~ upper working temperature 220-300~ All 19F NMR spectra were recorded at 188.29 MHz on a CMX-200 spectrometer. The 19F NMR spectra, with and without proton decoupling, were recorded using direct polarisation with 7r/2 pulses of 3/xs duration. Spinning speeds up to 12 kHz were used. Relaxation delays of 4 s were sufficiently long to ensure quantitative peak intensities. Chemical shifts were measured with respect to CFC13 on a sample of C6F6 without any irradiation at the proton frequency. In Fig. 18.20, the 19F NMR spectrum obtained at a spinning speed of 12 kHz is shown. Following the work of Dec et al. [117] we have assigned chemical shifts to structural features. The results are given in Table 18.2. The peaks indicated with a question mark in Table 18.2 do not correspond to any known or possible pentad for this kind of copolymer. They may be attributed to resonances of unsaturated monomer units [120]. Fig. 18.21 shows a computer deconvolution of the 19F spectrum. Lorentzian lineshapes were used to simulate the peaks. Relative integrated intensities for each peak of the sample are given in Table 18.2. Corrections for the intensity of the spinning sidebands were made. An analysis of the

FLUOROPOLYMERS

701

4

3

5

7 9

1

10

6

C" !

|

100

50

,

!

,

0

-

i

|

|

!

|

-50

-100

-150

-200

-250

6(ppm)

,,

,

-300

Fig. 18.20. Fluorine high-resolution (MAS) spectrum of a commercial sample of Viton. Direct polarisation without proton decoupling was used. The asterisks indicate spinning sidebands. The spinning speed was 12 kHz. See the text for further details of the experimental conditions. The peak numbers are referred to in Table 18.2, which lists the assignments. Table 18.2. Peak deconvolution and structural assignment of a V D F - H F P copolymer Peak

Chem. shift a (ppm)

Area (%)

1 2 3 4 5 6 7 8

-56.6 -62.5 -71.0 -75.1 -89.7 -103.9 -110.0 -114.3

1.9 1.1 6.4 19.5 32.3 4.2 12.9 2.4

9 10

-118.1 -183.7

11.0 8.3

Structural assignment b

--CF2---CH2---C*F2---CH2---CF2---CF2~CH2----C*F2~CF(CF3)--CF2~ --CF2--CH2--C*F2---CF2--CH2--

---CF2mCH2--C*F2--CH2--CH2~ --CF2mCF2---C*F(CF3)mCH2---CF2~

The fluorine(s) concerned are indicated by an asterisk. From direct polarisation experiments without proton decoupling. Therefore, there is no Bloch-Siegert effect.

a

b

relative 19F signal intensities determines the composition as VDF" HFP = 72"28 for this copolymer. Resolution in the 19F spectra is not improved by application of r.f. proton.decoupling at ambient temperature. In this kind of rubbery sample a spinning speed of about 10 kHz, combined with molecularlevel motion of the polymer chains, is enough to efficiently average 1H-I9F

702

R . K . H A R R I S , G. A. MONTI AND P. HOL ST E IN

I

A

v ~

0

''''I''''I

-50

....

-100

I''''I''''I''''I

F/pp

-150

....

I'''

Fig. 18.21. Deconvolution of the 19F high-resolution spectrum of Viton shown in Fig. 18.3.

dipolar interactions. However, the Bloch-Siegert effect [121] results in the chemical shifts for decoupled spectra appearing ca. 1.7 ppm to low frequency of the corresponding shifts in coupled spectra. Fluorine-19 spin-lattice relaxation times in the laboratory frame, T1, and 19F and 1H spin-lattice relaxation times in the rotating frame, T i p , w e r e measured under high-resolution conditions (i.e., selectively) at ambient probe temperature (--23~ and are reported in Table 18.3. Relaxation times T1 w e r e obtained using the inversion-recovery technique (Tr-r-~r/2-acq), while the 19F T i p relaxation times were measured by means of the variable-time spin-lock technique, and 1H Tip was obtained via 19F resonance by a variable 1H spin-lock followed by {1H ~ 19F} cross polarisation. In all cases decoupling of the complementary nucleus was not implemented during the variable time allowed for relaxation. The 19F relaxation times all show single-exponential decay of the magnetisation as a function of Table 18.3. 19F and 1H spin-lattice relaxation times for a Viton sample (ppm) -56.6 -70.6 -75.1 -89.9 -110.4 -118.0 -183.9 a

19F T1 (ms) (-+ 10 ms)

19F Tip (ms) a

1H Tip (ms) a

375 381 379 361 388

2.16 1.61 1.64 0.98 0.86 0.83 1.60

1.1 1.2 1.2 0.9 1.3

The errors quoted are statistical.

_ 0.04 _ 0.05 ___0.01 ___0.01 ___0.01 ___0.01 --_ 0.01

__+0.1 ___0.1 +_ 0.1 __. 0.1 _ 0.1

703

FLUOROPOLYMERS

the relaxation delays. The values obtained for Tx are very similar for all the resolved lines, whether they are associated with VDF or HFP units. We conclude that spin diffusion is effective enough to produce a single T1 value over the entire sample. The relaxation times T~o for the lines associated with CF/CF3 of the HFP units (resonances at - 7 1 , - 7 5 and -184 ppm) are slightly higher than for those associated with CF 2 fluorines. However, the signals at - 9 0 , -110 and -118 ppm also have contributions from HFP units. Proton Tlo relaxation times associated with all the 19F peaks show similar values, as expected. It is known that some copolymers of VDF and HFP show different degrees of crystallinity, and the presence of HFP decreases the crystallinity of the copolymer. This effect increases when the amount of HFP in the copolymer is increased [122-125]. However, our Tlo and WISE [126,127] results (see Section 6.6 for the latter) do not discriminate between amorphous and crystalline phases of the copolymer studied. The evolution of the 19F magnetisation obtained from the standard CP experiment is shown in Fig. 18.22 as a function of the contact time. The maximum intensity of the magnetisation is reached around 900 ms. This relatively short value occurs in spite of the high mobility of the polymer and even when the 19F~lH dipolar coupling is additionally averaged by MAS. This is presumably because we are dealing with two abundant nuclei and with similar and strong dipolar-couplings, resulting in

1.00

ooOOO(X)

o oo r 1 6 2

~---~-~ 9

o

z~

*o

lo~u

~ o

:3

9

9 o

A 9

0.10

9

o

9

-75

ppm CFs

o

-90

ppm CF2 (PVDF)

9

- 110 p p m CFz

[]

- 118 p p m CFz

A

-184

p p m CF

9

o,_ In c o c o~,

0.01 -

0.0

o

I

I

2.0

4.0

I

6.0

contact t i m e ( m s )

Fig. 18.22. Evolution of the 19F magnetisation as a function of the contact time in a standard cross-polarisation experiment for a commercial Viton sample.

704

R.K. HARRIS, G. A. MONTI AND P. HOLSTEIN Table 18.4. Effective

relaxation times of a Viton sample

(ppm)

Teff (ms) a Xlp

-75.1 -89.9

2.3 ___0.1 1.6 __ 0.1 1 . 2 _ 0.2 1.2 _ 0.2 2.4 _ 0.1

- 110.8

-118.0 -183.9 a

Teff --lp

The errors quoted are statistical.

an effective magnetisation transfer process. Effective proton-fluorine Tip values are given in Table 18.4. The values were obtained by fitting the decay of the magnetisation in the cross-polarisation experiments by the simplified equation:

M(r) =

Mo (1 - THF/Tlo )

(1 - exp[--(1/THF -- 1/Tlo)'r]) exp(-'r/Tlo).

The experimental data obtained using the T O R Q U E sequence [128] (Section 6.6) reveal in more detail the evolution of the 19F magnetisation. They show that the peaks associated with CF and CF3 fluorines gain magnetisation more slowly than the peaks associated with CFe groups. To have a reasonable idea of the cross-polarisation times, TI-IF, the T O R Q U E experimental data should be fitted, using T1H and T~p values from independent measurements. We have obtained 13C high-resolution spectra by means of 1H ~ 13C and 1 9 F ~ 13C cross polarisation. In both cases 13C spectra were obtained with 1H d e c o u p l i n g , 19F decoupling, or both 1H and 19F simultaneous decoupling during acquisition. The 13C spectra of the commercial Viton sample are shown in Figs. 18.23 and 18.24. Cross polarisation works as a discriminating technique that allows us to observe only ~3C atoms bonded to protons or bonded to fluorines. The high mobility of the Viton sample makes cross polarisation only effective for 13C atoms directly bonded to protons in the case of XH ~ 13C cross polarisation or bonded to fluorines in the case of 1 9 F ~ 13C cross polarisation (in contrast to the case for crystalline PVDF). The resonance lines were assigned by comparison with the 13C spectrum of PVDF [2]. The peaks around ~ c - 120 ppm correspond to CF3, CF2 and CF groups; peaks around 8c = 40 ppm correspond to C H 2 groups. The two peaks observed in the latter region reflect conformational effects of PVDF. The position of the main peak in this region is shifted by 10 ppm with respect to the main resonance of VDF homopolymer. The position of the small peak is coincident with the resonance of pure PVDF.

FLUOROPOLYMERS

705

1

~'

............

I

200

'

'1

100

'

I

0

[i(ppm) Fig. 18.23. 1H ---) 13C cross-polarisation spectra of P(VDF-co-HFP)" (a) 1H and 19F decoupling;

(b) IH decoupling; and (c) 19F decoupling. The experimental conditions were" proton zr/2 pulse 5/zs; contact time i ms; proton and fluorine decoupling power equivalent to 50 kHz; spinning speed 4 kHz; and acquisition time 25 ms.

Doverspike et al. [66] reported a deuteron N M R orientational lineshape study of perdeuterated (99%) PVDF and the copolymer of vinylidene fluoride with tetrafluoroethylene having 80 mol% VDF. The experiments examined only the crystalline part of the drawn and poled samples (crystallinity of P(VDF-co-TFE) estimated by X R D to be 50%). The maximum remanent polarisation attained with deuterated PVDF homopolymer film was 1.0/~C/cm 2, and for the copoplymer was 3.0/zC/cm 2. A Gaussian distribution about the draw direction characterises the chain-axis reorientation distribution in the stretched samples. The PVDF (draw ratio 3) has a Gaussian distribution of width 22 ~ (half width at e -1 of maximum), whereas, the P(VDF-co-TFE) has a distribution of width 18 ~ reflecting the better alignment of the more highly stretched copolymer film (draw ratio of 4). The authors found that as the magnetic field is rotated in the plane perpendicular to the stretch direction, the deuterium spectra of the poled copolymer sample do not change. It is concluded that the occurrence of electrical polarisation in the absence of orientation dependence of the deuterium lineshape indicates molecular reorientation through 180 ~ in the copolymer. Schmidt et al. [129] carried out deuteron N M R studies on a 70/30 mol%

706


c) '

I

200

'

I

100

'

I

0

'

,5(ppm) Fig. 18.24. 19F--->13C cross-polarisation spectra of P(VDF-co-HFP) with: (a) 1H and 19F decoupling; (b) 19F decoupling; and (c) 1H decoupling. The experimental conditions were: proton ~r/2 pulse 5 txs; contact time 1 ms; proton and fluorine decoupling powers equivalent to 50 kHz; spinning speed 4 kHz; and acquisition time 25 ms.

random copolymer of deuterated vinylidene fluoride and normal trifluoroethylene for temperatures above and below the ferroelectric-paraelectric phase transition of the crystalline portion of the copolymer. The observed narrowing of the deuterium lineshape in the paraelectric crystalline phase was interpreted as a result of motional narrowing caused by C n 2 H bonds making tetrahedral angles with the chain axis at all times, but rotating about that axis. A calculation was presented for the deuteron quadrupolar NMR spin-lattice relaxation caused by kink-3-bond motions along disordered helical chains in the paraelectric phase.

18.7

Composites and miscellaneous materials

In this section, papers reporting work on some materials containing both fluoropolymers and small molecules will be mentioned briefly. There are a number of NMR reports on ion-exchange membranes of the Nation | type. In these ionomers, perfluorinated sulfonates are attached as pendant groups to PTFE backbones. Several of the publications [130-136]

FLUOROPOLYMERS

707

deal with proton relaxation and bandshape measurements for water contained in the material, yielding information on mobility and domain formation, usually as a function of temperature. One paper [133] extends such a study to 2H spectra, which yielded two lines for Nation (due to unaveraged quadrupolar coupling) but only one for the related Dow system (XUS), suggesting that hydrogen bonding is weaker in the latter than in the former. Other papers [134,135,137-139] report 19F NMR measurements (T1, Tlo, T2 and bandshapes) taken under variable-temperature conditions. These give information about the mobility of the backbone and side chains. One article [139] reports a study of Nation/liquid crystal composite membranes, the presence of the liquid crystal apparently not appreciably affecting the mobility of the polymer matrix. A series of publications [134-136] have appeared reporting NMR studies on Nation using IH, 2H, 170 and 19F resonances at elevated pressures (with temperature held constant), and giving the activation volumes deduced. All the work on Nation involves static samples, and neither heteronuclear nor homonuclear decoupling was employed. Composite polymer electrolytes based on PVDF (at least 35%) containing lithium salts were studied [140] by a number of techniques including 7Li NMR. Spin-lattice relaxation measurements were used to show that localised lithium motion is significantly impeded in some samples but not in others. Fluorinated charcoal has been studied [141] by 19F MAS and 13C--->19F CPMAS spectroscopy. Dipolar dephasing and variable contact experiments yielded information on asignments and on quantification. Four types of carbon site were recognised, namely graphitic (C), CF, CF 2 CF2 and CF3. Fluorinated charcoal has been studied [141] by 19F MAS and 13C--> 19F CPMAS spectroscopy. Dipolar dephasing and variable contact experiments yielded information on assignments and on quantification. Four types of carbon site were recognised, namely graphite (C), CF, CF2 and CF3.

18.8

Postscript

While the early broadline and relaxation studies of fluoropolymers were very informative, particularly with regard to mobility at the molecular level, modern high-resolution techniques for both 19F and 13C are now being applied and show considerable potential for an expansion in the areas of applicability. The more sophisticated pulse sequences (e.g., for multi-dimensional and multiple-quantum spectra) have scarcely been used to date, with a few notable exceptions. The unusually favourable properties of the 19F nucleus offer the prospect of more detailed studies than are feasible with polymeric systems

708


containing only 1H and 13C as NMR-active nuclei. Expanded activity in this area is to be expected.

Note added in proof Recent (1997) papers report (i) 19F MAS studies [142] of Kel-F, which showed that substantially improved resolution is obtainable at elevated temperatures, (ii) an examination [143] of the ferroelectric phase transition for P(VDF/TrFE) using 13C CPMAS N M R , and (iii) a 13C CPMAS investigation [144] of perfluorooctyl acrylate/methylacrylate copolymer blends. Also, molecular motion in liquid-crystalline fluoropolymers of both main-chain and side-chain types has been extensively studied [145] by 1H ~ 13C and 19F ~ 13C CPMAS N M R , including two-dimensional isotropic/anisopropic chemical shift and WISE experiments. The reader's attention is also drawn to two reviews [146,147] summarising the work of Veeman and Maas, discussed in Section 18.5, on P M M A / P V D F copolymers and blends.

Acknowledgements One of us ( G . A . M ) thanks C O N I C E T (Argentina) for a post-doctoral fellowship, during the tenure of which this article was written. We are grateful to the Deutsche Akademischer Austauschdienst and the British Council for support to enable the collaborative work to occur. We also thank the U.K. E P S R C for research grant L02906.

References 1. R.D. Kendrick and C.S. Yannoni, J. Magn. Reson. 75 (1987) 506. 2. C.H.K. Douwel, W.E.J.R. Maas, W.S. Veeman, G.H.W. Buning and J.M.J. Vankan, Macromolecules 23 (1990) 406. 3. K.L. Wooley, C.A. Klug, K. Tasaki and J. Schaefer, J. Am. Chem. Soc. 119 (1997) 53. 4. Encyclopedia of Chemical Technology, 2nd Edition 9, 1966. 5. F. Anderson and J.O. Punderson, "Organofluorine Chemicals and their Industrial Applications", Chaper 12: Poly(tetrafluoroethylene) and Related Fluoroplastics. Ellis Horwood Chichester, UK, 1979. 6. E.S. Clark and H.W. Starkweather, J. Appl. Polymer Sci. 6 (1962) $41. 7. C.W. Bunn and E.R. Howells, Nature 174 (1954) 549. 8. M. Iwasaki, J. Polymer Sci. Part A 1 (1963) 1099.

FLUOROPOLYMERS

709

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Ctzapter 19


Hydrogen-Bonded Polymers Fumitaka Horii and Kenji Masuda Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan

19.1

Introduction

There are many kinds of hydrogen-bonded polymers in which hydrogen bonds are formed between OH groups, OH groups and carbonyl groups, amido groups or isocyanate groups, etc. In this chapter, the structure and hydrogen bonding of poly(vinyl alcohol) is mainly described as analyzed by CP/MAS ~3C NMR spectroscopy, since other representative hydrogen-bonded polymers, such as polyamides, polypeptides, protein and polysaccharides, are described in detail in other chapters. Poly(vinyl alcohol) (PVA), which is highly crystalline even in almost atactic form and hydrophilic due to the existence of OH groups [1, 2], has recently received much attention as one of the potential raw materials for highperformance hydrogels and high-tenacity fibers. However, it is very important, in developing such materials, to characterize intra- and intermolecular hydrogen bonding of PVA in the dissolved and solid states in detail. Infrared (IR) spectroscopy has been used frequently to investigate the hydrogen bonds in terms of the absorption band at --~3000 cm-~, ascribed to the OH stretching or its overtone at ~6700 c m - 1 [3-6]. In particular, the latter absorption band was resolved into three lines assigned to the OH groups associated with intramolecular, intermolecular and no hydrogen bonding in the order of increasing wavenumber. However, it may be difficult to further separate the contributions from the crystalline and noncrystalline regions into these three lines. High resolution solid-state NMR spectroscopy is also a very powerful method for characterizing the solid structure and the local motion of different solid polymers. We recently characterized the crystalline-noncrystalline structure for different crystalline and liquid crystalline polymers, such as polyolefins [7-12], polyesters [13-15], polyether [16], polyurethanes [17, 18] and polysaccharides, including cellulose [19-29], amylose [30, 31] and dextran [32]. On the basis of these analytical methods, we also investigated the intra- and intermolecular hydrogen bonds of PVA in both crystalline and noncrystalline regions as well as in the frozen solution state. In this chapter,

714

FUMITAKA HORII AND KENJI MASUDA

we review mainly these e x p e r i m e n t a l results of the h y d r o g e n bonding f o r m e d in different P V A samples with different tacticities.

19.2

CP/MAS

13C NMR spectra of PVA

Figure 19.1 shows C P / M A S 13CN M R spectra m e a s u r e d at r o o m t e m p e r a t u r e for dried P V A films with different tacticities [33]. H e r e , the degree of polymerization (DP) and the triad tacticity of each sample are as follows: D P = 1590, m m = 0.19, m r = 0.48 and r r - 0.33 for S-PVA; D P = 1700, m m = 0.23, m r = 0.50 and rr = 0.27 for A - P V A ; D P = 300, m m = 0.57, m r = 0.35 and rr = 0.08 for I - P V A . These films were subjected to annealing above

CH2

CH

I[[ II

S-PVA

I~PVA

Solution ,

,

,

III ,

I

80

,

,

,

III ,

I

60

,

,

,

,

I

40

I

I

v

I

I

I

t

20

ppm from Me4Si

Fig. 19.1. CP/MAS ~3C NMR spectra of different dried PVA samples measured at room temperature.

HYDROGEN-BONDED POLYMERS

715

O-ring

~sample iliiiiiii!i!iiiiiiiiiiiii~!iii: i!Li~i~i!!i!!iiiiii~ili!i!i!!i!

1 Fig. 19.2. Schematic diagram of a MAS rotor with an O-ring seal.

150~ In addition, a cylinder-type MAS rotor shown in Fig. 19.2, (originally developed by Horii et al. [33] and now available from JEOL), was used for the dried samples to prevent the absorption of moisture during NMR measurements. The CH resonance line of each sample shown in Fig. 19.1 clearly splits into a triplet, lines I, II and III, and the relative intensity of the three lines depends greatly on the triad tacticity. The extent of the line splitting is much higher compared to the case of the splitting due to the triad tacticity in the solution-state 13C NMR spectrum, as shown in a stick-type spectrum at the bottom of Fig. 19.1. On the basis of these results, lines I, II and III were initially assigned to the CH carbons with two, one, and no intramolecular hydrogen bond(s) in the triad sequences of the PVA chain with planar zigzag structure [34], as shown in Fig. 19.3. Here, the OH group bonded to the CH carbon I may form intramolecular hydrogen bonds with two OH groups on both sides in the mm sequence. In contrast, the OH groups associated with CH carbons II and III will form one and no hydrogen bond with the neighboring OH groups in mr and rr sequences, respectively. According to the crystal structure of PVA [35, 36], the O...O distance is estimated to be 0.252 nm in the intramolecular hydrogen bonds. Therefore, it is assumed that strong deshielding that induces a large downfield shift should occur for the CH carbons I and II, in analogy with the case of hydroxybenzaldehyde crystals

716


mr t

H

I

H

t

H U

mm

I

rr

Fig. 19.3. Schematic diagram for intramolecular hydrogen bonding in the triad sequences of PVA.

[37]. In these crystals, marked downfield shifts of resonance lines were observed for the carbons chemically bonded OH groups with decreasing O...O distance, when this distance is less than ---0.27 nm in the intramolecular hydrogen bonding between the hydroxyl and carbonyl groups. Therefore, the largest downfield shift should appear for the CH carbon I in solid PVA, while the medium-level downfield shift will be observed for the CH carbon II. In contrast, the CH carbon III is not associated with the intramolecular hydrogen bonding, but the O H group chemically bonded to this carbon may form the intermolecular hydrogen bonding with O H groups in the neighboring chains. However, the intermolecular hydrogen bonding will induce no appreciable downfield shift for the CH carbon, because the O...O distance is more than 0.27 nm in the intermolecular hydrogen bonds of PVA crystals. Therefore, it seems plausible to assume that resonance lines of the CH carbons I, II and III appear in the order of increasing field as shown in Fig. 19.1. In contract, it was proposed from the evaluation of 13C N M R results for solid ethylene-vinyl alcohol copolymers that the large split of the CH resonance line would be possibly induced by the substituent effect due to the introduction of O H groups to polyethylene chains [38]. More recently, ab initio gauge-included atomic orbital (GIAO) calculations were carried out for solid PVA and the triplet of the CH resonance line seemed to be well interpreted in terms of the formation of the intramolecular hydrogen bonding [39]. However, an appearance of a similar triplet was suggested by the similar calculation for the planar zigzag chain of polypropyrene, which has methyl groups as substituents in place of OH groups. Therefore, it was concluded in this case that the main cause of the triplet of the CH resonance line would be the substituent effect. To confirm the assignment of the triplet of the CH

HYDROGEN-BONDED POLYMERS

717

line of solid PVA, further experiments should be conducted on the basis of the solid-state NMR theories and computer-aided analyses. In the following sections, we describe some experimental results supporting the initial assignment that lines I, II and III are ascribed to the CH carbons bonded to OH groups associated with two, one and no intramolecular hydrogen bond(s). One of the most important experiments is to separate the contributions from the crystalline and noncrystalline regions, because the degree of crystallinity is less than 0.5 in each sample. Such a separation will be also useful for elucidating the relative intensities of lines I, II and III for different PVA samples shown in Fig. 19.1. In fact, the relative intensities of lines I, II and III are not in accord with the relative fractions of mm, mr, and rr sequences, e.g. the relative intensity of A-PVA significantly deviates from 1:2:1 that can be assumed from the triad tacticity.

19.3

Spectra of the crystalline and noncrystalline components

To separate the contributions from the crystalline and noncrystalline components, ~3C spin-lattice relaxation times T~c were measured for the PVA samples by the CPT1 pulse sequence [40]. In Fig. 19.4, the logarithmic peak intensities of line II are plotted against the decay time for A-PVA films at room temperature [33]. The total decay curve, indicated by open circles, was successfully resolved into three components with different T~c values by the least-squares method, as shown by the solid and dashed lines in Fig. 19.4. Similarly, three components were also recognized for other resonance lines of A-PVA as well as all lines of S-PVA and I-PVA. Since the glass-transition temperature of PVA is about 70 or 85~ [1], the segmental motion of PVA chains must be restricted at room temperature. In this situation, the longer T~c indicates less molecular mobility of the component. Therefore, the longest Txc component was assigned to the crystalline component, whereas the medium and shorter T~c components are ascribed to the less mobile noncrystalline and mobile noncrystalline components, respectively. The detailed estimation of the degree of crystallinity supports these assignments [33]. The T~c values for the crystalline component are more than 5 times larger than the values for the noncrystalline component. Using this difference in T~c, CP/MAS ~3C NMR spectra of the two components are recorded separately by selective measurements of the crystalline component by the CPT1 pulse sequence and the following spectral subtraction method. Figure 19.5 shows the CH resonance lines of the crystalline and noncrystalline components of A-PVA films thus obtained [33]. In this figure, the results of the computer lineshape analyses for these resonance lines are also shown. Here,

718

FUMITAKA

HORII AND KENJI MASUDA

(

CH(If)

5.0 4-)

o

------4.0 I'-"E2"-~:~,

TIC=65.0 s

3.0-

r

N N k

I-.-i,

2.01.0-

A

\ TIC:I4 6s \

TIC=I .2s I

j

l

20

i

\

9

X

X

a q i

40

x

i

x

x

i

x

\

x i

\

60

i

80

i

I

100

Time/s Fig. 19.4. Semilogarithmicplot of the peak intensity of the resonance line II of dried A-PVA

films as a function of time. The solid line is the composite decay curve for three components with different T i c values, which are shown by broken lines (Ref. [33]). each line was assumed to be described as Gaussian. The composite curve of the three lines I, II and III, shown by a broken line, is in good agreement with the experimental spectrum for each component. As is clearly shown in Fig. 19.5, the integrated fractions of lines I, II and III are still in discord with the fractions 0.23:0.50:0.27 of the mm, mr and rr sequences for both components. This discordance will be interpreted by assuming that some of O H groups in the m sequences do not form intramolecular hydrogen bonds but they are associated with the intermolecular hydrogen bonding. A detailed discussion will be given in the later section.

19.4

Effects of casting solvents for film preparation

Figure 19.6 shows the CP/MAS 13C N M R spectra of unannealed A - P V A films prepared from aqueous, dimethyl sulphoxide (DMSO) and hexafluoroisopropanol (HFIP) solutions [41]. It is found that the relative intensities of the CH triplet of the sample prepared from the aqueous solution are signifi-

HYDROGEN-BONDED POLYMERS II

719

IiI

I

(b)noncrystall

90

80

70

60 ppm from Me4Si

Fig. 19.5. CH resonance lines of (a) crystalline and (b) noncrystalline components in A-PVA

films (Ref. [33]). cantly different from those for the samples prepared from DMSO and HFIP solutions. Table 19.1 files the T~c values of the respective resonance lines for A-PVA films prepared from different solvents, which were measured by the CPT1 pulse sequence. There are also three components with different T~c values, in accord with the results of annealed A-PVA described in Section 19.3. As for the longest Tic values, A-PVA films prepared from the aqueous solution have the longest Txc values, while the sample prepared with HFIP has the shortest T~c values. Such differences in Tic may be due to the difference in the size of the crystallites. Using the difference in T~c between the crystalline and noncrystalline components in these samples, the respective spectra of the crystalline and noncrystalline components are also recorded separately. According to these results, line I is increased in intensity and line III is concomitantly reduced in intensity in the order of H20, HFIP and DMSO as casting solvents in both crystalline and noncrystalline components. This fact suggests that intramolecular hydrogen bonds will be more preferably produced in this order in the crystalline and noncrystalline regions. More detailed discussion will be made in the later section.

720


CH2

CH Ill

III a)

(b

I

. . . .

100

!

80

'

~

'

I

60

. . . .

i

40 2O ppm from Me4Si

Fig. 19.6. CP/MAS 13C NMR spectra of unannealed A-PVA films prepared from different solutions: (a) H20; (b) DMSO; and (c) HFIP solutions.

Table 19.1. 13Cspin-lattice relaxation times of the respective resonance lines for different PVA samples, measured at room temperature Sample

T1c/S CH I

A-PVA a A-PVA b A-PVA c

78.0 50.1 40.2

7.9 8.5 3.7

II 1.9 0.1

69.0 52.9 36.1

15.0 13.0 4.6

CH2

III 3.0 2.0 0.5

aUnnannealed sample prepared from H20 solution. bUnannealed sample prepared from DMSO solution. CUnannealed sample prepared from HFIP solution. -, not measured.

79.0 67.6 38.1

29.7 12.5 5.5

5.0 1.8 0.5

68.0 52.0 38.1

11.3 8.7 5.6

0.7 1.0 1.1

HYDROGEN-BONDED POLYMERS 19.5

721

Structure in the hydrate state

CP/MAS 13C N M R measurements are also powerful in characterizing hydrated polymer samples if the water content is not reduced by high centrifugation due to MAS during N M R measurements. We originally developed the MAS rotor with an O-ring seal, shown in Fig. 19.2, for hydrated samples [30-33] and successfully obtained CP/MAS 13C N M R spectra and spin-relaxation times for different polymer samples without any loss of water. Figure 19.7 shows CP/MAS 13C N M R spectra of A - P V A films with different water contents using the MAS rotor with an O-ring seal [33]. Here, the respective samples were exposed to atmospheres of different relative humidities at 24~ in a desiccator for about 1 week to obtain the equilibrium state. The water content is expressed as (g of H 2 0 / g of dry PVA) x 100%. As clearly shown in Fig. 19.7, an additional resonance line, which is termed line IV, appears between lines II and III for the sample with a water content CH2

CH

II III water content PE

0%

3%

18%

27%

l•''.I''.•I''''l'''.i''''I''''l'''••''''I''''l''''I''''l''''I''''I''''1''''I'''' 80

60

40 20 ppm from Me4Si

Fig. 19. 7. CP/MAS 13C NMR spectra of annealed A-PVA films with different water contents

(Ref. [33]).

722


CH2 CH

....

! ....

, ....

I ....

80

, ....

I ....

, ....

I ....

60

, ....

I ....

, ....

! ....

v ....

! ....

v ....

!

40 20 ppm from Me4Si

Fig. 19.8. Dipolar-decoupled MAS 13C NMR spectra of the rubbery component in annealed A-PVA films with a water content of 18%, which were measured by the modified 13C spinecho method (Ref. [33]).

of 24%. Since T i c values of this component are of the order of 0.1-0.2 s, line IV can be assigned to the rubbery component where the intra- and intermolecular hydrogen bonds may be broken by water molecules. In fact, the real line shape of line IV, which was selectively observed by using the 13C spin-spin relaxation time T2c of this component as shown in Fig. 19.8 [33], was in good accord with the solution-state 13C NMR spectrum. Figure 19.9 shows the result of the lineshape analysis for the spectrum of the crystalline component of A-PVA with a water content of 18% [33], which was carried out in the same way as for dried PVA films. Although an additional Gaussian curve must be introduced upfield for line III, the composite curve of the four lines, which is described by a broken line, reproduces well the experimental spectrum of the crystalline component. The additional upfield line can be assigned to the component free from the intra- and intermolecular hydrogen bonds, which may probably appear as a result of the enhancement in molecular mobility by water. In Fig. 19.8 the integrated fractions of lines I, II and III are also shown. The fractions of lines I and II significantly increase compared with those for the dry sample shown in Fig. 19.5. This may suggest that the probability of intramolecular hydrogen bonding is increased upon addition of water.

H Y D R O G E N - B O N D E D POL YME R S

0.52 II

80

70

723

0.33 III

60 ppm from Me4Si

Fig. 19.9. Lineshape analysis for the CH resonance line of the crystalline component of annealed A-PVA films with a water content of 18%. The broken line is the composite curve of the four components shown by the solid lines and the numerical values are the integrated fractions of lines I, II and III (Ref. [33]).

19.6

Fibers: effects of drawing

It is well known that ultrahigh molecular weight linear polyethylene samples can be drawn up to 100-200 times (super drawing) when they are prepared as films, or spun as fibers, from the gels produced at an appropriate polymer concentration [42-44]. The Young's modulus of the samples thus obtained reaches 235 GPa, which corresponds to the Young's modulus along the molecular chain axis for polyethylene crystals [45]. This fact suggests that the polymer chains are highly extended by the super drawing. PVA also adopts the planar zigzag-conformation in the crystalline region as polyethylene, and the Young's modulus along the molecular chain axis for PVA crystals is significantly higher (250 GPa [46]) than for polyethylene crystals. Therefore, different methods were tried to produce high tenacity PVA fibers, but such trials are still unsuccessful possibly because of the difficulty in controlling intra- and intermolecular hydrogen bonds in the case of PVA. Here, we describe briefly the CP/MAS ~3C NMR results of PVA fibers with different draw ratios, which were spun from aqueous or DMSO solutions [47]. Figure 19.10 shows the CH resonance lines of the crystalline components of PVA fibers spun from the 10 wt% DMSO solution, which were selectively measured by the CPT1 pulse sequence. This figure also shows the results of the lineshape analysis by the computer-aided least-squares method. In these fibers, it is necessary to introduce two Gaussians, lines IIIb and IIIf, for line

724

F U M I T A K A H O R I I AND KENJI M A S U D A

!H

III b ;IIlf ~

..

x=4.1

t

_~.

_ z,,, ^

annealed

spun

t'v'X~

j L n v l , , , v l t , , , l , , , , l , , , , I v v v , l , v , , l , , , ,

80

70

60 ppm from Me4Si

Fig. 19.10. CH resonance lines of the crystalline components of PVA fibers spun from the 10% DMSO solution (Ref. [47]).

III, as in the case of hydrated PVA samples shown in Fig. 19.9. Since the chemical shift of line IIIf is in good accord with that of the upfield line for the hydrated PVA, line IIIf can be assigned to the CH carbons chemically bonded OH groups, free from the intra- and intermolecular hydrogen bonding. As clearly shown in Fig. 19.10, the fraction of OH groups, free from hydrogen bonds, is increased significantly with increasing draw ratio. Similar results were obtained for PVA fibers spun from the aqueous solution. In contrast, much different effects of drawing were observed for the noncrystalline component of PVA fibers, as shown in Fig. 19.11. Here; are the CH resonance lines of the noncrystalline components of PVA fibers spun

725

H Y D R O G E N - B O N D E D POL YME R S

II

/

P 0.9. These results show that when the AA unit is isolated in the D M A A sequences, the molecular motion of the PEG is not strongly restrained by the intermolecular interaction with AA units. On the other hand, when consecutive AA units are distributed in the network, intermolecular interactions between the PEG and consecutive AA segments are enhanced and this results in the strongly restrained molecular motion of PEG [43]. By adding a small amount of HC1 into the gel, it shrinks and the DpEG decreases as shown by the arrows in Fig. 20.13. The complex formation arises from an intermolecular hydrogen bond between the oxygen atoms of the PEG and the carboxylic groups of the AA units in the undissociated state are shown in Scheme 20.1. It can be said that the addition of HC1 leads to an increase in the amount of undissociated carboxylic groups on going from the lefthand to the righthand side in Scheme 20.1. This may enhance the formation of the complex. The complex formation and the undissociation induce the shrinkage of the gel and lead to slow diffusion of PEG. The T2 value reflects the molecular motion [10, 12]. In the molecular motion of PEG in D 2 0 solution, the segmental motion is a dominant factor compared with the translational motion and rotational motion of a whole molecule. The 1H T2 of PEG ( M w - 4250) was measured by the CPMG method [10] at 303 K varying q and fAA of the poly(DMAA-co-AA) gel. Fig. 20.14 shows the plot of the 1H T2 values obtained against q. The 1H T 2 values for PEG in the gels with fAA < 0.5 decrease as q is decreased. This means that the segmental motion of the PEG molecule decreases with decreasing size of the network. The segmental motion of PEG may not be strongly

761

POLYMER GEL SYSTEMS 0.8

" ..... '

1

l"!

N r

[--,

EEl

N El

0.6

I::F! 51

0.4

,....,,

I

0.2

0

,,

v

t

10

~

t

z

J

,t

!

50

~

t

I

,

100

Degree of Swelling q Fig. 20.14. Dependence of 1H T2 of the PEG with Mw of 4250 in the poly(DMAA-co-AA) gels on the degree of swelling (q) at 303 K. The fAA are ([3) 0, ([]) 20, ([]) 50, (D) 90 and (ll) 100.

restrained by intermolecular hydrogen bonds with the AA units in the gels for fAa ( 0.5 as indicated by the experimental results on DpEo. The 1H T2 values of PEG for fAA ) 0.9 are much smaller than those for fAa ( 0.5. This result is similar to that of DpEo. The complex between PEG and the AA units in the network polymer restrains the segmental motion of PEG as well as the translational motion. From the above results, the complex should be stabilized through hydrogen bonding between the PEG chain and the consecutive AA units in the network polymer.

20.6.2

13C chemical shift

The hydrogen bonds may play an important role in the poly(DMAA-co-AA) gel-PEG system as described above. It is important to investigate hydrogen bonds formed between the AA units in the gel and PEG. It is known that the formation of hydrogen bond leads to a change in the 13C chemical shift. When the exchange rate between hydrogen bonded and unbonded form is larger than I / i ~ - 6~bl, where ~ and ~ub are the 13C chemical shifts in

762

H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA

hydrogen bonded form and unbonded form, respectively, the observed 13C chemical shift (6) can be expressed by -- Pb ~, + Pub

(20.7)

(~ub,

where Pb and Pub a r e the mole fractions of the carbons in the hydrogen bonded form and unbonded form, respectively [44]. Matsukawa and Ando studied the PAA gel-PEG system by a solid-state NMR technique. Fig. 20.15 shows the expanded 13C NMR spectra for (a) the PEG solution; (b) the PAA gel (q = 18); (c) the PAA gel soaked in 0.5 wt% of PEG solution (q = 7); and (d) the PAA gel soaked in 1 wt% of -C_H2C_H20-

a)

-C_OOH

b)

c)

d)

WM

1184

:182 ~

178

:176

I' I' I ~ I' i' I' I ~ I'I' I'r] 5.74 74 72 70 68 ~ F_,4

Fig. 20.15. Expanded 13C NMR spectra of (a) the PEG solution; (b) the PAA gel; (c) the PAA gel soaked in 0.5 wt% of the PEG solution (q = 7)" and (d) the PAA gel soaked in 1 wt% of the PEG solution (q < 5). Spectra (a), (b) and (c) were measured by normal solution NMR and spectrum (d) was measured by the PST/MAS NMR.

763

POLYMER GEL SYSTEMS

PEG solution (q < 5). The spectra (a), (b) and (c) were measured by a solution NMR technique, and spectrum (d) was measured by solid-state PST/MAS NMR method. The 13C chemical shift of the carboxylic carbon in PAA shows the low frequency shift from 178.62 ppm in the spectrum (b) to 178.56 ppm in the spectrum (c) and 178.45 ppm of spectrum (d). However, no signal is observed at the chemical shift position to be expected for the hydrogen bonded form. The chemical shift difference (A6= 178.62- 178.45 = 0.17 ppm) is smaller than that expected for the hydrogen bonded form. The amount of the hydrogen bonded form between the oxygen atom of the PEG and, the carboxylic carbon of the PAA in the complex, is so small that the average exchange rate between the hydrogen bonded and unbonded forms is larger than 1/16b- 6ubl. This is consistent with the result measured by potentiometric titration that the ratio of the number of binding groups to the number of potentially interacting groups in solution of PEG(Mw = 3000) and PAA(Mw = 1840000) is 0.05 [45]. From the results on the D~,EG and 1H T2 for the poly(DMAA-co-AA) gel-PEG system, it can be said that the polymer complex is formed by intermolecular interaction between the PEG and consecutive AA units in the network polymer. Therefore, the intermolecular interaction is caused by a small amount of hydrogen bonding between the oxygen atoms of PEG and the carboxylic acid groups

CHC! 3

C~H, I v ==C-- OCH=

-OCHs

2

PMLG

%

2

C-Oest~r

1,4-dio~me

C-Oax~d e

pp~ 2

0

Fig. 20.16.

150 13C

t00

50

PST/MAS NMR spectrum for the PMLG gel in CHC13.

764

H. Y A S U N A G A , M. K O B A Y A S H I A N D S. MATSUKAWA

50

30

20

,

15

/ 10

__~

jj'

!

"'

1~0

'

1

5

~,

'

~7~.

'

~

i

i;'6

~ '

"

I

'

s7~

'

'

I

~"-' '

.~72

'

'

I

rTo

' ' l ' ' ' l ' - ' ~ ' l " " ' l ' ' " i ' ' ' l ' ' ' i ' ' ' l

62

Go 5o

~

54

.~.

~

5o

4e

'~l'"~

34

32

~"

' ' u ~ l ' ' ' l ' ' ' l ' ' j l ~ ' ' ' ' l

ao

~n

~

~4

2~. ~o

T F A (%)

Fig. 20.17. 13C PST/MAS N M R spectra for the P M L G gel in the various volume fraction of T F A in a mixture of CHC13 and TFA.

POLYMER GEL SYSTEMS 177.5

|

765

!

177 O.--

176.5

.B" . . . . . . .

ID-

(a)

.E

-

176 175.5 175 174.5 174

O

173.5 E 0., I~

!

I

I

I

i

t

I

58

(b)

,....;

o e~o

57

--1

56

u., 0

55

== r/l

o

54

".r

53

~.o-

e---

e- ......

5Z

o

-O- - -.,

!

32

r,J r,.)

31

m .-.I

,,.

,,I

I o

- .. . . . . . . .

I

-.

I

I

I

(c) _

-

30 29 o,_-o . . . . .

28

-

27

Lll, ="& ,

26 0

-0 -

f

I

zo

I

I

40

60

,

-- "

"1

I

80

100

Volume Fraction of TFA (%)

Fig. 20.18. 13C chemical shifts for the PMLG gel as a function of the volume fraction of TFA in the mixture" (a) amide carbonyl carbon (O) and ester carbonyl carbon (O); (b) Ca(O) and OCH3(O)" and (c) C,/(O) and Ct3(O ).

of the PAA gel, which are formed and rapidly broken. This temporary hydrogen bonding should cooperatively form the polymer complex.

20.7

Poly(y-methyl L-glutamate) gel

The properties of the polymer gels, swollen by organic solvents, are studied in addition to water-swollen polymer gels [46]. In this section, we are concerned with poly(y-methyl L-glutamate) (PMLG) crosslinked by the esteramide exchange reaction in 1,4-dioxane using diaminododecane as a

766

H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA 0

...... ,

,

,

i

,

.

,

i

','

.,

,

i

,

,

,

i

,

,

,

turbid C) transparent

40 v

.=_

:30

r.~ o

o

20

(

10

0 0

,

20

40

60

80

,

100


Fig. 20.19. Dependence of the degree of swelling (q) of the PMLG gel on the volume fraction of TFA in the mixture (fTFA). Appearance of the PMLG gel is expressed by ( 9 for transparent gel and by (O) for turbid gel.

crosslinker, and swollen by chloroform (CHC13) and trifluoroacetic acid (TFA). 20.7.1

Conformationchange

It is known that PMLG takes an a-helix form in CHC13, and the random coil form in a mixture of TFA and CHC13 [47]. When the solvent composition is changed in the crosslinked PMLG gel, its volume and conformation are changed. A 13C PST/MAS NMR spectrum for the PMLG gel in CHC13 is shown in Fig. 20.16 together with peak assignments. The peak intensity for the side chain carbons is more intense than that for the main chain carbons. The side chains undergo faster molecular motion, compared with those of the main chain. The 13C PST/MAS NMR spectra for the PMLG gel are shown as a function of the volume fraction of TFA (fTFA) in a mixture of CHC13 and TFA at room temperature in Fig. 20.17. The obtained 13C chemical shifts of individual peaks plotted against fTFA are shown in Fig. 20.18. The amide carbonyl (amide C = O ) and C~ carbons resonate at 176.5 and 57.6ppm, respectively, in CHC13 (fvFa = 0). This indicates that PMLG network takes the a-helix form as determined by reference data of solid polypeptides [48-

767

P O L Y M E R G E L SYSTEMS 3.5

.~

09 v

.-

I

I

) (a) C--Oester

I

extreme narrowing region

O

2.5 2

O

()

1.5

O

O

()

O

0.5 3

I I

(b) C=Oamide

2.5

I

O (

---

I

O

I I

slow motion region

0

~7 1.s 0

0.5 0

o

I

I

5

1o

I

15

C 20


Fig. 20.20.

13C T1 of (a) ester carbonyl carbon and (b) amide carbonyl carbon as a function of fTrrA measured at 20~ (O) and 40~ (O).

50]. By the addition of TFA to the PMLG gel system, the 13C chemical shifts of the amide C - - O and C~ carbon transitions move to low frequency to 174.1 and 54.4 ppm, respectively, at fTFA = 0.2. They are independent of fTFA above 0.2. Such transitional low frequency shifts show that the main chain conformation of the PMLG network changes from the a-helix to the random coil form. However, the 13C chemical shifts of the side chain carbons also show small change at fTFA = 0.2. In Fig. 20.19, the degree of swelling for the PMLG gel is plotted as a function of fTFA. The degree of swelling is transitionally decreased at 0.05-0.2 of fTFA and is gradually increased for fTFA > 0.2. This shows that the helix-coil transition leads to the shrinkage of the PMLG gel. 20.7.2

13C T1 as a function of solvent composition and temperature

Fig. 20.20 shows the TFA content dependence of the 13C T1 values for the amide C---O and ester C - - O carbons of the PMLG gel measured by PST/MAS NMR combined with the IR method at 20 and 40~ As the temperature is decreased, the T1 for the amide C---O carbon increases and

768

H. YASUNAGA, M. KOBAYASHI AND S. MATSUKAWA

that for the ester C = O carbon decreases. The results shows that molecular motion of the amide C = O carbon is in the slow motion region and that for the ester C - - O carbon is in the extreme narrowing region according to the BPP theory [12]. Therefore, the increase in T1 for the amide C - - O carbon, and the decrease for the ester C - - O carbon with increasing fTFA, can be interpreted as being due to the decrease of motion of the PMLG network. In the range of fTVA = 0.05--0.2, T1 for the amide C = O carbon decreases with increasing fTFA and that for the ester C = O carbon does not change apparently. The content of the random coil form is increased gradually With increasing fTFA in this range (Fig. 20.18), and the degree of swelling is constant (Fig. 20.19). The results suggests that the molecular motion of the main chain increases when the conformation of the PMLG main chain changes from a-helix to random coil.

References

10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Y. Osada, M. Hasebe, Chem. Lett. (1985) 1285. T. Tanaka, Phys. Rev. Lett. 40 (1978) 820. T. Tanaka, Sci. Am. 244 (1981) 110. Y. Hirokawa and T. Tanaka, J. Chem. Phys. 81 (1984) 6379. R.A. Komoroski (Ed), High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk. VCH, New York, 1986. S.R. Hartmann and E.L. Hahn, Phys. Rev. 128 (1962) 2042. A. Pines, M.G. Gibby and J.S. Waugh, J. Chem. Phys. 59 (1973) 569. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M.J. Albright, The 20th Meeting on NMR. Tokyo, 1981, p. 68. E.R. Andrew, Progress in Nuclear Magnetic Resonance Spectroscopy, J.W. Emsley, J. Feeney, L.H. Sutcliffe (Eds), Vol. 8, Part 1, p. 1, Pergamon Press, Oxford, 1971. T.C. Farrar and E.D. Becker, Pulse and Fourier Transform NMR--Introduction to Theory and Methods. Academic Press, New York, 1971. F.A. Bovey, High Resolution NMR of Macromolecules. Academic Press, New York and London, 1972. N. Bloembergen, E.M. Purcell and R.V. Pound, Phys. Rev. 73 (1948) 679. J.M. Guenet, Thermoreversible Gelation of Polymers and Biopolymers. Academic Press, London, 1992. D.R. Paul, J. Appl. Polym. Sci. 11 (1976) 439. A. Takahashi and S. Hiramitsu, Polymer J. 6 (1974) 103. M. Kobayashi, I. Ando, T. Ishii and S. Amiya, Macromolecules 28 (1995) 6677. T.K. Wu and D.W. Ovenall, Macromolecules 6 (1973) 582. Y. Inoue, R. Chujo and A. Nishioka, J. Polym. Sci., Polym. Phys. Ed. 11 (1973) 393. Y. Inoue, R. Chujo, A. Nishioka, S. Nozakura and H. Iimuro, Polym. J. 4 (1973) 244. T.Terao, S. Maeda and A. Saika, Macromolecules 16 (1983) 1535. M. Kobayashi, I. Ando, T. Ishii and S. Amiya, J. Mol. Struct. 440 (1997) 155. D.A. Torchia, J. Magn. Reson. 30 (1978) 613.

POLYMER GEL SYSTEMS

769

23. H. Yasunaga, M. Kobayashi, S. Matsukawa, H. Kurosu and I. Ando, Ann. Rept. NMR Spectroscopy, 34, G.A. Webb. (Ed.), Academic Press, London, 1997, p. 39. 24. H. Yasunaga, H. Kurosu and I. Ando, Macromolecules 25 (1992) 6505. 25. T. Shibuya, H. Yasunaga, H. Kurosu and I. Ando, Macromolecules 28 (1995) 4377. 26. H. Kurosu, T. Shibuya, H. Yasunaga and I. Ando, Polym. J. 28 (1996) 80. 27. Y. Hotta, H. Kurosu, H. Yasunaga and I. Ando, Polym. Gels Networks (1998) in press. 28. H. Yasunaga and I. Ando, Polym. Gels Networks 1 (1993) 83. 29. H. Yasunaga and I. Ando, J. Mol. Struct. 301 (1993) 125. 30. H. Yasunaga and I. Ando, Polym. Gels Networks 1 (1993) 267. 31. H. Yasunaga and I. Ando, J. Mol. Struct. 301 (1993) 129. 32. C. Pichot, A. Hamoudi, Q.T. Pham and A. Guyot, Eur. Polym. J. 14 (1978) 109. 33. J. Schaefer, Macromolecules 4 (1971) 98. 34. E. Klesper, A. Johnsen, W. Gronski and F.W. Wehrli, Makromol. Chem. 176 (1975) 1071. 35. K. Yokota, A. Abe, S. Hosaka, I. Sakai and H. Saito, Macromolecules 11 (1978) 95. 36. K.L. Smith, A.E. Winslow and D.E. Petersen, Ind. Eng. Chem. 51 (1959) 1361. 37. E. Tsuchida and K. Abe, Adv. Polymer Sci. 45 (1982) 2. 38. O.E. Philippova, N.S Karibyants and S.G. Starodubtzev, Macromolecules 27 (1994) 2398. 39. S. Matsukawa and I. Ando, Macromolecules 29 (1996) 7136. 40. S. Matsukawa and I. Ando, Macromolecules 30 (1997) 8310. 41. R.E. Cameron, M.A. Jalil and A.M. Donald, Macromolecules 27 (1994) 2708. 42. P.G. de Gennes, Macromolecules 9 (1976) 594. 43. I. Iliopoulos and R. Audebert, Macromolecules 24 (1991) 2566. 44. J.A. Pople, W.G. Schneider and H.J. Bernstein, High-resolution Nuclear Magnetic Resonance. McGraw-Hill, New York, 1951, p. 218. 45. Y. Osada, J. Polymer Sci., Polym. Chem. Ed. 17 (1979) 3485. 46. C. Zhao, S. Matsukawa, M. Kobayashi and I. Ando, J. Mol. Struct. (1998), in press. 47. Y. Suzuki, Y. Inoue and R. Chujo, Makromol. Chem. 181 (1980) 165. 48. H. Saito and I. Ando, Ann. Rept. NMR Spectrosc. 21 (1989) 210. 49. I. Ando, T. Yamanobe and T. Asakura, Prog. NMR Spectrosc. 22 (1990) 349. 50. A. Shoji, S. Ando, S. Kuroki, H. Yoshimizu, I. Ando and G.A. Webb, Ann. Rept. NMR Spectrosc. 26 (1993) 55.


Chapter 21


Biodegradable polymers Yoshio Inoue Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan

21.1

Introduction

Today, a variety of and large amount of synthetic polymeric materials are produced and are supporting our comfortable daily life as well as modern high technologies. The major reasons for their success are their strength, durability, resistance to chemical and biological corrosion and low production cost. On the one hand these properties have created various fields of applications, but on the other have given rise to the problem of waste polymers. Currently, almost all plastic wastes are treated by either landfill or incineration. As great concern in conserving the global environment is growing worldwide, other methods of plastic waste treatment without raising environmental problems are eagerly sought. Recycling is one choice for reducing the polymer waste. But, recycling is only effective when waste materials are easily collectable and can be recycled to useful materials without significant deterioration of properties. Recycling also includes the chemical degradation of plastics to monomeric materials which are renewable to give polymeric materials. At present, only a few kinds of plastics, such as polystyrene, polyolefins and polyesters, are recycled on atrial scale. Another choice is using environmentally friendly biodegradable polymers. The term biodegradable polymer is used here in the sense of any polymer which is degraded to carbon dioxide, water and biomass using environmental micro-organisms. Hence, biodegradable polymers contribute to reducing environmentally released plastic waste. Some biodegradable polymers, such as polysaccharides, can be treated as compost and biologically recycled to produce natural products. Sometimes, according to a broad definition, biodegradable polymers also include biomedically useful polymeric materials which degrade and are absorbed into the animal body and again contribute to polymer waste reduction. Biodegradable polymeric materials can be classified into three categories based on their origins, i.e., chemically synthetic materials, natural products and composites of both chemical and natural products. Several aliphatic

772

YOSHIO I N O U E

polyesters, such as poly(glycolic acid), poly(lactic acid), poly(E-caprolacton) and poly(ethylene adipate), are well known examples of biodegradable polymers [1]. Poly(vinyl alcohol) [2, 3] and lower molecular weight poly(ethylene oxide) [4-6], which are typical water-soluble synthetic polymers, are also known to be biodegradable. There are many kinds of natural biodegradable polymers. They are classified into three types according to their chemical structures, i.e., polysaccharides, polypeptides/proteins and polynucleotides/nucleic acids. Among them, polysaccharides, such as cellulose, chitin/chitosan, hyaluronic acid and starch, and proteins, such as silk, wool, poly(y-glutamic acid), and poly(E-lysin), are well known and particularly important industrial polymeric materials. Recently, a series of bacterially synthesized polyesters called poly(hydroxyalkanoic acids) (PHA) have attracted much attention, because they are naturally occurring biodegradable thermoplastics (for review articles, see Refs. 1, 2, 7-10). Now, various types of micro-organisms are known to accumulate PHAs as an intracellular storage material for biological energy and carbon source [11]. Until now, more than 90 different PHAs with different chemical structures have been reported as constituents of biosynthetic PHA [10]. They are mainly produced by the feeding of designed precursor substrates, including unnatural synthetic compounds, as the substrate specificity of PHA synthases, key enzymes of PHA biosynthesis, is unexpectedly broad. Thus, it is expected that new PHA constituents will continue to be developed by using new substrates or new types of bacteria, resulting in new functional biodegradable and biocompatible polymeric materials. The productivity of these polyesters by some typical PHA-producing micro-organisms, such as Alcaligenes eutrophus and Pseudomonas oleovorans, increases by cultivating them under proper conditions, including a sufficient amount of carbon sources but lacking one of the growth factors such as nitrogen, phosphorus and oxygen. In addition to PHA-producing micro-organisms, a variety of aerobic and anaerobic PHA-degrading bacteria and fungi have been isolated from various environments, such as farm and forest soil, bottom sediments in lakes and rivers, sewage sludge and sewage sludge supernatent [12-18]. These microorganisms have as a rule, little or no ability to biosynthesize PHAs, but they can hydrolyze environmental PHAs by secreting extracellular PHBdepolymerases. They utilized the hydrolyzed products as nutrients. Thus, PHAs are expected to be fully biodegraded into carbon dioxide and water in a variety of environments. The third category of biodegradable polymeric materials is composites of both chemical and natural products. They are designed to have superior properties compared to those of the component materials, or to supply

BIODEGRADABLE POLYMERS

773

deficiencies of components to each other and, furthermore, to control biodegradability. There are many types of biodegradable polymeric composites [2]. The development of biodegradable composite materials is one of the growth areas of polymer research. The properties of solid-state biodegradable polymeric materials, such as mechanial strength and biodegradability, should be affected not only by chemical structure but also by the physical and the morphological state of materials. Almost all biodegradable polymeric materials are usually used in the solid state. Hence, high resolution solid-state NMR is expected to be a powerful method for the study of structure/morphology-properties relationships of solid-state biodegradable polymeric materials. Solid-state 13C NMR spectra should provide an independent means of discriminating between chemically equivalent carbon nuclei distributed among different environments, such as different crystalline and noncrystalline and/or amorphous regions of semicrystalline materials, and the miscible and immiscible domains of polymer blends. In this chapter, solid-state structure and properties relative to the morphologies of several chemically and bacterially synthesized biodegradable polymeric materials are described based mainly on the results obtained for bacterially synthesized polyesters by high resolution solid-state 13C NMR spectroscopy. This chapter briefly discusses polymer blends, which also includes polysaccharides and proteins, since more details are given in other chapters of this book. Several books on biodegradable polymers have been published [1, 2], and many review articles on structure and properties of bacterially synthesized polyesters have also been published elsewhere [7-10, 19-22].

21.2 21.2.1

Poly(hydroxyalkanoic acid)s Chemical structure and some physical properties of bacterial polyesters

Among a variety of bacterially synthesized poly(hydroxyalkanoic acid)s (PHAs), poly(3-hydroxybutyric acid), P(3HB), CH3

is the most popular and widely distributed homopolyester produced efficiently by various kinds of micro-organisms [10]. P(3HB) is the first example of

774

YOSHIO INOUE

PHAs discovered by Lemoigne [23] more than 70 years ago. The P(3HB) is produced and stored inside the bacterial cell walls in granules. Alcaligenes eutrophus, which was the first strain used for the semi-industrial production of PHAs, can accumulate large quantities of P(3HB) as discrete intercellular granules by careful control of the fermentation process, i.e., up to 80% of the weight of the dried cell can be in the form of P(3HB) granules [7]. P(3HB) is recovered from the cell by various procedures such as solvent extraction. As will be shown in the next section, this recovery process changes amorphous P(3HB) granules into plastic. The resulting P(3HB) material is a highly crystalline thermoplastic, whose several characteristics, such as melting point, degree of crystallinity and glass-rubber transition temperature, are comparable to those of isotactic polypropylene [24]. However, it is stiff and brittle. Furthermore, its melting temperature (ca. 170~ is near the temperature at which it thermally decomposes [25-27], limiting its processing in the molten state and, hence, its extensive industrial applications, although a number of mouldings, extrudates, films and fibres have been produced [7]. There are two possible ways to improve thermal processability and mechanical properties of P(3HB), i.e., copolymerization of 3HB with other monomers and blending P(3HB) with another polymeric materials. There are many kinds of bacterially synthesized copoly(hydroxyalkanoic acid)s [10]. A well-known typical copolyester is poly(3-hydroxybutyric acidco-3-hydroxyvaleric acid), P(3HB-co-3HV), CH3

I CH3

CH2

which is produced by fermentation with a high yield comparable to those of P(3HB), so it is an industrially important polymeric material. We can now synthesize bacterially a range of P(3HB-co-3HV)s with 3HV mole fractions ranging from 0% [P(3HB)] to 100% [P(3HV)] [28, 29]. These P(3HB-co3HV) samples are essentially random copolymers [28]. Several mechanical and physical properties of P(3HB-co-3HV)s have been found to vary widely with their comonomer composition [19, 20, 22]. Some of such composition dependences of properties should be ascribed to composition-dependent morphological changes in copolymers in the solid state. Hence, it is very interesting to investigate the composition dependence of the solid-state structures of P(3HB-co-3HV)s in detail. In the first section of this chapter, some solid-state properties of P(3HB-


775

co-3HV)s are described in relation to their morphology elucidated by solidstate a3C NMR. Furthermore, properties and morphologies of some related bacterial copolyesters, such as poly(3-hydroxybutyric acid-co-4-hydroxybutyric acid), P(3HB-co-4HB), CH3

and poly(3-hydroxybutyric acid-co-3-hydroxypropionic acid), P(3HB-co3He), CH3

are shown in order to discuss the molecular origins of the similarities and differences of their properties and morphologies. It will be exemplified that even small differences in the chemical structures of comonomer units induce significant changes of morphology and morphology-dependent properties. The second method to improve the properties of P(3HB) is a blending strategy, which has been widely applied [20-22]. In addition to improving the desired properties, the blending is also expected to reduce the marketing price of P(3HB) materials, which is one of the major reasons limiting its industrial applications, if the blending partners selected are less expensive biodegradable polymers. As the degree of mixing of the polymer blend systems influences the physical and chemical properties, it is important to know how morphologically heterogeneous the blend is. Solid-state NMR can provide molecular level detailed information on the mixing state of polymer blend systems. In the later sections of this chapter, the results of NMR studies of some P(3HB)-based polymer blend systems are shown.

21.2.2 Solid-state structure of poly(hydroxyalkanoic acid)s studied by NMR 21.2.2.1

Structure of poly(hydroxyalkanoic acid) granules in vivo

As mentioned above, P(3HB) isolated from bacterial cells is highly crystalline, with melting point of about 170-180~ and a glass-transition temperature of about 5-10~ while in the intact cell it is found in granules whose physical states have attracted much attention. As described below, some studies including solid-state NMR analysis of dried granules indicate the presence of

776

YOSHIO INOUE

highly crystalline structures, leading to the expectation that the same highly crystalline structures are also present in native granules. High resolution solution-state 13C NMR spectra of aqueous suspensions of A. eutrophus cells containing P(3HB) intact granules show the well resolved P(3HB) resonances even near room temperature [30], indicating that the P(3HB) molecules in the bacteria are in a mobile state to a great extent. Nascent P(3HB) granules are found to be isolated as a milky suspension [31]. The mobile state was found to remain stable in an isolated granule suspension of P(3HB-co-3HV) for several months at room temperature. Both transmission and scanning electron microscopy show that their structures are made of two distinct components, a solid shell composed of overlapping lamellar crystals and a soft noncrystalline core. Glycerol triacetate, which is a minor component of the granule, is found to be a good solvent [31]. The glycerol derivatives should contribute to plasticized poly(hydroxyalkanoic acid) granules as reported [32], although the amount of the glycerol derivatives (combined amount of proteins and lipids is up to 2% by weight [33]) may not be sufficient to fully plasticize the entire P(3HB) molecules [31]. Thus, it is not yet clear what maintains the core P(3HB) as a noncrystalline mobile state. A model of a state of P(3HB) granule core has been proposed in which the core is in a metastable gel state due to interactions between high molecular weight P(3HB) molecules and a plasticizing medium [31]. The influence of various types of treatments, such as freeze drying, separation of the granule from the cell debris by centrifugation, incubation of the granule at higher temperature etc., on the mobility and crystallinity of P(3HB) in whole cells of A. eutrophus H16 and native granules has been studied using solution-state 13C NMR spectroscopy and X-ray powder diffraction. The results have been correlated with the known biological effects of these treatments [34]. In this investigation, NMR spectroscopy is used as a highly sensitive diagnostic means to monitor the physical changes induced by different treatments. That is, when the P(3HB) included in a granule sample is completely converted from the mobile elastmer state to the immobile solid state, its resonances are predicted to disappear completely in solution-state NMR, while a sample consisting of a mixture of elastmer and rigid solid should show resonances with reduced intensities, but with the same linewidth as those of the elastomer. A sample consisting of polymers with lower chain mobility should show resonances with broader linewidth without loss of integrated intensity. The strains used in this study were grown on a minimal salt medium including glucose as the sole carbon source at 30~ under phosphate limitation which stimulates P(3HB) accumulation. These experiments discriminate between four different states of the P(3HB) granule: (1) the native state found

B I O D E G R A D A B L E POLYMERS

777

in live cells is a mobile elastomer which gives well-resolved NMR signals and no X-ray diffraction pattern; (2) freeze-drying can lead to a nonnative but relatively mobile amorphous state which has been partially characterized by NMR but not less X-ray diffraction studies. The dehydrated amorphous relatively-mobile granules are found to be rehydrated to a state which is spectroscopically indistinguishable from the native state, while the crystalline material cannot rehydrate to a native state; (3) extended centrifugation of the native granules in aqueous suspension, or treatment with hydrophobic detergents under certain conditions, is found to induce crystallization; (4) heating to 90~ or refrigeration has no detectable effect on mobility but leads to inactivation of the granule. Based on these results, the authors concluded that at least water is responsible for P(3HB) plasticization in vivo, and that only native mobile P(3HB) is susceptible to depolymerases, and another, probably hydrophobic, component was suggested to be involved either as a plasticizer or a nucleation inhibitor [34]. Transmission electron microscopy, differential scanning calorimetry, wideangle X-ray powder diffractometry and Fourier transform infrared spectroscopy have been used to investigate the crystallization behavior of P(3HB) granules from A. eutrophus isolated by enzymatic purification [35]. From the results of these investigations, water is suggested to be responsible for keeping the core of nascent P(3HB) granules in a noncrystalline state, and a model was proposed for the biosynthesis where emerging P(3HB) chains in an extended conformation are simultaneously hydrogen bonded to water molecules. The crystallization kinetics of native P(3-hydroxyalkanoic acid) (PHA) granules isolated from the strains of A. eutrophus and P. oleovorans have been studied by measurements of the glass-transition temperature with a differential scanning calorimeter [36]. The comparison is made between PHA in vivo and the isolated polymer. It is demonstrated that the native granules do not contain a plasticizer and the amorphous state of in vivo PHA can be explained by straightforward crystallization kinetics. High resolution solid state 13C NMR spectroscopy is useful to characterize the composition and solid-phase morphology of dried P(3HB) granules. A series of commercial powder preparations of extracellular microbial polysaccharides, such as gellan, welan and rhamsan, have been characterized by cross-polarization (CP) magic-angle sample spinning (MAS) a3C NMR spectroscopy [37]. The spectra indicate that samples contain a noncarbohydrate component exhibiting four inequivalent carbon nuclei attributable to those of P(3HB). It is suggested that P(3HB) may be a covalent adduct of these polysaccharides, i.e., a bacterial polyester may exist as a substituent associated with certain extracellular microbial polysaccharides in addition to its

778

YOSHIO INOUE

well-known role as an intracellular energy reserve [37]. I n fact, short-chain complexed P(3HB) is a ubiquitous constituent of cells and has been isolated from the plasma membranes of bacteria, from a variety of plant tissues, and from the plasma membrane, mitochondria and the microsomes of animal cells [38]. Solid-state NMR spectra of whole cells containing poly(hydroxyalkanoic acid) have been observed [39, 40]. The CP/MAS 13C NMR spectrum of P(3HB-co-3HV) containing 21 mol% 3HV in the freeze-dried cells is found to be qualitatively similar to that of a solution- or melt-cast film of the isolated copolymer [41]. The metabolic pathways of P(3HB) and polyphosphate in the micro-organism A. eutrophus H16 have been studied by several analytical techniques including ~H, 13C and 31p NMR spectroscopy [42]. Solid-state CP/MAS 13C NMR spectroscopy has been used to monitor the biosynthetic pathways of P(3HB) and other cellular biomass components from 13C-enriched acetate. The CP/MAS 13C NMR spectra of lyophilized intact cells grown on (1-13C) acetate (6% 13C)indicate that the carbonyl carbon of the acetate is selectively incorporated into both the carbonyl and methine carbons of P(3HB) and into the carbonyl carbons of proteins. From the 31p NMR analysis of A. eutrophus cells in suspension, the syntheses of both intracellular P(3HB) and polyphosphate are found to be closely related to each other. In contrast, under aerobic and anaerobic conditions, P(3HB) is degraded, but little polyphosphate is degraded. The rate of P(3HB) degradation is faster under anaerobic rather than aerobic conditions. Furthermore, under anaerobic conditions, 3hydroxybutyrate and acetate are found to be produced as the major extracellular metabolites. From these results, the pathways of P(3HB) and polyphosphate metabolism in A. eutrophus are suggested [42]. As shown here, solid-state NMR spectroscopy combined with solution NMR is a useful method to study not only metabolically important events in polyester-containing micro-organism cells but also to investigate the crystallization mechanism during polyester-isolation procedures. 21.2.2.2 Solid-state structure of P(3HB-co-3HV) Some important physical properties of P(3HB-co-3HV) have been found to show notable changes with their comonomer composition changes [19, 20, 22]. For example, Fig. 21.1 shows the plot of melting point (Tm) versus 3HV mole fraction (Fu) of random copolymer samples [28]. The melting point was measured by differential scanning calorimetry (DSC) at a heating rate of 20~ In order to eliminate the effect of thermal history, all samples for the DSC measurements were heated first to 200~ sufficiently higher than the highest melting point (namely, that of P(3HB) homopolymer), then


779

200

150

o

I00

50

0.0

'

'

0.2

t

|

o,q

0,6

Fv

t

0.8

1,0

Fig. 21.1. Plots of melting point (TM) vs. 3HV mole fraction (Fv) of bacterially synthesized random P(3HB-co-3HV)s. The circles indicate Tm values for P(3HB-co-3HV) samples which show a single DSC melting peak. The Tm values indicated by triangles are those for P(3HB-co3HV) samples which were bacterially synthesized as mixtures composed of two main copolymer components with different 3HV mole fractions, and so show well-resolved two DSC melting peaks. For these mixed samples, the 3HV contents of two components were determined by analyzing the solution XH NMR spectra based on the statistical model (for details of spectral analysis, see Ref. [28]). Hence, for these samples, the Tm values are plotted against the 3HV fractions of the component copolymers. (Reproduced from Ref. [28] with permission.)

quenched to room temperature at a rate of ca. 200~ and left at least 5 days [28, 43]. The melting point becomes a minimum at the F~ value of about 40 mol%, indicating clearly the occurrence of a crystalline phase transition in this composition range. According to the results of X-ray diffraction studies and intramolecular potential energy calculation of the crystalline structures, P(3HB) [44, 45] and P(3HV) [46] have crystalline structures similar to each other, that is, both are orthorhombic of the space group P21212x. The unit cell parameters for P(3HB) and P(3HV) are, respectively, a - 0.576 nm, b - 1.320 nm, c (fiber period) = 0.596 nm; and a = 0.932 nm, b = 1.002 nm, c (fiber period) = 0.566 nm. The fiber period of P(3HB) is very close to that of P(3HV). The crystalline structure has been refined by applying the whole-fitting method to the powder X-ray diffraction data [47]. Furthermore, it has been found by X-ray analysis that P(3HB-co-3HV)

780

YOSHIO INOUE

copolymers containing less than 40 mol% 3HV unit crystallize in the P(3HB) crystalline lattice, while those containing more than about 40 mol% of the 3HV unit crystallize in the P(3HV) crystalline lattice [48, 49]. That is, the transition from the P(3HB) to the P(3HV) crystalline lattice occurs at a 3HV unit composition of ca. 40 mol%, where the minimum of the melting point is observed. Upon a phase transition, only the a parameter of the crystalline unit cell of P(3HB) increases slightly, while the b and c parameters remain almost unchanged, irrespective of comonomer composition. X-ray data also show that the degree of crystallinity of P(3HB-co-3HV)s remains high, 5270%, with respect to the samples with comonomer compositions of 095 mol% 3HV [49]. As shown in Fig. 21.1, the melting point of P(3HB-co-3HV) is depressed rapidly as the 3HV content increases from 0 to 40mo1%. This behavior cannot be predicted by the Flory equation [48, 50], which was derived based on the assumption that copolymer crystals are composed of only one kind of comonomer component and the others exist only in the noncrystalline region. The analysis of the crystalline structure of P(3HB-co-3HV) samples with 3HV content up to 30 mol% indicate that only a small fraction of the 3HV units are included in the P(3HB) crystalline lattice as defects [51, 52]. A possible model of the crystalline structure which can explain the experimental results obtained by X-ray, solid-state 13C NMR and DSC is the one based on the assumption that both the 3HB and 3HV units are simultaneously cocrystallized into the same crystalline lattice [48]. Such a so-called isomorphism is expected to occur because the chemical structure of both of the monomeric units are not so significantly different from each other. In order to confirm the possibility of cocrystallization and to estimate how much of the minor comonomer component enters into a crystalline lattice of the major one, the solid-state structures of P(3HB-co-3HV)s with 3HV content ranging from 0%(3HB homopolymer) to 93.1 mol% were analyzed by 67.9 MHz high resolution solid-state 13C NMR spectroscopy [53, 54]. All the samples used for this NMR study are random copolymers and not mixtures of random copolymers as confirmed by the solution 13C NMR sequence analysis and by DSC melting-point measurements [43, 54]. Before taking NMR measurements, possible effects of the thermal history and recrystallization are avoided as mentioned above. Figure 21.2 shows the 13C CP/MAS NMR spectra of a series of P(3HBco-3HV) samples, where the 3HV content is indicated, for example, as P(3HB-18.3%-3HV). Each carbon resonance shows more or less a discontinuous chemical shift change between the spectra of P(3HB-31.6%-3HV) and P(3HB-55.4%-3HV). A relatively large chemical shift change of about 2 ppm,


781

P(BHB)

HB-18.3%-3HV)

t

PI3HB-316%-3HV)

P13HB-40.7%-3HV)

P (3HB-55.4%-3HV)

....L

~

P(3HB-93. 1%-3HV)

L '

'

'

i-'

180

'

'

'-!

''

160

'

'

i

'

140

'

'

'-I'

120

'

'

'

I

"'

'i00

'"'

!

80

'

'

'

'

I ' "

60

''

I

40

'

'

'"''I"'""'I'"

20

0

Fig. 21.2. 67.9 MHz 13C CP/MAS NMR spectra of a series of P(3HB-co-3HV) samples (2-ms contact time, 5-s pulse repetition time, 1000 FID accumulations. 's' indicates spinning side band). (Reproduced from Ref. [53] with permission.)

is observed for the main-chain methine carbon resonance. The chemical shifts for the resonances of the HB and HV units are listed in Table 21.1. As the CP efficiency in the crystalline region is in general larger than that in the noncrystalline region in the CP/MAS NMR spectra of semicrystalline polymers, the chemical shift changes of resonances which appear in the spectra are shown in Fig. 21.2 and Table 21.1 and should reflect predominantly the differences in the crystalline environment, that is, the crystalline

--.a b~

Table 21.1. Chemical shifts of 13C resonances of P(3HB-3HV) samples in the solid state Chem shift/ppma

X/%c

CO

0.0 18.3 31.6 40.7 55.4 93.1

170.0 169.8 169.8 169.8 169.9 169.8

CH(V) b

CH(B) b

LB

LB

72.5 72.6 72.6

LV

70.8 70.9 70.7

LV

68.6 68.5 68.6 68.7

CH:(B) b

CH2(V)

LB

LB

LV

42.9 42.9 42.9 42.9 67.5 67.2

LV

402 d 394 d 40.6 40.3

40.5 40.6 40.3

s-CH2(V) b

CH3(B) b

CH3(V) b

LB

LB

LB

LV

LV

21.4 21.1 20.9 21.0

267 d 26.9 27.2 29.0 28.7

LV

9.7 9.9 19.5 19.4

10.5 10.7 10.5

appm from TMS. bB, V, LB, and LV indicate the HB unit, the H V unit, the P(3HB) lattice, and the P(3HV) lattice, respectively, s-CH2 indicates side-chain methylene of H V units, c x indicates H V mol% content of P ( 3 H B - 3 H V ) copolymers, dBroad peak. (Reproduced from Ref. [53] with permission).


783

lattice structure. Considering the results of X-ray analysis [49], and comparing the observed chemical shifts for the copolymer samples with the different 3HV content, the main peaks appearing in the CP/MAS spectra of P(3HB) and copolymers with 3HV contents of 18.3 and 31.6 mol% are reasonably assigned to the contributions from the nuclei in the crystalline phase of the P(3HB) lattice and those of the copolymers with 3HV contents of 55.4 and 93.1 mol% from the nuclei in the crystalline phase of the P(3HV) lattice. In the spectrum of P(3HB-co-40.7%-3HV), doublet peaks are clearly observed for the main-chain methine carbon resonances of the 3HV unit, indicating the coexistence of both types of crystal phase. This result suggests that the phase separation is induced in the crystalline region on crystallization. The composition range of the crystal transition, where both the P(3HB) and P(3HV) crystal phases coexist, was investigated by wide-angle X-ray scattering and DSC for copolymer samples compositionally fractionated with a solution of an acetone-water solvent nonsolvent system [55]. The results of the X-ray analysis obtained for the fractionated P(3HB-co-3HV) samples with the 3HV contents range from 40.9 to 85.0 mol% indicate that the samples whose 3HV content range is 40.9-55.2 mol% show the coexistence of both the P(3HB) and P(3HV) crystal phases. The samples of 3HV content lying within this relatively wide range are found to show always the same mixed X-ray patterns of the P(3HB) and P(3HV) crystal phases. At this range, the crystal lattice parameters of the P(3HB) phase are found to expand from the original ones of P(3HB), a = 0.576 nm, b - 1.320 nm and c - 0.596 nm [44, 45] up to the maximum values for the P(3HB-co-55.2%-3HV) sample; a = 0.602 nm, b - 1.343 nm and c = 0.604 nm [54] the latter parameters are not the same as those for the P(3HV) lattice [46]. To study quantitatively the crystal morphology, the comonomer compositions in the crystalline, as well as in the amorphous, phases are measured by analyzing the solid-state NMR spectra. To make quantitative measurements, first, the I3C spin-lattice relaxation time T1 was measured by Torchia's pulse sequence [56]. The T1 decaying curves for all carbon nuclei are found to be practically reproduced by assuming slow and fast two-component relaxation processes. The slow and fast processes should be ascribed to the crystalline and noncrystalline regions, respectively. The T1 values for the protonated main-chain carbons are longer than ca. 60 s, while those for the side-chain methyl carbons are shorter than 5 s. The dipolar-decoupled (DD) MAS 13C NMR spectra (without CP) are measured in order to estimate the relative peak intensities of the methyl resonances of the 3HB and 3HV units in the crystalline and the amorphous phases. For this purpose, the pulse repetition time was adjusted to be longer than at least 5 times the longest T1 value of the methyl resonances.

784

YOSHIO INOUE P ( 3~B}

____~

P {3HB-93.1~-3HV)

~CH 3 IV)

PP14

13C DD/MAS NMR spectra of P(3HB) and P(3HB-co-3HV). (Reproduced from Ref. [53] with permission.) Fig. 21.3. Methyl resonances in the 67.9 MHz

Figure 21.3 shows the methyl resonance region of the 67.9MHz 13C DD/MAS NMR spectra of P(3HB) and P(3HB-co-3HV) samples. Each of the 3HB and 3HV methyl resonances was resolved into two peaks contributed from the crystalline and amorphous phases, respectively. Hence, from these measurements, the relative contributions from the crystalline and noncrystalline regions can be quantitatively estimated. The resulting chemical shifts and the relative peak intensities are listed in Table 21.2. These data clearly indicate that both comonomer units of P(3HB-co-3HV) are distributed among the crystalline and noncrystalline regions irrespective of the crystalline lattice types. That is, cocrystallization occurs in this copolyester system. From the relative intensity data shown in Table 21.3, the degrees of crystallinity are estimated to be ca. 60-70%, which are independent of comonomer composition and again indicate the occurrence of cocrystalliza-

T a b l e 21.2. Chemical shifts and peak intensities of methyl carbon resonances in DD/MAS NMR spectra

3HV unit b X/% ~

0.0 18.3 31.6 40.7 55.4 93.1

LB

11.32 11.56

LV

10.74 11.01 10.78

0 ~D

Rel peak intensity/%

Chem shift/ppm a 3HB unit b NC

10.11 10.29 9.73 10.31 10.22

LB 21.34 21.24 21.34 21.42

LV

21.89 20.77

3HV unit b NC 20.06 20.45 20.74 20.62 19.98 19.73

LB

6.2 24.5

LV

14.2 42.5 68.1

3HB unit b NC

19.4 25.7 18.0 13.2 25.8

LB 72.3 59.4 54.0 4.1

LV

20.1 16.5

NC 27.7 21.1 14.2 19.1 27.9 6.1

appm from TMS. oNC, LB and LV indicate noncrystalline component and crystalline components in the P(3HB) and P(3HV) lattices, respectively, cX indicates 3HV mol% content of P(3HB-3HV) copolymers. (Reproduced from Ref. [53] with permission).

> >

0

786

YOSHIO INOUE

Table 21.3. Comonomer composition, crystallinity, and heat of fusion for P(3HB-3HV) samples

xa/% 0.0 18.3 31.6 40.7 55.4 93.1

xa/%

xa[%

Eb

0.0 10.2 14.2 58.6 72.1 100.0

47.8 64.4 48.6

0 0.246 0.242 0.484 0.481 0

32.1 80.8

hc/%

AHd/kj mo1-1

72.3 59.4 60.2 62.9

11.2 6.2 3.6 4.8

58.9 68.1

7.6 10.0

ax, Sc, and Xa indicate 3HV mol% content of whole P(3HB-3HV) copolymers, that in the crystalline region, and that in the non-crystalline region, respectively, bE indicates the crystallizing ability of the secondary component, cA indicates the degree of crystallinity, dAHf indicates the heat of fusion of the pure crystal. (Reproduced from Ref. [53] with permission).

tion. The more important finding is that the comonomer composition of the crystalline phase is not the same as those of the whole copolymer molecule. The mole fraction of the minor comonomer component unit in the crystalline lattice of the major one is less than that in the whole copolymer. The fraction of the minor component incorporated into the crystalline lattice of the major component increases with increasing composition of the minor component in the whole copolymer. The crystallizing ability of the minor component as the crystalline component in the lattice of the major component can be discussed quantitatively by the term E defined as follows: when a crystal has the P(3HB) lattice, i.e., X is smaller than ca. 40%, E is given by E

=

(1 -

X)Xr

X(1 - X ~ ) ' where X and Xr indicate the 3HV mol% content of the whole copolymer and that in the crystalline region, respectively. When X is larger than ca. 40%, i.e., a crystal has the P(3HV) lattice, then the value of E is given by E=X(1 (1

- Xc) -

XlXc

As can be seen from the above definition, the E value represents the fraction of the minor component that is capable of crystallization. The E value of the main component is defined to be unity. When the comonomer composition in the crystalline region is the same as that of the whole copolymer molecule,


787

the E value become equal to unity. When the minor component is fully excluded from the crystal, the E value becomes zero. The experimental results are also shown in Table 21.3. The E values for the 3HV units in the P(3HB) lattice are about 0.24 and those for the 3HB units in the P(3HV) lattice are about 0.48. Thus, the ability of the 3HB unit to crystallize in the P(3HB) crystalline lattice is found to be twice as large as that of the 3HV unit to crystallize in the P(3HB) crystalline lattice. This is reasonable, because the side-chain of the 3HV unit is more bulky than that of the 3HB unit. Recently, VanderHart et al. [57] characterized a series of bacterially produced P(3HB-co-3HV) copolymers with a 3HB unit content of 0-27 mol% by CP/MAS ~3C NMR and determined the degree of cocrystallization of the two comonomer components. They also find that the 3HV minor component is incorporated into the P(3HB) type crystalline phase over the 3HV composition range of 0-27 mol% and that the ratio of the 3HV content in the crystalline phase to the overall 3HV composition increases with increasing 3HV content. They indicate for the copolymers with 3HV compositions of 21 and 27 mol% that the 3HV content in the crystalline phase is roughly 2/3 of the overall 3HV content. In general, most of the random copolymers form crystals composed of the major comonomer units of more crystallizable comonomer units alone, as incorporation of the minor component units into the crystalline phase need a large amount of excess free energy. So the cocrystallization of polymers is a rare phenomenon and a very few examples, such as poly(vinylidene fluoride)/vinylidene fluoride-tetrafluoro-ethylene copolymers system [58] and high-density polyethylene/linear low-density polyethylene [59], have been reported. Hence, the occurrence of cocrystallization found for the P(3HBco-3HV) copolymer is one of the rare examples. When the copolymer is composed of two types of units of different repeating-unit length, it is not expected to form an isomorphous crystal. However, for binary random copolyesters of ethylene terephthalate (ET) and 1,4-cyclohexenedimethylene terephthalate (CT), cocrystallization is found to be realized to some extent [60]. The comonomer contents in the crystalline phases are determined by high resolution solid-state X3C NMR spectroscopy. In the copolymers with an ET unit composition of 80-100 mol%, only the ET components form the crystalline phase, but in those with a CT unit composition of 66-100 mol%, a detectable amount of ET units are incorporated into poly (CT) crystalline lattice. The ET-CT copolyester system shows a minimum melting temperature at the intermediate comonomer composition range, quite similar to the case of 3HB-3HV copolyesters. According to the unit-cell dimensions, the sequence length of four ET repeating units is almost

788

YOSHIO INOUE

the same as that of three CT repeating units. This similarity should be the main reason that cocrystallization is realized for the ET-CT system.

21.2.2.3 Theoreticalstudy of cocrystallization of P(3HB-co-3HV) An involvement of different kinds of comonomer component units in the crystalline lattice of the main component unit of a copolymer should influence the bulk properties of a solid-state copolymer. As found for P(3HB-co-3HV) copolymers, the comonomer composition in the crystalline phase is not always the same as that in the noncrystalline phase. Hence, it is of great interest to investigate theoretically a comonomer composition in a crystalline phase. The cocrystallization ability of P(3HB-co-3HV) copolymers has been investigated from two different viewpoints, i.e., thermodynamically [61] and in terms of molecular mechanics [62]. Thermodynamic equations are formulated for the isomorphic behavior of A-B type random copolymer systems, in which both A and B comonomer units are allowed to cocrystallize in the common lattices analogous to, or just the same as, those of the corresponding homopolymers poly(A) or poly(B). It is assumed that, in the lattice of poly(A), the B units require free energy relative to the A units and vice versa. On the basis of the derived thermodynamic equations, phase diagrams are proposed for the A-B random copolymers with cocrystallization. The melting point versus comonomer composition curve predicted by this diagram is very consistent with that experimentally observed for the P(3HB-co-3HV) copolymers, as shown in Fig. 21.1. It is suggested that the minor comonomer unit with a less bulky structure cocrystallize thermodynamically simpler than that with a more bulky structure. The content of the minor comonomer units in the crystalline phase of the major ones is predicted to increase with respect to that in the whole copolymer. This prediction is also consistent with the results of 13C NMR observations. The theory also predicts that the content of the minor comonomer units in the crystalline phase decreases with a rise in the crystallization temperature [61]. To confirm this prediction, the contents of comonomer units in the crystalline as well as in the amorphous phases were again measured by high resolution solid-state 13C NMR spectroscopy for the P(3HB-co-3HV) copolymers crystallized at various temperatures [54]. The copolymer samples used were quenched from the melt into the crystallization temperature and left for 5 days at this temperature and then left at room temperature for more than 5 days. For the copolymer with a 3HV content as a whole of 18.3%, in which the 3HV unit is the minor component and this copolymer was crystallized in the P(3HB) lattice, the 3HV content in the crystalline phase (Xc) was found to decrease from 7.6 to 7.2% and the melting point


789

was found to rise from 112.8 to 127.1~ with a rise in the crystallization temperature from 0 to 80~ A more striking change was observed for the copolymer with the 3HV content as a whole of 55.4% (in this case the 3HB unit is the minor component and this copolymer was crystallized in the P(3HV) lattice). This copolymer shows an increase in the Xr value from 59.2 to 68.9% and a rise in the melting point from 68.7 to 78.2~ with a rise in the crystallization temperature from 0 to 40~ All of the results are consistent with the predictions based on the thermodynamic theory. The abilities of different monomeric units of the copolymer to cocrystallize into the same lattice should depend on the degree of structural similarity between them. The minor component should at least have the chemical structure causing no significant distortion in the backbone geometry of the homopolymer chain of the major component. To investigate how the involvement of the 3HV and 3HB units as the minor components in the homopolymer chains of P(3HB) and P(3HV), respectively, disturb their backbone conformations, the conformational properties have been theoretically studied for the P(3HB-co-3HV) copolymers on the basis of MM2 molecular mechanics calculations [62]. For this purpose, the model polymer chains consisting of 15 monomer units, are considered. The optimized conformations were obtained by starting from those of the X-ray crystal structures. The MM2 calculation is confirmed to be reliable, as each rotational angle along the backbone in the region extending from the 3rd to the 12th unit of the energetically optimized geometry for the P(3HB) homopolymer model having 15 monomer units, i.e., the B15 model, is found to agree well with that for the crystalline P(3HB) determined by X-ray method [45], as shown in Fig. 21.4. Here, the rotational angles are defined in Fig. 21.5. Thus, the 3rd to 12th region of B15 is assumed to be a better analogue of the P(3HB) homopolymer chain. The similar regular structure along the backbone is again found in the same region for the P(3HV) homopolymer model chain having 15 monomer units, i.e., the V15 model. The MM2 calculations reveal that the isolated model chains of both B15 and V15 have a tendency to form stable 21-helices, as found in the crystals by X-ray analysis for both the P(3HB) and P(3HV) homopolymers [44-46]. The effects on the B15 and V15 chain conformations of the incorporation of one or two 3HV and 3HB units, respectively, have been investigated. First, the central (8th) unit of B15 model chain is replaced by an HV unit, this copolymer model is called B14Vl[8], and the conformation was optimized starting again from the X-ray structure of P(3HB). The rotational angles of the optimized conformation are shown in Fig. 21.6. Special attention was paid to the conformational changes in the region

790

YOSHIO I N O U E

+10

CD

+5

(I)

"1:3 CD cM

c o o

-5

-10 [

1

/

1

5

I0 Unit

15

no.

Fig. 21.4. Each rotational angle along the backbone in the B15 model after optimization from

the initial X-ray structure versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray structure. (O)qJ; (A)~b; (D)o~; (O)0. (Reproduced from Ref. [62] with permission.)

from the 3rd to 12th unit, which corresponds to the regular structure region of the parent homopolymer model B15. The rotational angles ~b and 0 at the 7th unit, i.e., around t h e - - ( T t h u n i t ) C H 2 - - C O ~ and I O ~ C ( C H 3 ) (8th unit) I bonds, respectively, show large changes by + 11.4 and - 1 1 . 1 from those of B15. Such large changes are not found in the other angles of the same unit as well as in those in the other units. These conformational changes are found to be independent of the position of the 3HV substitution in the 3rd to 12th region. Thus, the substitution from 3HB to 3HV influences only the angles ~b and 0 of the nearest neighboring 3HB unit preceding the incorporated 3HV unit. This conformational change is attributable to the steric interaction between the ethyl side group of 3HV and the nearest carboxyl group. The overall


e

c=o CH2

791

~

.O-

CH3

Fig. 21.5. 3HB monomer unit and definition of rotational angles.

increase in steric energy introduced by changing one monomer unit from 3HB to 3HV can be relaxed by allowing rotation around the angles ~b and 0 of the 7th unit only. Hence, the conformational distortion induced by incorporating the 3HV unit is limited in the vicinity of the substituted unit and not propagated to more distant units. As a result, the 2~-helical structure of the parent homopolymer is almost unchanged in the B14Vl[8] copolymer model. This is also found to be valid in the di- an tri-3HV substituted copolymer models. For example, the optimized structures of the disubstituted models B13Vl[7]Vl[8] and B13Vl[6]Vl[8], in which the 3HV units are introduced at the 7/8th and 6/8th units, respectively, are shown in Fig. 21.7. In both cases, large conformational changes are found only in the nearest units preceding the incorporated 3HV units, indicating that the conformational properties of the resulting structures should not depend on the position of substitution. The copolymer models rich in 3HV units have been examined in the same way. For example, the result for the V14Bl[9] model, in which the central (9th) 3HV unit of V15 is replaced by a 3HB unit, is shown in Fig. 21.8. It is interesting to note that no apparent change in the backbone conformation occurs even around the incorporated 3HB monomer unit in contrast to the results for the B15-based models. The results indicate that substitution of one 3HB unit for one 3HV unit, i.e., the substitution of the methyl side chain for an ethyl one, causes no significant steric hindrance to the backbone of the model compound. The results of calculations for model compounds of di- and tri-substituted copolymers with various distributions of 3HB units indicate also that no significant conformational change is induced by incorporating 3HB units into the 3HV helix. Thus, it is concluded that 3HB units can

792

YOSHIO INOUE +15

+10

+5 Cf~ "[3

C

O3

0

O3

C 0 .m

O3

0

rr"

-5

-10

-15

~ 1

10

S Unit

15

no.

Fig. 21.6. Each rotational angle along the backbone in B14V1 [8] model after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the Xray data for P(3HB). Symbols are as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.)

be incorporated into the 3HV-rich copolymer models without any appreciable distortion of the polymer backbone conformation when the MM2 optimization starts from a reasonable initial structure [62]. From the results of MM2 calculations, it is expected that the incorporation of the 3HB unit as the minor component into the P(3HV) type lattice may cause no apparent influence on the crystalline P(3HV) morphology. This is

793

B I O D E G R A D A B L E POLYMERS + 1.5

_b

+15

+10

+1o

+5

,.-,

cn

+$

c m

~

t~ c o

0

o

o

o

-10

-10

-15

-15 1

5

10

Unit

no.

15

1

5

lO

Unit

no.

Fig. 21.7. Each rotational angle along the backbone in (a) B13Vl[7]Vl[8] and (b)

B13V116]V118] models after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray data for P(3HB). Symbols as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.)

consistent with the X-ray analysis, which shows that the lattice parameters are almost independent of the comonomer composition in the range of higher 3HV content. On the contrary, incorporation of the 3HV unit into the P(3HB) type lattice causes a substantial influence on the P(3HB) lattice. The MM2 predictions shown here are not inconsistent with the experimental conclusions based on the results of solid-state 13C N M R spectroscopy. The conformational aspects of P(3HB), as well as of P(3HB-co-3HV) in chloroform solution, have been investigated on the basis of the 1H spinspin couplings and N O E data [63]. The preferred conformation around the backbone of the methine-methylene bonds of both the 3HB and 3HV units in solution is found to be the same and coincides with that in the crystalline state [45, 46]. The different sequences and/or different monomer units do

794

YOSHIO INOUE

+10

+5 ol

o3 CO

co c o o~ r o c~

-5

x. -10 1

1

!

5

10 Unit

15

no.

Fig. 21.8. Each rotational angle along the backbone in the V14Bl[9] model after optimization versus the monomer unit number. A zero value for the rotational angle corresponds to the X-ray data for P(3HV). Symbols as given in Fig. 21.4. (Reproduced from Ref. [62] with permission.) not affect the short-range chain conformations in solution. The ~H NMR results indicate that the P(3HB-co-3HV) copolymers with different 3HV mole fractions have a higher possibility of forming isomorphic crystals.

21.2.2.4 Solid-state structure of P(3HB-co-4HB) It is very interesting to compare the crystallization behavior of P(3HB-co4HB) with that of P(3HB-co-3HV). The 4HB unit has no side chain which interacts sterically with neighboring monomeric units. The number of backbone carbon atoms in the 4HB unit is four, whereas that in the 3HB and 3HV units is three. Hence, a crystallization behavior different from that of P(3HB-co-3HV) is expected for P(3HB-co-4HV). The tendency of comonomer composition dependences of several physical properties of P(3HB-co-4HB) is quite different from that of P(3HB-co-3HV)


795

[49]. The melting temperatures of P(3HB-co-4HB) samples decreases slightly from 178 to 150~ as the 4HB content increases from 0 to 18 mol% and are almost constant in the range from 18 to 49 mol% 4HB. On the contrary, the enthalpy of fusion decreases monotonously with an increase in the 4HB unit in the composition range of 0-49 mol%. The degree of crystallinity, measured by an X-ray method, decreases from 55 to only 14% as the 4HB content increases from 0 to 49 mol%. The X-ray data indicate the existence of the P(3HB)-type of crystalline lattice at low 4HB content. A P(3HB-co-4HB) sample containing 82 mol% 4HB unit was found to show a sharp DSC melting endotherm at about 45~ [64, 65], which is close to the melting point (54~ observed for the P(4HB) homopolymer [66]. P(3HB-co-4HB) samples with 4HB contents over the range 85-100mo1% are found to have the non-P(3HB) type of crystalline lattice with a degree of crystallinity of 30-40% [65]. These experimental results suggest that the 3HB and 4HB units cannot cocrystallize in the same lattice. The factors which prevent the formation of cocrystals in the P(3HB-co4HB) copolymer, were investigated by molecular mechanics calculations for the model copolymer chains consisting of 15 monomer units [64]. Conformational analysis by the MM2 method of a P(3HB-co,4HB) model chain, including only one 4HB unit at the central position, revealed that 4HB unit cannot be included within the P(3HB) crystal lattice mainly because of the difference in the numbers of the main-chain carbon atoms of the repeating monomer units. The results of MM2 calculations also reveal that the 3HB unit, included within the P(4HB)-type lattice, takes a conformation which significantly deviates from a complete planar-zigzag conformation, which is the putative preferred one for the crystalline P(4HB) [64]. This is chosen in order to relax the steric hindrance between the methyl side chains of the 3HB units and the adjacent carbonyl oxygen atoms. Thus, the structural difference between the 3HB and the 4HB units is too large for them to cocrystallize into the same P(3HB)- or P(4HB)-type crystalline lattice.

21.2.2.5 The solid-state structure of P(3HB-co-3HP) There are several possible important factors which affect the cocrystallizability of comonomer units with shorter side chains in copoly(hydroxyalkanoic acid). These are such as the number of main-chain carbon atoms, the existence and the size of side-chains, steric configurations, crystallization temperature and the rate of crystallization. From a chemical point of view, the 3HP unit is expected to be easily incorporated into the P(3HB)-type crystalline lattice as a crystal component, since it has no side chain and its main chain consists of the same number of carbon atoms as the 3HB unit.

796

YOSHIO INOUE

To investigate whether, or not, cocrystallization occurs, in the P(3HB-co3HP) copolymer, the solid-state structure was analyzed by high resolution 13C NMR, X-ray diffraction, DSC and optical polarizing microscopy, for a series of P(3HB-co-3HP) samples with the 3HP content ranging from 0 mol% (bacterially synthesized 3HB homopolymer) to 100mol% (chemically synthesized 3HP homopolymer, i.e., polypropiolactone, PPL) [67]. The P(3HB-co-3HP) samples containing 0-37 mol% 3HP unit form the distinct spherulite when the melts of samples were cooled rapidly to the crystallization temperature on the optical polarizing microscope. The growth rate of spherulite deceases with an increase in the 3HP content. The spherulite formation could not be observed for copolymers with a higher 3HP content, while all copolymers show a DSC melting endotherm. The copolymers containing up to 37 mol% 3HP show melting peaks of about 160~ which is close to that of the P(3HB) homopolymer. The value of the enthalpy of fusion rapidly decreases with an increase of the 3HP content from 0 to 37 mol% 3HP, indicating a decrease of crystallinity with an increase of 3HP content. The crystal lattice parameters observed for these copolymers by X-ray diffraction are found to be almost identical to those of the P(3HB) homopolymer. The degree of crystallinity determined from the X-ray diffraction decreases steeply from 60 to 23% as the 3HP content in the copolymers increases from 0 to 37 mol%. The trends of composition dependence of the thermal properties and the X-ray diffraction are very similar to those for P(3HB-co4HB) but not to those for P(3HB-co-3HV), as already mentioned above. Thus, it is reasonable to assume that in the P(3HB-co-3HP) samples containing up to 37 mol% 3HP unit, the 3HB units exist in the crystalline as well as in the noncrystalline regions, while almost all of the 3HP units exist only in the latter. To further investigate this point, the 13C NMR relaxation times have been measured for the films cast from a chloroform solution. The CP/MAS 13C NMR spectrum of a P(3HB-co-37%3HP) sample is shown in Fig. 21.9, in which the signal intensities of the 3HB units, relative to the 3HP units, are much larger when compared to the corresponding relative intensities found in solution-state NMR. As described above, the CP efficiency is larger in the more rigid crystalline region than in the more mobile amorphous region. As a result, the signals arising from the crystalline region are emphasized in the CP/MAS 13C spectrum. The temperature (ca. 45~ at which the CP/MAS 13C NMR spectrum was measured, is much higher than the glass-transition temperature of this sample (ca. -5~ [68]. Hence, Fig. 21.9 indicates that fewer fractions of the 3HP units are incorporated into the crystalline region. In the CP/MAS 13C NMR spectra of the P(3HB-co13%3HP) and P(3HB-co-27%3HP) samples, the signal intensities of the

797


C=O

CH3 (B)

C H (B) I CH (B) CH 3

O ii

-(- o - cI , -

O

/3 3HB

,C

'

,

i

I

200

I

"'

,'

~

'

I

I00

'

-):--

a

3HP

H2 (P-B)

]

0

,

i

f

i

6 (ppm)

Fig. 21.9. 67.9 MHz 13C CP/MAS NMR spectra of P(3HB-co-37%-3HP). Most of the unassigned small signals are spinning sidebands and some others may arise from the rotor with polyimide end caps (2-ms contact time, 5-s pulse repetition time, 500 FID accumulations). "B" and "P" indicate that the signals arise from the 3HB and 3HP units, respectively. (Reproduced from J. Mol. Struct. 441 (1998) 119 with permission.)

carbons arising from the 3HP units are too weak to be detected, suggesting that the existence of a crystalline 3HP component is less probable in these samples. Figure 21.10 shows the partially relaxed solid-state 13C NMR spectrum of a P(3HB-co-37%3HP) sample measured by Torchia's pulse sequence [56]. Figure 21.11 shows the decaying curves of signal intensities estimated from the spectra shown in Fig. 21.10. In general, a spin-lattice relaxation process of a semicrystalline polymer, observed at temperature much higher than its glass-transition temperature, is decomposable as a rough approximation into two components, a slow and a fast relaxation processes due to the rigid crystalline and mobile amorphous region, respectively. Figure 21.11(e, f) indicate that the relaxation process of the carbon nuclei in the 3HP units is composed of only one component with a single relaxation time. However, as shown in Figs. 21.11(a-d), the T1 decaying curves for the 3HB carbons show a nearly biexponential behaviour. Hence, the T1 values of the 3HB units are obtained by the analysis assuming that the relaxation process for each carbon nucleus is composed of two components. In Fig.

798

Y O S H I O

I N O U E

3000 2500 2000 1500 1000 800

500 300 200

.~_._~

100 50

30

V < " ~ ' ~ ' " - " ~ " ~ " '""

J~ - ~ "

,

20

SSB ~ --,

II--I--I

i

I

~ I

200

" -- ,

""'"~

SSB CHz(P-~ ) ICI-h(B)lICI42(P-#)

r/ms --

"

)

]

i

150

' ....

,"

I ,

i

l

I00

~

~ --

I

I ~

50

"i I

~

~

~

I

' l

,I

I ,

I [

1--

0

6 in ppm

Fig. 21.10. 6 7 . 9 M H z lSc C P I M A S N M R spectra of P(3HB-co-37%-3HP) measured by Torchia's pulse sequence (t: pulse delay time). (Reproduced from Ref. [67] with permission.)

21.11(a-d), the semiexponential plots at the side of a longer delay time show linear decays, so that the T~ values of the slow components are determined from the slope of these parts. The T~ values of the fast components are determined from the residual intensities obtained by substracting the estimated intensities of the slow components from the original intensities. The plots of the residual intensities also become linear as shown in Fig. 21.12, supporting the validity of a two-component analysis. The T~ values of P(3HB-co-3HP)s are listed in Table 21.4, in which those for P(3HB) and the chemically synthesized PPL are also included. The methyl carbon of the 3HB unit shows a shorter Ta value, reflecting its faster internal

799


o....o

(a)

"0""0.

:5

(b)

..

~

.G

"-..

.__--

_

ttl IlJ

.~

........O........ CH

0

10

20

30 0

10

20

r~ s

3( r~ s

( 5 %o...o. d

(c) J lO

._c >,

........o ........ CI-13 (d)

........O...

slow)

IZ

2 ._c ---O~ 0 )

CH2(slow)

[

I

1

l

10

20

30 0 r/s

9

.,

(e)

5 d .c_

---o---

20

10

r/s

30

0

----o~

CH=(c~)

Cm(~3)

I1/

.r

!

1

0

10

r/s

20 0

10

r/s

20

Fig. 21.11. Semiexponential plots of the signal intensities for each carbon: (a) carbonyl; (b)

methine; (c) methylene; (d) methyl in the 3HB units; (e) c~-methylene; and (f)/3-methylene in the 3HP units of a P(3HB-co-37%-3HP) sample. (Reproduced from Ref. [67] with permission.)

rotation. All of the 3HB units in P(3HB) and P(3HB-co-3HP)s show the two fast- and slow-relaxing components. PPL also shows two components. The 13C NMR T1 values (4 and 2 s) observed for carbons of the 3HP units in P(3HB-co-37% 3HP) are close to corresponding to those of the fast-relaxing component of the 3HB methylene carbon (5 s) in the same copolymer as well as in PPL. Thus, the T1 data indicate again that almost all of the 3HP units in P(3HB-co-37%3HP) exist only in the amorphous region. The T~ value of the fast-relaxing component of each carbon in the 3HB

800

YOSHIO INOUE

(b)

(a) C=0(fast)

5 O

._c

:~

r =0.990

0J

_

o

5 d .c

lO

20

o

lO

20

r/s

L

r/s

' (c)

CI-h(Fast) )w

r :

Ct-I3(fast)

0.904

r =0.974

I/I C

.c_

l

0

I0

0

20 r/s

10

20

r/s

Fig. 21.12. Semiexponential plots of the signal intensities of the fast components for each carbon: (a) carbonyl; (b) methine; (c) methylene; and (d) methyl in the 3HB units of a P(3HBco-37%-3HP) sample (r: correlation coefficient for a linear plot). (Reproduced from Ref. [67] with permission.)

Table 21.4. 13C spin-lattice relaxation times (qS~ in s) of PHB, PPL and P(3HB-co-3HP) on the assumption of a biexponential decay obtained by 67.9 MHz 13C high-resolution solid-state N M R measurements Sample

C--O

CH(B)

CH2(B)

CH3(B)

CH2(P-ce)

CH2(P-/3)

PHB

fast slow

10 170

15 80

12 180

4 22

-

-

P(3HB-co13%3HP)

fast slow

8 150

11 70

10 170

2 17

n.d. n.d.

n.d. n.d.

P(3HB-co27%3HP)

fast slow

5 160

5 70

9 160

2 17

n.d. n.d.

n.d. n.d.

P(3HB-co37%3HP)

fast slow

4 120

5 70

5 120

2 12

4 N.D.

2 N.D.

PPL

fast slow

N.D. 130

-

-

-

7 112

4 83

n.d. indicates that the intensities of resonances were too weak to be detected. N.D. indicates that the slow component did not exist. (Reproduced from Ref. [67] with permission).


801

unit is found to decrease as the 3HP content increases, indicating that the mobility of the 3HB unit in the amorphous region increases with the content of the 3HP unit. The less bulky 3HP unit may make the copolymer segment more flexible in the amorphous region. All of experimental results shown above indicate that a cocrystallization of 3HB and 3HP units in the same crystal lattice does not occur in the P(3HBco-3HP) samples containing up to 37 mol% 3HP. A possible explanation of the fact that a cocrystallization is obstructed in this copolymer system may be the chain flexibility of the segments included the 3HP comonomer units. This allows the 3HP-containing segments to take several conformations, other than the 21-helix, found in the 3HB crystalline lattice. In fact, it has been reported for PPL that there are several crystalline conformations, including helix and planar zigzag conformations [69-71]. Among them, the paracrystalline state (planar zigzag) of PPL seemed to be very stable [71]. The imbalance of the solid-state morphology composed of 3HB and 3HP units, which has the tendency to form different crystal forms, i.e., 21-helix and planar zigzag, respectively, may be one of the possible reasons that cocrystallization does not occur.

21.2.3 Molecular dynamics in the solid state of poly(hydroxyalkanoic acid)s studies by NMR Segmental mobility of polymer chains is one of the important properties of polymer materials. It has intimate relations to several physical properties of polymer materials. NMR relaxation measurements are wellknown as powerful techniques for elucidating the segmental motions of polymer materials not only in solution but also in the solid state. Segmental motions of P(3HB) and P(3HB-co-27%3HV) in chloroform-d solution have been studied by measuring 13C NMR relaxation times and NOE factors as a function of temperature [72, 73]. Analysis of the relaxation data on the basis of the Dejean-Laupretre-Monnerie (DLM) model, which describes the dynamics of polymer chains [74], indicates that the local dynamics of a comonomer unit, e.g., 3HB, are independent of the presence of a nearby 3HV unit and vice versa that segmental motion of the P(3HBco-27% 3HV) copolymer described by cooperative conformational transitions [73] is similar to that for the P(3HB) homopolymer [72]. These motional characteristics of the P(3HB-co-3HV) copolymer chain are consistent with the conformational characteristics derived by the analysis of 1H spin coupling as shown in Section 21.2.2.3 [63] and are consistent with the occurence of cocrystallization in this copolymer system. 13C NMR relaxation is a tool for the elucidation of the rates and mechan-

802

YOSHIO INOUE

isms of backbone segmental motion and side-chain internal rotations in solution, it is also useful for the study of the amorphous bulk state of polymers, because polymer chains in the amorphous phase are expected to exhibit a liquid-like rapid molecular motion on the NMR timescale at temperatures well above the glass-transition temperature. As already mentioned in Section 21.2.2.1, the poly(hydroxyalkanoic acid)s in vivo, which exist in the mobile state, show liquid-like well-resolved a3C NMR spectra [30]. The differences in solid-state 13C spin-lattice relaxation times, reflecting differences in the segmental motions, have been used widely to discriminate between the carbon nuclei in the crystalline phase from those in the amorphous phase, as already exemplified for P(3HB) and P(3HB-co-3HV) [53] (in Section 21.2.2.2) and for P(3HB-co-3HP) [67] (in Section 21.2.2.5). The compositional dependence of the segmental motions of P(3HB-co4HV) has been found to be different from that of P(3HB-co-3HV) [75], reflecting the fact that cocrystallization occurs in the latter (see Sections 21.2.2.2 and 21.2.2.3) but not in the former (Section 21.2.2.4). The 67.8 Hz CP/MAS 13C NMR spectra of P(3HB-co-4HB) with the 4HB unit contents of 11, 33 and 49 mol% in the powder form have been observed with a contact time of 10 ms [75]. From the analysis of NMR and DSC, the crystallinity of the P(3HB-co-4HB) samples is found to decrease with an increasing fraction of the 4HB units. In the CP/MAS 13C NMR spectrum of a P(3HB-col l%4HB) samples, the resonances appear at almost identical chemical shifts with those in the P(3HB) homopolymer, and the 4HB resonances are hardly detectable, indicating that the CP is ineffective for the carbon nuclei of the 4HB units in this copolymer due to rapid molecular motion with a frequency comparable to that of CP [76, 77]. In contrast, in the spectra of the P(3HBco-33% and 49To4HB) samples, the 4HB resonances with highly sharp lines appear at chemical shifts, which are approximately consistent with those found in their solution 13C NMR spectra, suggesting that the segmental motion of the 4HB units becomes too rapid to induce motional narrowing of the linewidth [76]. These results indicate that the 4HB units of the P(3HBco-4HB) samples investigated here exist in the highly mobile amorphous phase. The segmental dynamics in the amorphous phase of bulk samples of bacterially synthesized P(4HB) homopolymer and P(3HB-co-18% and 69%4HB) random copolymers have been studied by measuring the 13C NMR spinlattice relaxation times (T1) and NOE factors at two frequencies, 75.4 and 125.7 MHz, as a function of temperature from 25 to 125~ [78, 79]. The glass transition temperature (Tg) and melting temperature (Tm) of these samples measured by DSC are P(4HB) (Tg = -48~ Tm=54~ P(3HB-co-


803

69%4HB) (Tg = -36~ Tm = 50~ and P(3HB-co-18%4HB) (Tg = -4~ Tm = 165~ The degree of crystallinity of P(4HB) and P(3HB-co-18%4HB) measured by DSC are 25 _ 5 and 30 +_ 5%, respectively. The temperature at which the NMR spectra were measured is well above Tg. Samples for NMR relaxation measurements were prepared by placing finely ground pieces of the polymer in NMR tubes, heating above the melting point until a homogeneous melt phase was produced, and holding them at room temperature for several days prior to conducting any experiments, 13C T1 relaxation times and NOE factors were measured by the standard inversion-recovery and the gated decoupling method, respectively. The unimodal dynamic models, such as the Cole-Cole distribution model [80], the Jones-Stockmayer model [81] and the Hall-Weber-Helfand model [82], which attribute relaxation to a single motional mode and were originally derived for the study of polymer dynamics in solution, have been found to be incapable of reproducing all of the experimental 13C relaxation data of the amorphous phases of P(4HB) and P(3HB-co-4HB)s [79]. But, the 13C T1 and NOE data of these samples measured as functions of temperature and magnetic field have been successfully interpreted by the Dejean-LaupretreMonnerie (DLM) model [74, 83]. The DLM model is derived from the original Hall-Weber-Helfand model by including fast librational motions of the backbone C ~ H vectors in addition to the backbone segmental motion. The mobility of both the 3HB and 4HB units was found to increase with increasing the content of flexible 4HB units in the copolymer [79]. This result is consistent with the increasing elastomeric behavior reported for the P(3HBco-4HB) copolymers with a high 4HB unit content [8, 84, 85]. Furthermore, the segmental motion of the 4HB unit in the amorphous region of both copolymers was found to be 2-4 times faster than that of the 3HB unit [79]. These results indicate that the segmental motions which affect the 13C relaxation processes in bulk polymers have local character. The activation energies for the backbone motion of P(4HB) and the 3HB and 4HB units in the P(3HB-co-4HB)s, derived from the DLM model analysis, are found to be similar and in the range 42-47 kJ/mol [79]. This range is typical of amorphous polymers at temperatures above Tg, but they are greater than typical ones for polymers in solution, possibly due to the increased apparent viscosity exerted by the amorphous matrix on the moving backbone segment [79]. The activation energy observed for the backbone motion of P(3HB) in chloroform solution is 17 kJ/mol [72]. As the segmental dynamics probed by a3C relaxation measurements of P(4HB) and the 3HB and 4HB units in two P(3HB-co-4HB) samples followed the Williams-Landel-Ferry empirical equation [86], they are considered to be involved in the glass-transition phenomena and so the decrease in Tg values

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on going from P(4HB) to P(3HB-co-69%4HB) and P(3HB-co-18%4HB) is considered to be in agreement with the slowing down of the segmental motion as described by the DLM model. The linewidths of the high resolution scalar decoupled a3C and 1H NMR have been studied for semicrystalline P(4HB) and P(3HB-co-18%4HB) in the amorphous phase and in the melt as functions of temperature and magnetic field strength [87]. There are two classes of origins, which exert linebroadening of the NMR spectra of amorphous or semicrystalline polymers, i.e., broadening due to a distribution of chemical shifts and the second one is that related to relaxation processes. The latter includes the natural linewidth. From measurements of the 13C spin-spin relaxation times by the standard Carr-Purcell-Meiboom-Gill pulse sequence under the same experimental conditions, the natural linewidth is found to be a minor contributor to the line-broadening observed in the 13C NMR spectra of the solid P(4HB) and P(3HB-co-4HB). Possible contributions from various line-broadening mechanisms have also been examined by using a variety of coherent averaging solid-state NMR methods [87]. The crystalline phase was found to play a crucial role in modulating the ~3C linewidths of the amorphous regions of P(4HB) and P(3HB-co-18%4HB). It was shown that magnetic susceptibility and chemical shift dispersion are the major causes for the line-broadening of the proton and carbon resonances of P(4HB) in the amorphous phase and the melt, respectively. For P(3HB-co-18%4HB) in the amorphous phase, incomplete motional narrowing due to a slow motional mode restricted in amplitude by presence of crystallites and/or chain constraints was found to be the major line-broadening factor. It is interesting to investigate effects of long side chains on the molecular dynamics and related physical properties of poly(hydroxyalkanoic acid)s. The poly(hydroxyalkanoic acid)s with longer side chains, such as poly(3hydroxyoctanoic acid) [P(3HO)], have quite different mechanical properties from P(3HB) and P(3HB-co-3HV), being thermal elastmers with low glass transition temperatures ranging from - 2 5 to -40~ [88] and a much lower crystallinity of about 25-33% [89]. P(3HO) produced by P. oleovorans from sodium octanoate as the sole carbon source is a copolyester composed of 85% octanoate monomer units (with n-pentyl side chains). The remaining monomer units are roughly equally distributed between valerate, caproate and decanoate units [88]. This sample has a Tu of -36~ and Tm of 61~ [88]. The chain dynamics of the amorphous part of this P(3HO) sample have been studied by measuring variable-temperature 13C NMR spin-lattice relaxation times T~ and the nuclear Overhauser enhancements at two magnetic fields [90, 91]. Well-resolved 13C spectra of


805

this polymer in the amorphous region were observed without CP and MAS even at room temperature, indicating that the chains in this region undergo rapid and nearly isotropic motion. The temperature dependence of the T~s of the side chain and backbone carbons reveals that these two moieties undergo different motions. The relaxation data of the backbone carbons have been analyzed by employing a number of motional models. As found for P(3HB-co-4HB), among the motional models, the DLM model was again found to be superior to the unimodal dynamic models, i.e., the relaxation data of the backbone carbons have been interpreted in terms of conformational transitions and librational motions of the backbone C ~ H vectors [91]. Temperature- and frequency-dependent TI and NOE data of the carbon nuclei in the n-pentyl side chain have been well described by internal rotations of limited amplitude superimposed on the backbone motion [91]. The temperature dependence of the linewidths of the protonated carbon resonances of P(3HO) has been discussed in terms of the semicrystalline nature of this polyester [91].

21.3 Polymer blends containing poly(3-hydroxyalkanoic acid)s studied by NMR 21.3.1 PHA containing polymer blends Blends of poly(3-hydroxyalkanoic acid)s (PHAs) with various natural and synthetic polymers have been reported as reviewed in Refs. [21,22]. By blending with synthetic polymers it is expected to control the biodegradability, to improve several properties, and to reduce the production cost of bacterially synthesized PHAs. The polymers investigated as the blending partners of PHAs include poly(ethylene oxide) [92, 93], poly(vinyl acetate) [94], poly(vinylidene fluoride) [95], ethylene propylene rubber [94, 96], poly(epichlorohydrin) [97, 98], poly(E-caprolactone) [99], aliphatic copolyesters of adipic acid/ethylene glycole/lactic acid [100] and of E-caprolactone/lactide [101], poly(vinylphenol) [102] and polymethacrylates [103]. The blends of bacterially synthesized polyesters with bacterially synthesized copolyesters, such as P(3HB) with P(3HB-co-3HV) [104-106] and P(3HV) with P(3HBco-3HV) [107], have been also studied. Some of these are biodegradable blends and the others are biodegradable/nonbiodegradable blends. PHA blends with natural biodegradable polymers such as starch [108], pullulan [109], dextran, amylose and alginate [110], have been reported. Furthermore, blends of bacterially synthesized fully-biodegradable P(3HB)

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YOSHIO INOUE

with chemically synthesized partially-biodegradable P(3HB) have also been reported [111-113].

21.3.2 NMR studies of PHA containing polymer blends The miscibility and molecular dynamics of several PHA containing blends have been studied by solid-state NMR spectroscopy. Some cellulose derivatives and P(3HB) and P(3HB-co-3HV) have been found to show good compatibility [114-116]. These are chemically modified natural and natural biodegradable polymer blend systems. Blends obtained by melts compounding P(3HB) with cellulose acetate butyrate (CAB, degrees of butyrate and acetate substitution are 2.50 and 0.18, respectively) have been found to be miscible over the whole composition range by DSC and dynamic mechanical spectroscopy [116]. Blends in the composition range 20-80 wt% of CAB (degrees of butyrate and acetate substitution are 1.6 and 1.0, respectively; Tg and T m measured by DSC are 129 and 169~ respectively) and P(3HB-co-10%3HV) (Tg and T m measured by DSC were - 2 and 166~ respectively), have been prepared by thermal compounding [117]. The blends containing 20-50% P(3HB-co10%3HV) are found to be amorphous, optically clear miscible blends, while those containing 60-80% P(3HB-co-10%3HV) are semicrystalline, partially miscible blends. High resolution 13C NMR spectra of CAB, P(3HB-co10%3HV) and 50%CAB/50% P(3HB-co-10%3HV) blend in the bulk melt have been observed at 100 MHz and at 165-185~ and 235~ (CAB only) by using a high-temperature solution-state probe. All of the P(3HB-co10%3HV) carbon resonances observed at 165~ are relatively narrow, indicating the presence of liquid-like motion, and their linewidths are largely unaffected as the temperature is increased to 185~ (temperature is limited to 185~ due to the thermal instability of this copolymer). In contrast, the CAB glucopyranosyl ring carbon resonances and butyryl and acetyl substituent carbon resonances are lost in the baseline at 185~ a characteristic for polymers with solid-like mobility. At 235~ the sugar ring carbon resonances are still spread in the baseline, but the resonances of the methyl and methylene carbons of the substituents are observed. In the spectrum of the CAB/P(3HB-co-10%3HV) 50/50 blend observed at 185~ the P(3HB-co10%3HV) carbon resonances show considerably broadened lines and the CAB butyryl methyl resonance with a more narrow line appears. The results indicate that P(3HB-co-10%3HV) has a decreased mobility, while the CAB side chain has increased mobility in the blend. Thus, even in a homogeneous melt, the blend components of CAB and P(3HB-co-3HV) can have much


807

different mobilities. This blend system has been characterized also by X-ray and mechanical analysis [117]. Poly(vinyl alcohol) (PVA) is a chemically synthesized well-known watersoluble polymer which is biodegradable [118]. In comparison with P(3HB), atactic PVA has excellent mechanical properties. Thus, by blending with PVA, it is expected to improve the mechanical properties of P(3HB). Both PVA and P(3HB) are semicrystalline polymers. So, PVA/P(3HB) blends are thought to be crystalline-crystalline polymer blend systems. It is not easy to analyze the miscibility of such blend systems, because in general miscibility is a property of the amorphous phase. Observations of glass-transition temperatures are usually employed to judge the miscibility of amorphous polymer blends. It is often difficult to judge from Tu data whether the blend system is miscible or immiscible, because the existence of the crystalline phase prevents the observation of Tu of blends with high crystallinity. The crystalline phase sometimes increases the Tg value. Measurements of NMR relaxation parameters can provide an alternative means to judge the miscibility of polymer blends on a molecular level. Crystallization and compatibility of the PVA/P(3HB) blend have been studied by high resolution solid-state 13C NMR spectroscopy, DSC, FTIR, and density measurements [119-121]. The polymer samples used were bacterial P(3HB) (Mw 360000), atactic PVA (degree of polymerization 2000; degree of saponification 99%; Tm 224.0~ triad tacticity mm - 22%, mr = 50%, rr = 28%) and syndiotactic PVA (degree of polymerization 1690; degree of saponification 99.9% ; Tm 246.1~ triad tacticity mm = 15%, mr 50%, r r - 35%). The PVA/P(3HB) blends were prepared by casting from solutions in hexafluoroisopropanol, which is a common solvent for PVA and P(3HB) [119]. The DSC thermogram of each blend sample shows three distinct endothermic peaks, representing the fusion of P(3HB) crystals, the fusion of the PVA crystal and the decomposition of P(3HB) chains in order of increasing temperature [119, 120]. Any peaks corresponding to glass transitions of both polymers are not observed, probably due to their high crystallinity [120]. Two blend systems, atactic (a)- and syndiotactic (s)PVA/P(3HB), show similar blend composition dependencies of melting temperatures, Tm, and heat of fusion. The Tm of the P(3HB) phase shows small decreases with increase of the PVA content, while the Tm of PVA phase remains constant in both blend systems [120]. Thus, it is likely that P(3HB) and PVA crystallize separately. The crystal structure of the PVA phase in the blends is considered to be the same as that before blending, while the lamellar thickness of the P(3HB) crystal is reduced by blending with PVA. The DSC peak representing the fusion of the P(3HB) crystal is not observed for the blends containing more than 70 wt% PVA.

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The heat of fusion of P(3HB) phase decreases with an increase of the PVA content in both of the blends, indicating the decrease of P(3HB) crystallinity. No P(3HB) crystals were formed in the blends containing more than 75 wt% a- and s-PVA. In the same composition range, the heat of fusion of the s-PVA phase also shows about 15% decreases, indicating the decrease of s-PVA crystallinity. These results indicate the partial compatibility of these blend systems [120]. The heat of fusion of the a-PVA phase could not be estimated precisely from the DSC thermograms because of overlapping of the peaks of the fusion of a-PVA and the decomposition of P(3HB). The DSC results suggest the presence of a specific PVA-P(3HB) intermolecular interaction, which reduces the degree of crystallinity of both components and leads to compatibility. A probable interaction expected for these blend systems is a hydrogen-bonding interaction between the P(3HB) carbonyl and the PVA hydroxyl groups. This possibility has been examined by observing the solid-state 13C NMR spectra of these blends [120]. Solidstate 13C NMR spectroscopy is a powerful tool for the investigation of hydrogen-bonding. In general, 13C nuclei participating in hydrogen bonds exhibit, more or less characteristic high frequency shifts [122]. In the high resolution X3C CP/MAS NMR spectra of isotactic-PVA, a-PVA and s-PVA, the fixation of the intramolecular hydrogen bonds in solids has been found to produce a large deshielding of the methine carbon resonance [123]. The miscibility of the semicrystalline PVA with the amorphous poly(N-vinyl-2-pyrrolidone) (PVPy) has been investigated by ~3C CP/MAS NMR and DSC [124]. The a3C CP/MAS NMR spectra show that the PVA-PVPy blends are miscible on a molecular level over the whole composition range, and that the intramolecular hydrogen bonds of PVA are broken and intermolecular hydrogen bonds between PVA and PVPy are formed in the blends. 13C CP/MAS NMR spectra, which emphasize the resonances from 13C nuclei in the crystalline phase, have been observed for a series of PVA/P(3HB) blends [120]. The chemical shift of the P(3HB) carbonyl carbon, which is expected to participate in the hydrogen-bonding interaction with a PVA hydroxyl group, is found to be independent of the composition of both the a- and s-PVA/P(3HB) blends, indicating that there are no detectable PVA-P(3HB) intermolecular hydrogen-bonding interactions in the crystalline phase. The relative intensities of the P(3HB) resonances are found to be smaller than those calculated from the P(3HB) content of the blend. This result means that the crystallinity of the P(3HB) phase decreases by blending with PVA. This is consistent with the observation of heat of fusion as shown above. Intermolecular interactions in the relatively mobile amorphous phase of PVA/P(3HB) blends have been investigated by solid 13C pulse saturation transfer (PST) MAS NMR spectroscopy [125]. Figure 21.13 shows the shift

809

BIODEGRADABLE POLYMERS 172

E

O. Q.

171 r--

(/3 O O

"~

O

170

clJ tO

169

0

'

~S

.

.

PVA

80

.

.

wt

%

.

90

100

Fig. 21.13. Composition dependence of the chemical shift for the carbonyl carbon of P(3HB) in the 13C-PST MAS NMR spectra of s-PVA/P(3HB) (O) and a-PVA/P(3HB) (O) blends. (Reproduced from Ref. [120] with permission.)

variation of the P(3HB) carbonyl resonances with blend composition [120]. A significant deshielding is observed for the blends containing higher than 80wt% PVA, indicating the formation of hydrogen bonds between the P(3HB) carbonyl and the PVA hydroxyl groups. This hydrogen-bonding interaction should cause compatibility between PVA and P(3HB) in the amorphous phase. The deshielding observed for the s-PVA/P(3HB) blend is slightly larger than that for the s-PVA/P(3HB) blend. This result seems to show that the capacity to make intermolecular hydrogen bonds differs with the tacticity of PVA. More hydrogen bonds are formed between s-PVA and P(3HB) than between a-PVA and P(3HB). It was reported for the triad tactic sequences of PVA that the oxygen atom bonded to the central methine carbon atom of the isotactic, or the heterotactic triad, can make two or one intramolecular hydrogen bond(s), respectively. However, no intramolecular hydrogen bond is expected for the syndiotactic triad [123]. If these observations are also valid for PVA in the blends with P(3HB), then s-PVA has a larger capacity than a-PVA to form intermolecular hydrogen bonds. The 1H relaxation time can provide information on the domain size in the solid polymer. If two component polymers, each of which has inherent 1H relaxation times in its pure state, in the binary blends are intimately mixed and the domain sizes of the two components are small enough for effective spin diffusion during 1H relaxation, both components exhibit the same ~H relaxation time. When the domain sizes are larger, that is, the components

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YOSHIO INOUE (a)

() .

5

0

0

0

"1"

0

00

3

1

0

100

5O wt %

a-PVA

(b)

CO

O

I-.

O

2

-

0

9

L

L

t

.

.

.

50 s-PVA

.

.

100 wt %

Fig. 21.14. Composition dependence of the proton T1 of P(3HB) (&) and PVA(O) in (a) aPVA/P(3HB) and (b) s-PVA/P(3HB) blends. (Reproduced from Ref. [120] with permission.)

exist in separated phases, each component shows a different relaxation time. A scale for effective spin diffusion during I H relaxation of a typical bulk polymer is 200-300 A (see Chapter 10). The domain size of PVA/P(3HB) blend systems was studied by observing the IH spin-lattice relaxation time, T I , by the CPMAS NMR technique [126]. Each of the pure polymers, P(3HB), a-PVA, and s-PVA, shows only one TI value, indicating that the crystalline and amorphous domains of these pure polymers are smaller than is the scale of effective spin diffusion. Figure 21.14 shows the variation of the TI values with the blend composition [120]. In the blends, the T1 values of PHB and PVA approach each other with increasing


811

PVA content, indicating that the compatibility between PVA and P(3HB) improves with increasing PVA content. In the s-PVA/P(3HB) blends containing more than 55 wt% s-PVA, and in the a-PVA/P(3HB) blends containing more than 80 wt% a-PVA, the T~ values of P(3HB) agree very closely with those of PVA. This result implies that the domain sizes of PVA and P(3HB) in these blends are less than 200-300 A. The s-PVA/P(3HB) blend system is compatible over a wider range of composition than is the a-PVA/P(3HB) blend system. This is consistent with the results obtained from the chemical shifts of the carbonyl carbon resonances. In short, the PVA/P(3HB) blend is compatible in the amorphous phase and the compatibility between them depends on the tacticity of PVA.

21.4

21.4.1

Other biodegradable polymers studied by solid-state NMR

Natural biodegradable polymers other than PHAs

There are several kinds of natural biodegradable polymers in addition to bacterial PHAs, such as proteins, nucleic acids and polysaccharides. Among them, particulary important polymers such as industrial materials are polysaccharides, such as starch, cellulose, chitin and chitosan. The solid-state structure and properties of starch and amylose [127], cellulose [128] and chitin [129], have been extensively studied by high resolution 13C NMR spectroscopy. The details of the NMR study of solid-state polysaccharides are described in Chapter 24 of this book. The molecular structure as well as the dynamics of natural polymers are important in understanding their properties. For intact plant materials such as lignin, cutin and suberin, the principal chemical constituent moieties have been identified and quantified by solid-state 13C NMR [130-133]. The dynamics of intact lime cuticle and its two major component polyesters, cutin and wax, have been studied by the MAS 13C NMR experiment [134]. By the measurements of 13C spin-lattice relaxation times and spinlattice relaxation times in the rotating frame which characterize respectively the megahertz- and kilohertz-regime motions, it is indicated that motional restrictions are present at the crosslinks of the cutin polymer and along the alkyl chains of the wax. The values of relaxation times, which differ for analogous carbon sites of cutin and wax individually, approach common values for the two materials in the intact lime cuticle. These results are considered to provide evidence for hydrophobic association within the plant cuticle of the long aliphatic chains of cutin and wax.

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YOSHIO INOUE

21.4.2 Chemicallysynthesized biodegradable polymers There are several types of biodegradable synthetic polymer with vulnerable chemical moieties susceptible to enzymatic attack. The most typical ones are aliphatic polyesters, such as poly(glycolic acid), poly(lactic acid), and poly(ecaprolactone). Highly isotactic-, syndiotactic- and atactic-homopolyesters of 3-hydroxybutyric acid [135] and its copolymer with valeric acid [136] have also been chemically synthesized and their solid-state structure and properties have been compared with those of their natural counterparts. Recently, aliphatic polyesters based on lactic acid and lactide have attracted much attention, because they can be formed from L-lactic acid produced from natural renewable sources and they decompose rapidly and completely under a typical compost condition. Solid-state 13C NMR spectra have been reported for CH3

CH3

I poly(lactide) poly(L-lactide) (PLLA) and poly(i>lactide) [137-139]. The crystallinity and morphology of highly crystalline, partially crystalline and amorphous PLLA have been studied by solid-state CP/MAS 13C NMR spectroscopy [140]. The amorphous domains show broad resonances with Gaussian lineshapes, while the crystalline domains show narrower resonances with a high degree of Lorentzian character. The splittings of the resonances indicate that there are at least five crystallographically inequivalent sites in the crystalline domains of highly crystalline PLLA samples. For the partially crystalline PLLA sample, spectral features of both amorphous and crystalline domains are observed. The short-range (nanometer) as well as the longrange order (micrometer) in PLLA appear to affect the CP/MAS 13C NMR spectrum. A simple method which can be used to determine the crystallinity and morphology of PLLA from their NMR spectra has been investigated [1401. Molecular dynamics of/3-propiolactone (PL) homopolymer and its block copolymer with/3-butyrolactone (BL) in the solid state have been investigated by broad-line and pulse NMR [141].


813

CH3

I --(--CH--CH2--CO--O--)x--(--CH2--CH2--CO--O--)y--

BL-PL block copolymer Proton spin-lattice relaxation times, T1 and Tip in the rotating flame have been measured in the temperature range from 113 K to the melting points (about 350 K), and have been interpreted in terms of molecular motion and phase structure of the polymers. Two regions with different mobilities in the amorphous phase of the 50/50 BL-PL block copolymer have been found. The temperature dependences of both of the relaxation times indicate the presence of molecular motions in the amorphous phase of the PL homopolymer (glass transition temperature Tg = 258.6 K) and BL-PL block copolymer (Tg = 263.8 K) far below their melting points. Rotation of the methyl groups in the block copolymer is observed, and spin-diffusion-limited relaxation is suggested. Poly(vinyl alcohol) (PVA) and poly(ethylene oxide), which are water soluble synthetic polymers, are also biodegradable. Solid-state CP/MAS 13C measurements have been performed for PVA samples with different tacticities in order to obtain information about the structure and hydrogen bonding in the crystalline and noncrystalline region [123,142-144]. The details of the NMR study of solid-state PVA are described in Chapter 19 of this book.

References

,

6. 7. 8. 9. 10. 11.

Y. Doi (Ed), Sei-Bunkaisei Kobunshi Zairyo (Biodegradable Polymeric Materials). Kogyo Chosakai Publishing Co., Tokyo, 1990. Y. Doi (Ed), Sei-Bunkaisei Plastic Handbook (Biodegradable Plastic Handbook). N.T.S. Co., Tokyo, 1995. T. Suzuki, Y. Ichihara, M. Yamada and K. Tonomura, Agric. Biol. Chem. 37 (1973) 747. M. Shimao, H. Saimoto, N. Kata and C. Sakazawa, Appl. Environ. Microbiol. 46 (1983) 605. B. Schink and M. Stieb, Appl. Environ. Microbiol. 45 (1983) 1905. N. Obradors and J. Aguilar, Appl. Environ. Microbiol. 57 (1991) 2383. P.A. Holmes, Phys. Technol. 16 (1985) 32. Y. Doi, Microbial Polyesters. VCH, Weinheim, 1990. H.M. Muller and D. Seebach, Angew. Chem. 105 (1993) 483. A. Steinbtichel and He.E. Valentin, FEMS Microbial. Lett. 128 (1995) 219. R.Y. Stanier, J.L. Ingraham, M.L. Wheelis and P.R. Painter, The Microbial World (5th edn). Prentice-Hall, New Jersey, 1986.

814

YOSHIO INOUE

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815

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B I O D E G R A D A B L E POLYMERS 121. 122. 123. 124. 125. 126. 127.

128.

129.

130. 131. 132. 133. 134. 135. 136. 137. 138. 139. 140. 141. 142. 143. 144.

817

T. Ikejima, N. Yoshie and Y. Inoue, Macromol. Chem. Phys. 197 (1996) 869. D.L. VanderHart, W.L. Earl and A.N. Garroway, J. Magn. Reson. 44 (1981) 361. T. Terao, S. Maeda and A. Saika, Macromolecules 16 (1983) 1535. H. Feng, Z. Feng and L. Shen, Polymer 34 (1993) 2516. T. Fujito, K. Deguchi, M. Ohuchi, M. Imanari and M. J. Albright, The 20th Meeting of NMR, Tokyo, 1981, p.68. M.J. Suilivan and G.Maciel, Anal. Chem. 54 (1982) 1615. For examples: (a) H. Saito and R. Tabeta, Chem. Lett. (1981) 713; (b) R.P. Veregin, C.A. Fyfe, R.H. Marchessault and M.G. Taylor, Macromolecules 19 (1986) 1030; (c) M.J. Gidley and S.M. Bociek, J. Am. Chem. Soc. 110 (1988) 3820. For examples: (a) R.L. Dudley, C.A. Fyfe, P.J. Stephenson, Y. Deslandes, G.K. Hamer and R.H. Marchessault, J. Am. Chem. Soc. 105 (1983) 2469; (b) D.L. VanderHart and R.H. Atalla, Macromolecules 17 (1984) 1465; (c) P.C. Belton, S.F. Tanner, N. Cartier and H. Chanzy, Macromolecules 22 (1989) 1615; (d) A. Isogai, M. Usuda, T. Kato, T. Uryu and R.H. Atalla, Macromolecules 22 (1989) 3168; (e) H. Yamamoto, F. Horii and H. Odani, Macromolecules 22 (1989) 4130. For examples: (a) H. Saito, R. Tabeta and S. Hirano, Chem. Lett. (1981) 1479; (b) H. Saito, R. Tabeta and K. Ogawa, Macromolecules 20 (1987) 2424; (c) B. Focher, A. Naggi, G. Torri, A. Cosani and M. Terbojevich, Carbohydr. Polym. 17 (1992) 97. G.E. Maciel, J.F. Haw, D.H. Smith, B.C. Gabrielson and G.R. Hatfield, J. Ag. Food Chem. 33 (1985) 185. N.G. Lewis, E. Yamamoto, J.B. Wooten, G. Just, H. Ohashi and G.H.N. Towers, Science 237 (1987) 1344. T. Zlotnik-Mazori and R.E. Stark, Macromolecules 21 (1988) 2412. J.R. Garbow, L.M. Ferrantello and R.E. Stark, Plant Physiol. 90 (1989) 783. J.R. Garbow and R.E. Stark, Macromolecules 23 (1990) 2814. P.J. Hocking and R.H. Marchessault, Macromolecules 28 (1995) 6401, and references cited therein. S. Bloembergen, D.A. Holden, T.L. Bluhm, G.K. Hamer and R.H. Marchessault, Macromolecules 22 (1989) 1663. H. Tsuji, F. Horii, M. Nakagawa, Y. Ikada, H. Odani and R. Kitamaru, Macromolecules 25 (1992) 4114. C. Howe, S. Sankar and A.E. Tonelli, Polymer 34 (1993) 2674. C. Howe, N. Vasanthan, C. MacClamrock, S. Sankar, I.D. Shin, I.K. Simonsen, and A.E. Tonelli, Macromolecules 27 (1994) 7433. K.A.M. Thakur, R.T. Kean, J.M. Zupfer, N.U. Buehler, M.A. Doscotch and E.J. Munson, Macromolecules 29 (1996) 8844. S. Glowinkowski, J. Kapturczak, Z. Pajak, P. Kurcok, M. Kowalczuk and Z. Jedlinski, Polymer 30 (1989) 519. H. Ketels, J. de Haan, A. Aerdts, and G. van der Velden, Polymer 31 (1990) 1419. F. Horii, S. Hu, T. Ito, H. Odani, R. Kitamaru, S. Matsuzawa and K. Yamamura, Polymer 33 (1992) 2299. S. Hu, M. Tsuji and F.Horii, Polymer 35 (1994) 2516.


Chapter 22


Polypeptides Isao Ando ~, Tsunenori Kameda ~, and Naoki Asakawa 2 1Department of Polymer Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo, Japan; 2Department of Biomolecular Engineering, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama, Japan

22.1

Introduction

Synthetic polypeptides consist of a repeating sequence of certain amino acids and their primary structures are not as complicated as those in proteins. The polypeptides are very important polymers in both polymer and protein science. The characteristic properties related to the structure lead to possible expansion for research in the field of polymer science, to provide very different moplecules from conventional synthetic polymers. For example, the concept of the liquid crystal is expanded by revealing the variety of structures and properties of liquid crystals. Furthermore, the polypeptides are sometimes used as biomimic materials. On the other hand, synthetic polypeptides are sometimes used as model biomolecules for proteins because they take the c~-helix,/3-sheet, w-helix structure, and so on, under appropriate conditions. From such situations, it can be said that synthetic polypeptides are "interdisplinary" macromolecules and are very important for research work in both polymer and protein science.

22.2

Conformations and 13C NMR chemical shifts

As is well known, most of the peptides, polypeptides and proteins considered here consist of repeating sequences of peptide bonds with 20 different types of substitutents at the C~ carbon. The limited conformations such as c~-, oJ-,/3-sheet, etc., are taken by a possible set of dihedral angles (oh, g') around the N ~ C ~ and C ~ C ( - - - O ) bonds. In the solution state, the NMR chemical shift of these biomolecules, with possible rotation around the bonds, becomes the averaged value because of rapid rotation about the peptide bonds on the NMR timescale. In the solid state, however, the chemical shift is characteristic of specific conformations because internal rotation around the peptide bonds is fixed. This shows that the NMR chemical shift can be used for elucidating the conformation of polypeptides and proteins in the solid state. It has

820

ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA 1801

I-~fl-sheet-~"l~

"~ 1 7 5 ~ ~ ~ - ~

a-heliXo

o ?h-o-

=O

~70 55 -O

.~ 50

a

,-

O

o---r

o

o-~ 0---o.

45 m 20 15 10

-O ~

~

) I'~'~J~l

5

J

10

,

'

i ~lj~l

I

50 100 DPn (DPn)

I

I j ~llll~}

I

500 1000 2800

Fig. 22.1. Plots of the 13C chemical shifts of (Ala)n against the number-average degree of

polymerization (DPn).

been experimentally and theoretically shown that the NMR chemical shift of polypeptides and proteins is a very important NMR parameter for determining the main-chain conformation. More recently, studies on the structural characterization of polypeptides and proteins by using such a methodology has been introduced [1]. As an example, it is shown that the 13C chemical shifts of the C~, Ct3 and amide ~ O carbons of poly(L-alanine)[(Ala)n], are related to particular conformations [2]. ~3C cross-polarization/magic-angle spinning (CP/MAS) NMR spectra of solid poly(L-alanine) shows that the C~, Ct3 and amide ~ O carbon signals are well resolved between the a-helix and/3-sheet forms. The 13C chemical shifts are plotted against the numberaverage degree of polymerization in Fig. 22.1 [2]. Clearly, 13C chemical shifts of (Ala)n (n > 16) are unchanged for the peptides of various molecular weights within experimental error and, thus, can serve to characterize the ahelix form. The chemical shifts of the C~ and carbonyl carbons of the ahelix are displaced significantly to high frequencies by 4.2 and 4.6 ppm, respectively, relative to those of the/3-sheet form, while the chemical shift of the Ct3 carbon of the a-helix is displaced to low frequencies by ---5 ppm with respective to that of the/3-sheet. For this reason, the value of the X3C chemical shift can be used to describe the local conformation. In addition, the 13C chemical shifts of randomly coiled (Ala)~ in trifluoroacetic acid solution have values between the a-helix and/3-sheet forms. The existence of such characteristic displacements of 13C chemical shifts

POLYPEPTIDES

821

Table 22.1. 13C chemical shifts of polypeptides characteristic of a-helix, /3-sheet and o~-helix

forms Sample a

Conformationb

(Gly*). (Gly*). (Gly*)5

/3

(Gly).

/3

(Gly).

31

(Ala, Gly*).

a

(Ala).

a

(Leu, Gly*),, (Leu).

a a

(Glu(OBzl), Gly*). (Glu(OBzl).

a a

(Asp(OBzl), Gly*),, (Asp(OBzl), Gly*),, (Asp(OBzl),,

a w a

31

al s

(Val, Gly*),, (Val).,

/3 a

/3

Carbonyl c

168.5 + 171.8 + 171.4 + 168.4 169.2 172.3 172.1 176.7 171.7 + 176.4 176.8 171.8 172.2 171.4 + 175.7 170.5 172.1 + 175.6 175.4 171.0 172.2 172.0 + 171.1 + 174.9 174.9 171.i 171.3 169.6 168.5 + 174.9 171.8 171.5

13C chemical shift

Ca

Ct3

43.2 43.2 41.7 43.2 44.3 43.2 42.0 52.2

14.6

52.4 52.8 48.2 49.3 55.4 55.7 50.5 56.5 56.4 56.8 51.2 51.1 53.2 50.8 53.4 53.6 50.9 50.5 49.2 58.0 65.5 58.4 58.2

14.9 15.5 19.9 20.3 39.0 39.5 43.3 25.3 25.6 25.9 29.0 29.7 33.8 33.2 33.8 34.2 33.8 32.9 35.1 32.0 28.7 32.4 32.4

Phenyl

Benzyl

127.5 --- 128.5

65.2 66.0

127.8 127.7 129.8 128.2 129.2 129.0 129.2

65.9 65.3 65.7 66.1 66.1 65.3 65.9

aSymbol * indicates carbonyul carbon enriched by 13C. ba-Helix is right-handed unless otherwise specified. CSymbol + indicates chemical shift of glycine rsidue.

is not limited to the Ala residue. Table 22.1 summarizes the 13C chemical shift values of various amino acid residues in the a-helix and/3-sheet forms [1]. It is seen here that the C~ and C - - O peaks of the a-helix form are all displaced to high frequencies with respect to those of the/3-sheet form, which is consistent with the data for (Ala),. Furthermore, it is significant to show

822

ISAO ANDO, TSUNENORI KAMEDA AND NAOKI ASAKAWA

Table 22.2. 13C chemical shifts of various conformations of poly(/3-benzyl L-aspartate) in the solid state (ppm from TMS; ---0.5ppm) Sample

Conformation Ca

Ct3

C~O (amide) C=O (ester)

Phenyl CH20

Polymer

ag-helix aL-helix to-helix /3-sheet

53.4 50.9 50.5 49.2

33.8 33.8 32.9 38.2; 35.1

174.9 171.1 171.3 169.6

167.0 169.0 167.8 169.5

129.8 129.2 129.0 129.2

/3-sheet /3-sheet

49.1 49.5

38.0 36.8

169.8 169.6

168.2 168.0

129.0 65.5 128.4 65.9

65.7 66.1 65.3 65.9

Oligomers DP,,5a DP,,10

aDP = degree of polymerization. the 13C chemical shift values for various conformations of poly(/3-benzyl Laspartate) (PBLA) such as the aR-, aL-, ~OL-helix and /3-sheet forms in Table 22.2, which are taken by appropriate treatments [3]. The absolute 13C chemical shifts of the C~ and C~ carbons are affected by the chemical structure of the individual amino acid residues, and can be used effectively for conformational studies on particular amino acid residues in polypeptides and proteins. On the other hand, the C ~ O chemical shifts do not seem to be affected by residue structure and can be used for diagnosing the main-chain conformation. 22.2.1

Origin of the conformation-dependent 13C NMR chemical shift

Such sizeable displacements of the 13C chemical shifts can be characterized by variations in the electronic states of the local conformation as defined by the dihedral angles (4), qJ) [lg, 2, 4]. The calculated contour map for the C~ carbons of an alanine dipeptide was made using the FPT I N D O method within the semiempirical MO framework as shown in Fig. 1.2 in Chapter 1

[3]. It should be noted that the negative sign of the shielding constant oindicates deshielding, and so shielding variations can be compared with the experimental chemical shift 6 where a positive sign denotes deshielding. Such a chemical shift map successfully predicts the ~3C chemical shifts and conformations of the L-alanine (L-Ala) residues in polypeptides and proteins. For example, the experimental ~3C chemical shift for the a-helix form appears at lower frequencies by ---5.5 ppm from that for the /3-sheet form. The calculated map reasonably predicts the experimental data. Most recently, the calculated I3C chemical shift maps of the C~ and Ct3 carbons of the L-AIa residue were obtained using the G I A O - C H F method with an ab initio 4 - 3 1 G basis set as shown in Fig. 1.3 in Chapter 1 [4], of

fl-sheet

cr-helix i

i

110 '

Hm

I

I

~b > 0 ~ and 0 ~ qJ > 180 ~ which includes/J-sheet structure, the combinations of allowed 0 values are selected from Table 23.2 The C ' ~ N bond orientation of the oriented B. mori silk fibroin fiber is also obtained from the 1 3 C ' - - 1 5 N dipolar splitting of [1-13C]Gly-[15N]Ala double labeled sample using solid-state 15N N M R [48]. The reasonable orientations are 39 or 141 ~ for the 0NC of Ala residue. These angles are in agreement with the angle, 0NC for Ala residue

PROTEINS

865

selected from Fig. 8.6, indicating a high accuracy in the selection process of the angle, 0 in Section 8. The combination of the bond orientations, 0NC, 0NH, 0CN, 0CO for Ala is shown in Fig. 8.7. The width of each line corresponds to the experimental error of -+5~ for each 0 value. There is only one overlapping area which satisfies all of the 0 constraints within experimental error as shown in Fig. 8.7 this is considered to be the/3-sheet structure. Using a least-squared fitting of 0 values, the best fitted torsion angles (4~, qJ) of the Ala residue for this region A are obtained as (-140, 142~ The Gly C a - h e l i x a ( i - 1)-Gly Cahelixa(i + 1) distance, which corresponds to a unit cell length along the c axis (fiber axis) is calculated to be 6.98 A by using the best fitted torsion angles (4), qJ) of Ala determined here, indicating a good agreement with the X-ray fiber diffraction data [30, 35]. A similar combination for the Gly site is obtained. Fujiwara et al. [69] applied solid-state NMR to a structural study on oriented [1-13C]AIa silk fibroin fiber from B. mori. They found that the Euler angles obtained from the simulated lineshapes of the Ala carbonyl group are slightly different from that of a typical anti parallel /3-sheet, [30] raising questions concerning the accuracy of the current models for silk II structure. However, our data are in agreement with the X-ray diffraction model within experimental error. As mentioned above, [15N]Ser, [15N]Tyr and [15N]Val B. mori silk fibroin were obtained by cultivation of the silk gland. The torsion angles of these residues have not been reported because these minor amino acid residues give basically no X-ray diffraction data. The solid-state NMR analysis described here is the only method for obtaining the torsion angles [64]. On the other hand, the R E D O R (Rotational Echo Double Resonance) technique for the detection of weak heteronuclear dipole interactions such as those due to the 13C and ~SN nuclei [70, 71], has been applied to the structure determination of a silk fibroin model compound [17]. In general, this does not require orientation of the samples in the analysis, but selective isotope labeling between specified nuclear pairs in the samples is required. It is possible to determine the dihedral angles using the atomic distances obtained from R E D O R experiments. 23.2.4

Angular dependent solid-state 2H NMR

In order to use solid-state 2H NMR for atomic coordinate determinations, the angle of the C2H bond vector relative to the fiber axis was determined for [2,2-2H2]Gly and [3,3,3-2H3]Ala labeled silk fibroin fibers from B. mori with 2H quadrupole echo NMR spectroscopy [65]. This structural information

866

TETSUO ASAKURA ET AL.

160

"

0

KHz

'

" -100

'

Fig. 23.9. (A) 2H quadrupole echo spectrum of (A) an oriented block and (B) unaligned silk fibroin fiber of [2,2-2Hz]Gly-labeled samples.

is used complementarily for the determination of the backbone chain conformation of the Gly and Ala residues obtained from solid-state 15N and 13C NMR studies. The 2H labeled silk fibroin samples are prepared by the oral administration of isotope labeled amino acids or 2H20 to fifth instar larvae as described above. Figure 23.9(A) shows the 2H quadrupole echo spectrum of an oriented block of [2,2-2H2]Gly labeled silk fibroin fibers when the fiber axis was set parallel to the magnetic field direction. The 2H quadrupole echo spectrum, Fig. 23.9(B), of unaligned [2,2-2H2]Gly labeled silk fibroin fiber is also observed as a powder pattern. Both spectra are split into doublets, which give the value of the quadrupole splitting, Avo as 117.8 kHz for the [2,2-2H2]Gly site; the values are the same in each case. The full rigid-lattice width of about 126 kHz should be observed when the motion is frozen [72]. Thus, it is concluded that the motion of the methylene groups of the Gly residue is almost frozen at room temperature, which is in agreement with the prediction from the intermolecular hydrogen bonding network in the silk fibroin backbone chain with an antiparallel/3-sheet conformation. The quadrupole splitting, Avo of 117.8kHz observed for the oriented block of [2,2-2H2]Gly labeled silk fibroin fiber is the same as the value in the spectrum of an unaligned sample as shown in Fig. 23.9(B). Therefore, if the latter quadrupole splitting is used to provide a value of the proportionality constant, (3/4)e2qo/h, which describes the relationship between the quadrupole splitting and the angle 0CD (the angle of the C2H bond vector of Gly relative to the fiber axis), 0CD is calculated as 90~ By taking into account an

PROTEINS

867

experimental error of +_0.2 kHz in the determination of the constant because of a slightly broader peak than the uniaxially aligned spectrum, the 0 values are calculated as 90 _+ 2 ~ 2H solid-state N M R spectra of an ordered block of [2,2-2H2]Gly labeled silk fibroin fibers were observed as a function of the angle between the fiber axis and Bo in order to check the validity of the angle, 0 - 90 ~ obtained here. A series of observed spectra is shown in Fig. 23.10 along with lineshape simulations assuming 0 = 90 ~ The details of the method of simulation are described elsewhere by Ulrich et al. [72, 73]. The agreement between the observed and simulated spectra is good. The solid-state 2H N M R spectrum of an oriented block of the [3,3,32H3]Ala labeled silk fibroin fiber was also observed. The quadrupole splitting, Avo, is 39.8 kHz and is the same as the value in the spectrum of the unaligned sample. This is the same as the case of the [2,2-2H2]Gly labeled silk fibroin fiber. The smaller value of the quadrupole splitting for the Ala site indicates the presence of three-fold fast rotation about the Cc~-C/3 axis [50, 72]. This

Experimental

Simulated

cx=90 200

100

0

KHz

-100

-200

200

100

0

-100

-200

KHz

Fig. 23.10. Experimental and calculated 2H solid-state NMR spectra of an ordered block of

[2,2-2H2]Gly-labeled silk fibroin fibers as a function of the angle between the fiber axis and Bo.

868


Fig. 23.11. A restricted (4~, qJ) region for Gly in the Ramachandran map.

has also been supported by the observation of a spin-lattice relaxation time (T~) minimum at about - 7 0 to -80~ in the 90 MHz 1H pulsed NMR study of B. mori silk fibroin fiber [74]. For a rapidly spinning methyl group, the direction of the three individual bonds is time averaged, and the effective bond vector is that of the methyl rotor axis, that is, the C~C2H3bond axis relative to the fiber axis. The angle of the C c~-helixa~Cc~-helix/3 bond vector of Ala with respect to the fiber axis was calculated to be approximately 90 ~ if the quadrupole splitting observed for the unaligned [3,3,3-2H3]Ala labeled silk fibroin sample is used to calculate the constant (3/4)e2qo/h. A restricted (4~, 0) region for Gly in the Ramachandran map ( - 1 8 0 ~ 6 > 0~ 0 ~ 0 > 180~ was obtained as the overlap of each region obtained experimentally from the NH, NC, CN, and CO bond orientations as described above. The 2H quadrupole echo spectrum of the oriented [2,22H2]Gly labeled silk fibroin fiber yields an angle, 0 = 90 -- 2 ~ for the C2H2 bond vector of Gly residue relative to the fiber axis. A further narrow

PROTEINS

869

restricted (4~, ~) region for the Gly residue in the Ramachandran map is obtained as shown in Fig. 23.11. Here the width of each line denotes the experimental error of +5 ~ in the determination of the angle, q, for each bond vector. An overlapping region is obtained for the antiparallel/3-sheet region. Thus, the angle constraints from solid-state 2H NMR can be used effectively to narrow the allowed region obtained from previous solid-state 13C and lSN NMR studies. A similar result is obtained for the Ala residue.

23.3

Spider silk

Spider dragline silk is a remarkable biopolymer: its mechanical properties combine high tensile strength and high elasticity as well as B. mori silk [75]. The spider silk most investigated is the dragline forcibly silks from the neotropical golden silk spider Nephila clavipes. The molecular weight of dragline silk was determined to be of the order of 200-350 kDa [76]. The primary structure of the major ampullate gland protein (dragline) is still not known completely. The results of Lewis [77] suggest that N. clavipes dragline silk is composed from two different proteins designated as spidroin I and II. In an independent study Mello et al. [76] have confirmed the existence of spidroin I. The primary structure of spidroin I contains (Gly-Gly-X)m segments (where X = Gln, Ala, Tyr, Ser, or Leu and m = 3-6 if minor sequence errors are tolerated) and Alan segments (with n = 4-7). In a simplified view, silk may be looked at as a block copolymer with glycine-rich and alaninerich domains. Diffraction results [78, 79] clearly indicate that dragline silk is a heterogeneous material with "amorphous" and "crystalline" domains. It is generally accepted that the "crystalline" domains consist of protein segments that adopt an ordered /3-sheet conformation. Controversy has, however, arisen as to whether these /3-sheets are formed by the alanine-rich [77, 80] or the glycine-rich [79, 81] segments of the protein. In addition, the molecular origin of the exceptional mechanical properties of spider silk is unclear. The local structure of dragline silk from the spider Nephila madagascariensis has been investigated by two-dimensional spin-diffusion NMR [20]. In order to obtain 13C labeled samples, [1-~3C]Ala or [1-13C]Gly was used, and a resulting enrichment of 60 % was found in the silk. Two-dimensional spindiffusion NMR experiments show that the alanine-rich domains of the protein form/3-sheet structure in agreement with one-dimensional NMR results from a different species of the genus Nephila [80] but at variance with diffraction results. The microstructure of the glycine-rich domains was found to be ordered (Fig. 4.11). The simplest model that explains the experimental finding

870


is a 31-helical structure. Random coil, planar fi-sheets, and a-helical conformations were not found in significant amounts in the glycine-rich domains. On the other hand, solid-state 2H NMR data from unoriented, oriented, and supercontracted fibers, indicate that the crystalline fraction of dragline silk consists of two types of alanine-rich regions, one that is highly oriented and one that is poorly oriented and less densely packed. A new model for the molecular-level structure of individual silk molecules and their arrangement in the fibers has been proposed [19].

23.4

Keratin

The keratins are fibrous proteins derived from human hair [82], wool, feathers, nails, etc., and chemical analyses indicate that no particular amino acid predominates but that there is a high content of polar residues, Cys, and Pro [83]. Wool keratin consists of several kinds of proteins and can be separated into three main fractions after reducing the disulfide bonds and protection of the resulting thiol groups with iodoacetic acid to form S-carboxymethyl kerateine (SCMK) [84, 85]. On the other hand, from the viewpoint of morphology, the native wool fiber consists of intermediate filaments (termed "microfibrils") composed of low-sulfur proteins which are embedded in a nonfilamentous matrix. The nonfilamentous matrix usually contains two classes of proteins; one is a high-sulfur protein and the other is a protein containing Gly and Tyr residues [83]. High resolution solid-state 13C NMR spectra of the wool keratin proteins were systematically measured to clarified their conformational features in the solid state [86-90]. In this stage, four kinds of SCMK samples extracted from wool [low-sulfur fractions (SCMKA), helix-rich fragments prepared from SCMKA by partial hydrolysis with c~chymotrypsin (SCMKA-hf), high-sulfur fractions (SCMKB), and high-GlyTyr fractions (HGT)] were used. The content of helix-forming amino acid residues such as Asp, Glu, Ala, and Leu increase but the helix-breaking amino acid residues such as Thr, Ser, Pro, Gly, and Cys decrease in the order of wool, SCMKA, and SCMKA-hf. On the other hand, SCMKB contains Thr, Ser, Pro, and Cys as its major components, and HGT contains Set, Gly, Tyr, and Phe as its major components. From these findings, it has been suggested that the conformational features of wool and the SCMKs may be different from each other. Figure 23.12 shows the ~3C CP/MAS TOSS NMR spectra of wool and four kinds of SCMKs. In Fig. 23.12, the spectral patterns of each sample are different from each other due to their amino acid compositions and higher order structures. Since the ~3C NMR chemical shift value of the main chain carbonyl carbon

PROTEINS

871

rj

tj rj ,~

4)-linked 2-amino-2-deoxy-D-glucose residues, is formed on deacetylation of chitin. As pointed out already, this polysaccharide takes an extended conformation similar to that of cellulose. Deacetylation of chitin is very easily evaluated in view of the 13C NMR spectra, as illustrated in Fig. 24.5. The three polymorphs of chitosan, "tendon-chitosan" (from crab shell), "L-2" (from shrimp shell), "Annealed"(from crab shell chitosan annealed at 22~ in the presence of water) are easily distinguished, consistent with the data for the polymorphs as obtained by a powder X-ray diffraction data [38, 39]. The observed non-equivalence of two chitosan chains, as viewed from the splittings of the C-1 and C-

898

HAZIME SAITO, SATORU TUZI AND AKIRA NAITO u I

"i

A Shrimp shell chitosan ~

,

i i .

9

B Crab shell chilosan

(annealed at 220"C) t

[

i:i

~

i:

!t, . I

Fig. 24.5. (A-C) 13C NMR spectra of three preparations of chitosans; and (D) chitin [38].

4 signals in the first two polymorphs, turned out to be removed by dehydration of water molecules loosely bound, during either annealing or complex-formation with transition metal ions. Conformational changes accompanied with metal-binding were also conveniently examined by means of 13C NMR spectroscopy [38, 39].

24.2.2 Conformation and dynamics of the gel network The network structure of gels is generally highly heterogeneous from the structural and dynamic points of view. The existence of solid-like domains from the cross-linked region is characteristic of the formation of the gel network. Such a domain in polysaccharide gels is ascribed to formation of cross-links due to the physical association of chains adopting an ordered conformation. It is now obvious that the secondary structure of such ordered polysaccharide chains is readily determined on the basis of the conformation-

899

P O L Y S A C C H A R I D E S AND B I O L O G I C A L SYSTEMS ca

?

ss

o

ca

ii

i

!

t

D. Gel (CP-MAS) 9o

~ :

:

9 . :

' A

C. Gel (MAS)

!

J~ ;1 f V

!

1~o

Fig. 24.6.

13C

~6o

i

;il

'

5'0

'

NMR spectra of curdlan gel recorded by a variety of experimental conditions

[21].

dependent displacements of the ~3C chemical shifts with reference to the corresponding peaks of crystalline polymorphs as described above. Naturally, there remain, however, considerable proportions of the polymer chains undergoing isotropic reorientational motions which are characteristic of the liquid-like domain. In fact, rather sharp 13C NMR signals were clearly visible from an elastic gel of curdlan by a conventional high resolution NMR spectrometer or by broad band decoupling (Fig. 24.6B) and ascribed to the existence of such a liquid-like domain [15, 16, 21, 40, 41], whereas these signals were completely suppressed for brittle gels of branched (1 ~ 3)-/3-Dglucans [42]. This means that the network structures of gels consisting of (1 ~ 3)-/3-Dglucans are different from each other with respect to the linear and branched forms. The C-1 and C-3 13C chemical shifts from the liquid-like domain of

900

H A Z I M E S A I T 0 , SATORU TUZI AND A K I R A NAITO

curdlan are appreciably displaced from those of oligomers taking the random coil conformation [15, 40, 41]. It is also interesting that the 13C NMR peakpositions of the solid-like domain (Fig. 24.6D) as well as those of the liquidlike domain (Fig. 24.6B) are very close to those of the hydrated curdlan (Fig. 24.6A). The amount of the triple helix form is nominal, if any, as indicated by the arrow of Fig. 24.6D, as long as the heating temperature is kept below 80~ On the other hand, we found that the 13C NMR signals characteristic of the triple helix from are dominant in the CP-MAS NMR spectra of gels consisting of branched (1--~ 3)-/3-D-glucans including HA-/3-glucan, schizophyllan and lentinan, although they take the single chain form when they are lyophilized from DMSO solution, as illustrated in Figs. 24.7(C) and 7(A), respectively. Of course, these signals are not visible by using a conventional spectrometer. This means that gelation of the branched glucan proceeds from partial association of these triple helical chains [18, 19]. It was shown that amylose gel contains two kinds of 13C NMR signals: the B-type signals from motionally restricted regions as recorded by the CP-MAS NMR technique and the signals identical to those found in aqueous solution [43]. The latter signals could be ascribed to flexible molecular chains adopting "7 O

~.d ~.

C. Hydrate(lyophillzedsample I~

9

!

p B. Hydrate(lyophllizedsample / j

150

100 f,p,.

510

"

()

Fig. 24.7. ;3C NMR spectra of HA-/3-g]ucan at (A) anhydrous; and (B) and (C) hydrated state

[211.

901

POLYSACCHARIDES AND BIOLOGICAL SYSTEMS

Table 24.1. 13C spin-lattice relaxation times of starch gel (33%) by DD- and CP-MAS NMR method (s, from Ref. [19])

Liquid-like domain Solid-like domain

C-1

C-4

C-3

C-2

C-5

C-6

0.36 9.2

0.29 11.8

0.30 11.9

0.32 11.9

0.29 11.9

0.16 2.1

a random coil conformation in the liquid-like domain. On the other hand, the former peaks are ascribed to the solid-like domain of cross-links, either double helical junction zones [43] or aggregated species of single helical chains as discussed already [21]. It is found that two such domains in the gel network are very clearly distinguished when the 13C spin-lattice relaxation times were compared as in starch gel as summarized in Table 24.1. It is interesting to note that the 13C T1 values of the liquid-like domain are in the vicinity of the T1 minimum, ~c ~ 10-8 s, whereas those of the solid-like domain are on the lower temperature side of the T1 minimum, ~'c > 10 -8 s. Here, ~c denotes the correlation time of motional reorientation. Obviously, the solid-like domain of the starch gel arises from the crystalline portion as obtained from the solid state in view of their 13C T1 values as compared with those of crystalline samples. On the contrary, the T1 values from the liquidlike domain arose from flexible molecular chains undergoing isotropic motions even if their mobility is restricted to some extent due to the presence of the cross-linked region. It is also interesting to examine whether or not the single and triple helical chains of the solid-like domain in the gel network from (1 ~ 3)-/3-D-glucans are distinguishable by this approach. It turns out that this approach is unsuccessful probably because such comparison should be made by relaxation parameters sensitive to lower frequency motions rather than the T~ values which are sensitive to high frequency motions. For this reason, we measured the ~3C resolved 1H spin-lattice realxation times in the rotating frame (Tlo) and the cross polarization time (TcH) of linear and branched (1 ~ 3)-/3-D-glucans, as summarized in Table 24.2. It is interesting to note that the TCH and Tlo values of curdlan taking the single helix form are significantly longer than those of schizophyllan and HA-/3-glucan taking the triple helix form. This means that the single helical curdlan is able to afford low frequency motions of the order of 10 -5 s in the solid-like domain, although the triple helical glucans are not. The well-documented network model of agarose gel arises from the junction zones consisting of associated double helical chains [44]. Nevertheless, it is seen that intense ~3C N M R signals of agarose gel are clearly visible from the liquid-like domain (Fig. 24.8, top trace) either by conventional N M R spectrometer or DD-MAS experiment, in addition to the intense signals from

902

H A Z I M E SAITO, S A T O R U T U Z I A N D A K I R A N A I T O

Table 24.2. The observed TCH and Tlo of linear and branched (1 ~ 3)-/3-D-glucans (from Ref. [19]) C-1

Curdlan Schizophyllan HA-/3glucan

,

' t20

C-3

C-5

C-2

C-4

C-6

TCH Tip

TCH Tip

TCH Tip

TCH Tip

TCH Tip

TCH Tip

ixs

ms

Ixs

ms

Ixs

ms

Ixs

ms

Ixs

ms

Ixs

ms

128

17.9

138

16.6

137

17.0

145

19.2

110

14.0

82.2

22.9 10.0

' ' '

67.3

3.32

62.8

4.18

53.1

4.30

56.4

4.09

32.0

5.69 22.1

56.6

5.04

35.4

5.38

64.4

6.27

47.8

5.34

50.7

5.80 53.0

l It O '

' ' '

I'

,

tO0

'"''

I'

90

,

' ' '

I ' ' ' ' 1 '

O0

70

'''

I"''''

60

I

50

'

'

"'

7.28

Pl~m

Fig. 24.8. 13C NMR spectra of agarose gel. Liquid-like domain (top) and solid-like domain (bottom) (Ref. [19]).

the solid-like domain (Fig. 24.8 bottom) recorded by CP-MAS experiment [19]. Note that the intense signal at l l 0 p p m in DD-MAS spectrum arises from the materials used for the probe assemby. A significant spectral change at 77-78.5 ppm should be ascribed to the conformation-dependent displacements of the peaks of the C-3 and C-4 carbons for (1 ~ 3)- and (1 ~ 4)linked galactosyl residues, respectively. Therefore, it is expected that the peaks which exhibit the conformation-dependent change of agarose are the

P O L Y S A C C H A R I D E S AND B I O L O G I C A L SYSTEMS

903

C-3 peak of (1 ~ 3)-linked galactosyl and the C-4 peak of (1 --~ 4)-linked 3,6anhydro-a-L-galactosyl residues, respectively. Usov [45] demonstrated that these peaks resonate at 81.9 and 77.0ppm, respectively. Therefore, the above-mentioned spectral change as manifested from the two types of spectral data, DD- and CP-MAS experiments, is ascribed to the conformational change from random coil to an ordered conformation [19]. It is now obvious that the network model by Arnott et al. is too simplified to account for the presence of the liquid-like domain as manifested by the NMR experiment. So far, several workers [14, 46, 47] have questioned the validity of the doublehelical junction zones and proposed an alternative model of gel network containing extended single helices. We also examined the 13C NMR spectra of dried agarose film prepared from N,N-dimethylacetamide solution, followed by drying at 80~ under anhydrous condition and its hydrate [26]. We found that the 13C NMR spectrum thus obtained is identical to that obtained from agarose gel. This finding is consistent with the view that the network structure of agarose gel consists mainly of single helical chains. It is now obvious that 13C NMR is a unique technique for the analysis of both the conformation and dynamics of polysaccharides in solutions, solids and gels. Especially, it is very useful to distinguish between the liquid- and solid-like domains by use of DD- and CP-MAS techniques. In addition, a systematic study of a converison diagram among polymorphs by a series of physical treatments is especially useful in order to clarify whether the polysaccharide chain under consideration takes a single or multiple chain both in the solid and in the gel.

24.3

24.3.1

Structural and membrane proteins

Conformation-dependent 13C Shifts

It is well recognized that all of the I3C chemical shifts of amino acid residues which are more than two residues away from a chain end in peptides and proteins adopting unfolded conformations in solution are effectively independent of all neighbouring residues except for proline [49]. Therefore, it is expected that the 13C chemical shifts of the backbone C~ and C = O and sidechain Ct~ signals of peptides and proteins are significantly displaced (up to 8 ppm) depending on their local secondary structure as defined by a set of torsion angles in the peptide unit (4~, q~) (Fig. 24.9), irrespective of there being a variety of neighbouring amino acid residues [1, 2]. To demonstrate this view, we have recorded the I3C NMR signals of polypeptides in the solid state whose secondary structures such as a-helix, /3-sheet, 31-helix, etc. are

904

HAZIME SAITO, SATORU TUZI AND AKIRA NAITO

C

N/

Fig. 24.9. Definition of the torsion angle in the peptide unit.

known from X-ray diffraction or other spectroscopic techniques [50-54]. In particular, the two major conformations, a-helix and /3-sheet forms, are readily distinguished from the peak-position of the ~3C NMR signals" the C~ and C ~ O 13C NMR signals of the a-helix form are displaced to high frequency by an amount of 3-8 ppm with respect to those of the/3-sheet forms, whereas the Ct3 signals of the a-helix was displaced to low frequency with respect to those of the/3-sheet form, as summarized in Table 24.3. In addition, it has been demonstrated that seven conformations including the random coil form can be distinguished by the conformation-dependent displacements of peaks as manifested from those of Ala residues [1-4]. This means that the local conformation of particular amino acid residues from any polypeptides, structural, globular and membrane proteins is readily evaluated, in an empirical manner, by means of the conformation-dependent displacements of 13C chemical shifts of respective residues with reference to the data base so far accumulated from a number of polypeptides, because the transferability of these parameters from a simple model system to more complicated proteins proved to be excellent. For globular proteins in solution, specific displacements of 13C NMR peaks with respect to those of the random coil have been utilized as a convenient probe to lead to a sequential assignment of the secondary structures of proteins [54, 55], as inspired by the success of the compilation of the conformation-dependent 13C chemical shifts as mentioned above. The existence of the conformation-dependent ~3C chemical shifts was theoretically evaluated as a contour map of 13C chemical shift (nuclear magnetic shielding constant) for the Ala residue from N-acetyl-N'-methyl-alanine amide [56, 57] as a function of the above-mentioned torsion angles.

905


Table 24.3.

13C chemical shifts characteristic of the c~-helix and/3-sheet forms (ppm from TMS)

(Ref. [2]) Amino-acid residues in polypeptides Ala Leu GIu(OBzl) Asp(OBzl) Val Ile Lysc Lys(Z) Argc Phe Met Gly

C-a

C-B

c~-

~-

helix

sheet

52.4 52.3 55.7 55.8 56.4 56.8 53.4 65.5

48.2 48.7 50.5 51.2 51.2 51.1 49.2 58.4 58.2 57.8 51.4

6.2

53.2 52.2 43.2

8.1 5.0

63.9 57.4 57.6 57.1 61.3 57.2

Aa

C=O

o~-

/~-

helix

sheet

Aa

4.2 3.6 5.2 4.6 5.2 5.7 4.2 7.1

14.9 14.8 39.5 43.7b 25.6 25.9 33.8 28.7

-5.0 -5.2 -3.8 (4.1) -3.4 -3.8 -4.3 -3.7

6.1

34.8 29.9 29.3 28.9 35.0 30.2

19.9 20.0 43.3 39.6 29.0 29.7 38.1 32.4 32.4 39.4 28.5

-0.8

39.3 34.8

-4.3 -4.6

-4.6

o~-

fl-

helix

sheet

176.4 176.2 175.7 175.8 175.6 175.4 174.9 174.9

171.8 171.6 170.5 171.3 171.0 172.2 169.8 171.8 171.5 174.9 172.7 176.5 175.7 170.4 176.8 175.2 169.0 175.1 170.6 168.4 171.6~ 168.5

Aa

4.6 4.6 5.2 4.5 4.6 3.2 5.1 3.1 2.2 5.3 6.2 4.5 3.1

aDifference in the 13C chemical shifts of the a-helix form relative to those of the/3-sheet form. bMistyping or erroneous assignment. This assignment should be reversed. CData taken from neutral aqueous solution, dAveraged values from the data of polypeptides containing 13Clabelled glycine residues.

24.3.2

Structural proteins

It w o u l d s e e m r e a s o n a b l e to e x p e c t t h a t the d i s p l a c e m e n t s of the 13Cc h e m i c a l shifts could be u s e d as an intrinsic p r o b e of local e n v i r o n m e n t of a given a m i n o acid r e s i d u e , if the t r a n s f e r a b i l i t y of the c o n f o r m a t i o n - d e p e n d e n t shifts of p o l y p e p t i d e s to m o r e c o m p l i c a t e d p r o t e i n systems is g u a r a n t e e d . F i b r o u s p r o t e i n s such as silk fibroin, collagen a n d collagen-like p o l y p e p t i d e s can serve as ideal s y s t e m s to justify this view, b e c a u s e several crystalline p o l y m o r p h s are available d e p e n d i n g on a variety of physical t r e a t m e n t s a n d the spectral p a t t e r n is very simple as c o m p a r e d with those of g l o b u l a r p r o t e i n s b e c a u s e of the limited n u m b e r s of a m i n o acid residues involved. Crystalline silk fibroins are k n o w n to exist in o n e of the p o l y m o r p h s , e i t h e r silk I or silk II a n d e i t h e r the c~-helix o r / 3 - s h e e t f o r m s , d e p e n d i n g on the species of s i l k w o r m , Bombix mori or Philosophia cynthia ricini respectively.

906

HAZIME SAITO, SATORU TUZI AND AKIRA NAITO

It is straightforward to distinguish the two polymorphs o f silk fibroin from B. mori by 13C NMR spectra (Fig. 24.10) [58], because the variety of amino

acid residues is limited to the following four kinds: Gly (42.9%), Ala (30.0%), Set (12.2%) and Tyr (4.8%). It is noteworthy that the C~ signals of Ala and Ser residues of silk I (designated as I) are displaced to high frequency by about 2 ppm as compared with those of silk II (designated as II), whereas the Cl3 signals of silk I are displaced to low frequency by 3.4-4 ppm with respect to those of silk II. Consistent with expectation, it was found that the C~, C~ and C - - O chemical shifts of the silk I and II samples are the same as those of (Ala-Gly), II and I, respectively, within experimental error [59]. It appears, however, that the 13C NMR signal of the Gly C~ carbon is not sensitive to the present conformational change, although the 13C chemical shift of the C - - O group is very sensitive to this change. The major advantage of the present 13C NMR approach is to be able to estimate the relative proportion of material which is not readily converted to each other (20-30%) [60]. In addition, distinction of the a-helical and/3-sheet signals in the Ala residue of P.c. ricini fibroin and the hydration-induced conformational change from the less stable ce-helix to /3-sheet region are also very conveniently examined by the 13C NMR approach [59].

L U

-..,~..-j.~-m

i ili

O i.L.

8-J~" ~ | "7

A S,,.K,

"

! Ji

i !

It

.,

:i

1

:

;

:

~

i

!

!ill ~ iAl i

9

i :

i

__J ._,1

150

._.d.

,

100

....L_

50

i._

0

ppm

Fig. 24.10. 1 3 C NMR spectra of crystalline fraction of B. mori fibroin taking (A) silk I" and (B) silk II forms [58].

907

P O L Y S A C C H A R I D E S A N D B I O L O G I C A L SYSTEMS

o

'i! i!

_

!: u ou )'i i~

I i

___jil.

9

i i

!

i

200

I

150

i

100

: : :

'1

i ,

:.

"':i

"I

i.

; i: ol

ii'.

9

i

9,.

~:

I

50

~-'-

:

:i

i

:"

:

i :

"

:

!

PPM

Fig. 24.11. 13C N M R spectra of collagen from (A) bovine achilles tendon; and ( B - D ) of model polypeptides taking collagen-like triple helical structure. (B) (Pro-Ala-Gly)." (C) (Pro-ProGly)lo; (D) (Hyp). [61].

In a similar manner, most of the 13C NMR signals of the collagen fibril, arising from the major amino acid residues, which amount to approximately 65% (Gly, 33 +- 1.3%; Pro, 11.8 +- 0.9%; Ala, 10.8 +- 0.9%; Hyp, 9.1 ___1.3%), can be readily assigned, on the basis of the peak positions from model polypeptides as indicated at the top of the individual peaks (Fig. 24.11) [53, 61]. The assignment of peaks was made by referring to the 13C chemical shifts of appropriate model polypeptides, because individual triple-helical

908

HAZIME SAIT0, SATORU TUZI AND AKIRA NAITO

chains of collagen are composed of a repeating pattern of (Gly-X-Y)~, where X and Y are frequently occupied by prolyl, 4-hydroxylprolyl or alanyl residues. The similarity of the ~3C chemical shifts of the respective amino acid residues between the collagen fibril (Fig. 24.11A) and model triple-helical polypeptides (Fig. 24.11B-D) thus confirmed previous conclusions as to the tertiary structure of collagen analyzed by X-ray diffraction studies [53]. The peaks B and C were ascribed to the C~ and Cv.~ carbons of the remaining amino acid residues (ca. 35%) such as Ser, Glu, Leu, Arg, Lys, Val, etc. by spectral simulation [61], although the Ct3 signals might be suppressed due to the presence of low-frequency molecular motions with a timescale of 104105 Hz interfered by the proton decoupling frequency [62]. It appears that distinction of the collagen-like triple helix from the assembly of the single 31 helices is not always feasible by means of the ~3C NMR data alone, because of the similarity in the torsion angles, ( - 8 0 ~ 150 ~ and ( - 7 2 ~ 153 ~ for the 31 and collagen-like triple helix, respectively. This problem was readily solved by a ~SN NMR study, because the 15N chemical shift is very sensitive to the presence or absence of ( G l y ) N ~ H . . . O - - C inter-chain hydrogen bonds which are essential for the stabilization of the triple helical conformation [63]. In fact, it was found that the Gly N ~ H ~SN chemical shift of (Pro-Pro-Gly)~o as a model of the collagen triple helix is displaced to high frequency by 4.9 ppm as a result of the formation of the inter-chain hydrogen bond stabilizing the triple helix when this peptide is fully hydrated. This is not the case for collagen in which this particular hydrogen bond is retained even if it is obviously dried. Therefore, the 15N peak-positions of the collagen fibril and (Pro-Ala-Gly)~ are found to be very close to that of this hydrogen bonded species. It is interesting to note that the ~3C spin-lattice relaxation times (T~'s) of the collagen fibrils and model polypeptides are substantially different among the carbons of a variety of amino acid residues. It is noteworthy that the T1 values of the Ct3 and Cv carbons of both the Pro and Hyp residues in collagen fibril and (Pro-Pro-Gly)~o are substantially reduced as compared with those of a variety of crystalline oligopeptides [61]. Such a significant reduction was interpreted in terms of the presence of rapid puckering motions in the pyrrolidine rings of the Pro and Hyp residues in the solid state with a timescale of 10 -8 s. Further, it is also pointed out that the dynamic feature of the side chains in the Ser and Tyr residues in silk fibroin and related polypeptides are conveniently examined by a similar reduction of the spin-lattice relaxation times [64]. We have recorded the ~3C CP-MAS and DD-MAS NMR spectra of the dry and hydrated barley storage protein, C-hordein (a fibrous protein of approximate molecular weight of 40,000) and its synthetic model peptides, (Pro)2(Gln)6 and (Pro-Gln-Gln-Pro-Phe-Pro-Gln-Gln)3 under dry and hy-


909

drated conditions, with the expectation to be able to relate to its viscoelastic property [65]. The spectral features of C-hordein as well as these peptides appreciably differ from each other depending on the extent of hydration, reflecting different domains that adopt different types of conformations as well as dynamics. In particular, considerable proportions of the peak intensities are lost in the CP-MAS and well-resolved 13C NMR signals emerge in the DD-MAS NMR spectra owing to the acquisition of molecular motions by swelling.

24.3.3

Membrane proteins

Membrane proteins are integral parts of a membrane and have at least one segment of peptide chain traversing the lipid bilayer. They are not soluble in ordinary solvents owing to the presence of both hydrophilic and hydrophobic regions in the same molecule. Thus, solution NMR studies are very difficult because of the high molecular weight complex. Crystallization is extremely difficult as compared with soluble proteins: whole membrane lipids are first solubilized and replaced by appropriate detergent molecules prior to crystallization.

24.3.3.1 Three-dimensional crystal: cytochrome c oxidase Tuzi et al. have recorded the 13C CP-MAS NMR spectra of a three-dimensional crystal of bovine heart cyctochrome c oxidase [66] which is a membrane protein of 400 kDa containing 70 detergent molecules per protein. The observed 13C NMR signals give rise to a spectral resolution comparable to that of crystalline lysozyme. In contrast to fibrous proteins, it is difficult to assign signals to individual carbons of these membrane- and globular proteins unless otherwise specifically 13C-labeled proteins were used, due to severe signal overlaps from many amino acid residues. It is emphasized that the 13C NMR signals are not seriously overlapped with the detergent signals, because the observed peak intensity of the polar heads in detergent BL8SY, C H 3 ( C H z ) 1 1 ~ ( O C H z C H z ) s O H , is only about 10% of the anticipated values at 1 ms contact time, owing to the presence of rapid tumbling motions in the crystal as detected by the spin-lattice relaxation times. The molecular motions of the detergent molecules attached to the proteins were found to be highly heterogeneous. It is pointed out that the whole three-dimensional structure of this protein was recently revealed by an X-ray diffraction study at 2.8 resolution [67, 68].

910

H A Z I M E SAITO, SATORU TUZI AND A K I R A NAITO

24.3.3.2 Two-dimensional crystal: bacteriorhodopsin Bacteriorhodopsin (bR) is the only protein present in the purple membrane (PM) of Halobacterium salinarium which is active as the light-driven proton pump to translocate protons from the inside to the outside of the cell. In addition to studies on the mechanism of proton pump activity by 13C NMR, this protein can also serve as an ideal model system to gain insight into the general aspects on conformation and dynamics of membrane proteins, because bR in PM is organized as two-dimensional crystals and a large scale preparation of 13C-labeled bR is exceptionaly simple as compared with other membrane proteins. ~3C NMR signals of specifically 13C-labeled bR are clearly distinguished from those of the unlabeled preparation (Fig. 24.12) [69]. It is more preferrable to utilize the 13C-labeled Ala residue for the sake of conformational probe, because the lSc chemical shifts of Ala residues have been most througly examined for a variety of local conformations (Table 24.4). The 13C NMR spectra of bR have been recorded under several conditions: lyophilized preparation, lyophilized preparation followed by hydration, and hydrated pellets of PM [69-71]. It was found that dehydration of bR by lyophilization resulted in substantial conformational distortion of the protein backbone as manifested from the obvious line broadening of 13C NMR signals [69], although such a distortion was partially recovered by a subsequent hydration experiment [69, 71]. Therefore, it is strongly recommended to use hydrated pellets to avoid ambiguity arising from such a conformational distortion. It has been demonstrated that the spectral pattern of [3-~3C]Ala-bR is significantly different when obtained by CP-MAS and DD-MAS NMR (Fig. 24.13) [70, 71], because the local flexibility of the peptide backbone is substantially different among the transmembrane a-helix, loop and N- or C-terminus regions, as inferred from the primary structure of bR (Fig. 24.14). We have found that seven Ala residues located both at the N- and C-terminus residues are missing in the 13C CP-MAS NMR but they are fully recovered in the 13C DD-MAS NMR spectra [70, 71]. This is because these Ala residues are located at these terminal regions and undergo rapid isotropic tumbling motions with correlation times of 10-8s which average out dipolar interactions essential for cross polarization. This is consistent with the fact that the XSc spectral pattern recorded by CP-MAS NMR (22 Ala residues) was unchanged even if the C-terminus moiety containing six Ala residues are cleaved by papain [70, 71]. This is of course not true for the ~3C NMR spectra recorded by DD-MAS NMR spectra (29 Ala residues). Clearly, both the 13C CP-MAS and DD-MAS NMR spectra of [3-13C]Ala bR resonating at 14-18 ppm arise from at least seven resolved 13C NMR signals. The individual peaks are ascribed to the portions of the transmem-

POLYSACCHARIDES

AND BIOLOGICAL

911

SYSTEMS

Ala C[3 13CH~

A

i

~N/~-I~c /

I

*

II

H

i

o

9

'20o . . . .

,~o

. . . .

wl,

,'oo

. . . .

~'o

Val C=O

'

'

'

'

g

~(ppm)

CHa~ /CH3 CH2

I

B

C 13

ssb

I

II

H

O

ssb

'2~

. . . .

,'5~ . . . .

,'oo'

'

'

'

"o

.

.

.

.

g

~(ppm)

Fig. 24.12. 13C CP-MAS NMR spectra of [3-13C]Ala-bR and [1-13C]Val-bR. Peaks designated by the asterisk from 13C-labeled lipids [69].

brane ai-helix, aii-helix, loop, N- or C-terminus, with reference to the conformation-dependent 13C chemical shifts as summarized in Table 24.4. The reference data for the ai-helix (so called a-helix) and an-helix were taken from the 13C N M R spectra of (Ala), in the solid and hexafluoroisopropanol (HFIP) solution. The presence of the latter was previously proposed by Krimm and Dwivedi to explain anomalously higher amide I frequency of 1665 cm -1 in the IR spectra of bR and can be extended to the assignment of 13C N M R signals by adopting the same reference sample [70, 71]. As indicated by the top trace, these types of a-helices are well distinguishable. The

912

HAZIME SAITt3, SATORU TUZI AND AKIRA NAITO

Table 24.4. Conformation-dependent 13C chemical shifts of Ala-residues (ppm from TMS) a

C,, C~ C--O

al-helix (aR-helix)

aIi-helix

aL-helix

/3-sheet

Collagenlike triple helix

Silk I

52.4 14.9 176.4

53.2 15.8 178.4

49.1 14.9 172.9

48.2 19.9 171.8

48.3 17.6 173.1

48.3 16.6 177.1

Random coil b

16.9

all. Sait6 and I. Ando, Annu. Rep. NMR Spectrosc. 21 (1989) 209. bS. Tuzi, S. Yamaguchi, A. Naito, R. Needleman, J.K. Lanyi and H. Sait6, Biochemistry 35 (1996) 520.

DDMAS

Loop i~^,,i VV/,.,

'

'

'

'

helix

i

ppll

~o

'

'

'

'~

.

.

.

.

;o

et n -helix CPMAS Loop I ~ ' t~ 'J

.

'

'

.

~

.

I txI-helix

.

.

.

ppnl

.

.

'~

.

.

.

.

.

/o

"

Fig. 24.13. (A) 13C DD-MAS; and (B) CP-MAS NMR spectrum of [3-13C]AIa-bR [71].

913

POLYSACCHARIDES AND BIOLOGICAL SYSTEMS Ser Thr ( ~ ( ~ ( ~

Val S~ Asp G4y P,r9 Met

G~

~1~3 Lys Ser

Pro Glu PGlum(~)

Met

Arg Vail Pro Asp Glu Set Leul Asp 10r Gin Thr Phe; / ~ al Arg Leu _ Leu ThrGly Gly ~ Leu - - -~-'- - t.e~rI..... lie" . . . . . . . . . . l"he. . . . ~. ~ ....Tbr.., ;e ,- - lle-. . . . . . . 9 Phe ~Leu Phe! Phe Leu Asp Leu eA~ 11c Leu GlY220 ValLys Leu II Vd Phe Leul Leu Thr !!Val ~Gly TYrlS011e Leu AsnArg Gly(~( Leu ~u Pro Thr 10 i Asp Tyr Val V~ Gly Leu Thr Thr Vd ~ m 5Ol~ Phe lie et Vai 180 Leu Asp Leu I Met Trp lle GIY' 20 ~ Set Thr ~Asp TJ~/Thr Pa Trp J l LeuThr,...Vaj110~ ~ (~a~ Tyr Gly lie 14o~ P Met Leu Tyr Leu T~' . . . . . . . VaIL. . . . . . . . . .I'D. . . . . .V.,.-.m..yir . . . ,:w.,.. . . . 4.4u- - - ~ L..''| ,,. Ty~TM 0 "/~ly Trp Trp | |Glu l r V Leu Pra Gly Tyr Asn Lys Tyr Gly Val Set Ser Leu GlyLeu Gin 130 Tyr G|u Pro 200 Thr Glu Met Gly G~(~G ~ hV-' Val Gly Pro70Phe Asp

~ ~

Gly Asp G~y (~2Ser

LyS3o L~. ,Y~I..~.oLO.,i...L.Y'(~. TF

ThrL~ Leu

Gy

Met 20

w

Trp . . . . . . "Ire'" Trp 10 Glu Pro Arg

G~

Thr

129.8, 18.5, 59.5, 139.3sh 51.4 H 2 N

E

\CH2CH2CH2NH2 1'

2'

a

2

3'

/ CH2CH2PPh2 (S)Q C H 2 1'

2'

1

2

3'

/ CH2CHzPPh2 (S).

C 4'

H

2

C

H

2

N

H

C

H

2

C

51.4 H

2

41.5 N

~>

5'

6.8

28.4

-

14.5

28.4

28.4

131.4,

19.2 , 59 .8 51.6

O:Z 9

\CH2CH2CH2SH a' 2' 3'

a a3C chemical shifts in ppm relative to tetramethylsilane (TMS)" sh = shoulder" estimated errors in the values of 13C chemical shifts are _+0.5 ppm. 0 b Numbers in parentheses correspond to samples after treatment with HC1. :Z > N 0 .< 0 > Z rID tao

940

GARY E. MACIEL

A ~9

B

pH

3.5

C

pH

5.0

~

e-

s

[

9'

,,

I ' ' ' '

150

I ' ' ' '

100

I ' '

50

'

'

I ' ' ' '

0

,

I'

-50

'

'

'

I

-100

'

15N (PPM)

Fig. 25.12. 15N CP-MAS NMR spectra of the polysiloxane-immobilized monoamine system, (S)-CHzCH2CHzNH2, at various pH's. Ref. 17.

Figures 25.22A and 25.23A show 298i CP-MAS spectra of (S)CHzCHzCHzPPh2 and (S)-CHzCHzPPh2, respectively. Figure 25.24 shows 298i CP-MAS spectra of the polysiloxane-immobilized thiol-monoamine system (A), and of the thiol-diamJne system without treatment (B) and after treatment with HC1 (C). Figures 25.22B, 25.22C, and 25.22D show 298i CP-MAS spectra of the diphenylpropylphosphine-monoamine, diphenylpropylphosphine-diamine, and diphenylpropylphosphinethiol systems respectively. Figures 25.23B, 25.23C and 25.23D show 29Si CPMAS spectra of the diphenylethylphosphine-monoamine, diphenylethylphosphine-diamine and diphenylethylphosphine-thiol samples, respectively. For purposes of avoiding the quantitation problems associated with CPMAS spectra, for which intensities in the spectra of Figs. 25.18-25.24 have not been corrected on the basis of the relevant spin dynamics (not characterized for these samples), a few 29Si MAS spectra were obtained via direct polarization (DP-MAS), i.e., with no cross polarization involved, so that 298i magnetization is generated directly via 29Si spin-lattice relaxation.

NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES

"

L'I

-_

941

2

1 2 A

--....

1

2

,,,.,

o~ re-

A.3

~-_o.

,

2

3

F

9 I " 150

'

''""1

'" 100

'

'

I" 50

'

'

0 I ' 0

'

'

'

t . . . . -50

"1 ' -100

"-

5 I'SN (PPM)

Fig. 25.13. ~SN CP-MAS NMR spectra of: the untreated (A) and Cd2+-treated (B) polysiloxane-immobilized monoamine samples; the nonprotonated (C) and protonated (pH = 1) (D) polysiloxane-immobilized diamine system; the untreated polysiloxane-immobilized triamine system (E); the untreated thiol-monoamine system (F); and the untreated thiol-diamine system (G). Ref. 17.

942

GARY E. MACIEL

r

I

A ._.j

A i-

D .Q L.

r(1) r-

c

1

150

100

50 0 15N (PPM)

-50

-100

Fig. 25.14. Dipolar-dephasing 15N CP-MAS spectra of the protonated (pH = 1) polysiloxaneimmobilized monoamine system (A), protonated (pH = 1) polysiloxane-immobilized diamine system (B) and protonated (pH = 1) polysiloxane-immobilized triamine system (C). Dephasing period shown in Ixs. Ref. 17.


943

~

t'--

t_ t~ v ,,,.,. oN gO t-" (I) t'-,.i

c

o

16C).0

,It

i

80.0

i

0.0

,It

i

-80.0

~531p (PPM)

Fig. 25.15. 31p CP-MAS NMR spectra of polysiloxane-immobilized phosphine systems. (A) diphenylpropylphosphine, (S)-CH2CH2CH2PPh2; (B)diphenylpropylphosphine-monoamine; (C) diphenylethylphosphine, (S)-CH2CH2PPh2" and (D) diphenylethylphosphine-monoamine. Asterisks indicate spinning sidebands. Ref. 8.

Figure 25.25 shows 298i DP-MAS spectra of (S)-CH2CH2CH2NH2 and (S)CHzCHzCHzC1. These spectra were obtained with repetition delays of 300 s, which was shown by a rough evaluation of T si to be more than adequate for avoiding intensity distortions.

944

GARY E. MACIEL

~._~

~a'.o

81o

4:o

o'.o

1'

2'

3'

iCH2CH2CH2SH

-4'.o

-~.o

a 1H (PPM)

Fig. 25.16. 1H CRAMPS spectra. (A) Polysiloxane-immobilized thiol system" (B) polysiloxaneimmobilized thiol-monoamine system; (C) polysiloxane-immobilized thiol-monoamine system treated with 0.11 M HC1; (D) polysiloxane-immobilized thiol-diamine system; and (E) polysiloxane-immobilized thiol-diamine system treated with 0.11 M HC1. Ref. 16.

25.3.6

Relaxation measurements

~H spin-lattice relaxation times were measured on a few selected samples via 13C, 29Ni or 31p detection in 1H ~ X (X = 13C, 29Si or 31p) CP-MAS experi-


945

2 1

/~

/J -'_O

4

_

1

2

3

4

i-CH2CH2CH2NH2

A

1.r

A (/) r

::) ,.Q i,_

>.,

C

u~ t(9 ..,.,.

~_o

8:0

41o

o:o

-41o

-8'.0

a 1H (PPM)

Fig. 25.17. 1H CRAMPS NMR spectra of the untreated (A), protonated (pH = 1) (B), Cd 2+treated (C), and Hg2+-treated (D) polysiloxane-immobilizedmonoamine system; and untreated

3-chloropropylpolysiloxane (E). Ref. 17.

ments with 1H inversion-recovery, at various magnetic field strengths. The T~ results are summarized for various functionalized polysiloxanes of types (S)-Y and X-(S)-Y in Tables 25.4 and 5, respectively.

4~

Table 25.3. 1H Chemical shifts of some functionalized polysiloxanes a.

Sample

MeO, E t O

H2, H:~

H3, H;

/H + b

0.8 1.1

/Cd2+ b

0.9 0.9

1.7 2.0 1.9 1.9

2.7 3.1 2.9 3.0

I. 1

2.0

3.6

1.4, 3.6-4.1

>

1.1

1.9

2.7

1.4, 3.7, 3.9

,

3,

\CH2CH2i~H2SH /H + b 1

2

3

4

/ CH2CH2CH2NH2NHCH2CH2NH2 (S)\

1'

2'

1.1

1.8

1.2

1.8

5

7.8

2.7 3.1br

3'

\CHzCH2i~H2SH /H + b a Chemical shifts in ppm relative to tetramethylsilane -+0.1 ppm. br = broad. After treatment with HCl(aq), Cd2+(aq) or Hg2+(aq).

b

3.7br

8.1br


i

50.0

i

0.0

'

I

-50.0

i

-100.0

947

!

-150.0

8 29Si (PPM)

Fig. 25.18. 298i CP-MAS NMR spectra of (A) silica gel" (B) primary amine system, (S)CH2CH2CHzNH2; (C) silylated p+rimary amine system; and (D) trimethylpropylammonium chloride system, (S)-CHzCHzCHzN(CH3)3C1-.Ref. 15. 25.4

25.4.1

Discussion

13C NMR spectra

Functionalized polysiloxanes of type (S)-Y. 13C NMR spectra of the polysiloxane-immobilized amine ligands (Fig. 25.1) show the absence of residual ethoxy or methoxy signals, whereas 13C spectra of the polysiloxane-immobilized 3-chloropropyl and 3-propylthiol systems (Fig. 25.2) show strong signals from residual ethoxy (18 and 60ppm) and methoxy (51 ppm) groups. This shows the catalytic effect that amino groups appear to have in promoting the hydrolysis/condensation reactions responsible for the formation of the gel/polymer systems. The 13C NMR spectrum of the untreated polysiloxane-immobilized monoamine ligand (Fig. 25.1A) shows three signals, at 10.7, 27.4 and 44.9ppm, whereas the spectrum of the corresponding protonated system (washed with 0.1 M aqueous HC1) of Fig. 25.1B displays three signals, at 10.5, 21.3 and 43.1 ppm. By analogy with literature reports on the corresponding modified

948

GARY E. MACIEL

o..

e-

~ r e"

E

l-

1

o.oo

I

-so.oo

1

-loo.oo

i

.lso.oo

8 ~Si (PPM)

Fig. 2.5.19. 29Si CP-MAS NMR spectra of different preparations of: the polysiloxane-immobilized monoamine system (different ratios of reactants) (A and B)" the 3-chloropropyl system, prepared via catalysis by 0.1 M HC1 (C) and by (n-Bu)2Sn(O2CCH3)2 (D); the polysiloxaneimmobilized diamine system (E)" the polysiloxane-immobilized triamine system (F)" and the polysiloxane-immobilized monoamine system after treatment with 0.10 M HC1 for 24 hours (G). Ref. 17.


949

A

B --__L

U

1

50.0

0.0

i

1

-50.0 29Si

Fig.

25.20.

298i CP-MAS

NMR CHzCHz"CHzNHCHzCO2CH3; (B) 1 2 3 CH2CH2CH2SH" Ref. 15. 1

2

.,

4

5

6

-100 .0

--___,,.'.

[

- 150.0

(PPM)

spectra of polysiloxane systems. 1 2 3 4 5 6 (S)-CH2CH2CH2SCH2COzCH3 and

(A) (C)

(S)(S)-

silicas [37-41, 53], these signals are assigned to C(1), C(2) and C(3) carbon atoms, respectively. Carbon atom C(2) manifests a substantial increase in shielding from 27.4 ppm to 21.3 ppm upon protonation (Figs. 25.1A and 25.1B), whereas C(1) and C(3) display only minor shifts (1-2 ppm) on protonation. This behavior is in agreement with previous results reported in the literature on aminopropylsilane(APS)-modified silica [6, 30, 38]. A low-shielding chemical shift to about 27.4 ppm for C(2) was found for the polysiloxane-immobilized amine ligand upon washing the acid-treated material with aqueous 0.1 M NaOH. The 21ppm C(2) chemical shift in the spectrum of Fig~ 25_5D is consistent with what one might have expected for the the 3-(trimethylammonium) propylpolysiloxane system and the 27 ppm chemical shift is consistent with the C(2) value reported for an APS-modified silica in which most of the residual silanol groups were capped by a silylation reaction, eliminating them from participation in hydrogen bonding or proton transfer [38]. Caravajal et al. [6] have indicated, on the basis of 13C chemical shift arguments, that the amino groups of APS grafted onto uncapped silica are involved in hydrogen bonding and/or BrOnsted protonation by acidic silanols of the silica surface and the large increase in shielding of C(2) is due to protonation of the amino group. The 13C NMR spectrum of the untreated polysiloxane-immobilized di-

950

GARY E. MACIEL

D

E

I

50.0

. . . . . . . .

1.

0.0

.

.

.

I'

-

-50.0

1

-

-100.0

9

-

-150.0

--

(PPM)

29Si

Fig. 25.21. 295i CP-MAS NMR spectra of polysiloxane-immobilized systems. (A) (S)1

2

3

1

2

3

4

5

1

2

3

45

CH2CH2CH2CI" (B) (S)-CH2CH2CH2N(CH2CHs)2; (C) (S)-CH2CH2CH202CCH2NH2; (D) 1 2 3 45 6 7 1 2 3 45 (S)-CH2CHzCH202CCH2NHCH2CO~; and (E) (S)-CHzCH2CH2OzCCH3. Ref. 15.

amine system (Fig. 25.1C) shows four signals at 11.7, 24.0, 42.9 and 53.2 ppm. As shown in Fig. 25.1D, the spectrum of the protonated form of this diamine ligand displays four signals, at 10.5, 20.8, 39.2 and 51.5 ppm. These were readily assigned, as shown in Fig. 25.1 and Table 25.1, on the basis of spectral data taken from the literature [30, 38]. The ~3C spectrum of the polysiloxane-immobilized triamine system (Fig. 25.1E) shows four signals, at 12.0, 24.3, 43.0 and 51.9 ppm, a pattern similar to that of the diamine system. The signal at 51.9 ppm is very intense, because it involves the four carbon atoms, C(4), C(5), C(6), and C(7), as identified in Fig. 25.1. The signal at 43.0 ppm is weak and results from one type of carbon, C(3). This interpretation implies that the chemical form, ~S)CH2CH2CH2NHCH2CH2NHCH2CH2NH2, shown in Fig. 25.1 is in fact the

NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 951

A A

o ~

v

r _=

-

__

!

0.0

J

1

~

-50.0

I

-100.0 29Si

~

v

I

i~

-150.0

(PPM)

Fig. 25.22. 29Si CP-MAS spectra of polysiloxane-immobilized systems. (A) diphenylpropylphosphine; (B) diphenylpropylphosphine-monoamine" (C) diphenylpropylphosphine_diam_ ine; and (D) diphenylpropylphosphine-thiolsample. Ref. 18.

form that was obtained from the synthesis employed. Had the other potential reaction product, (S)-CH2CH2CH2N(CH2CH2NH2)2, been produced in the synthesis, then one would have expected a ~3C spectral pattern different from that observed for the diamine ligand, i.e., a pattern in which a signal due to carbon atoms attached to the tertiary amine nitrogen should appear at 5758 ppm [54]. The ~3C CP-MAS spectra of 3-chloropropylpolysiloxane samples prepared with HC1 or (n-Bu)2Sn(OCOCH3)2 as catalyst, shown in Figs. 25.2A and 25.2B, indicate that the proportion of the residual ~ O M e (51 ppm) and ~ O E t (18 and 60ppm) moieties on the 3-chloropropylpolysiloxane prepared by HC1 catalysis are higher than those on the sample prepared by

952

GARY E. MACIEL

A ~ r

E', .D

E

c

,

!

0.0

~

!,

,!

-50.0

!

-100.0

!

!

,

!

-150.0

5 29Si (PPM)

295iCP-MAS spectra of polysiloxane-immobilized ligand systems. (A) diphenylethylphosphine; (B) diphenylethylphosphine-monoamine" (C) diphenylethylphosphine-diamine; and (D) diphenylethylphosphine-thiol sample.Ref. 18. Fig. 25.23.

(n-Bu)2Sn(OCOCH3)2 catalysis. The 13C CP-MAS spectrum of (S)CH2CH2CH2SH, shown in Fig. 25.2C, displays two functionalized-polysiloxane signals, at 12.6 and 28.7 ppm, corresponding to C(1), and C(2), C(3) sites, respectively, in agreement with data on silica systems grafted with 3mercaptopropyltriethoxysilane [41]. The signals at 19.0, 60.8, and 51.9ppm are assigned to unhydrolyzed ethoxy (CH3 and CH2) and methoxy groups, respectively. The 13C NMR chemical shifts summarized in Table 25.1 show that the polysiloxane-immobilized monoamine systems prepared in this study display chemical shifts that are similar to those of the bulk self-polymerized 3aminopropyltriethoxysilane (APS) polysiloxane system (i.e., with no Si(OEt)4 employed) [30]; but the chemical shifts measured in this study are slightly different from those of APS-modified silica [30]. The apparent low-


!

50.00

I

I

0.00

t.

1.

-50.00

,

!

-100.00

I

!

953

I

-150.00

29Si (PPM)

Fig. 25.24. 295iCP-MAS spectra of polysiloxane-immobilized thiol-monoamine system (A) and of polysiloxane-immobilized thiol-diamine system without treatment (B) and treated with 0.11M HC1 (C). Ref. 16.

shielding shifts of all peaks in the 13C NMR spectra of the polysiloxaneimmobilized amine systems, compared with those of APS-modified silica, may be due at least in part to the bulk magnetic susceptibilities of these materials. The same reason, as well as small structural differences, might account for the observed slight differences in the 29Si chemical shifts of

954

G A R Y E. M A C I E L

A t--

t~ .13 t__

< e-

e-' .m

0.0

I

I

30.0

I

I

60.0

L

!

90.0

I

,!

120.0

~

!

_

150.0

29Si (PPM)

Fig. 25.25. 298i DP-MAS spectra of polysiloxane-immobilized ligand systems. (A) Experimental and (B) deconvoluted spectra of polysiloxane-immobilized monoamine system. (C) Experimental and (D) deconvoluted spectra of polysiloxane-immobilized 3-chloropropyl system. Ref. 17.

pendent groups, compared to previous results on modified silica systems [37411. The relatively broad peaks in the 13C NMR spectra of the immobilized amine ligands (Fig. 25.1), compared with those of the 3-chloropropylpolysiloxane or (S)-CH2CH2CH2SH spectra (Fig. 25.2) and of the 3-(trimethylammonium)propyl-polysiloxane (Fig. 25.5D) suggest that one source of line broadening in spectra of the nonprotonated amine ligand may involve hydrogen bonding between the amine groups and surface silanols or with other ligand groups, as shown below:

7< Table 25.4. 1H spin-lattice relaxation results on functionalized polysiloxane systems of the type, (S)-Y.

13C_Detected

Sample

260 MHz TH(s) a

(S)-CH2CH2CH2NH2

(S)CH2CH2CH2NHCH2CH2NH2

29Si-Detected (298i chemical shifts) b 150 MHz

260 MHz

TY(S) a

TH(s) b

0.69 d 0.76 d

0.67 0.72

200 MHz TH(s) b

(S)-CH2CH2CH2SH

(S)-CH2CH2CH2PPh2 (S)-CHzCH2PPh2

(10.9 (27.3 (47.8 (60.0

ppm) ppm) ppm) ppm)

0.56 0.57 0.56 0.55

(12.6 (28.7 (51.9 (60.8

ppm)0.75 ppm) 0.73 ppm) 0.75 ppm) 0.71

0.59

31p-DetectedC

150 MHz TH(s) b

150 MHz T~(s) c

> XJ >

0.68 0.75

0.48 0.43

0.47 0.44

~z N >

0.67 1.0 0.50

0 :Z

0.56

0.70 1.0 0.52

/H + /Cd 2+

(S)-CH2CH2CH2C1

~Z

0 nl :Z -]

0.69, 0.72

0 2: > IN

0.71, 0.72

0.95 d 1.1 d

0.90 1.2

0.93 1.2

1.0 1.1

1.0 1.1

,-o 0

a Measured via 1H-13C cross polarization, with 1H frequency indicated. Estimated error: ---8%. b Measured via ~H-29Si cross polarization, with 1H frequency indicated. Estimated error: ---8%.

c Measured via ~H-31P cross polarization, with 1H inversion-recovery, at 150 MHz. Estimated error: +-6%. d Only one 1H relaxation behavior observed for all carbon signals measured.

V(D

Table 25.5. XH spin-lattice relaxation results on functionalized polysiloxanes of the type, X-(S)-Y

Sample

aaC-Detected"

1 2 3 /CH2CH2CHaNH2 ('').S.~ 1' 2' 3' CH2CH2CH2SH 1 2 3 4 5 / CH2CH2CH2NHCH2CH2NH2

~.S.\ ~'

"29Si-Detected (29Si chemical shifts) b

3ap-DetectedC

260 MI-Iz

150 MHz

260 MI-Iz

150 MI-Iz

150 MHz

~(s)

~(s)

~(s)

~(s)

~(s)

(-60 ppm)

(-100 ppm)

(-60 ppm)

(-100 ppm)

-PPh2

-P(O)Ph2

0.76

0.79

0.81

0.88

0.94

0.90

0.65

0.65

0.64

0.64

0.63

0.77

0.77

0.74

0.73

0.74

2' 3'

CH2CH2CH2SH /CH2CH2CH2PPh2

<s>.

\CH2CH2CHzNH2 /CH2CH2PPh2

<s>.

\CH2CH2CH2NH2

" Measured via 1H-13C cross polarization, with aH inversion-recovery, at the aH frequency indicated. Only one proton spin-lattice relaxation behavior noted for each sample. Estimated error: -+8%. b Measured via 1H-29Si cross polarization, with aH inversion-recovery, at the aH frequency indicated. Estimated error" -+8%. ~ Measured via all-alP cross polarization, with aH inversion-recovery. Estimated error: -+8%.


I~./H

/H

H~ /H

../H

~N\H,,,,, o,H ,/~,/~,/////J//,

/////////~///// III

IV

V

Possible reasons why hydrogen bonding (or related proton transfers, leading to hydrogen-bonded structures like VI or VII) could lead to line broadening include the following: (1) Proton-transfer at rates comparable to the inverse of the relevant 13C chemical shift differences due to

////I/////)/t VI

/

Y"/////////SJ//

VII

hydrogen bonding or protonation of amines [6, 38, 39]. (2) Hydrogen bonding and proton transfer enhance the possible number of chemically different structures, which can result in inhomogeneous line broadening due to the dispersion of isotropic chemical shifts. (3) Hydrogen bonding networks can impart a kind of three-dimensional rigidity to an otherwise flexible system, thereby "freezing-in" a distribution of configurations (e.g., conformations) and preventing or attenuating the line-narrowing that could otherwise result from motional averaging of different isotropic chemical shifts (such as the case for 3-chloropropyl siloxane system). Another likely source of line broadening in the spectra of amine systems is the effect of the 14N quadrupole interaction in interfering with MAS averaging of the 14N~13C dipolar interaction [55-60], an effect that is especially strong when unprotonated amine groups, which have large 14N quadrupolar couplings, are present; apparently this effect is dramatically attenuated in quaternary ammonium forms, in which the quadrupole coupling constant is markedly reduced. The quadrupolar effect of 35C1 or 37C1 on 13C is largely absent due to the rapid motion of the 3-chloropropyl moiety. The 13C CP-MAS spectrum of polysiloxane-immobilized diphenylpropylphosphine, (S)-CH2CH2CH2PPh2, shown in Fig. 25.4A, displays signals at 14.9, 19.1, 27.9, 60.5,128-141 ppm. The strong signal in the 128-141 ppm region is assigned to the phenyl carbons of ~PPh2 groups. The signals at 14.9 and 27.9 ppm are assigned to C(1) and C(2), C(3), respectively, where

958

G A R Y E. M A C I E L

C(1) is attached to silicon. The peaks at 19.1 ppm and 60.5 ppm are from the

CH3 and CH2 carbons, respectively, of residual unhydrolyzed ethoxy groups. Previous studies have shown that the ~3C chemical shifts of phosphine oxide moieties are close (within about _+2 ppm) to those in corresponding phosphine moieties [61]. Hence, the 13C signals in phosphine oxide moieties are not resolved from the corresponding 13C signals of the phosphine moieties in these spectra. The strong phenyl carbon signals in the 128 to 141 ppm region of the 13C NMR spectra shown in Fig. 25.4 might have been expected to consist of four components at about 128, 131,133 and 141 ppm, due to the various aromatic carbon sites, according to a previous study on diphenylethylphosphine moieties immobilized on silica gel via surface modification [61]. The 13C chemical shifts anticipated for the various aromatic carbon sites are shown below for immobilized phosphine and phosphine oxide moieties [61]: 128 131~128 Ii II

.._

--'CH2CH2P~

131

C6H5 "

,,c0H i--CH2CH2P= 0 128 (PPM) 128 131

The 13C CP-MAS spectrum of polysiloxane-immobilized diphenylethylphosphine, as shown in Fig. 25.4B, exhibits peaks very similar to those of the propyl analog, with similar chemical shifts (Table 25.1). The 13C NMR spectra shown in Fig. 25.4 reflect the fact that the relative amplitudes of the four anticipated chemical shift components depend upon the relative amounts of the phosphine and phosphine oxide moieties in the samples. Of course, only qualitative conclusions can be drawn from comparisons of the various 13C CP-MAS spectra, because the CP spin dynamics were not studied in detail for these samples. The 13C NMR spectra of the polysiloxane-immobilized primary amine ligand system before and after silylation of accessible silanols with hexamethyldisilazane, given in Fig. 25.5, show very similar peak maxima positions for each of the three methylene carbons (Table 25.1), although there is apparently a small peak narrowing for C(2) and C(3) in the (CH3)3 Si-capped


959

sample, for which Fig. 25.5B shows the distinct (CH3)3 Si peak near 0 ppm. This comparison is of interest primarily for examining any chemical/structural changes in the CH2CH2CH2NH2 group, especially as reflected in the C(2) ~3C signal, as the surface silanols are "capped" by Me3Si groups to reduce the number of surface hydroxyls that might otherwise be expected to interact with amino groups. A similar strategy was used for 3-aminopropyltrimethoxysilane-derivatized silica [38]. The spectra in Figs. 25.5A and 25.5B display very similar positions of peak maxima for each of the three methylene carbons (Table 25.1), although there is apparently a small peak narrowing for C(2) and C(3) in the (CH3)3Si-capped sample. The slightly broader peaks for the uncapped material may reflect a greater structural inhomogeneity associated with hydrogen bonding between surface silanols and at least some of the pendant ligand groups. The 13C NMR spectrum of the polysiloxane-immobilized diethylpropylamine system in Fig. 25.5C shows signals at 12.6, 21.8 (shoulder at 27.2), 47.8 and 57.0 ppm. The shoulder at 27.2 ppm may come from C(2') of residual, unreacted 3-chloropropyl groups, due to incomplete reaction of 3-chloropropylpolysiloxane with diethylamine. The C(1') and C(3') peaks of the residual precursor ((S)-CH2CH2CH2C1) are overlapped with the C(1) and C(4) peaks of the product, polysiloxane-immobilized diethylpropylamine system. This interpretation of the 27.2 ppm shoulder is in agreement with the elemental analysis data, which show that unreacted polysiloxane-immobilized 3-chloropropyl material remained in the product. Peaks in the 13C NMR spectrum of the polysiloxane-immobilized diethylpropylamine system (Fig. 25.5C) are in general sharper than those of the corresponding polysiloxane-immobilized primary amine system (Fig. 25.5A). The reason for the apparently enhanced broadening in the spectrum of the immobilized primary amine system may be that the geometrical arrangements and/or mobilities of the aminopropyl moieties of these two systems may differ from each other in terms of whether the amine group is "free" or hydrogen bonded, where this kind of dual behavior would yield a range of slightly different chemical shifts. In the case of the polysiloxane-immobilized diethylpropylamine system, the absence of the possibility of amine-amine hydrogen bonding may eliminate one source of structural inhomogeneity, resulting in somewhat smaller linewidths. The absence of amine-amine hydrogen bonding may also permit increased mobility of the pendent ligand groups, rendering some degree of motional averaging of a diversity of isotropic chemical shifts associated with geometrical (e.g., conformational) variations. The 13Cspectrum of the polysiloxane-immobilized propyltrimethylammonium chloride system (shown in Fig. 25.5D), displays four peaks, at 10.8, 18.7, 54.5 and 69.1 ppm, which are assigned to C(1), C(2), C(4) and C(3),

960

GARY E. MACIEL

respectively (as identified in Fig. 25.5D, 25.5B and Table 25.1). The small shoulder at 60.5 ppm is assigned to CH2 carbons of unhydrolyzed, residual ethoxy groups. The corresponding CH3 carbon signal of the unhydrolyzed ethoxy groups, at 18 ppm, is overlapped with the C(2) peak, at 18.7 ppm, of the product. The 13C CP-MAS spectrum of this sample (Fig. 25.5D), in which the nitrogen atom cannot possibly be involved in hydrogen bonding with surface silanols or other ligand groups, also displays at least one line that is relatively sharper than the peaks in the spectrum of the corresponding primary amine system (Fig. 25.5A), due to the same reasons discussed for the (S)-CHzCHzCHzNEt2 sample. The 13C NMR spectrum of the polysiloxane-immobilized system that includes a pendent mCHzCHzCHzNHCHzCOzMe group, shown in Fig. 25.6A, displays in the sp 3 carbon region four signals, at 11.1, 22.4, 43.7 and 53.2 ppm, and a shoulder at 60.8 ppm, which were assigned [37, 39, 45] to C(1), C(2), C(3), C(6) and C(4), respectively, as shown in Fig. 25.6A and Table 25.1. This spectrum also includes a "doublet" with maxima at 168.4 and 171.9ppm, due to two carbonyl carbon resonances. These two peaks were assigned on the basis of 13C NMR data reported previously [37, 45, 55] to a "free" carbonyl and a carbonyl group that interacts via hydrogen bonding with an amine group or with a surface silanol. It had been shown previously that hydrogen bonding to the oxygen atom of a carbonyl group decreases the shielding of the carbonyl carbon [37, 45, 55]. In order to find out if 14N quadrupolar interactions have some effect [56-60] on the broadening of the carbonyl peak, 13C CP-MAS spectra of these two products were obtained at a higher field, 64.8 MHz for 13C. Since the 14N quadrupole line-broadening effect decreases at higher field, a sharper peak would be expected at 64.8 MHz if the main effect of the line-broadening of the carbonyl peak arises from the 14N quadrupolar interaction. In fact, similar broad doublets were observed for the carbonyl peak at 64.8 MHz. Therefore, in both cases, the line-broadening of the carbonyl signals mainly comes from the involvement in hydrogen bonding of the carbonyl group, and is not the result of a 14N quadrupole line-broadening effect. The 13C CP-MAS spectrum of the polysiloxane-immobilized ligand system bearing mCHzCHzCHzSCHzCOzMe groups, shown in Fig. 25.6B, has one relatively sharp carbonyl resonance at 172.8 ppm. The carbon signals at 9.7, 28.3, 35.8 and 53.2ppm were assigned to C(1), C(2) and C(3), C(4) and C(6), respectively, as shown in Fig. 25.6B and Table 25.1. The lack of a very large linewidth of carbonyl signal in the 13C spectrum of (S)CHzCHzCHzSCHzCOzCH 3 (Fig. 25.6B) may suggest the absence of a distribution of hydrogen-bonding interactions of the carbonyl group. One might have expected to see a broad carbonyl signal if some substantial (but not

NMR CHARACTERIZATION OF FUNCTIONALIZED POLYSILOXANES 961 dominant) faction of the carbonyl groups of this system were also involved in hydrogen bonding with surface silanols [62]. The 13C NMR spectrum of the polysiloxane-immobilized system that bears ~CHzCHzCH2OzCCH2NH2 (propylglycinate) groups, given in Fig. 25.7A, shows resonances at 10.4, 26.5, 55.0 (broad), 65.0 and 175.0 (broad) ppm, which were assigned [37, 39, 45] to C(1), C(2), C(5), C(3) and C(4), respectively (as defined in Fig. 25.7A and Table 25.1). The peak at 65.0, which was assigned to C(3), is consistent with the resonance position of methylene carbons attached to an ether oxygen atom, as reported for similar materials [39]. The carbonyl signal at 175 ppm is broad and appears to be a doublet, ostensibly with a high-shielding component due to the "flee" carbonyl and a lower-shielding component due to carbonyl groups involved in hydrogen bonding with amine or silanol groups. In the reaction of the polysiloxaneimmobilized 3-chloropropyl system with sodium glycinate, the basis of the preparation of this sample, there are in principle, two possible products: (S)CHzCHzCHzOzCCHzNH2, from the substitution of the chloride group by the carboxy group, or (S)-CHzCHzCHzNHCHzCOzNa, from substitution of the chloride group by the amine group of glycinate. The 13C spectrum of Fig. 25.7A clearly indicates that, in the reaction of the 3-chloropropylpolysiloxane with sodium glycinate, the chlorine atoms are replaced by the carboxylate group and not by the amine group. If the latter material had been produced in the synthesis, we would have expected a 13C spectral pattern similar to that of the polysiloxane-bearing ~ N H C H z C O z M e case shown in Fig. 25.6A, in which the C(3) resonance occurs at about 43.7 ppm; this chemical shift is not observed in the spectrum of the product (Fig. 25.7A). The 13C NMR spectrum of the polysiloxane-immobilized system bearing iminodiacetate ligand groups displays a pattern (Fig. 25.7B) that is similar in some respects to that of Fig. 25.7A for the glycinate polysiloxane system (i.e., it shows signals in the 66 and 172 ppm regions). The presence in the spectrum of Fig. 25.7B of strong signals of the starting 3-chloropropylpolysiloxane material (10.9, 27.3 and 47.8 ppm) and only weak or largely masked peaks for the expected product (66.4 and 172 ppm) indicate that in the iminodiacetate case the substitution reaction was incomplete under the conditions used. The incompleteness of the reaction was also indicated by elemental analysis data showing a large amount of chlorine present in the product. The 13C NMR spectrum of a polysiloxane-immobilized acetate system, bearing -CHzCHzCHzOzCCH3 groups, was obtained as a model for the C(3) peak of a system that contains the immobilized -CHzCHzCH2~O~C(O)CI-~2 group; its spectrum, shown in Fig. 25.7C, displays the 66.6ppm carbon resonance of the C(3) methylene group in this moiety. This is consistent with the 13C spectra of the glycinate and iminodiacetate polysiloxane systems,

962

GARY E. MACIEL

where signals in the range, 65-66 ppm, are seen for the methlyene groups attached to an ether oxygen of an ester moiety. The assignments for other carbons of the polysiloxane-immobilized propylacetate are as shown in Fig. 25.7C and Table 25.1. 13C NMR spectra of functionalized polysiloxanes of type, X-(S)-Y. The 13C CP-MAS spectra of X-(S)-Y samples, shown in Figs. 25.8-25.11, can largely be interpreted as linear combinations of X-(S) and (S)-Y spectra. Of course, there are distortions of intensities from what would predict from a simple spectral addition, because of variations in conformational averages (thus, chemical shifts) and in CP spin dynamics (thus, intensities). The 13C chemical shifts derived for X-(S)-Y polysiloxanes are summarized in Table 25.2. It is interesting to note that while the presence of a pendant amino group on a propylthiol-functionalized polysiloxane apparently is effective in promoting hydrolysis/condensation processes to the extent that no residual ethoxy or methoxy signals are apparent in the 13C CP-MAS spectra of Fig. 25.8, such amino groups do not lead to elimination of methoxy and ethoxy signals in the spectra of phosphine-functionalized polysiloxanes with incorporated amino groups (Fig. 25.10).

25.4.2

15N NMR spectra

The 15N CP-MAS spectra of the polysiloxane-immobilized monoamine samples treated with aqueous HC1 or NaOH solutions of various pH values (Fig. 25.12) show that, for the pH = 7 case, the spectrum has a 15N signal at 25 ppm, with a small shoulder at 34 ppm. For the pH = 1 case, the spectrum shows a peak at 45 ppm, with a weak shoulder at 33 ppm. For the pH - 13 case, the signal is a sharper, more intense peak at 25 ppm, accompanied by a small shoulder at 33 ppm. This behavior, in which the intensity of the shoulder at 34 ppm increases as the pH is changed from 13 to 3.5, is consistent with the idea that there is a simple analogy between hydrogen bonding and protonation [51]. The presence of the shoulder at 34 ppm between the free amine peak at 25 ppm and the ammonium cation peak at 45 ppm may indicate the involvement of the NH2 group in hydrogen bonding with acidic surface silanols and perhaps with other amine groups. According to 15N NMR data in the literature [53, 63-66], one can assign the signals at 45, 34 and 25 ppm in the spectra of Fig. 25.12 to the ammonium cation form (VI, VII or VIII), hydrogen bonded amine forms (III or IV), and the nonhydrogen-bonded amine form (IX and X), respectively.

NM R C H A R A C T E R I Z A T I O N OF F U N C T I O N A L I Z E D P O L Y S I L O X A N E S

H\|

N!/H VIII

..........;.,. IX

963

H /I-I X

At pH = 1 (Fig. 25.12A), the cation form (VIII) is favored, whereas at pH = 13 (Fig. 25.12E), the ionic amine form (X) would be favored; the proportion of various hydrogen bonded forms depends on the pH value. The ~SN NMR spectrum of the Cd(II)-treated sample derived from the polysiloxane-immobilized monoamine ligand, shown in Fig. 25.13B, consists of a broad signal, with most of its intensity at lower shielding than for the corresponding uncomplexed ligand system (Fig. 25.13A) between that of the ammonium cation (45 ppm) and hydrogen bonded forms (33 ppm) (vide supra). The observed slight broadening of the C(2) and C(3) signals in the 13C spectra of the Cd(II)- and Hg(II)-complexed polysiloxane-immobilized monoamine system (not shown here), compared with the spectrum of the corresponding uncomplexed polysiloxane-immobilized amine system (Fig. 25.1A), may result from an increase in chemical structural heterogeneity, indicating that not all ligand sites are accessible to metal ions, thereby remaining uncoordinated and therefore having somewhat different chemical shifts from those groups involved in coordination. This inaccessibility of metal ions to some amino sites may be due to a combination of steric hindrance and electrostatic repulsions between metal ions approaching amino groups and metal ions already complexed to the immobilized amino groups. This view, that some of the ligands remain uncoordinated, is in agreement with the suggestions of other workers for other polysiloxane systems [13]. Another possible cause of the increase in chemical structural heterogeneity might be the presence of different complexation forms of the amine groups with the metal ions, e.g., the number of ligands complexed to each metal ion. The broad signal observed in the 15N NMR spectrum of the Cd(II)-treated sample derived from the polysiloxane-immobilized monoamine ligand (Fig. 25.13B) also suggests that a substantial fraction of the immobilized monoamine that has been treated with aqueous Cd(II) solution is involved in coordination to Cd(II). The breadth of the peak in Fig. 25.13B suggests that a portion of the ligand groups remain uncoordinated to metal ion in the sample. Of course, there may be a rapid exchange of C d 2+ ions between different amino sites, which could result in another line-broadening mechanism. The ~SN NMR spectrum of the polysiloxane-immobilized diamine system

964

GARY E. MACIEL

(Fig. 25.13C) shows two signals, at 22.2 and 37.0ppm. On the basis of 15N NMR data taken from the literature [63-66], these peaks are assigned to primary and secondary amine groups, respectively. The spectrum of the protonated form of the polysiloxane-immobilized diamine ligand (washed with 0.1 M aqueous HC1) is given in Fig. 25.13D; it shows that both signals move to lower shielding, at 45.0 ppm and a shoulder at 52 ppm, upon protonation. These signals are assigned on the basis of 15N data taken +from the literature [65 ~ 66] to the ammonium cation sites of types ~ N H 3 and + ~ N H 2 ~ , respectively. The 15N NMR spectrum of the polysiloxane-immobilized triamine system presented in Fig. 25.13E shows two signals, at 18.2 and 33.6ppm, which are expected for the primary and secondary amine groups, respectively. In comparing peak intensities of the diamine system (Fig. 25.13C) with those of the triamine system (Fig. 25.13E), one sees that there is a significant increase in the secondary amine intensity relative to that of the primary amine peak in the triamine case, compared with the diamine case. This is consistent with the presence of two secondary amine groups for each primary amine group in the triamine ligand system. Furthermore, the absence of a tertiary amine signal, which would be expected to occur at about 50-55 ppm [63-66] and to survive 1H-15N dipolar dephasing very well (vide infra), confirms the absence of structure XII in the product and shows that structure XI is the product of the reaction between diethylenetriamine and the 3chloropropylpolysiloxane precursor; this result is consistent with the 13C results described above.

.~H2 H XI

"~

H2

XII

The 15N NMR spectrum of the polysiloxane-immobilized thiol-monoamine system, presented in Fig. 25.13F, shows a broad signal at about 28 ppm. The breadth and the lower-shielding intensity of the peak shown in this spectrum, in comparison with that of the corresponding polysiloxane-immobilized monoamine ligand (Fig. 25.13A), may be associated with hydrogen bonding [54] involving the thiol group. The 15N NMR spectrum of the polysiloxaneimmobilized thiol-diamine ligand system (Fig. 25.13G) shows two signals, at 22.0 and 37.2 ppm; these are assigned, according to 15N NMR data in the literature [42, 63], to primary and secondary amine groups. A similar pattern


was seen above for the polysiloxane-immobilized diamine ligand system, with no thiol component. 25.4.3

31p N M R results

Solid-state 31p NMR spectroscopy is useful for obtaining information on the phosphine ligands, e.g., the phosphorus oxidation state. The 31p CP-MAS NMR spectra of the polysiloxane-immobilized diphenylpropylphosphine (Fig. 25.15A), the diphenylpropylphosphine-monoamine (Fig. 25.15B), the diphenylethylphosphine (Fig. 25.15C), and the diphenylethylphosphine-monoamine (Fig. 25.15D) systems all reveal the presence of the following two types of structural moieties, which have been identified on the basis of previously reported 31p NMR chemical shifts [61, 67-70]" (A) phosphine moieties: -17 ppm for diphenylpropylphosphine samples and -11 ppm for diphenylethylphosphine samples; and (B) phosphine oxide moieties: 3436 ppm for diphenylpropylphosphine samples and 34 ppm for diphenylethylphosphine samples. 31p NMR spectra of related samples (not shown here) also display the presence of both the phosphine moiety and phosphine oxide moiety. The ratio of the peak amplitudes of phosphine moiety to phosphine oxide moiety varies from 0.1 to 5.5 among the samples. 31p spectra were obtained twice on the samples, with a 1-year measurement interval; no changes in the relative amounts of phosphine and phosphine oxide moieties were observed over this 1-year period, which indicates that further oxidation of phosphine moiety to phosphine oxide moiety did not occur during sample storage. Simulations [69, 70] of the spinning sideband patterns of 31p spectra obtained at 80.9 MHz with a low MAS speed (1-2 kHz) (not shown here) were carried out on all the polysiloxane-immobilized phosphine samples [71] to provide values of the principal elements of the chemical shift tensor, which often are valuable in elucidating chemical structure. Two model compounds, diphenyl-isopropylphosphine oxide and 1,6-bis(diphenylphosphino)hexane, were used to provide CSA tensor elements that would be approximately representative for the polysiloxane-immobilized phosphine oxide and phosphine moieties, respectively. The experimental 31p spectra were then simulated on the basis of the principle tensor elements obtained on the two model compounds: 117.1, 86.8 and - 9 3 . 7 p p m for diphenyl-isopropylphosphine oxide and 7.3, -29.8 and -40.9 ppm for 1,6-bis(diphenylphosphino)hexane. The simulations showed that the 31p chemical shift anisotropies of both phosphine and phosphine oxide moieties are qualitatively similar within each of these two categories for all the samples examined, implying very similar

966

GARY E. MACIEL

local phosphine structures (e.g., bond angles, bond lengths) within each category. 25.4.4

1H NMR results

1H CRAMPS spectra of the polysiloxane-immobilized monoamine system, its protonated form and two of its metal complexes, and of the polysiloxaneimmobilized monoamine and 3-chloropropyl systems, given in Fig. 25.17, show that for the protonated monoamine ligand (Fig. 25.17B) and products of treatment with aqueous metal ion solutions (Figs. 25.17C and 25.17D) there are strong, broad signals for the ~ N H ~ resonance at about 7-8 ppm. The methylene proton signals are also broad and partially overlapped, and probably obscure the weak silanols signals. In contrast to these spectra, the IH NMR spectra of the polysiloxane-immobilized 3-chloropropyl system (Fig. 25.17E) and thiol system (Fig. 25.16A) show sharp signals. Besides the methylene proton signals at about 1.1, 2.0 and 2.7 or 3.6 ppm, these spectra also show the presence of residual unhydrolyzed methoxy and ethoxy groups at about 1.4 and 3.6-4.1 ppm. The broadening of peaks in the 1H NMR spectra of the various polysiloxane-immobilized monoamine samples (Fig. 25.17) are presumably caused by some combination of effects of the types discussed above for 13C spectra of these systems. Also, the effects of metal-ion complexation (e.g., "freezing in" a variety, or range, of structures and isotropic chemical shifts) can be analogous to what were discussed above for hydrogen-bonding effects. The explanation that hydrogen bonding may be one source of line broadening in the 1H CRAMPS spectra of polysiloxane-immobilized amine ligands is supported by the 15N NMR results. In Fig. 25.12 one can see that the intensity of the shoulder at 34 ppm in the 15N CP-MAS spectrum of the polysiloxaneimmobilized monoamine system increases as the pH is changed from 13 to 3.5. This is consistent with the idea that there is a simple analogy between hydrogen bonding and protonation; the presence of the shoulder at 34 ppm between the free amine peak at 25 ppm and the ammonium cation peak at 45 ppm may indicate the involvement of the NH2 group in hydrogen bonding with acidic surface silanols and perhaps with other amine groups. The 1H CRAMPS spectrum of the polysiloxane-immobilized thiol-monoamine ligand system (Fig. 25.16B) shows no signal due to NH2 protons. This may be in part because the ~H signal of the NH2 groups is broadened by the 14N quadrupole effect on MAS averaging of the 1H-14N dipolar interaction, and in part because of effects of proton exchange of NH2 protons. Quadrupolar line broadening effects of 14N on attached protons have previously been observed in 1H CRAMPS spectra of amino acids and other systems (72-74).


The spectrum of the corresponding protonated material (Fig. 25.16C) shows a broad ~Nt-I~3 signal at 7.8 ppm; the observation of ~NH~3 signals under conditions in which m N H 2 signals are not observed may reflect the higher local electric symmetry around nitrogen of the ~Nt-I~3 group, and correspondingly smaller quadrupolar coupling constant and line broadening effect, as well as altered proton exchange behavior. The methylene proton signals of the protonated ligand system show a slight shift to lower shielding in comparison with those of the parent ligand. Separate silanol proton signals are not seen in the spectra due to the strong overlap with broad methylene signals. In the 1H NMR spectra of the polysiloxane-immobilized thiol-diamine system and its protonated form shown in Figs. 25.16D and 25.1E, the breadths of spectral lines of the thiol-amine systems, compared with those of the thiol system, may be due to a diversity of hydrogen bonding of the amine groups with surface silanols and thiol groups. 25.4.5

29Si NMR spectra. The polysiloxane framework

298i CP-MAS NMR spectra are expected to provide information on the nature

of attachments of the various silicon atoms, as well as the relative populations of the ligand-bearing silicon moieties and nonligand-bearing silicon moieties. Of course, conclusions on these populations can be only qualitative in the absence of detailed studies of the relevant spin dynamics, which would be necessary for quantitative interpretations. Nevertheless, since the same set of experimental set-up parameters was used for all 298i CP-MAS experiments, and since all of the samples in this study are physically similar (amorphous polysiloxane solids with organic ligand moieties having substantial mobilities), comparisons of relative amplitudes should be at least qualitatively valid. In general, peak linewidths in the 298i NMR spectra are larger than those in the 13C NMR spectra of corresponding samples. This observation is due to the fact that, in a polysiloxane framework, each type of silicon site, such as the single silanol (Q3), (~SiO)3SiOH, has many structural variations in terms of bond angles, chemical environments (e.g., neighbors of the silicon atom) and the absence or detailed form of hydrogen bonding (e.g., hydrogen bonded to silanols or water on the surface), etc. Such variations in one specific type of silicon site give rise to corresponding variations in chemical shifts, resulting in inhomogeneous broadening of the peaks in the 298i NMR spectra. Compared to the silicon atoms in a polysiloxane framework, the 13C atoms of a specific type of site in an organic group (e.g., in m C H z C H z C H z S H ) have much narrower local-structural variations (at least in a time-average sense) than those of silicon atoms, which results in smaller linewidths in the 13C NMR spectra.

968

GARY

E. MACIEL

The 2 9 8 i CP-MAS spectra displayed in Figs. 25.18-25.24 in general show two regions of major intensity, centered at about -100 to -105 and about - 6 0 p p m . These two peak positions correspond to to S i ( ~ O ~ ) 4 and R S i ( ~ O ~ ) 3 units, respectively, where R is an organic group containing the pendent functionality. The -105 ppm pattern is composed of at least three contributions, at about -91, -100 and -108 ppm, due to the following types of species: (~SiO)2Si(OR')2 (Q2), (~SiO)3SiOR' (Q3), and (~SiO)4Si (Q4), respectively, where R' = Et or H. The relative importance of Et and H for R' in the Q2 and Q3 sites can be inferred from the 13C NMR spectra (vide supra), which show that there are few remaining ethoxy groups in systems with amino groups present, and more residual ethoxy groups in the 3-chloropropylpolysiloxane system and the corresponding propylthiol system. The lower-shielding pattern is composed of at least one peak at about - 6 4 and a shoulder at -57 ppm, due to species containing ligand groups, RSi(OSi--~)3 and RSi(OSi~)2OR', respectively, where R ' = H, Me or Et. These assignments are in agreement with previous results on modified silica systems [3741]. For comparison purposes, the 29Si CP-MAS spectrum of silica made by the sol-gel process through the HCl-catalyzed hydrolytic condensation of Si(OEt)4 is given in Fig. 25.18A. This spectrum shows one major pattern centered at about -100ppm, composed of a shoulder at - 9 0 ppm, a clear peak at - 9 9 p p m and a shoulder at -109ppm. These components were assigned to (~SiO)2Si(OH)2, (~SiO)2Si(OEt)2 and (~SiO)2Si(OH)OEt, \ . \ . ./ - 9 0 p p m ; (~S10)3SiOH, and (~S~O)3SiOEt, - 9 9 p p m ; and Si(OSIs)4, -109 ppm. The presence of unhydrolyzed residual ethoxy groups indicated b~r the 13C spectrum (not shown here) leads to the silicon assignments: (~SiO)2Si(OEt)2, (~SiO)3Si(OH)OEt and (~SiO)3SiOEt. When a compound of type RSi(OR')3 (where R is an organic ligand group and R' = CH3 or CzHs) is added to the initial reaction mixture of Si(OEt)4, H20 (and HC1 in some cases) and MeOH, the 2 9 8 i NMR spectrum of the resulting product shows two major patterns centered at about - 6 0 and -102 ppm, as seen in Fig. 25.18B for the case R = CHzCHzCHzNH2. In this figure the - 6 0 ppm signal consists of a main peak at - 6 4 p p m , with a shoulder at - 5 7 ppm, .J .J . corresponding to RSi(OS1s)3 and RSi(OSI~)zOH structures, respectively [6, 33, 36, 41]. Since the 13C NMR spectrum of the polysiloxane-immobilized primary amine system (Fig. 25.1A) indicates that there are no, or few, unhydrolyzed, residual ethoxy groups in the product, the silicon signal at / - 5 7 p p m corresponds only to RSi(OSis)zOH structures, not including ./ RSi(OS~s)2OEt. For those samples m which the ~3C NMR spectra indicate that there are unhydrolyzed, residual ethoxy groups (see Table 25.1 and .

.


969

Figs. 25.1, 25.2 and 25.3), the silicon signal at - 5 7 p p m corresponds to RSi(OSi~)2OR' ( R ' = H, Me or Et). In the 29Si NMR spectrum of the 3-aminopropylpolysiloxane sample after silylation of the accessible silanols by (Me3Si)2NH (Fig. 25.18C), the peak at 12.1 ppm corresponds to Me3SlmOS1s groups. It can be seen, by companng ~pectra of Figs. 25.18B and 25.18C, that the relative intensity of the Si(OSi ~)4 peak at - 1 0 9 p p m is increased and the relative intensity of RSi(OSi~)2OH at - 5 7 ppm is decreased in the spectrum of 3-aminopropylpolysiloxane after silylation (Fig. 25.18C) compared with that of the amine ligand system before silylation (Fig. 25.18B); but there is still substantial (slightly decreased) intensity of the single silanol peak at - 9 9 p p m after silylation. It was estimated by spectral deconvolution of the spectra of Figs. 25.18B and 25.18C (not shown here) that ---76% of the silanols of the R(~SiO)2SiOH sites are silylated, while ---15% of the silanols of the (TSiO)3SiOH sites are silylated, which indicates that hydroxyl groups attached to ligand-bearing silicon atoms are more accessible and/or more reactive toward the silylating reagent, (Me3Si)2NH. The estimated percentage of . . . . . unreacted sdanols, both R(TS10)2SIOH and (TS10)3SIOH, m the sflylauon of the polysiloxane-immobilized primary amine system is 55%. In underivatized silica gel, approximately 58% of the silanols are not accessible for reaction with hexamethyldisilazane under roughly the same conditions as employed in the studies described here [33]. Figure 25.18D shows the 29Si CP-MAS NMR spectrum of the polysiloxane-immobilized trimethylpropylammonium chloride system, (S)C H 2 C H 2 C H ~ ( C H 3 ) 3 C I - , prepared via catalysis by HC1. This spectrum shows a higher intensity (relative or absolute) of the single silanol peak at - 9 9 ppm and a lower intensity of the Si(O~Si~)4 peak at -109 ppm, compared with those of the immobilized primary amine system (Fig. 25.18B), where the amino group has served as a basic catalyst in the polymerization. This implies that the polymerization was incomplete in the preparation of the polysiloxane-immobilized trimethylpropylammonium chloride system, and that this material is less cross-linked. The incomplete polymerization may be due to electrostatic repulsion among the positive charges of the polysiloxane-immobilized trimethylpropylammonium cations, which might prevent the small polysiloxane entities formed initially from getting sufficiently close together to undergo further polymerization. The same reason may account for the formation of a precipitate, instead of a clear homogeneous gel in the preparation. Of course, the absence of an "internal" amine catalyst may also be partly responsible for the incomplete polymerization. The 29Si NMR spectrum of the polysiloxane-immobilized monoamine sys9

,/Y

9

\

9

~

.

.

\

970

GARY E. MACIEL

tem made from a mixture with a high (2:1) Si(OEt)4-to-(EtO)3Si(CH2)3NH2 molar ratio (Fig. 25.19A) shows a lower intensity of the RSi(O~SI~)3 s~gnal at - 6 4 ppm, relative to the Si(O~Si--~)4 signal, in comparison to the ratio seen in the spectrum of the polysiloxane-immobilized monoamine sample prepared from a mixture with a 1:1 molar ratio (Fig. 25.19B). This suggests that the material prepared from a reaction mixture with a lower relative Si(OEt)4 content is less cross-linked, i.e., has fewer Si(OuSi~)4 sites, but more RSi(O~Si~)3 sites. In the 29Si NMR spectra of the polysiloxane-immobilized 3-chloropropyl systems made by using HC1 or organotin catalysts, shown in Figs. 25.19C and 25.19D, the sample prepared with a 0.1 M HC1 catalyst (Fig. 25.19C) shows stronger peaks at -100 ppm and -57 ppm than in the spectrum of the polysiloxane-immobilized 3-chloropropyl sample prepared with the organotin catalyst (Fig. 25.19D). This indicates that with HC1 as catalyst the crosslinking of both organosilane-containing moieties and silica-like moieties was less complete; i.e., there are more RSi(OSi--~)2OR' and (~SiO)3SiOR' sites. The 13C spectra of the 3-chloropropylpolysiloxane samples (Fig. 25.3) also show substantial amounts of residual ethoxy and methoxy groups. The 29Si spectrum of 3-chloropropylpolysiloxane prepared using the organotin catalyst also shows an extra peak at -82 ppm, perhaps due to --~Si--OmSi(OR')3 sites ( R ' = H, Et). Because there are not any residual methoxy or ethoxy groups in the polysiloxane-immobilized amine systems, as indicated by 13C NMR spectra (Figs. 25.1 and 25.2), the -57 and -100 ppm peaks of the 29Si NMR spectra in Figs. 25.19A and 25.19B represent RSi(OSi--~)2OH and HOSi(OSi~)3 moieties, respectively. 29Si NMR spectra also indicate that there are substantial amounts of crosslinking in the polysiloxane-immobilized amine systems in both the organosilane-containing moieties (as evidenced by the RSi(OSi--~)3 peak at - 6 4 ppm) and silica-like moieties (as evidenced by the Si(OSi~)4 peak at -108 ppm). The rather high relative intensities of the -57 and - 6 4 p p m peaks in Figs. 25.19C and 25.19D reveal the presence of substantial amounts of RSi(OSi~)2OR' and RSi(OSi--~)3, with R ' = H, Me or Et, in the two 3-chloropropylpolysiloxane samples; i.e., the degree of crosslinking in both the organosilane-containing and silica-like moieties is less in the 3-chloropropylpolysiloxane samples than in polysiloxane-immobilized amine systems. 29Si NMR spectra (Figs. 25.19C and 25.19D) and 13C NMR spectra (Fig. 25.3) show that (n-Bu)2Sn (OCOCH3)2 is a more effective catalyst than HC1 (aq) for condensation of the 3-chloropropylpolysiloxane system, and 13C NMR spectra (Fig. 25.2) show that (n-Bu)2Sn(OCOCH3)2 is a better catalyst than HC1 for the hydrolysis of both Si(OEt)4 and CI(CH2)3Si(OMe)3. Compared to the 29Si spectra of polysilxoane-immobil9

o

. ~

9


971

ized 3-chloropropyl systems (Figs. 25.19C and 25.19D), the 29Si spectra of polysiloxane-immobilized monoamine ligand systems (Figs. 25.19A and 25.19B) show much stronger Q4 intensity relative to Q3, indicating that the condensation in the latter case is more complete, and the material has more crosslinking Of course, since the signal intensities in CP-MAS spectra are affected by CP dynamics, the results obtained from 29Si CP-MAS spectra are only qualitative. The 29Si CP-MAS NMR spectrum of the polysiloxane-immobilized diamine system, shown in Fig. 25.19E, displays features similar to those of the spectrum of the polysiloxane-immobilized monoamine ligand system (Fig. 25.19B), in the terms of relative intensities of 29Si signals in the - 6 0 p p m region and -105 ppm region, because the same molar ratios (1" 1) of Si(OEt)4 to (R'O)3SiCH2CH2CH2X (X = NH2, R' = Et; X = NHCH2CH2NH2, R' = Me) were used for both preparations. However, in both the - 6 0 ppm and -105 ppm regions, there is evidence of more crosslinking (higher relative intensities at - 6 4 ppm vs. - 5 7 ppm, and -108 ppm vs. - 1 0 0 p p m ) in the diamine ligand system than in the monoamine ligand system; therefore, the diamine ligand serves as a better catalyst for crosslinking than the monoamine ligand does. These results indicate that the hydrolysis/polymerization processes are very similar, with the amine groups acting catalytically and yielding analogous polysiloxane frameworks. The 29Si CP-MAS spectrum of the polysiloxane-immobilized triamine system is shown in Fig. 25.19F. This spectrum displays features that are similar to those of the spectrum of its precursor, 3-chloropropylpolysiloxane prepared via (n-Bu)2Sn(OCOCH3)2 catalysis (Fig. 25.19D). The 295i spectrum of the polysiloxane-immobilized triamine system (Fig. 25.19F) shows higher intensities at - 6 4 and -108 ppm, relative to those at - 5 7 and -100 ppm, respectively. These results indicate that under the reaction conditions of preparing the polysiloxane-immobilized triamine system, the polysiloxane framework is basically well-formed and has a rather high degree of crosslinking in both the organosiloxane and silica-like regions, even though the polysiloxaneimmobilized triamine system is prepared from the 3-chloropropylpolysiloxane, which we have found to contain a substantial quantity of unhydrolyzed alkoxy groups (hence, uncrosslinked silicons; see Fig. 25.2). This result is an indication that the triamine ligand acts as a good catalyst for hydrolysis of ~ S i ~ O E t or --~Si~OMe and for the crosslinking in both organosiloxane and silica-like regions. Comparison of the 295i NMR spectrum of the polysiloxane-immobilized monoamine system after treatment with 0.1 M aqueous HC1 solution (Fig. 25.19G) to the spectrum of the untreated sample (Fig. 25.19B), reveals that the spectrum of the sample treated with HC1 (Fig. 25.19G) has higher relative

972

G A R Y E. MACIEL

intensity in the -105 ppm region in comparison to the - 6 0 ppm region; in this sense the spectrum in Fig. 25.19G is a much more "silica like" spectrum, and shows increases in the relative intensities of peaks at -57 ppm, - 9 0 and -100 ppm and decreases of relative intensities at - 6 4 ppm and at -108 ppm, i.e., much less crosslinking in both organosilane-containing moieties and silica-like moieties. These results, together with elemental analysis data showing that 12.3% of the carbon and 10.9% of the silicon of the original polymer were extracted into the aqueous HC1, were explained by postulating that some leaching of small oligomeric material containing ligand species into the solution is facilitated by aqueous HC1. This presumably results from hydrolysis of some S i ~ O ~ S i linkages, especially those near the immobilized ligand. This kind of leaching of small oligomeric species by acidic aqueous solution has been discussed by others for amine ligands grafted onto silica surfaces [75]. The 29Si NMR spectra of the polysiloxane-immobilized thiol (Fig. 25.20C), thiol-monoamine (Fig. 25.24A) and thiol-diamine (Fig. 25.24B) systems, and the HCl-treated polysiloxane-immobilized thiol-diamine system (Fig. 25.24C) display two major regions of signals, centered at about -105 and - 6 0 ppm relative to TMS. These regions correspond t o S i ( ~ O ~ ) 4 and R S i ( ~ O ~ ) 3 units, respectively, where R is an organic (thiol, monoamine or diamine) ligand. As indicated above, the -105 ppm spectral region is composed of at least three peaks or shoulders, at about -109, -100 and - 9 1 p p m , which \ 9 \ . \ . correspond t o ( ~ S I ~ O ) 4 S i ; ( T S I ~ O ) 3 S i O H and/or (~SI~O)3SiOEt; and (~Si--)2Si(OH)2, (--~Si--O)2Si(OH)(OEt) and/or (--~Si--O)2Si(OEt)2 sites, respectively; these features can be readily distinguished in the 29Si CP-MAS spectrum of the polysiloxane-immobilized thiol system in Fig. 25.20C. The structures given below show the kinds of ligand-containing species that one can readily envision on the surface of the polysiloxane framework. Details of the attachment of silicon to the polysiloxane framework are intentionally left unspecified.

R R'O~/i/OR'

R\ /OR'

,

otiS\

i

I

0

XIII

0

I

XIV

\

R I

-/Sko-']

Solid State NMR of Polymers, Volume 84 (Studies in Physical and Theoretical Chemistry)

Solid State NMR (Topics in Current Chemistry)

Solid State NMR (Topics in Current Chemistry, Volume 306)

New Techniques in Solid-State NMR (Topics in Current Chemistry)

Solid State NMR Spectroscopy: Principles and Applications

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Multinuclear Solid-State NMR of Inorganic Materials

NMR of Polymers

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Solid State Electrochemistry (Chemistry of Solid State Materials)

Physical Acoustics in the Solid State

Physical acoustics in the solid state

Physical Acoustics in the Solid State

Concepts Of Theoretical Solid State Physics

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Advances in Heterocyclic Chemistry, Volume 84

New Directions in Solid State Chemistry

New Directions in Solid State Chemistry

Physical Chemistry of Polymer Solutions: Theoretical Background

Physical Chemistry, Volume 1

Physical Chemistry, Volume 1

Solid State Organometallic Chemistry: Methods and Applications

Solid-state Hydrogen Storage Materials and Chemistry

Physical Properties of Polymers

Computational Chemistry of Solid State Materials

Magnetic Ceramics (Chemistry of Solid State Materials)

Solid State NMR of Polymers, Volume 84 (Studies in Physical and Theoretical Chemistry)

Solid State NMR (Topics in Current Chemistry)

Solid State NMR (Topics in Current Chemistry, Volume 306)

New Techniques in Solid-State NMR (Topics in Current Chemistry)

Solid State NMR Spectroscopy: Principles and Applications

Organic and Physical Chemistry of Polymers

Solid State Organometallic Chemistry

Solid State Chemistry

Solid State Chemistry

Multinuclear Solid-State NMR of Inorganic Materials

NMR of Polymers

Dielectric Physics (Studies in physical and theoretical chemistry)

Solid State Electrochemistry (Chemistry of Solid State Materials)

Solid State Electrochemistry (Chemistry of Solid State Materials)

Physical Acoustics in the Solid State

Physical acoustics in the solid state

Physical Acoustics in the Solid State

Concepts Of Theoretical Solid State Physics

Physical Principles of Solid State Devices

Solid State Chemistry: An Introduction

Advances in Heterocyclic Chemistry, Volume 84

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New Directions in Solid State Chemistry

Physical Chemistry of Polymer Solutions: Theoretical Background

Physical Chemistry, Volume 1

Physical Chemistry, Volume 1

Solid State Organometallic Chemistry: Methods and Applications

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