Photophysics, Photochemistry, and Optical Properties of Polyimides

Prog. Polym. Sci. 26 (2001) 259±335 www.elsevier.com/locate/ppolysci Photophysics, photochemistry, and optical propert...

Author: Hasegawa M. | Horie K.

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Prog. Polym. Sci. 26 (2001) 259±335

www.elsevier.com/locate/ppolysci

Photophysics, photochemistry, and optical properties of polyimides M. Hasegawa a,*, K. Horie b a

Department of Chemistry, Faculty of Science, Toho University, Miyama 2-2-1, Funabashi, Chiba 274-8510, Japan Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-8656, Japan

b

Received 11 April 2000; revised 20 June 2000; accepted 24 October 2000

Abstract This review discusses charge transfer (CT) interactions in wholly aromatic polyimides (PI). Discussion was extended to the in¯uence of CT interactions on PI properties, i.e. photophysics, photochemistry, optical properties, and other properties. Section 2 deals with the electronic states and photophysics in PIs. Most of the experimental and theoretical data evidenced the presence of both intra- and interchain CT interactions in wholly aromatic PIs. A model compound approach made it possible to understand the detailed intramolecular photophysics including the intramolecular CT process. However, it is still a long way from obtaining quantitative information such as the concentration (population) and thermodynamic parameters of intermolecular CT complex (CTC) present in the PI solids. Wholly aromatic PIs show a broad/structureless ¯uorescence in a long-wavelength range, originating from intermolecular CTC. A few papers revealed that the CT ¯uorescence intensity increased corresponding to the degree of chain packing/staking. Thus, the CT ¯uorescence was applied to studies on local ordering and binary PI blend miscibility. In Section 3, the in¯uence of CTC on photoconductivity, mechanical and other properties is discussed. Low molecular weight donor-loaded PI exhibited prominent photoconductivity enhancement. Transient absorption measurements revealed that the CTC formed between the added donor and the acceptor moiety in the PI chains is responsible for the photoinduced charge generation process. The donor-free PI ®lm also showed a similar transient absorption spectrum to the donor-loaded PI ®lm, suggesting that even wholly aromatic PI itself can form CTC. The CTC sites in PIs may act as physical crosslinks. But it is not yet clear how the thermal and mechanical properties (glass transition temperature, thermoplasticity or melt viscosity, modulus, etc.) of PI ®lms are affected by CTC formation. A paper reported that CT-inhibited PIs, such as semi-aromatic PIs, tend to show a much higher voltage holding ratio, which is desirable for liquid crystal displays, than the corresponding wholly aromatic PIs. Section 4 reviews photosensitive PIs and their precursor systems. In this section, photochemistry in an intrinsically photosensitive polyimide containing a benzophenone unit and photodegradation in ¯uorine-containing PIs are also described. For benzophenone-containing PIs, the model compound approach was carried out to depict a detailed energy diagram including some photochemical kinetic parameters. The use of a series of model compounds elucidated a prominent conformation effect on the photoinduced hydrogen abstraction ef®ciency. * Corresponding author. E-mail address: [email protected] (M. Hasegawa). 0079-6700/01/$ - see front matter q 2001 Elsevier Science Ltd. All rights reserved. PII: S0 0 7 9 -6 7 0 0 (0 0 ) 0 0 04 2 - 3

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M. Hasegawa, K. Horie / Prog. Polym. Sci. 26 (2001) 259±335

The control of transparency and refractive indices and birefringence of PI ®lms and molecular-design were focused in Section 5. This section also describes why the control of such optical properties has been demanding for practical applications. Finally, non-linear optical (NLO) properties of several PIs and their related systems, modi®ed by attaching NLO chromophores, were reviewed. q 2001 Elsevier Science Ltd. All rights reserved. Keywords: Polyimides; Charge transfer; CT ¯uorescence; Photoconductivity; Melt viscosity; Photosensitive polyimides; Photocrosslinking; Hydrogen abstraction; Optically transparent polyimide; Refractive indices; Birefringence; In-plane orientation; Non-linear optics

Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Electronic states and photophysics of polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Donor±acceptor chain sequence in aromatic polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. CT transitions in polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.1. CT absorptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2.2. CT ¯uorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Photophysical processes in polyimides and their model compounds . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Model compound approach for the s-BPDA±PDA system . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Effect of the conformation around the N-phenyl linkage . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Energy diagram for s-BPDA±PDA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Theoretical approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Fluorescence studies on higher-order structures in polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Local ordering and CTC structure in polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. Ordered structure formation in the imidization process . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. PI/PI blend miscibility studied by CT ¯uorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.4. Perylenetetracarboxydiimide ¯uorescence yield associated with the CT ability of polyimide matrices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.5. PI/PI blend miscibility probed by PEDI ¯uorescence . . . . . . . . . . . . . . . . . . . . . . . . . . 3. Polyimide properties in¯uenced by CT interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Photoconductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Thermal, mechanical, and melt viscosity properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Voltage holding ratio in alignment layers for liquid crystal displays . . . . . . . . . . . . . . . . . . . . . 4. Photochemistry of polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Photosensitive polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.1. Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1.2. Negative working organo-soluble benzophenone-containing PIs and their photocrosslinking mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Photodegradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Model compounds approach for photochemistry of benzophenone-containing polyimides . . . . . . 5. Optical properties of polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Optically transparent polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.1. Control of CT character in the PI chain sequence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.2. Use of ¯uorinated monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1.3. Use of alicyclic monomers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Control of refractive indices in polyimides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. In-plane orientation responsible for birefringence in polyimide ®lms . . . . . . . . . . . . . . . . . . . . . 5.4. NLO properties in polyimide ®lms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

261 262 262 264 264 267 273 273 276 277 279 282 282 287 290 292 294 297 297 300 303 304 304 304 306 311 313 317 317 318 320 323 323 325 326 330 330


261

1. Introduction Polyimides (PIs) are a class of representative high-performance polymers possessing the cyclic imide and aromatic groups in the main chains. The most familiar polyimide is widely known as Kapton H w (Fig. 1), developed by Du Pont in the 1960s. After marketing of this polyimide, many other polyimides with some modi®ed properties were commercially developed. At present, they are extensively used in microelectronics, photonics, optics, and aerospace industries not only for their considerably excellent thermal stability but also for their good mechanical properties, low dielectric constant, low coef®cient of thermal expansion, and high radiation resistance. Signi®cant effort has been expended to improve their properties further by chemical modi®cation of the chain backbones and higher-order structure control [1±6]. On the other hand, in a view point of applying the inherent high-temperature characteristics of wholly aromatic polyimides, much attention has been paid to functional polyimides, for example, photosensitive polyimides [7]. Fundamental studies are becoming increasingly indispensable to produce high-performance novel photo- and opto-functional polyimides. This paper reviews the accumulated fundamental research results for photophysics, photochemistry,

Fig. 1. Reaction scheme for polyimide synthesis and chemical structures and abbreviations of monomers, poly(amic acid), and polyimides.

262


Table 1 Concept of photo- and opto-related phenomena and typical examples of photophysical, photochemical, and optical functions (reproduced from Ref. [8])

Functions

Examples

Photophysical functions

Organic photoconductors Electroluminescene Excitation energy Transfer Scintillators

Photochemical functions

Photoresists Photochromism Photochemical hole burning Photoresponsive polymers Photocatalysts Photoenergy conversion

Optical functions

Optical ®bers, waveguides Nonlinear optical materials Phase and frequency modulation devices Liquid crystals display devices Near-®eld optics

and optical properties of polyimides on the basis of a common key term ªcharge-transfer interactionsº and points out unresolved important problems. Major photo- and opto-processes in polymer systems can be classi®ed as listed in Table 1 [8]. 2. Electronic states and photophysics of polyimides 2.1. Donor±acceptor chain sequence in aromatic polyimides Wholly aromatic PIs like Kapton, insoluble in common organic solvents, are usually synthesized through a two-step method: PI precursors, high molecular weight poly(amic acid)s (PAAs) are readily synthesized from ring-opening polyaddition of an extra-pure dianhydride and an equimolar diamine in aprotic amide solvents such as N,N-dimethylacetamide (DMAc) and N-methyl-2-pyrrolidone (NMP), then the as-cast dried PAA ®lms are subjected to thermal or chemical imidization. The reaction scheme and the abbreviations of polymers and monomers are shown in Fig. 1. As already known, the PAA polymerization rates are dominated by the electron af®nity (EA) of dianhydrides as an electron acceptor and the ionization potential (IP) of diamines as an electron donor [9]. This suggests that the PAA polymerization proceeds via a charge transfer (CT) complex intermediate


263

Fig. 2. Relationship between the ionization potential of hydrocarbons and the peak wavenumber of the CT absorption and CT ¯uorescence bands in PMDA±hydrocarbons CT complexes in nonpolar solvents.

[9]. Indeed, one often observes the formation of a yellow-green solution due to the CT complex (CTC) at the initial polymerization stage, followed by yield of a viscous light-yellowish PAA solutions. Do wholly aromatic PIs also possess such CT interactions? Here, let us take up Kapton-type PI, often called ªPMDA±ODAº using the abbreviations of the monomers (PMDA pyromellitic dianhydride and ODA 4,4 0 -oxydianiline). The pyromellitimide fragment in the PMDA±ODA backbone has an electron-accepting ability close to that of the PMDA monomer [10]. On the other hand, one can notice that the electron-donating ability of the diamine residue, possessing two carbonyl group substituents on the nitrogen atom, should be much lower than that of the ODA monomer. Estimation of the electrondonating ability in the diamine residue could provide a criterion for whether PMDA±ODA can have the CT interactions. PMDA is a well-known low molecular weight strong electron acceptor in a large number of studies on 21 ) vs the values IP [11±15] CTCs. Fig. 2 shows the plot of the peak wavenumber in CT absorption bands (l max of the donor components in the PMDA±aromatic hydrocarbon systems reported in the literature [10,16±26]. A good linear relationship was observed according to Mulliken's theory for weak CTCs [27±29]: hnCT IP 2 EA 1 CC : constant

1

These studies showed that hydrocarbon electron donors stronger than benzene IP 9:24 eV form CT complexes unexceptionably in non-polar solvents. On the other hand, no discernible CT bands are observed for weaker hydrocarbons (higher IP) such as pyridine IP 9:25 eV; benzaldehyde IP 9:49 eV; binzonitrile IP 9:62 eV; and acrylonitrile IP 10:91 eV: As shown in Fig. 2, the Stokes 21 21 ± l ¯u ) is also almost proportional to IP. This will be discussed later. The results suggest shift (l abs intermolecular CTCs can be formed if the diamine portion in PMDA±ODA has an electron-donating ability higher than benzene. Gordina et al. [30] observed a new absorption band peaking around 450 nm in p-chloranil-doped PMDA±ODA ®lm. They concluded that this new band is attributed to intermolecular CTC between pchloranil EA 2:45 eV as a typical electron acceptor and the ODA moiety in the PI. Fig. 3 illustrates a 21 and IP in the chloranil±hydrocarbons pairs in non-polar solvents as good linear relationship between l abs

264


Fig. 3. Relationship between the ionization potential of hydrocarbons and the CT absorption peak wavenumber in chloranil± hydrocarbons systems.

reported in the literature [31±34]. This plot as a sort of ªcalibration curveº allows to estimate the donor ability of the ODA moiety in PMDA±ODA. The estimated value IP 8:19 eV close to IP of anisole (8.20 eV) satis®es the necessary condition for CTC formation in PMDA±ODA. However, an important point to note is that the CTCs in PI solids are most likely in a non-equilibrium. This suggests that thermal annealing of PIs at higher temperatures, which can make the chain packing denser, in turn, may cause the spectral red-shits of absorption and ¯uorescence bands by reduction of the donor±acceptor distance. 2.2. CT transitions in polyimides 2.2.1. CT absorptions Bikson et al. [35] observed a hypsochromic shift in the transmission spectra for a series of PIs derived from a ®xed diamine and three kinds of dianhydride, changing from PMDA, s-BPDA, and ODPA. They concluded that the color of PI ®lms is related to the extent of the diimide fragment conjugation (diimide coplanarity). Dine-Hart and Wright [36] discussed ®rst the color of PI ®lms in terms of CT interaction using PMDA-derived low molecular weight model compounds; the model crystals of PMDA±cyclohexylamine (white), PMDA±aniline (pale yellow), and PMDA±4-aminodiphenylether (bright yellow) signi®cantly changed in color with the electron-donating abilities of the monoamine fragments [36,37]. In the corresponding model compounds derived from s-BPDA (3,3 0 ,4,4 0 -biphenyltetracarboxylic dianhydride), similar color change occurs (Hasegawa, unpublished results). Low molecular weight CTCs in non-polar solvents show distinct CT absorption bands, located in the longer wavelength region in most cases, whereas no separated CT bands are observed in aromatic PI ®lms such as PMDA±ODA, as well as in the corresponding model compound [37]. This is probably due to overlapping of the absorption bands based on local electronic transitions and conjugations. The absence of distinct CT bands disturbs the presence of CT interactions from Mulliken's Plot. Instead, Kotov et al. [38] examined the correlation between the cut-off wavelength (absorption edge) and IP of the corresponding compounds of the diamine residue. A linear plot was obtained, suggesting that the coloration of PI ®lms is caused by CT interactions. They also pointed out that both the intra- and intermolecular CT interactions can contribute to the coloration and that they are dif®cult to separate experimentally. Ishida et al. [37] described that the longer wavelength absorption band (tail) in the PMDA±ODA ®lm


265

Fig. 4. (a) UV±vis absorption spectra of PMDA±ODA thin ®lm; and (b) the trimethyl phosphate solutions (1.5 £ 10 24 M) of the model compounds; N,N 0 -bis(phenoxyphenyl)pyromellitimide; and (c) N,N 0 -dicyclohexylpyromellitimide. Reproduced with permission from Macromolecules 1980;13:826. q 1980 American Chemical Society [37].

originates probably in the intramolecular CT transition. The PMDA±ODA thin ®lm (0.1 mm thick) exhibited an absorption spectrum very similar to that of the corresponding model compound, PMDA±4aminodiphenylether, in solution as shown in Fig. 4. Instead of the insoluble PMDA±ODA, this model compound was used to study the concentration dependence of absorbance (Abs) at 320, 360, and 400 nm in the solutions. Fig. 5 clearly displayed a 1.0 power law, corresponding to the intramolecular transition. On the other hand, Erskine et al. [39] showed direct evidence that the lower-energy absorption in the PMDA±ODA ®lm results not from intramolecular but from intermolecular CT interactions. Fig. 6 represents the hydrostatic pressure dependence of the transmission spectra of the Kapton ®lm (7.5 mm thick). It is clearly shown that the absorption edge red-shifted with increasing pressure. In addition, these spectral shifts were strictly reversible. They reasonably explained the pressure-induced spectral red-shift in terms of the CT theory as follows: the CT transition energy is expressed as hnCT 4b0 b1 1 E2 0:5 1 2 S201 21

2

E W1 2 W0 < IP 2 EA 2 e2 =rDA

3

266


Fig. 5. Log Abs±Log c plots at 320 and 360 nm for the trimethyl phosphate solutions and at 400 nm for the dichloroacetic acid solution of N,N 0 -bis(phenoxyphenyl)pyromellitimide. Reproduced with permission from Macromolecules 1980;13:826. q 1980 American Chemical Society [37].

b0 W01 2 W0 S01

4

b1 W01 2 W1 S01

5

where e and rDA represent the electron charge and the donor±acceptor distance, respectively. W0 and W1 denote the energies of the non-bond and perfectly charge-separated structures, respectively; W01 is the interaction matrix, which is approximately proportional to the overlap integral S01. In a weak CTC, S01 , 1 and 4b0 b1 p E2 : An increase in pressure leads to a decrease in rDA and to an increase in S01, resulting in an overall decrease in the CT transition energy. The intermolecular CTC formation was also con®rmed from blending two semi-aromatic PIs. 1 The 50/50 blend of a PI from aliphatic dianhydride and aromatic diamine with a PI from aromatic dianhydride and aliphatic diamine caused the formation of a new absorption band in the difference spectrum [40]. This blend system was highly miscible in contrast to the facts that PI/PI pairs with high molecular weights are immiscible in most cases. An attractive intermolecular CT interaction between the different semi-aromatic PI chains may be responsible for the good miscibility. The results of Ishida et al. and Erskine et al. lead us to conclude that PMDA±ODA includes both intraand intermolecular CT interactions in the solid state, although each contribution is unknown. This point will be discussed again later. There are important points to pay attention to when one discusses the PI ®lm coloration. The color of PI ®lms is very sensitive not only to the monomer structures but also to processing factors such as kinds of solvent, monomer purity, atmosphere, and cure conditions [41,42]. The use of NMP as a solvent for PAA polymerization tends to form more intensely colored PI ®lms than when DMAc and DMF are used, probably relating to a trace of oxidized residual solvent. Even if extra-pure reagent grade of diamines were used, a trace amount of some oxidized impurities contained in diamines lead to the occasionally severe coloration of the resultant PAA and PIs. A semi-aromatic PI derived from s-BPDA and an aliphatic diamine, trans-1,4-cyclohexanediamine (t-CHDA), in spite of a CT-inhibited PI, provides a colored ®lm owing to some brown oily impurity partially contained in the commercial t-CHDA. But, a completely colorless s-BPDA±t-CHDA PI ®lm can be obtained through careful puri®cation of t-CHDA 1 PI/PI blend ®lm was prepared upon thermal imidization of PAA/PAA blend ®lm cast from the mixed solution. Transamidation between the PAAs is actually suppressed since the solution mixing period is very short (5 min) in this experiment.


267

Fig. 6. Effects of pressure on the transmission spectra of Kapton ®lm (7.5 mm thick) at room temperature. Reproduced with permission from J Polymer Sci, Part C 1988;26:465. q 1988 John Wiley & Sons [39].

[43], in contrast to colored ®lms of the corresponding wholly aromatic PI, s-BPDA±PDA. This result supports the CT hypothesis as a main reason for coloration of wholly aromatic PIs. An increase in annealing promotes coloration of wholly aromatic PIs. This annealing effect probably results not only from the dense chain packing (increase in the intermolecular CTC population) but also from the thermal decomposition of thermally weak bonds such as terminal amines. As mentioned above, CT-inhibited sBPDA±t-CHDA polyimide provides a colorless ®lm when imidized at T , 2508C in vacuum. But, successive annealing at higher temperatures in vacuum or in N2 atmosphere causes appreciable coloration of the PI ®lm, suggesting that a trace of thermally decomposed products may be the origin of coloration in s-BPDA±t-CHDA. Thermogravimetric analysis (TGA) in Fig. 7 indicates that semiaromatic s-BPDA±t-CHDA has a much lower thermal stability than the corresponding wholly aromatic s-BPDA±PDA [43]. This is attributed to the much lower bonding energy at the cyclohexyl group. Thus, it should be noted that the color of PI ®lms is signi®cantly affected by many factors. The transparency of PI ®lms will be described in Section 5.1 in more detail. 2.2.2. CT ¯uorescence In a number of low molecular weight CT systems, CT ¯uorescence was also observed by excitation within the CT absorption bands. The characteristics of CT ¯uorescence are a spectral red-shift and a

Fig. 7. TGA curves of s-BPDA±PDA and s-BPDA±t-CHDA ®lms cured at 3508C/1 h in a nitrogen ¯ow at a heating rate of 108C min 21.

268


Fig. 8. Absorption and ¯uorescence spectra for PMDA±naphthalene CT complex in carbon tetrachloride. Reproduced with permission from Spectrochim Acta A 1967;23:2487. q 1967 Elsevier Science [44].

¯uorescence yield decrease with an increase in CT ability. As a typical example, the CT ¯uorescence spectrum of PMDA±naphthalene system [44] is illustrated in Fig. 8. In general, the CT ¯uorescence bands are located in a lower energy region than the corresponding CT absorption bands as a result of the Stokes shifts, suggesting that fully separated CT ¯uorescence may be observed for wholly aromatic PIs. Barashkov et al. [45] mentioned that the PI ®lm of PMDA±ODA was non-¯uorescent, whereas the PAA ®lm showed a blue ¯uorescence peaking around 470 nm. For this reason, the ¯uorescence of PIs have been paid no attention for a long time since the appearance of Kapton. Wachsman and Frank [46] observed a slightly structured appreciable ¯uorescence of the PMDA±ODA polyimide ®lm, centered at 575 nm, and stated that this ¯uorescence may originate in CT interactions. Hasegawa et al. [40,47±49] ®rst demonstrated experimentally that wholly aromatic PIs ®lms show CT ¯uorescence, on the basis of Mulliken's CT theory. The ®rst approach was to measure the ¯uorescence spectra of several commercial PI ®lms (50 mm thick). All the commercial samples used exhibited broad and structureless ¯uorescence in the long-wavelength range as shown in Fig. 9. Another spectral feature is that the ¯uorescence intensity tends to reduce as the peak positions shift toward lower energy. This phenomenon is characteristic of low molecular weight CTC systems. However, one may have a question that this PI ¯uorescence may be due to some ¯uorescent species in these commercial PI ®lms, which contain many unknown additives. Fig. 10a exhibits the ¯uorescence spectra of labo-made additive-free PI ®lms with a ®xed diamine component (PDA). It should be noted that a good linear relationship, according to the expression: hnCT 2EA 1 C

6

is observed between the ¯uorescence peak wavenumber and EA of the dianhydrides used (Fig. 10b), suggesting that this is CT ¯uorescence. However, it is not easy to separate the intermolecular and intramolecular CT ¯uorescence as for the CT absorption. Wachsman and Frank [46] observed a ¯uorescence intensity enhancement with increasing ®nal cure temperature for the PMDA±ODA system and proposed that this behavior is caused either by coplanarization between the benzimide and the N-phenyl molecular planes via intramolecular conformational rearrangement, or by intermolecular CTC formation promoted by denser molecular aggregation. Similar cure temperature effect was also commonly observed in other wholly aromatic PI systems [49]. Hasegawa et al. selected s-BPDA±PDA polyimide, which gives a comparative strong ¯uorescence


269

Fig. 9. Uncorrected ¯uorescence spectra of several commercial PI ®lms (50 mm thick) upon excitation at 300 nm.

Fig. 10. (a) Corrected ¯uorescence spectra of labo-made PI ®lms derived from a series of dianhydrides and a ®xed diamine (PDA) upon excitation at 350 nm at room temperature; and (b) relationship between the ¯uorescence peak wavenumber and the dianhydride electron af®nity (EA). The data of EA were taken from Ref. [9].

270


Fig. 11. Corrected ¯uorescence lex 300 nm; excitation lem 550 nm; and absorption spectra of s-BPDA±PDA thin ®lm cured at 2508C/1 h in vacuum on a quartz substrate.

among wholly aromatic PIs, to investigate the ¯uorescence behavior in detail. To compare strictly the excitation and UV±vis absorption spectra, very thin ®lms (Abs , 0.15) were prepared. Fig. 11 displays the ¯uorescence, excitation, and UV±vis absorption spectra of an s-BPDA±PDA thin ®lm cured under a mild thermal condition (2508C/1 h in vacuum) [50]. Here, the ¯uorescence and excitation spectra were corrected for the wavelength-dependent detector sensitivity and for the wavelength distribution of Xe lamp intensity, respectively. One notices that the UV±vis absorption spectral shape is completely identical with the excitation spectrum, suggesting that the CT emission occurs much more ef®ciently via non-radiative energy transfer from the local photoexcitation of each chromophore in the PI backbone to the CTC sites than the direct excitation of the CT absorption bands. The ®lms of s-BPDA±PDA (50 mm thick) were prepared under a variety of thermal cure (annealing) conditions as listed in Table 2. More severe thermal conditions that give more intensive molecular mobility led to higher density PI ®lms and to an increase in the CT ¯uorescence intensity without signi®cant spectral changes in the shape and position. Fig. 12 clearly indicates a correlation where the CT ¯uorescence of these PI ®lms enhances in accordance with an increase in ®lm density representing an extent of molecular packing [40,49,51]. s-BPDA±PDA undergoes no appreciable thermal decomposition under the present conditions (T , 4008C in vacuum) as evidenced by IR measurements and TGA. The s-BPDA±PDA ®lm chemically imidized at room temperature using a dehydrating reagent Table 2 Thermal cure histories and densities of s-BPDA±PDA ®lms Sample no.

1 2 3 4 5 6 7

Cure history 2008C/5 h

3008C/1 h

4008C/1 h

W W W W ± ± ±

± W ± W W W ±

± ± W W ± W W


271

Fig. 12. Relationship between ®lm density and CT ¯uorescence intensity lex 350 nm for s-BPDA±PDA ®lms cured in vacuum under various thermal conditions listed in Table 1.

(acetic anhydride±pyridine mixture) also provides essentially the same ¯uorescence spectrum, but weaker than those of the thermally cured PI ®lms, corresponding to the looser molecular packing. Accordingly, it is evident that the observed ¯uorescence intensity enhancement in Fig. 12 is not attributed to the formation of a trace of thermally decomposed unknown ¯uorescent products. In addition, the CT ¯uorescence intensity of the resultant PI ®lms, cured at a ®xed thermal condition (T 250 or 3308C), increased linearly with the draw ratio in the uniaxial stretching of the PAA ®lms [49]. This orientation effect emphasizes again that the CT ¯uorescence is very sensitive to the local structures associated with chain stacking and aggregation. Thus, these results led to the conclusion that the local structure-sensitive CT ¯uorescence of s-BPDA±PDA is from the intermolecular CTC. For the s-BPDA±PDA ®lm cured at 2508C (imidization is almost completed on the IR spectrum), successive annealing at 3308C altered the excitation spectrum signi®cantly. Fig. 13 shows the formation of a new ¯uorescence excitation band peaking at 465 nm in addition to a broad excitation band peaking around 350 nm. This annealing-induced new excitation band formation corresponds to a new absorption band appeared in the difference spectrum as an origin of ®lm coloration [49]. The new excitation band is not related to thermal decomposition since a relationship similar to the plot in Fig. 10b, according to Mulliken's theory, was also observed for the annealing-induced excitation bands of a series of PIs [49]. It

Fig. 13. Uncorrected ¯uorescence (2, 4, 5) and excitation spectra (1, 3) of s-BPDA±PDA ®lms (50 mm thick) cured at 2508C/2 h (1, 2) in vacuum and successively annealed at 3308C/2 h (3, 4, 5). Fluorescence spectra were taken upon excitation at 350 nm (2, 4) and 465 nm (5).

272


can be assumed that the new excitation band at 465 nm re¯ects an abrupt increase in the intermolecular CTC population, which allows the direct CT band excitation. However, strictly speaking, the ¯uorescence upon excitation at 465 nm slightly differs from that obtained via the 350 nm excitation in the peak position and band width. Additionally, these ¯uorescence components show different draw ratio dependence on the intensity of each other [49]. This may mean the presence of at least two different con®gurations of CTC. Thus, it is far from being fully understood, for the CTC structures present in PIs, as discussed again later. However, it is most likely that the intermolecular CTC mainly contributes to the ¯uorescence in the solid state of wholly aromatic PIs, from the fact that the ¯uorescence intensity of the PI ®lm prepared at 4008C for 1 h (rapid cure) was approximately 10 times higher than that cured upon the mildest condition (2008C/5 h), as shown in Fig. 12. In other words, the intramolecular CT ¯uorescence is much weaker than the intermolecular one. This may be rationalized in terms of a model compound result in which the benzimide±phenyl coplanarization leads to an essentially non-¯uorescent character as will be shown later. The intermolecular CTC ¯uorescence hypothesis is also supported from the miscibility-sensitive CT ¯uorescence intensity in a PI binary blend system composed of non-¯uorescent and ¯uorescent PI, as discussed later. Another important thing is the intermolecular CTC concentration ([CTC]) in PI solids. In addition to the bonding energy of CTC in PIs, [CTC] must also in¯uence signi®cantly the PI physical properties. Nevertheless, to our knowledge, no approaches to determine [CTC] were so far reported in the literature. This parameter can be estimated from the peak wavelengths and molar extinction coef®cients of the intermolecular CTC absorption bands. Although no distinct CT absorption peak is observed for the PMDA±ODA, the ®lms annealed at higher temperatures have excitation spectra with a distinct peak as in the s-BPDA±PDA system. Fig. 14 shows CT ¯uorescence peaking at 590±620 nm and an excitation band at 520±540 nm in the PMDA± ODA ®lm, depending on the cured conditions. One notices a mirror image between the excitation and ¯uorescence bands with energy gaps about 2300±2400 cm 21. These values are much lower than the Stokes shift (,7000 cm 21) in the CTC in CCl4, composed of PMDA and 1,2,4-trimethylbenzene IP 8:27 eV or mesitylene IP 8:39 eV in which the IP values are approximate to that of the ODA residue in PMDA±ODA (see Fig. 2) [20]. The much higher Stokes shift for the CTC in CCl4 can be explained by taking into account the structural relaxation allowed in solution during the excited lifetime and the consequent stabilization, in contrast to the restricted structural relaxation of the CTC in the PMDA± ODA solid. Accordingly, it is likely that the CT absorption peak in the PMDA±ODA ®lm is located in the same wavelength region as the excitation peak at 520±540 nm. As in the s-BPDA±PDA system, the long-wavelength excitation band has a trend to enhance with an increase in cure temperature, which makes the molecular packing denser, as illustrated in Fig. 14. Again, it is reasonably assumed that the long-wavelength excitation band originates in the intermolecular CTC. If only the intermolecular CTC contributes to the absorption in this wavelength region, its concentration in the PMDA±ODA ®lm, [CTC], is roughly estimated from a simple equation using a conveniently determined e unit with respect to the repeating unit: Abs eCTC CTCL eunit repeating unitL

7

where e CTC and L represent the molar extinction coef®cients (in M 21 cm 21) for the intermolecular CT complex (CTC) absorption and ®lm thickness (in cm), respectively. N,N 0 -di-n-pentyl pyromellitimide± anthracene CT complex in chloroform possesses eCTC 1200 M21 cm21 at the CT absorption


273

Fig. 14. Uncorrected ¯uorescence and excitation spectra of PMDA±ODA ®lm (50 mm thick) cured at (1) 908C/3.5 h 1 1508C/ 8 h 1 1908C/4 h 1 2408C/5.5 h stepwise; (2) 2508C/12 h; (3) 3508C/12 h; and (4) 4308C/2 h in vacuum.

maximum [10] (in acceptor (such as p-chloranil)±aromatic hydrocarbons CTC systems in non-polar solvents, the value of e ranges at 2000±3000 M 21 cm 21 in many cases, see, e.g. Ref. [52]). which can be determined by the Benesi±Hildebrand method [53]. If one assumes that this e CTC value is also roughly applicable to the CTC in the PMDA±ODA ®lm, using [repeating unit] , 3 M and e unit , 10 M 21 cm 21 at 520 nm, which was represented with respect to the repeating unit [38,45], CTC 0:025 M in the PMDA±ODA ®lm is obtained. Note that the conventional e unit value in this wavelength region depends strongly on not only the cure conditions but also on many other processing factors as pointed out in Section 2.2.1. Discussion on the intramolecular CT has been given from the approaches of the PI model compounds and molecular orbital (MO) calculations by several research groups as described in the following sections. 2.3. Photophysical processes in polyimides and their model compounds 2.3.1. Model compound approach for the s-BPDA±PDA system The spectra of dilute PI solutions must provide information on the intramolecular CT. But in many cases, the very limited solubility of PIs disturbs this approach. The use of low molecular weight model compounds may solve this problem, if they re¯ect the electronic states of PIs well. It is most important to learn the nature of the lowest energy excited state for elucidating the emission mechanisms. A comparison of the spectra of the model compounds corresponding to each structural unit in the PIs allowed to estimate the degree of conjugation and to assign the lowest energy excited state. In this viewpoint, sBPDA±PDA was studied using various model compounds in detail [50]. Fig. 15a exhibits the ultraviolet absorption spectra of the diamine (PDA) residues in dilute solution. Keep in mind that the s-BPDA± PDA ®lm displays a broad structureless absorption band in the range 300±450 nm with a small shoulder at 350 nm (see Fig. 11). In contrast, the diamine residue has only an absorption below 280 nm with a slightly structured benzenoid band around 250±270 nm, indicating no contribution to the lowest energy band in s-BPDA±PDA. On the other hand, one notices that N,N-dicyclohexyl-substituted biphenyltetracarboxydiimide, CHA±s-BPDA±CHA, possesses a strong absorption band over 300±400 nm (Fig. 15b). Its spectrum is more intensive and located in the longer wavelength region than the twofold spectrum of the corresponding monomeric compound, N-cyclohexyl phthalimide. The results indicate

274


Fig. 15. UV±vis absorption spectra of the model compounds of s-BPDA±PDA in hexa¯uoro-2-propanol (HFP) solution: (1) SA±AN; (2) SA±PDA±SA; (3) PA±CHA; (4) CHA±s-BPDA±CHA; (5) PA±AN; (6) PA±PDA±PA; and (7) AN±s-BPDA± AN. HFP was used here because of the limited solubility of the model compounds (especially, AN±s-BPDA±AN) to common organic solvents.

that there exists a certain degree of the electronic conjugation through the biphenyl linkage in CHA±sBPDA±CHA [54]. In comparison, N,N-diphenyl-substituted AN±s-BPDA±AN showed a similar but a slightly more intensive spectrum in almost the same region, suggesting that the conjugation between the benzimide and the N-phenyl groups is not signi®cant. On the other hand, another structural model, PA± PDA±PA, was also shown for comparison in Fig. 15c. This compound gave a spectrum similar in shape and location to that of PA±AN in a much shorter wavelength region than those of two biphenyldiimide compounds. Accordingly, these results led to the conclusion that the biphenyldiimide structural unit, which corresponds to CHA±s-BPDA±CHA, mainly contributes to the lowest energy band in s-BPDA± PDA [50]. From the comparatively high molar extinction coef®cient (e ca. 10,000 M 21 cm 21) a strong band at


275

Fig. 16. Fluorescence and excitation spectra and the ¯uorescence anisotropy ratio as a function of wavelength within the ¯uorescence and excitation bands for CHA±s-BPDA±CHA molecularly dispersed in PMMA c 1024 M:

320 nm in AN±s-BPDA±AN is observed; this absorption band can be assigned to (p, p p) transition [55]. However, it is known that carbonyl compounds often possess (n, p p) bands as the lowest energy transition (S1). This means that bipenyldiimide may include a (n, p p) band in the absorption tail. Fluorescence polarization measurements can be helpful for assignment of S1 in this case. In general, the degree of ¯uorescence polarization does not alter signi®cantly in the S1 ! S0 transition successive to the S0 ! S1 transition under the special condition without both rotational diffusion of the ¯uorophore itself and non-radiative (also radiative) energy migration between ¯uorophores. On the other hand, the excitation to the higher energy singlet excited states (S2, S3, etc.) frequently causes a considerable change in the degree of ¯uorescence polarization. Fig. 16 shows the ¯uorescence and excitation spectra of the model compound, CHA±s-BPDA±CHA, in an inert polymeric matrix, poly(methyl methacrylate; PMMA) at a very low concentration (10 24 M). At this concentration, the energy migration is practically suppressed. The ¯uorescence anisotropy ratio (r), which represents the degree of ¯uorescence polarization, is given by r Ivv 2 GIvh =Ivv 1 2GIvh

8

where Ivv and Ivh represent vertically and horizontally polarized emission intensities obtained by excitation with the vertically polarized beam, respectively, and G Ihv =Ihh is a correction factor for detector sensitivity to the polarization directions. The value of r ranges from 20.2 to 0.4. In many cases, a value close to the upper limit under the special condition mentioned above. Fig. 16 shows that the value of r is close to the upper limit and remains constant within both the 320 nm excitation (absorption) and the ¯uorescence bands. The results led to the conclusion that the 320 nm absorption band corresponds to the S0 ! S1 (p, p p) transition [50]. Note that CHA±s-BPDA±CHA has a sharp ¯uorescence peaking at 385 nm in the PMMA matrix and in solutions such as dichloromethane. The corresponding PI, s-BPDA±t-CHDA ®lm also gives a similar but slightly red-shifted ¯uorescence to that of this model compound. It should be noted that the ¯uorescence of the wholly aromatic s-BPDA±PDA ®lm includes no 385 nm ¯uorescence component from the biphenyldiimide fragment (see Fig. 11). A similar situation was also seen in a wholly aromatic PI containing naphthalene-1,4,5,8-tetracarboxydiimide unit which is usually highly ¯uorescent [56]. The results suggest the presence of a quenching mechanism for the biphenyldiimide ¯uorescence in s-BPDA±PDA. Such a quenching mechanism must be a key to clarify the photophysical processes in wholly aromatic PIs.

276


Fig. 17. UV±vis absorption spectra of four model compounds in dichloromethane c 1:0 £ 1024 M:

2.3.2. Effect of the conformation around the N-phenyl linkage A series of model compounds with different N-substituents were compared for the absorption and ¯uorescence spectra. Note the position of the substituents on the N-phenyl group in biphenyldiimides. The bulky ortho-substituents (methyl and isopropyl groups in this case) cause considerable distortion between two molecular planes owing to steric hindrance; consequently, a decrease in conjugation. Fig. 17 clearly shows that these ortho-substituted compounds have similar absorption spectra in shape to that of the N-cyclohexyl substituted one (CHA±s-BPDA±CHA), owing to the almost inhibited benzimide± N-phenyl conjugation. On the other hand, for the meta-substituted compound without such steric hindrance, a spectral red-shift was observed with a spectral deformation of the 320 nm band, although the extent of the conjugation is not as high as mentioned previously. These substituent effects on the most stable dihedral angle around the N-phenyl linkage was con®rmed by the MO calculations. Tokita et al. [57] calculated the potential surfaces of meta-ethyl and orthodiethyl-substituted N-phenylphthalimides using the ab initio method (gaussian 92, basis set: STO-3G) and determined the most stable dihedral angles (v ms) to be 33 and 738, respectively. Ando et al. [58] carried out the ab initio calculation using a larger basis set (gaussian 94: 6-31G pp) and obtained that vms 598 for N-phenylphthalimide and vms 908 for N-(2,2 0 -dimethylphenyl)phthalimide. This calculated value for N-phenylphthalimide is in good agreement with an experimental value v 608 based on the crystallographic analysis of PA±ODA±PA as a model of PMDA±ODA [59]. LaFemina et al. [60] measured the absorption spectra of the PMDA±ODA PI ®lm and its model compounds and obtained vms 308 using the spectroscopically parameterized CNDO/S3 model. These calculations also led to a qualitatively consistent conclusion in which the coplanar state formation is practically forbidden for the ortho-substituted compounds owing to a considerable high energy barrier at v 08; whereas the ortho-substituents-free compounds, in which the N-phenyl rotation is allowed owing to a very low barrier (a few kcal mol 21), possess a certain degree of conjugation. Fig. 18 displays the ¯uorescence spectra of these compounds in dilute dichloromethane solution. For all the compounds, the ¯uorescence excitation spectra completely agreed with the corresponding absorption spectra, indicating that these ¯uorescences are not from some unknown ¯uorescent species. Whereas CHA±s-BPDA±CHA had a sharp and strong ¯uorescence at 385 nm (¯uorescence yield Ff


277

Fig. 18. Fluorescence and excitation spectra of four model compounds in dichloromethane (Abs , 0.2). The numbers indicated represent the compounds shown in Fig. 17.

0:04; three other compounds showed very broad and much weaker ¯uorescences. The ¯uorescence of the meta-substituted compound with the strongest conjugation is weaker and red-shifted than that of the ortho-substituted ones. These spectral features lead us to propose that these broad ¯uorescences result from an intramolecular CT state [50]. The ortho-substituted compounds, which can form no coplanar structure, showed unexpectedly no 385 nm ¯uorescence from the biphenyldiimide unit in spite of its inhibited benzimide±N-phenyl conjugation. This probably means that the intramolecular CT can take place at the excited state even in the signi®cantly distorted conformation. In the preceding section, we mentioned that the enhancement of CT ¯uorescence in the s-BPDA±PDA ®lm with increasing cure temperature probably occurs by an increased intermolecular CTC population rather than by the coplanarization between the benzimide and N-phenyl planes. This hypothesis is based on the practically non¯uorescent tendency of the coplanarization-allowed meta-substituted model compound in solution. 2.3.3. Energy diagram for s-BPDA±PDA CHA±s-BPDA±CHA also exhibits a green phosphorescence over 500±600 nm with a long life time ( a few seconds) only in a degassed rigid glass at 77 K. The phosphorescence excitation spectrum agreed with the 385 nm ¯uorescence excitation spectrum. According to a criteria to judge the T1 nature from the singlet±triplet splitting and the phosphorescence lifetime, T1 can be assigned as a (p, p p) transition. Similar phosphorescence was also observed in PMMA matrix at room temperature in air, but there was no phosphorescence for the meta-substituted compound even in a degassed rigid glass at 77 K, as for the s-BPDA±PDA ®lm at room temperature. This means that the phosphorescence in s-BPDA± PDA is also subjected to quenching, in addition to the 385 nm ¯uorescence [50]. On the basis of the overall results, an energy diagram for CT-allowing AN±s-BPDA±AN can be depicted as shown in Fig. 19. In the proposed mechanism, the intermolecular CT ¯uorescence emission occurs via two different pathways: ®rst, the photoinduced electron transfer from a local excited state at the biphenyldiimide unit to the spatially adjacent ground-state PDA residue, and second, the direct excitation at the CT absorption band. The ®rst process is possible even at very low CTC concentration as in the PI ®lm cured at a low temperature such as 2008C. Such a photoinduced electron transfer mechanism will be theoretically discussed again later. In fully aromatic s-BPDA±PDA, both the ¯uorescence from S1 (p, p p) and phosphorescence from T1 (p, p p) are not observed practically. The results are probably attributed to the considerably fast CT process from S1 (p, p p).

278


Fig. 19. Energy diagram for photophysical processes in AN±s-BPDA±AN.

The photophysical kinetic parameters can be estimated on the basis of this energy diagram. For CTinhibiting CHA±s-BPDA±CHA, the ¯uorescence yield, F f, of the 385 nm ¯uorescence is expressed as

Ff 385 nm kf =kf 1 kds 1 kisc kf tf

9

where kf, kds, kisc, and t f are the rate constants (in s 21) of the 385 nm ¯uorescence emission, deactivation, and intersystem crossing, and the singlet lifetime in seconds, respectively. From Eq. (9), the measured t f ( ca. 1 ns) and F f ( 0.05) in PMMA derived kf 5 £ 107 s21 and kds 1 kisc 109 s21 : On the other hand, For CT-allowing AN±s-BPDA±AN, it is reasonable to assume that its S1 (p, p p) state is similar, in the energy and electronic con®guration, to that of CHA±s-BPDA±CHA, but additionally possess the intramolecular CT process from S1 (p, p p) to D 1A 2 state. Accordingly, the most probable energy diagram for AN±s-BPDA±AN as the best model of wholly aromatic s-BPDA±PDA polyimide is depicted in Fig. 19. Since AN±s-BPDA±AN shows no 385 nm ¯uorescence even in PMMA, it is reasonable to assume:

Ff 385 nm kf =kf 1 kds 1 kisc 1 kCT , 1024

10

where kCT denotes the rate constant for intramolecular CT process. Eq. (10) gives kCT . 5 £ 1011 s21 : Thus, this estimated value con®rmed that the intramolecular CT process is much faster than other competitive processes. The intramolecular CT ¯uorescence yield is represented by:

Ff D1 A2 FCT kf D1 A2 =kf D1 A2 1 kd D1 A2 < kf D1 A2 tD1 A2

11

where F CT is the intramolecular CT transition yield, which is regarded as almost unity. Using the experimental values, Ff D1 A2 8 £ 1023 and tD1 A2 2 ns; one obtains kf D1 A2 4 £ 106 s21 and kd D1 A2 5 £ 108 s21 : The estimated values means that the deactivation process occurs ef®ciently via the intramolecular CT (D 1A 2) state. This effective thermal deactivation pathway is most likely common to wholly aromatic PIs and probably related closely to the high UV light resistance characteristic of wholly aromatic PIs. More recently, it was reported that N-(2-methylphenyl)-2,3-naphthalimide exhibits a dual ¯uorescence band consisting of a structured ¯uorescence similar to that of N-alkyl-2,3-naphthalimide and a CT-like broad ¯uorescence [61]. This result is qualitatively consistent with the energy scheme drawn for AN±s-BPDA±AN.


279

Fig. 20. Orbital charge density for HOMO and LUMO in PMDA±ODA. Reproduced with permission from J Chem Phys 1989;90:5154. q 1989 American Institute of Physics [60].

2.3.4. Theoretical approaches It is known that the UV±vis absorption of the PMDA±ODA ®lm consists of three peaks centered at 6.4 eV (194 nm), 5.9 eV (210 nm), 4.4 eV (280 nm), and a very weak absorption tail around 3.3 eV (376 nm) [37,45,60,62,63]. LaFemina et al. [60,64], on the basis of the experimentally measured absorption spectrum, computed the electronic transition energies in PMDA±ODA using the spectroscopically parameterized CNDO/S3 model and compared with the experimental results. Fig. 20 schematically depicts the orbital charge density for HOMO and LUMO in the PMDA±ODA. It should be noted that the charge at HOMO and LUMO is localized on the ODA and PMDA residues, respectively. This means that CT can take place via the one-electron HOMO ! LUMO transition. Bredas and Clarke [65] earlier observed similar charge segregation behavior on the valence effective Hamiltonian nonempirical method for PMDA±ODA. Matsumoto [66,67] also con®rmed this phenomenon from the INDO/S MO calculation for a large model compound, PA±ODA±PMDA±ODA±PA. The results of LaFemina et al. [60,64] led to 3.0 eV (413 nm) as the HOMO±LUMO transition with a considerably low oscillator strength, corresponding the very weak lowest energy absorption tail in the PMDA±ODA ®lm. The superjacent (second) LUMO also displays similar charge localization on the PMDA fragment, indicating that the HOMO±superjacent LUMO transition also causes CT. This transition at 4.4 eV (280 nm), which corresponds well to the actual absorption peak, occurs with a much higher transition probability (oscillator strength) than the HOMO±LUMO transition at 3.0 eV. In addition to these calculated transitions, the 320 nm band due to the pyromellitimide fragment [37,45], which is actually observed as a shoulder in the PMDA±ODA spectrum, should also be involved. Nonetheless, no 320 nm transition was shown in the calculated transitions. Fig. 21a shows an energy diagram drawn by the present authors according to the descriptions mentioned above. LaFemina et al. [68] also observed the very weak CT ¯uorescence upon excitation at 290 nm Ff 9:7 £ 1027 and proposed a simple emission mechanism in PMDA±ODA as shown in Fig. 21b. The preceding section described the prominent N-substituent effect on the emission behavior. Their monomeric compounds, N-substituted phthalimides also show a very similar spectral features; N-cyclohexylphthalimide emits a normal ¯uorescence in dichloromethane solution and N-phenylphthalimide leads to a much weaker and broader ¯uorescence above 500 nm, whereas N-phenylphthalimide (PA± AN) and N-cyclohexylphthalimide (PA±CHA) give a very similar absorption spectra peaking at 300 nm (see Fig. 15b and c) [50]. The comparison of these absorption spectra suggests that photoirradiation at 300 nm causes the excitation of the p electron localized on the phthalimide fragment. Tokita et al. [57] examined the electronic structure of the simpler models, N±arylphthalimides, by means of a semi-empirical

280


Fig. 21. Schematic diagrams for: (a) HOMO±LUMO and HOMO±second LUMO transitions; and (b) CT emission in PMDA± ODA. Reproduced with permission from Polymer 1990;31:840. q 1990 Elsevier Science [68].

MO calculation (mopac 5.0, potential function: AM1) and con®rmed this consideration. Fig. 22 displays the orbital charge density at LUMO, HOMO, second HOMO, and third HOMO in N-(3-ethylphenyl)phthalimide. As in the PMDA±ODA system, the charge segregation behavior is clearly shown between HOMO and LUMO. Similar HOMO±LUMO charge segregation was observed even in N-(2,6-diethylphenyl)phthalimide possessing a signi®cantly distorted conformation (not shown). The charge localized on the phthalimide fragment at the third HOMO indicates that the 300 nm excitation corresponds to the third HOMO±LUMO transition with a rather high transition probability than the HOMO±LUMO transition (direct excitation of the CT band). Possible successive processes are


281

Fig. 22. p Electron densities of ®rst, second, and third HOMO and LUMO for N-(3-ethylphenyl)phthalimide). White and black circles are positive and negative wave functions, respectively. The area of the circles is proportional to the p electron densities.

Fig. 23. Electronic con®gurations at the ground- and excited states for N±arylphthalimides.

282


proposed in Fig. 23. CT can occur via the HOMO±third HOMO transition from S1. Such a photoinduced electron transfer mechanism is commonly observed in the low molecular weight excited complex (exciplex) systems; the excitation of acceptor and donor lead to an increased EA of acceptor and a decreased IP of donor, and consequently promotes CT at the excited state. It is most likely that the same mechanism is included in the emission processes even for wholly aromatic PI systems. Even in orthosubstituted N-(2,6-diethylphenyl)phthalimide, similar charge orbital behavior was observed, corresponding to the fact that even the highly distorted conformation showed no normal phthalimide ¯uorescence but only a broad and weak ¯uorescence in the longer wavelength region. Salley and Frank [69] reviewed CT phenomena in aromatic PIs and discussed the relationship between the ¯uorescence behavior and the possible PI chain stacking modes including the preferred layer packing (PLP) where neighboring PI chains take the face-to-face arrangement between the imide groups and mixed layer packing (MLP) where the imide±(diamine) aromatic group (CT) interaction is allowed between the neighboring PI chains. The presence of both crystal structures were con®rmed in low molecular weight model compounds, depending strongly on the model compound chemistry, as will be described in the next section, actual higher-order structures in PI ®lm are not so simple that still remains unknown. 2.4. Fluorescence studies on higher-order structures in polyimides 2.4.1. Local ordering and CTC structure in polyimides Discussion in the preceding sections pointed out that the intermolecular CT ¯uorescence is sensitive to the degree of molecular packing of PI chains. Thus, monitoring of the CT ¯uorescence could be a promising tool to research local ordering, particularly, in the amorphous region for wholly aromatic PIs. In this section, local ordered structure formation probed by the CT ¯uorescences will be described. Among PIs, PMDA±ODA has been best investigated for its higher-order structures. Highly crystalline PMDA±ODA ®ber was well characterized by wide-angle X-ray diffraction (WAXD) [70,71]; it has an orthorhombic unit cell with the lattice parameters of 0.631 (a-axis), 0.397 (b-axis), and 3.2 nm (caxis). In contrast, the morphology of the PMDA±ODA ®lm is less understood. Several research groups revealed that the initial imidization (cure) temperature, Ti, is very critical in developing the morphology of PMDA±ODA [72±74]. This prominent Ti effect is associated with its highly restricted molecular motion even above the Tg, arising from the strong interchain interactions and the inherent chain stiffness of this polymer. The higher-order structures formed in the PMDA±ODA are rather complex, unlike common semi-crystalline polymers such as poly(ethylene terephthalate) that have a simple crystal/ amorphous two-phase structure. In the PMDA±ODA, a sign of crystallization begins to appear at Ti's exceeding 4008C as seen in the WAXD patterns in Fig. 24 [74]. On the other hand, even at Ti , 4008C; a prominent local ordered structure formation takes place in the amorphous region. For example, Isoda et al. [72] illustrated a prominent increase in the small-angle X-ray scattering (SAXS) intensity with a peak shift to the lower scattering angle (2u ) with increasing cure temperature for the free-standing PMDA± ODA ®lm (Fig. 25a). They analyzed these SAXS patterns using a one-dimensional model based on the ordered/less-ordered layered structure depicted in Fig. 25b. In combination with the measured ®lm densities, the good curve ®tting results derived that the ordered phase volume fraction increased from 12 to 19% (corresponding to the increased SAXS intensity) and the average thickness of the ordered Ê (corresponding the lower 2u shift of the SAXS peak) with varying lamellae increased from 13 to 23 A cure condition from 2508C/12 h to 4308C/2 h in vacuum. Note that the ordered phase has a denser chain packing r 1:49 g cm23 than the less-ordered phase (1.39 g cm 23) but differs from the crystal phase


283

Fig. 24. Re¯ection and transmission mode WAXD patterns of PMDA±ODA ®lms cured at different temperatures. Reproduced with permission from Polyimides: fundamentals and applications, 1996, p. 207. q 1996 Marcel Dekker, Inc. [74].

[71] (1.580 g cm 23). On the other hand, Russell [73] obtained a con¯icting pro®le in which the SAXS intensity reduced with a lower scattering vector-shift with an increase in Ti for the same PI ®lm (Fig. 25c). He explained the decreased SAXS intensity as a result of a decrease in the electron density modulation (difference) between the ordered/less-ordered regions, arising from the lateral segment packing proceeding concurrently in both these regions. However, the increased extent of the chain packing is evident in the non-crystal region with increasing Ti. Wachsman and Frank [46] observed a signi®cant increase in the CT ¯uorescence intensity with an increase in Ti from 200 to 4508C, coinciding with the increased chain packing as demonstrated from the above-mentioned SAXS measurements. However, a very important thing to be re®ned here is to assign whether the intermolecular CT ¯uorescences are emitted from the amorphous or from the crystal region. This should be clari®ed by comparing the ¯uorescences of a highly crystalline PI, which is available in the ®ber form, and its amorphous ®lm. Some indirect evidence supports the amorphous-origin CT ¯uorescence hypothesis, although to our knowledge there is no direct evidence based on their comparison owing to a great technological dif®culty of getting pure PI crystals. Baklagina et al. [75,76] determined the crystal structure of the PMDA±PDA ®ber with lattice parameters of a 5:6; b 8:5; The main feature of the most probable crystal structure in the PMDA±PDA ®ber is the and c 12:3 A: PLP arrangement where the pyromellitimide fragments and the phenylene groups constitute separate layers. 2 Although it is ambiguous whether such layered crystal structure can be always formed preferentially even in the ®lm state, if it was accepted, the CT ¯uorescence requiring the pyromellitimide± phenylene face-to-face stacking should come from the amorphous region in the ®lm. Similar PLP-type 2 Another PI ®ber is known to take the mixed layer packing crystal structure where the diimide fragment can interact with the diamine residue, see Ref. [76].

284


Fig. 25. (a) Desmeared SAXS curves of PAA and PMDA±ODA polyimide ®lms cured off substrate under different conditions: (1) 908C/3.5 h 1 1508C/8 h 1 1908C/4 h 1 2408C/5.5 h stepwise; (2) 2508C/12 h; (3) 3508C/12 h; and (4) 4308C/2 h. (b) Schematic diagram of a possible layered morphology based on one-dimensional model analysis. Reproduced with permission from J Polymer Sci, Polym Phys Ed 1981;19:1293. q 1981 John Wiley & Sons [72]. (c) Smeared SAXS curves of PMDA±ODA polyimide ®lms cured off substrate under different temperatures. Reproduced with permission from J Polymer Sci, Polym Phys Ed 1984;22:1105. q 1984 John Wiley & Sons [73].

crystal structure was observed for the corresponding model compound [77]. This model crystal showed a signi®cantly different ¯uorescence spectrum at much lower wavelength than the PMDA±PDA ®lm, similar to that of the isolated molecule in PMMA, and in comparison with the s-BPDA±PDA ®lm and its model crystal (Hasegawa, unpublished results). In addition, for the PMDA±ODA ®lm, the CT ¯uorescence reduced somewhat at Ti 4308C [48], coinciding with crystallization (a decrease in amorphous fraction) in the sample (see Fig. 14). On the other hand, the presence of the MLP-type crystal structure


285

Fig. 26. DMTA curves of PMDA±ODA polyimide ®lms cured off substrate under different conditions: (1) 908C/3.5 h 1 1508C/ 8 h 1 1908C/4 h 1 2408C/5.5 h stepwise; (2) 2508C/12 h; (3) 3508C/12 h; and (4) 4308C/2 h. Reproduced with permission from J Polymer Sci, Polym Phys Ed 1982;20:837. q 1982 John Wiley & Sons [80].

was also suggested in low molecular PI model compounds by means of solid-state 13C NMR [78] and single-crystal WAXD [79]. In rigid PI systems such as PMDA±ODA and s-BPDA±PDA, it is known that the one-step cure process at Ti is more effective to promote the ordering than the two-step cure process with the same ®nal temperature, since the former gives stronger molecular motion [51]. An increase in Ti also signi®cantly in¯uences the Tg and softening behavior above the Tg. Dynamic mechanical thermal analysis (DMTA) is useful for monitoring such properties. Fig. 26 obviously indicates that the Tg became unclear concurrently with a higher temperature shift of the Tg [80]. The disappeared glass transition cannot be simply explained in terms of only the increased crystallinity, a c, (decreased amorphous fraction) since a c is not still too high even in the sample prepared at Ti 4308C (roughly estimating from the ®lm density, 1.426 g cm 23, ac , 20%: An increase in interchain interactions in the amorphous region must also contribute signi®cantly to the non-softening behavior. The intermolecular CTC formation could be a candidate for the origin of the increased intermolecular interaction, as demonstrated by the CT ¯uorescence enhancement. Semi-rigid s-BPDA±PDA also shows similar effect of Ti or ®nal cure temperature, Tf, on the morphology [74,81±83]. A sign of crystallization is observed at Ti or Tf 4008C: Unfortunately, there are so far no reports on the SAXS measurements for the s-BPDA±PDA ®lm. An increase in Tf also led to a decrease in the anisotropy ratio, r, for the CT ¯uorescence concurrently with the increases in ®lm density and CT ¯uorescence intensity. This result can be rationalized in terms of an increased ef®ciency of the non-radiative energy migration between the CTCs, caused by the annealing-induced CTC population increase [49]. The resultant morphology of the s-BPDA±PDA polyimide ®lm is also in¯uenced by the kind of the precursors (water-releasing PAA or alcohol-releasing poly(amic alkyl ester)s, PAE), which give the same PI chemistry [40,82±85,86]. A common observation is that at Ti , 3508C the PAE-derived PI ®lms show a lower ordered structure than the PAA-derived ones, but this situation is reversed when Ti

286


Fig. 27. (a) Changes in ®lm density; and (b) intermolecular CT ¯uorescence intensity of s-BPDA±PDA ®lms derived from PAA and PAE in the stepwise heating process (maintained for 10 min at each step).

was elevated up to 4008C. This fact is due to the higher molecular mobility as a result of the lower molecular packing in the PAE-derived PI cured at Ti , 3508C; as demonstrated from the more drastic change in the CT ¯uorescence intensity (Fig. 27) and a wider ¯uorescence peak wavelength range [40,86]. Actually, a slight ¯uorescence spectral shift and a ¯uorescence lifetime difference were observed for the PAA- and PAE-derived s-BPDA±PDA ®lm [86]. These may mean a slight structural difference in the CTC, like a difference in the intermolecular donor±acceptor spacing. The striking morphological development in s-BPDA±PDA is based on the chain linearity and coplanarity. Most of low molecular weight biphenyl compounds have a dihedral angle (v ) at the biphenyl linkage, ranging from 0 to 508 in the crystalline form, and there are many cases with v 08 (coplanar state) as reported in the literature [87]. This means that s-BPDA±PDA can also take the coplanar form


287

with a help of intermolecular force. Accordingly, it is likely that the coplanarization proceeds in the higher temperature annealing process, although no experimental evidence are known yet. In contrast, its isomeric PI, asymmetric BPDA±PDA (a-BPDA±PDA) showed no ¯uorescence enhancement with increasing Ti (Hasegawa, unpublished results), corresponding to the unchanged ®lm densities [81]. This result is most likely attributed to its highly bent and distorted chain structure and the inhibited coplanarization based on a considerably high rotational barrier at v 08 [81]. The ¯uorescence was also annealing-insensitive for dimethyl-substituted s-BPDA±PDA on the PDA residue (Hasegawa, unpublished results). Accordingly, a planar structure without bulky substituents is required for CTC formation. 2.4.2. Ordered structure formation in the imidization process When a PI ®lm is prepared via the one-step cure at a high Ti, e.g. 4008C, two processes are included: imidization (chemical structure change) and rearrangement of the formed PI chains. These should be discussed separately. In many cases, the former is almost completed till the sample temperature reaches 2508C. The chemical structural change on imidization is readily detected by monitoring the imide carbonyl infrared absorption bands. However, hardly any attention has been paid to higher-order structure formation occurring during imidization. A few researchers demonstrated that a certain degree of local ordering takes place even in a region of partial imidization. Vladimirov et al. [82,83] examined by means of FT-IR spectroscopy the effects of thermal cure conditions (Ti and one-step/two-step), ®lm thickness, kind of solvent (DMAc or NMP), the amount of residual solvent, and kind of precursor on the resultant morphology in detail. They found some infrared bands sensitive to ordered structure formation in the ®nger print region for s-BPDA±PDA. Fig. 28a exhibits the FT-IR spectra (1050±400 cm 21) of the s-BPDA±PDA ®lm (40 mm thick) as a function of Ti. One can see that one of the structure-sensitive bands, the 550 cm 21 band, grows up with an increase in Ti; on the other hand, the 529 cm 21 band is insensitive to Ti. Therefore, the latter is suitable for an internal standard in following the local ordered structure formation. The 550 and 529 cm 21 bands were assigned as the C±H bending vibration in the 1,2,4-trisubstituted benzene (BPDA moiety) and in the 1,4-disubstituted benzene (PDA moiety), respectively, on the basis of the accumulated spectral data for the substituted benzenes and the comparison with the s-BPDA±t-CHDA spectrum. Accordingly, the observed 550 cm 21 band growth is independent of the chemical structural change by imidization. The relative band intensity A550 =A529 was plotted in Fig. 28b. Similarly, the relative intensity also increased with an increase in ®lm thickness at a constant Ti. This thickness dependence can be rationalized in terms of a plasticizing effect of residual solvent molecules, which stays for longer periods in a thicker ®lm, as a result, giving a chance of the PI chain rearrangement. Thus, the 550 cm 21 band intensity is associated with local ordering. It should be noted that the ordering begins even at Ti 1708C where the partially imidized ®lm is completely amorphous and the extent of imidization (20±25%) is rather low. These results led to a hypothesis that a conformational order as an extended chain conformation favorable for crystallization is formed even in the partially imidized ®lm. Fig. 28b also depicts that the relative intensity almost leveled off above Ti 2508C: However, further increase in Ti up to 4008C caused much more extreme spectral changes: a high-frequency shift and narrowing of the 550 cm 21 band without the relative intensity change, concurrently with a prominent band splitting of the 1020 cm 21 band. These spectral changes correspond to crystallization as evidenced by the WAXD patterns [74,81]. Thus, these spectral criteria made it possible to differentiate the conformational ordering at the initial stage of imidization and crystallization due to the PI chain rearrangement after full imidization. CTC formation is a sort of local ordering. Hasegawa et al. [51] monitored local ordering during the

288


Fig. 28. (a) FT-IR spectra of s-BPDA±PDA ®lms (40 mm thick) imidized at different temperatures; and (b) relative band intensities (A550/A529) as functions of ®lm thickness and imidization temperature. (W) DMAc-cast 40 mm thick ®lm; (K) NMPcast 40 mm thick ®lm; (A) DMAc-cast 10 mm thick ®lm; and (X) solvent-free 40 mm thick ®lm.

imidization process for s-BPDA±PDA using the intermolecular CT ¯uorescence. Fig. 29 exhibits the change in the ¯uorescence spectra during the isothermal imidization at 1708C. Pure PAA ®lm shows a comparatively strong ¯uorescence peaking at 490 nm, which obviously differs from the CT ¯uorescence of s-BPDA±PDA polyimide. The PAA ¯uorescence abruptly reduced at 20±30% imidization in spite of a high residual PAA fraction. As imidization proceeds furthermore, a ¯uorescence corresponding to the polyimide CT ¯uorescence peaking at 530±540 nm grew gradually. Fig. 30 shows the changes in the peak ¯uorescence intensities with the progress of imidization at various Ti's. Even at Ti 1508C; prolonged heating brought about a very high conversion exceeding 90%, but the CT ¯uorescence intensity increased only slightly, suggesting very weak molecular packing. On the other hand, isothermal imidization at higher Ti's allowed a pronounced intensi®cation of the CT ¯uorescence. It should be noted that the ICT ±imidization% curves at each Ti begins to branch from about i 50%: This result elucidated that CTC begins to be formed even in the partially imidized ®lms, coinciding with the above-mentioned


289

Fig. 29. Change in the ¯uorescence spectra of s-BPDA±PDA ®lm during isothermal imidization at 1708C: (1) the extent of imidization 0% (PAA); (2) 6.1%; (3) 24.1%; (4) 50.4%; (5) 86.3%; and (6) after annealing at 3008C.

conformational order yielded on the way to full imidization, as concluded from the changes in the structure-sensitive infrared band intensity. The arrows indicated in Fig. 30 represent the extents of imidization (ig) where the apparent Tg of the partially imidized ®lm reached Ti (vitri®cation). These values were determined from an extrapolation to the zero value of the apparent ®rst-order rate constants for imidization di=dt1 2 i21 as a function of the imidization% for Ti 150 and 2008C. It is clearly indicated that at Ti 1508C; vitri®cation occurs at only initial imidization stage; on the other hand, it does not take place up to i 60% when imidized at Ti 2008C: Thus, an increase in Ti leads to an increase in ig, and as a result, to an enhanced chance of molecular motion required for molecular rearrangement (CTC formation) of the PI segments. Taking into account the vitri®cation concept [88] is very important for precise property control in PI systems without suf®cient molecular mobility even above the Tg's, such as s-BPDA±PDA and PMDA±ODA. This is a reason why once PI possesses a less-ordered structure formed at a low Ti, it can be no longer be completely canceled even by subsequent annealing at higher temperatures.

Fig. 30. Fluorescence peak intensities as a function of the extent of imidization (i%) for s-BPDA±PDA ®lms at different imidization temperatures. The arrows represent the values of i% where vitri®cation occurs (ig).

290


2.4.3. PI/PI blend miscibility studied by CT ¯uorescence Random copolymerization by combining various monomers is a very useful and convenient way of controlling the PI properties precisely. If copolyimides were sequence-controlled, they are expected to show several improved properties compared to the corresponding random copolymers [89,90]. An alternative approach is simple physical blending between different homo PIs, although it includes a disadvantage that most polymer blends give rise to phase separation unfavorable for property control. Miscible PI blends tend to exhibit similar properties to the corresponding random copolyimides as in other polymer blend systems but sometimes provide appreciably better properties [91]. Molecular composites [92,93], where rigid and ¯exible components are miscible and contribute separately, for example, to mechanical reinforcement and toughening, respectively, make it possible to enhance simultaneously con¯icting properties such as modulus and toughness. Accordingly, the blend method is still important for producing new PI materials [94]. As discussed in the previous sections, the ¯uorescences of wholly aromatic PIs, which are sensitive to local ordering, were concluded to be those arising from the intermolecular CTC. According to this criterion, the intermolecular CT ¯uorescence intensity should re¯ect the PI/PI blend morphology (miscibility). To study the miscibility using the CT ¯uorescence, Hasegawa et al. [95] established a binary blend system composed of s-BPDA±PDA with PMDA±ODA or PMDA±PDA. These PMDA-derived PIs are practically regarded as non-¯uorescent polymers since their ¯uorescence is roughly 1000 times weaker than that of s-BPDA±PDA. In principle, if phase-separated, the blend remains ¯uorescent owing to the presence of the s-BPDA±PDA-rich domains. On the other hand, if miscible, CTC formation between the s-BPDA±PDA segments should be hindered, consequently, the s-BPDA±PDA ¯uorescence would be signi®cantly weakened. PI/PI blend ®lms were prepared via thermal imidization of the PAA/PAA blend ®lms dried at 608C/2 h (as a standard casting condition), because of the limited solubilities of wholly aromatic PIs. The sBPDA±PDA/PMDA±PDA blend ®lms were optically transparent over the whole blend compositions. D , of the component PI ®lms (1.7710 for s-BPDA±PDA and Also, the average refractive indices, nav 1.7370 for PMDA±PDA) are different enough to form an opaque blend ®lm if phase-separated DnD . 0:01 [96]. This means that the blends are at least homogeneous on the visible wavelength level. In contrast, the s-BPDA±PDA/PMDA±ODA blend provided opaque ®lms at the whole blend compositions, indicating the phase-separation. Comparison with the DMTA curves of these blends (50/50) suggests that the s-BPDA±PDA/PMDA±PDA blends have much better miscibility but still include very small s-BPDA±PDA-rich domains (partially miscible). Fig. 31 displays the CT ¯uorescence intensity changes with the blend composition for s-BPDA±PDA/PMDA±ODA and s-BPDA±PDA/ PMDA±PDA blends together with the intensities predicted from the additive property for the perfect phase-separation case in these blends. In the partially miscible blends, the CT ¯uorescence intensity drastically decreased with the addition of the non-¯uorescent PMDA-type PIs, coinciding with the homogeneous morphology. On the other hand, the phase-separated blends showed very unusual composition dependence of the intensity. It is interesting to note that at 80/20 the intensity of the blend exceeded the predicted value for the perfect phase-separation. A possibility of the phase separationinduced internal stress effect was suggested for this unusual behavior. From the above mentioned results, it is expected that the morphology in the blends (50/50) of homo sBPDA±PDA with PMDA±PDA;ODA copolymer varies from inhomogeneous to homogeneous with increasing PDA content in the copolymers. In fact, the phase-contrast microscope observation indicated that a modulated phase structure formed by the spinodal decomposition in the blend ®lm at PDA 0


291

Fig. 31. Blend composition dependence of the CT ¯uorescence intensity for (X) s-BPDA±PDA/PMDA±ODA and (W) sBPDA±PDA/PMDA±ODA blends. The broken and dotted lines represent the ideal curves for the completely phase-separated blends.

mol% reduced gradually in the domain sizes with increasing PDA content, then ®nally the blend became completely transparent at PDA 100 mol%. The CT ¯uorescence intensity reduced in accordance with the homogenization of the blends as illustrated in Fig. 32. In addition, the higher frequency shift of the ¯uorescence was observed at a higher PDA content range, suggesting that the peak position is also sensitive to the miscibility [95]. The blend morphologies can be controlled by casting temperature, Tcast, for the mixed PAA solutions. In the s-BPDA±PDA/PMDA±ODA blend (50/50), the increase in Tcast from 60 to 1008C caused a gradual decrease of the domain size in the modulated phase morphology, and the blend ®lm turned transparent at Tcast 1208C: Some possible reasons for this Tcast dependence were listed: (1) an increase in the solvent evaporation rate and consequent rapid solidi®cation from the homogeneous PAA/PAA solutions; (2) molecular weight decrease of the component PAAs; (3) structural changes of PAAs due to partial imidization during the drying process; (4) temperature-dependent thermodynamic miscibility such as the upper critical solution temperature type phase diagram; and (5) entire copolymer formation from two homo PAAs or compatibilizer-active partial copolymer formation through transamidation. The mechanism of the Tcast-dependent miscibility was discussed in the literature [95,97]. Fig. 33 clearly indicates that, with an increase in Tcast, the CT ¯uorescence reduced in accordance with the blend homogenization. Very similar Tcast dependence was also observed for the ®lm densities which is related

Fig. 32. Changes in the CT ¯uorescence intensity and its peak position for s-BPDA±PDA/PMDA±PDA;ODA blends (50/50) with varying PDA content in the copoymer.

292


Fig. 33. Effect of casting temperature on the CT ¯uorescence intensities of the resultant 50/50 blends: (1) s-BPDA±PDA/ PMDA±ODA; and (2) BPDA±PDA/PMDA±PDA. Open and closed marks denote optically trasparent and opaque ®lms, respectively.

to the miscibility-dependent crystallinities of the component PIs. DMTA measurements con®rmed that the CT ¯uorescence intensity re¯ects the PI/PI blend miscibility at the molecular level [95]. The advantages of the present technique were again emphasized when the polymer chains in blends are considerably stiff, therefore, representing no Tg's, since the so-called Tg criterion for miscibility evaluation is not applicable. Note that rigid PIs such as PMDA±PDA, s-BPDA±PDA, and PMDA±ODA ®lms cured at a low Ti (e.g. 2508C) still represent the Tg's in the DMTA curves, but do not at all in the ®lms cured at T . 4008C as often performed for commercially produced PI ®lms. 2.4.4. Perylenetetracarboxydiimide ¯uorescence yield associated with the CT ability of polyimide matrices A ¯uorescent dye, perylenetetracarboxydiimide, (DBu-PEDI, Fig. 34) is known to possess valuable functionalities as organic photoelectric mutual conversion devices (solar cell [98,99] and electroluminescence [100]), layered organic photoconductive devices for electrophotography [101,102], and dye for

Fig. 34. Chemical structures of the perylenetetracarboxydiimides used.


293

Fig. 35. Visible absorption and ¯uorescence spectra of PEDI-bound s-BPDA±PDA ®lm.

laser [103]. When DBu-PEDI molecules were dispersed in aromatic PI ®lms, the dye ¯uorescence is not disturbed even by the visible absorption of the PI matrices, since the absorption and ¯uorescence bands of the dye are both located in a longer wavelength region as shown in Fig. 35. Hasegawa et al. [104] revealed that DBu-PEDI molecularly dispersed in PAA and PI ®lms shows unique ¯uorescence properties. First, the originally strong ¯uorescence of DBu-PEDI was ef®ciently quenched in the PAA ®lms. The systematic chain structure variation of the PAA matrices led to a conclusion that the aromatic amide groups in the PAA chains functions as a quencher. This quenching mechanism is also supported by the recovered strong ¯uorescence of DBu-PEDI with the progress of imidization (decrease in the aromatic amide fraction). This result suggests that the PEDI ¯uorescence could be a promising tool for monitoring imidization process in very thick ®lms which prevents the conventional infrared spectroscopic measurements. Second, DBu-PEDI dispersed in PI matrices shows signi®cant dependence of the ¯uorescence yield on the PI chain structures. In this section, we focus on the second characteristics. In Fig. 36, one can see a good linear relationship between the normalized DBu-PEDI ¯uorescence intensities, IN / Ff 3 in PI ®lms and the electron af®nity, EA, of the dianhydride monomer components [9] in a series of PIs with a ®xed diamine (PDA). This result reminds one of Mulliken's CT theory. A similar tendency was observed for the ODA series. A quenching mechanism via the non-radiative energy transfer from PEDI p to the CTCs in PIs is unlikely, because all the PIs used have practically no CT bands in the longer wavelength region than the PEDI ¯uorescence. A reasonable quenching mechanism is proposed in Fig. 37: (1) the excitation of PEDI; (2) the electron transfer from the ground state diamine moiety to the excited state PEDI; and (3) subsequent electron transfer from the excited state PEDI to the ground state diimide moiety to form the intramolecular CT state (D 1A 2) in the PI chains and the groundstate PEDI (quenching). The PIs possessing higher intramolecular CT ability should lead to more ef®cient electron transfer in process (3), and consequently to the higher quenching ef®ciency of the 3

PEDI ¯uorescence intensity was normalized with respect to the absorbance, I If =1 2 exp22:303Abs; where If is the relative ¯uorescence intensity at the second vibrational (0±1) peak near 590 nm, and Abs the absorbance at the excitation wavelength (495 nm).

294


Fig. 36. Relationship between the normalized PEDI ¯uorescence intensity and electron af®nity of dianhydride components in a series of PIs with a ®xed diamine (PDA).

PEDI ¯uorescence. The intramolecular CT excited state formed in the PI chains is preferably non¯uorescent as described in the previous section. Similar multistep electron transfer was observed in a porphyrin connected to two quinones and in several porphyrin±quinone±carotenoid systems [105±108]. The probability of exciplex formation (electron transfer ¯uorescence quenching) in polar solvents can be estimated from the free energy changes, DGet, given by the Rehm±Weller equation [109,110]: DGet 23:06Eo D1 =D 2 Eo A=A2 2 E0±0 2 C kcal mol21

12

where E o(D 1/D) and E o(A/A 2) (in volt vs SCE) are the oxidation potential of donor and the reduction potential of acceptor, respectively, E0±0 is the excitation energy of ¯uorophore, and C is the Coulombic term. However, this treatment was not applied to the present system since no E o(A/A 2) value of PEDI was available and E o(D 1/D) of the diamine residue in the PI segments is also unknown, although only E o(A/A 2) ( 20.74 V) of the pyromellitimide is available [111]. The relation observed in Fig. 36 resembles to the chain structure-dependent CT ¯uorescence yield of PIs. This means that PEDI ¯uorescence is applicable for PI/PI blend miscibility judgement. 2.4.5. PI/PI blend miscibility probed by PEDI ¯uorescence The binary blends of s-BPDA±PDA with PMDA±ODA were again taken up here. For the present

Fig. 37. Schematic diagram for a mechanism of PEDI ¯uorescence quenching.


295

Fig. 38. Changes in the normalized PEDI ¯uorescence intensity with varying initial Mw of PMDA±ODA in the 60/40 blends Tcast 608C : (a) s-BPDA±PDA/PMDA±ODA; and (b) s-BPDA±ODA/PMDA±ODA. Open and closed marks denote optically transparent and opaque ®lms, respectively.

purpose, only BPDA-derived PIs were labeled by copolymerizing a trace amount of bifunctional PEDI (Fig. 34). In principle, when the miscibility is good, the different PI chains interpenetrate, as result, the PEDI ¯uorescence intensity should be signi®cantly reduced owing to the intimate contact between the PEDI moieties connected to the BPDA-derived PI chains and the PMDA±ODA segments as quencher. On the other hand, if phase-separated, the PEDI ¯uorescence should behave as in the homo BPDAderived PI matrix. This technique is more reliable than the previously described CT ¯uorescence approach since the PEDI ¯uorescence is well characterized and much stronger (higher sensitivity). Hasegawa et al. [97] reexamined the factors in¯uencing the PI/PI blend miscibility using the PEDI ¯uorescence in combination with DMTA. The results showed that the normalized PEDI ¯uorescence intensity, IN, in the s-BPDA±PDA/ PMDA±ODA blend (50/50) decreased as Tcast is increased, corresponding well to the blend morphological changes described previously. The IN ±Tcast curves were very similar in shape to the ICT ±Tcast curves (see Fig. 33). DMTA measurements demonstrated a tendency of a gradual increase in the miscibility caused by the Tcast increase. The blend ®lms turned from opaque to transparent over Tcast 908C: At Tcast 1208C the ®lm transparency remained unchanged, but the value of IN decreased appreciably, suggesting how the PEDI ¯uorescence is sensitive to the slight miscibility change [97]. It was demonstrated that, among the factors in¯uencing the miscibility as listed in the preceding section, the initial PAA molecular weights (Mw) play an important role in the resultant PI/PI blend miscibility. Fig. 38 exhibits the initial Mw dependence of IN for the blends (60/40) of a ®xed Mw of s-BPDA±PDA or s-BPDA±ODA with various Mw of PMDA±ODA. According to a general

296


Fig. 39. Dependence of DMTA curves on the initial Mw of PMDA±ODA (indicated in the ®gures) in s-BPDA±ODA/PMDA± ODA blend (60/40).

thermodynamic consideration for the Mw dependence of the combinatorial entropy (DSmix), the IN gradually reduced as the Mw of PMDA±ODA is decreased in accordance with a gradual decrease in the turbidity of the blend ®lms. In addition, a decrease in Mw of BPDA±PDA led to a downward shift of the IN ±Mw curves (not shown). In the s-BPDA±ODA/PMDA±ODA blend containing a common structural unit (ODA), the blend ®lm turbidity varied more drastically with Mw. For this blend system, the lower IN at the whole Mw's studied corresponds to the higher miscibility than for the BPDA±PDA/ PMDA±ODA system with no common structural units. The typical DMTA curves as a function of Mw of PMDA±ODA are shown in Fig. 39. One can see a pronounced variation from double for a higher Mw (205,000) to single glass transition pro®le for a lower Mw ( 25,100), indicating the drastic miscibility changes [97]. It is known that a well-mixed PAA/PAA solution undergoes homogenization during prolonged storage even at low temperatures (e.g. 08C) without the practical Mw decrease of component PAAs [112,113]. This process is believed to be based on copolymer formation via transamidation, although the composition and fraction of the formed copolymer are both unknown [90,114]. To study the effects of storage temperature and period, the PAA solutions of BPDA±PDA and PMDA±ODA were vigorously mixed at room temperature very promptly (for 10 min) and stored for a given period at 220 and 208C in the dark, then cast at 608C, followed by thermal curing for the IN measurements. The 50/50 blend gave opaque ®lms within our experimental storage periods at 2208C, corresponding to the fact that the higher IN values remained practically unchanged until 10 days as shown in Fig. 40. Note that the initial mixed solutions before casting were all transparent. On the other hand, storage at 208C caused a signi®cant morphological change from turbid to transparent with a gradual IN decrease. Similar behavior was also observed for the 70/30 and the 30/70 blends (not shown). The striking miscibility change occurring during the storage at 208C was also demonstrated from the changes in the DMTA curve pro®les and the ®lm densities [106].


297

Fig. 40. Changes in the normalized PEDI ¯uorescence intensity of the resultant PI/PI blend (50/50) of s-BPDA±PDA/PMDA± ODA during storage of the mixed PAA/PAA solutions at 20 and 2208C. Open and closed marks denote optically transparent and opaque ®lms, respectively.

Thus, the PEDI ¯uorescence is a powerful tool for judging the PI/PI blend miscibility. The present technique was successfully applied to the a-BPDA±ODA containing the PMDA±PDA-based blend without Tg, which has an extremely low linear coef®cient of thermal expansion (CTE) close to that of silicon wafer concurrent with an improved ®lm toughness [115]. 3. Polyimide properties in¯uenced by CT interactions In this section, we discuss how the CT interactions in¯uence other physical properties than the absorption (color) and ¯uorescence behavior. 3.1. Photoconductivity High-temperature polyimides are used as an excellent electrical insulator in microelectronic devices. This is due to the fact that the CTCs formed in PI ®lms are classi®ed into a category of weak CT complex, and therefore, have practically no contribution to the charge-separated structure at the ground state. Fainshtein et al. [116] showed that the increases in pressured and temperature cause a decrease in the electrical resistance in the dark for PIs derived from a ®xed diamine (ODA) with PMDA, BTDA, and 3,3 0 ,4,4 0 -diphenylsulphonetetracarboxylic dianhydride. These results were explained in terms of an electron conductance mechanism based on interchain CTC formation. On the other hand, even in weak CTCs, the completely charge-separated state can be formed at the excited state. Accordingly, photoconductivity could be a ®rst candidate among physical properties in¯uenced strongly by the CT character in wholly aromatic PIs. Photoconductivity of polymers has been widely reported, in particular, with great interest on low molecular weight electron acceptorsloaded poly(vinyl carbazole) for xerographic applications [117,118]. Kapton or additive-free labo-made PMDA±ODA has been mainly investigated for its dark- and photocurrent behavior using X-ray, UV and visible light as excitation sources [119±124]. Iida et al. [122] examined how the photoconductivity is affected by ordered structure formation in PMDA±ODA in combination with WAXD measurements. Comparison of a labo-made PI ®lm with a commercial

298


Fig. 41. Photocurrent response as a function of applied electric ®eld for 7.5 mm thick Kapton ®lm and N,N 0 -dimethylaniline (DMA)-loaded Kapton ®lm upon excitation at 480 nm. Reproduced with permission from Macromolecules 1987;20:973. q 1987 American Chemical Society [125].

Kapton ®lm possessing a much higher ordered structure and the 30% uniaxially stretched ®lm with a less-ordered structure suggested that ordering lowers the photoconductivity. Freilich [125] reported that electron donor-loaded Kapton ®lm results in a striking photocurrent enhancement by ®ve orders of magnitude as compared to the donor-free ®lm. Fig. 41 displays the photocurrent of the 7.6 mm thick Kapton ®lm containing 7.6 wt% DMA as a function of applied electric ®eld (10 5 ±10 6 V cm 21) upon illumination at 480 nm. The difference absorption spectrum between DMA-loaded and DMA-free Kapton ®lms showed a new broad band peaking at 460 nm, which coincides with the photocurrent action spectrum in position. Also, a linear relationship was observed between the transition energy and the ionization potential of the dopants. From these results, he concluded that this photocurrent enhancement results from CTC formation between the pyromellitimide fragment in the PI backbone and DMA. However, as pointed out by Freilich, it should be noted that the maximum photoresponse wavelength depends on ®lm thickness (absorbance). To assign the photogenerated charge carrier in the PMDA±ODA ®lm, Freilich performed earliest transient spectroscopy on a Nd 31:YAG laser ¯ush photolysis system. Fig. 42a shows the transient absorption spectrum taken at 35 ps after excitation for N,N 0 -di-n-pentylpyromellitimide (0.1 M), which is a model of the electron-accepting fragment in PMDA±ODA, containing 1 M DMA in dichloromethane. The spectrum peaking at 728 and 659 nm is virtually identical with that of radical anion of this pyromellitimide compound as formed through electrochemical reduction. As shown in Fig. 42b, the DMA-loaded Kapton ®lm also displayed a similar transient absorption spectrum. Accordingly, the transient species can be assigned as the radical anion formed via electron transfer from DMA to the pyromellitimide portion in the PMDA±ODA backbone. The pyromellitimide radical anion can be formed even in the additive-free PMDA±ODA ®lm. Lee et al. [126] also observed a similar transient absorption spectrum (after just a 20 ps pulse) peaking at 720 and 660 nm with a sharp peak at 550 nm in the pure PMDA±ODA ®lm. The peaks at 720 and 660 nm correspond to the pyromellitimide radical anion; on the other hand, the 550 nm peak may be attributed to the radical cation of the ODA residue. This result provides direct evidence that wholly aromatic PIs undergo the ultrafast CT at the excited state as discussed in the preceding section. The photogenerated radical anion in the DMA±Kapton ®lm decays very rapidly, as in the model system, with a short lifetime of 150 ps, suggesting the presence in the back electron transfer or geminate recombination. The quantum ef®ciency of the photocurrent generation as a function of applied electric


299

Fig. 42. Transient absorption spectra taken with a delay of 35 ps after excitation at 355 nm: (a) N,N 0 -di-n-pentylpyromellitimide±N,N 0 -dimethylaniline CTC in dichloromethane; and (b) N,N 0 -dimethylaniline-loaded Kapton ®lm. Reproduced with permission from Macromolecules 1987;20:973. q 1987 American Chemical Society [125].

®eld for the DMA±Kapton ®lm showed a good ®tting on the Onsager description and gave the adjustable and the initial quantum yield of the bound ion parameters of initial thermalization radius r0 13 A pair F0 1 [125]. The low r0 value means the presence of the effective geminate recombination or back electron transfer unfavorable for photocurrent. Lee et al. [127] measured the photoconductivity of a semi-aromatic PI derived from PMDA and an alicyclic diamine, 4,4 0 -methylene bis(cyclohexylamine) (MBCHA). Unexpectedly, this CT-inhibiting semi-aromatic PI showed somewhat higher photoconductivity than CT-allowing PMDA±ODA over the whole electric ®eld examined. For the PMDA±MBCHA ®lm cured at 2008C/1 h, prolonged annealing at 2008C increased the photocurrent by one order of magnitude. In contrast, dimethyl-substituted semiaromatic PI derived from PMDA and 4,4 0 -methylene bis(3-methylcyclohexylamine) (MBMCHA) exhibited no photoconductivity, regardless of the annealing. They explained that the annealing effect in PMDA±MBCHA is associated with molecular packing, whereas the dimethyl-substituted PI remains loosely packed owing to the steric hindrance even if annealed. On the basis of these results, a mechanism of charge carrier photogeneration was proposed for the PMDA±MBCHA: the pyromellitimide fragments weakly interact with the alicyclic portions of different chains under an assumption that the PI chains take the mixed layer packing arrangement. Then, upon photoirradiation, the electron transfer occurs from the ground-state alicyclic portions to the excited state pyromellitimide fragment, ®nally charge carriers are produced by ®eld-assisted thermal dissociation of the photogenerated exciplex to form separated ion pairs [127]. Unfortunately, there exists no experimental evidence whether the PMDA±MBCHA can take the mixed layer packing preferably. Also, the main active species for the photoconductivity of PMDA± MBCHA still remains unknown since no transient measurements were performed for this semi-aromatic PI, although the radical anion may be listed as a candidate for the photogenerated carriers as inferred from the model experiments in solution. In the DMA-loaded PMDA±ODA system [125], the addition of

300


Fig. 43. Photocurrent response as a function of applied electric ®eld for 1 mm thick PI ®lms: (W) PMDA±DCHM lex 350 nm; (A) TMPD-doped PMDA±DCHM lex 400 nm; (S) PMDA±ODA lex 420 nm; and (K) TMPD-doped PMDA±ODA lex 460 nm: The absorbance of all samples ranges 0.3±0.5. Closed marks represent dark currents.

electron donors into the PMDA±MBCHA ®lm also signi®cantly enhanced the photocurrent compared to that of the neat PI ®lm, owing to CTC formation between the dopant molecule and the pyromellitimide portions in the PI backbone [128]. It is noteworthy that the photoconductivity of the PMDA±MBCHA ®lm loaded N,N,N 0 ,N 0 -p-phenylenediamine (TMPD) as a donor was one order of magnitude higher than that of the TMPD-loaded PMDA±ODA ®lm as shown in Fig. 43 [128]. This result might suggest that the absence of the intrachain CT process in the PMDA±MBCHA ®lm consequently promoted the capability of intermolecular CTC formation between TMPD and the pyromellitimide portions. 3.2. Thermal, mechanical, and melt viscosity properties Weak CTCs look like exciplex in the viewpoint that both undergo charge separation only at the excited state. However, the latter has no attractive interaction between the donor±acceptor molecules at the ground state, whereas the former possesses a few kcal mol 21 of bonding energies even in solution at the ground state, comparable or lower than the hydrogen bonding energies [129]. It is well known that the interchain hydrogen bonding behaves as crosslinks, and consequently, often increases the Tg's as shown in polycarboxylic acids, aramides, etc. CTCs in PIs may also be the case. An electron donor±acceptor miscible blend between poly[(N-alkylcarbazol-3-yl)methyl methacrylate] and poly{2-[(3,5-dinitrobenzoyl)oxy]ethyl methacrylate} is a typical example. The interchain donor±acceptor attractive interactions resulted in a pronounced positive deviation from the additive property in the Tg ±composition curve [130,131]. DSC thermograms con®rmed that the blend includes the CTCs formed exothermically. From the structure±Tg relationship in a series of PIs, Fryd [132] pointed out that the interchain CTC formation should contribute somewhat to enhancing the Tg's of PIs, although the chain stiffness (rigidity) must be certainly an important factor. The introduction of ¯exible ªhingeº groups into the diimide portion has a greater impact on lowering the Tg's than introduction into the diamine residue, for PIs with the same chain ¯exibility (rotational freedom) as listed in Table 3. In addition, the meta linkage is also very effective in decreasing the Tg's (Table 4), despite the fact that it introduces no additional


301

Table 3 Effect of the position of ¯exible parts on Tg. Reproduced with permission from Polyimides: synthesis, characterization, and applications, 1984, p. 377. q 1984 Plenum Press [132] Dianhydride

Diamine

Tg (8C)

PMDA ODPA PMDA BTDA PMDA BTDA

p,p-ODA p-PDA p,p-DABP p-PDA m,m-DABP m-PDA

399 342 412 333 321 300

rotational freedom. He deduced that these effects originate from more effectively reduced electronwithdrawing ability of the hinge-containing diimide fragments and the disturbed chain packing due to the decreased chain linearity, both of which should consequently prevent the intermolecular CTC formation. This hypothesis may be acceptable due to the pronounced Ti dependence of the Tg for the fully cured PMDA±ODA ®lms as shown in Fig. 26 [80]. However, the present consideration is not applicable when one compared the Tg's of CT-allowing wholly aromatic s-BPDA±PDA and CT-inhibiting semi-aromatic s-BPDA±t-CHDA, since they give almost identical Tg under the same cure conditions [40]. In these rigid systems, the effect of chain Table 4 Effect of the para/meta linkages on Tg. Reproduced with permission from Polyimides: synthesis, characterization, and applications, 1984, p. 377. q 1984 Plenum Press [132] Dianhydride

Diamine

Tg (8C)

BTDA BTDA BTDA BTDA BTDA BTDA HFPDPA HFPDPA HDFODPA

p,p-DABP m,m-DABP p,p-BABB p,p-MDA m,m-MDA p,p-BABDM p,p-ODA m,m-ODA p,p-ODA

290 264 286 290 234 256 222 178 186

302


stiffness on the Tg seems to be more preferential than that of the interchain force. No signi®cant contribution of CT interactions to Tg is exposed on comparison between the Tg's of a-BPDA±PDA and s-BPDA±PDA; the former, which has a highly bent and distorted structure is unfavorable for dense molecular packing, showing an unexpectedly higher Tg than the latter, which is a semi-rigid chain structure preferable for the interchain CTC formation. The higher Tg behavior for a-BPDA-derived PIs was rationalized in terms of a more strongly inhibited internal rotation around the biphenyl linkage in the a-BPDA fragment than in the s-BPDA portion [81]. The interchain CT interactions in PIs tend to in¯uence the softening behavior above Tg rather than the Tg's themselves. Note that with increasing Ti or Tf the glass transitions in the DMTA curves for the PMDA±ODA, s-BPDA±PDA, and s-BPDA±ODA ®lms became ambiguous with occurrence of the rubbery plateau, and ®nally disappeared after heat treatment at T $ 4008C [40,80,81]. As mentioned previously, crystallization (decrease in the amorphous fraction) certainly contributes to the glass transition disappearance, but the low magnitudes of crystallinity are not enough to explain it clearly. The enhanced interchain CT interactions should also be taken into account. If CTCs survive even above Tg, they are capable of behaving like physical crosslinks responsible for the glass transition disappearance tendency. Interchain CT interactions also may improve the mechanical properties of PI ®lms. Sulzberg et al. [133] reported that 1:1 blending of anisylimino group-containing polycarbonate and nitro group-containing polycarbonate resulted in a higher tensile modulus (360,000 psi) than those of the component polymers (335,000 psi for the donor polymer and 260,000 psi for the acceptor polymer). Similar mechanical property improvement was also observed in the 1:1 blend composed of N,N-dimethylamino-group and dinitro group pendant poly(propylene terephthalate) polymers [134]. Unfortunately, the CT effect on the mechanical properties at room temperature has never been discussed in wholly aromatic PI systems. St. Clair et al. [135] found that the addition of a small amount of a low molecular weight diimide compound into high molecular weight thermoplastic PIs has resulted in considerably decreased melt viscosities without signi®cant Tg decrease. For example, PA±PDA±PA (see insertion in Fig. 15c for its structure) among the diimide additives examined was especially effective. For PIs from BTDA and 3,3 0 diaminobenzophenone (LARC-TPI) and copolyimide derived from 4,4 0 -bis(3,4-dicarboxyphenoxy)diphenyl sul®de dianhydride (BDSDA) with ODA and meta-phenylenediamine (m-PDA), the melt viscosities at 3508C drastically lowered by addition of 5% of this additive. Two possible mechanisms were proposed for the pronounced results: the ®rst is that the additives possessing a higher molecular mobility can have stronger interactions with the PI chains than the chains have with each other. Namely, the PI± additives CT interaction can break up the interchain CT interactions. In other words, this mechanism means that the CT interactions survive even in the molten state above the Tg. The second hypothesis is an Mw decrease of the PI caused by transimidation during melt mixing above the Tg, although measurements of the Mw were not conducted. Other properties of PIs were also improved by the diimide additives [135]. The dielectric constant, e 0 , of PI from BDSDA and 2,2 0 -bis[4-(4-aminophenoxy)phenyl]hexa¯uoropropane (BDAF) reduced appreciably by additives shown in Fig. 44. The results are as listed in Table 5. Furthermore, the addition of 10 wt% AN±BDSDA±AN into BDSDA±ODA;m-PDA polyimide caused a decrease in the saturation moisture content from 1.52% for the neat PI to 1.27%. Positron annihilation spectroscopy proved that a decreased free volume is responsible for the reduced water uptake. Also, a slightly increased in modulus and a decreased CTE were caused by the 5% addition of AN±6FDA±A into LARC-TPI in both the undrawn and drawn states.


303

Fig. 44. Chemical structures of additives used for lowering the dielectric constants.

3.3. Voltage holding ratio in alignment layers for liquid crystal displays Low molecular weight liquid crystals (LC), in contact with the surface of PI ®lms unidirectionally rubbed with a velour cloth, form a monodomain aligned parallel to the rubbing direction [136,137]. Ê thick) have become a major material for the Therefore, rubbing-treated very thin PI ®lms (about 5000 A alignment layers for liquid crystal displays (LCD). The proposed mechanism for the LC orientation include the microgroove formation during rubbing [138] or interactions between the LC molecules and the oriented PI chains which act as a sort of molecular template [136,139]. The second mechanism is convincing from the fact that the PI chains located on the top surface highly oriented during the rubbing process [140,141], although the details are still poorly understood. However, we do not discuss the surface orientation of PI chains further, since these subjects are beyond the scope of the present review article. One of the most important necessary conditions in recent LCD such as thin ®lm transistor and super twisted nematic modes is to have higher voltage holding ratio Rv . 95% even in a high-temperature Table 5 Dielectric constants at 10 GHz and 10% weight loss temperatures of the additive-containing PIs. Reproduced with permission from Polyimides: materials, chemistry, and characterization, 1989, p. 243 q 1989 Elsevier Science [135] Additive

Concentration (%)

10% weight loss (8C)

e 0 at 10 GHz

Control AN±6FDA±AN AN±6FDA±AN AN±6FDA±AN AN±ODPA±AN PA±BDAF±PA PA±BDAF±PA

0 3 10 15 5 5 10

517 519 529 507 521 520 521

2.84 2.86 2.80 2.73 2.65 2.61 2.59

304


Table 6 Voltage holding ratio (%) for a variety of wholly and semi-aromatic PIs (reproduced from Ref. [142]) Dianhydride

PMDA s-BPDA H±PMDA H±s-BPDA

Diamine PDA

ODA

BAPP

BDAF

CHDA

40 40 94 ±

43 44 97 90

66 78 92 72

38 41 42 37

88 52 95 ±

range (608C), which is required for the display stability of LCD. Kikuchi et al. [142] synthesized a large numbers of PIs and measured the Rv values to clarify a relation with the PI structures using a cyanocontaining LC. The results are summarized in Table 6. Wholly aromatic PIs have a trend to show low values (40±70%). On the other hand, in addition to wholly aliphatic PIs, semi-aromatic PIs exhibited very high values exceeding 90% except for the PIs using BAPP and BDAF as diamine components. These results lead to a speculation that the Rv is related to CT interactions in PIs, although the detailed mechanisms are still poorly understood. Rv depends on not only the PI structure but also on the LC used. Much work has been carried out to improve this parameter through the chemical modi®cation of LCs themselves, rather than through the survey of low-Rv PIs. 4. Photochemistry of polyimides As discussed in the preceding sections, wholly aromatic PIs have a high UV±vis radiation resistance owing to the presence of the ultrafast CT process at the excited state and successive ef®cient thermal deactivation. But, exceptionally, BTDA-derived PIs can undergo photoinduced hydrogen abstraction when adequate hydrogen donors were included. This section deals mainly with photochemistry in BTDA-derived PIs and how their photoreactivities are affected by CT interactions. 4.1. Photosensitive polyimides 4.1.1. Review PI systems introducing well-molecular-designed low molecular-weight photoreactive groups, called


305

photosensitive polyimides, are mainly utilized as high-temperature photoresist materials for passivation layers and interlayer dielectrics, etc. A variety of photosensitive PIs were reported and reviewed in the literature [7,143,144]. We ®rst mention here very brie¯y the outline of photosensitive PI systems. The previous usual method for PI patterning included several processes; photoresist-coating onto PI ®lm, resist patterning, resist developing, polyimide etching by toxic chemicals such as hydrazine, ®nally resist removing. The use of photosensitive PIs makes it possible to shorten signi®cantly the PI patterning processes, compared to the usual procedure. Photopatterning is commonly based on the solubility difference between exposed and unexposed portions as a result of the main chain (chain scission and crosslinking) or side chain (pendant) reactions. An increased or a decreased solubility leads to the positive or negative pattern, respectively. Photopatternable PI systems are classi®ed into the PAA-exposure type, where patterning is performed at the PAA stage, then cured, and the PI-exposure type in which PIs themselves have both photosensitivity and developability by adequate solvents. Familiar negative working PAAs include the radical polymerizable acryloyl groups bound to the carboxyl groups of PAAs through the ester linkage [145] or tertiary amine±COOH ionic bonds [146]. These negative working photosensitive PAAs are now commercially available. However, in general, the negative type has a lower resolution than the positive type because of swelling during development. Hence, positive working systems have been becoming more important for the microprocessing of semiconductors. An attempt is a PAA containing o-nitrobenzyl ester groups. It decomposes into the original PAA and free nitrosobenzaldehyde upon deep-UV irradiation. Since the exposed part (the formed PAA) is soluble in an aqueous alkaline solution, this system displays positive working function [147]. PAA systems, where photosensitive compounds were physically blended, were also reported. In these cases, naphthoquinonediazide-4-sulfonyl ester derivatives [148] and 1,4-dihydropyridine derivative are known as photoreactive compounds [149]. The photosensitive PAA systems have inevitable disadvantages in the successive imidization step, i.e. a signi®cant shrinkage toward the thickness direction due to the evaporation of the residual solvent, water as by-product, and unreacted and reacted photoreactive groups and deformation of the ®ne patters obtained at the PAA. For these reasons, much effort has also been undertaken to give photoreactivity to the originally radiation-resistant PIs. PI from cyclobutanetetracarboxylic dianhydride (CBDA) and ODA provides positive patterns when the ®lm was irradiated with a Xe lamp in the range 230±254 nm via the chain scission due to the reverse reaction of CBDA photodimerization [150]. Recently, chemical ampli®cation-type positive working PI system were also developed. The reported systems are composed of the combination of a photoacid generator (e.g. p-nitrobenzyl-9,10-dimethoxyanthracene-2-sulfonate) and a PAA in which the groups responsible for alkaline-solubility (e.g. ±OH group) were capped with an acid-removable protecting group such as tert-butoxycarbonyl group [151]. Nakano [152] prepared negative working organo-soluble PIs from s-BPDA or PMDA with a variety of asymmetric diamines connecting acryloyl, cinnamoyl, and benzophenone carbonyl groups. Omote et al. [153] reported that a naphthoquinone diazide moiety-pendant ¯uorine-containing PI gives negative pattern for an organic solvent developer or positive one for an alkaline aqueous solution developer. Yu et al. [154] introduced an epoxy side group into this soluble ¯uorinated PI and obtained the chemically ampli®ed negative working PI. Pfeifer et al. [155] developed intrinsically negative working PIs from BTDA and various ortho-methyl-substitute diamines. We focus these systems and describe in more detail and discuss a relation of their photoreactivities with CT interactions.

306


Fig. 45. Basic structure of benzophenone-type photosensitive PI.

4.1.2. Negative working organo-soluble benzophenone-containing PIs and their photocrosslinking mechanisms As mentioned above, PAA-exposure type suffers from a serious de®ciency for the ®lm shrinkage in the subsequent imidization process. In addition, if the patterned PAA coatings were thermally cured at higher temperatures on a metal substrate, in many cases, the PI±metal laminates have a considerable residual stress responsible for delamination and ®lm cracking. Organo-soluble photosensitive PI systems, which can be solution-cast on a metal substrate, overcome this problem. Pfeifer et al. [155] developed such a negative working PI system combining a certain degree of thermal stability and good image resolution as formulated in Fig. 45. Note that these PI systems have ortho-alkyl substituents on the diamine residue. They mentioned that meta-substituents reduce signi®cantly the photosensitivity. This result is associated with the intramolecular CT affected by the N-phenyl conformation. This point will be discussed later in detail. According to the previously reported results of very familiar hydrogen abstraction of triplet excited state benzophenone (BP) [156] and photocrosslinking observed in BP-connecting vinyl polymers

Fig. 46. Proposed photocrosslinking mechanism in benzophenone-containing PI.


307

Fig. 47. Chemical structures of the benzophenone-containing PIs used.

[157,158], Lin et al. [159] proposed a reasonable photocrosslinking mechanism: triplet BP moieties undergo hydrogen abstraction from a spatially adjacent benzyl-positioned hydrogen of the diamine moiety and successively the formed radicals couple to yield a crosslink (Fig. 46). In addition to the BP carbonyl group, some photoreactions of the imide carbonyl groups present in this PI should be taken into account, since N-substituted phthalimides are also photoreactive [160,161]. These compounds undergo intramolecular photocyclization [162], but it can not contribute to the intermolecular crosslinking even if it occurred, as con®rmed from the fact that a BTDA-absent PI derived from s-BPDA and tetraethylmethylenedianiline as a hydrogen donor (TEMDA, see Fig. 47) did not cross-link even after prolonged irradiation. Also, even if the polymers contain the benzophenonediimide units, a hydrogen donor-absent BTDA-type PI also showed no photoreactivity. Thus, the presence of both the benzophenone units and hydrogen donors are required for photocrosslinking. For copolyimides derived from BTDA with TEMDA and 1,3bis(aminopropyl)tetramethylsiloxane (BADS) as a non-hydrogen donor, Lin et al. determined the gel dose, Dg. In Fig. 48, one can see that the reciprocal gel dose (sensitivity) increases linearly with increasing TEMDA content in the copolymers. They also observed that the benzophenone infrared band around 1680 cm 21 reduced during photoirradiation, whereas the symmetric and asymmetric

Fig. 48. Effect of hydrogen-donating diamine content on the reciprocal gel dose for benzophenone-containing copolyimides (BTDA±TEMDA;BADS) in: (X) air; (A) nitrogen; and ( £ ) oxygen. Reproduced with permission from Macromolecules 1988;21:1165. q 1988 American Chemical Society [159].

308


Fig. 49. Change in the Mw of BTDA±DEDPM during exposure (365 nm) at room temperature.

imide carbonyl stretching bands at 1783 and 1727 cm 21, respectively, remained unchanged in the intensity. Also, ESR spectra of the BTDA±TEMDA ®lm exposed to 365 nm radiation at 20 K showed a strong ketyl radical signal at 3268 G [159], indicating that the photoinduced hydrogen abstraction is indeed responsible for the benzophenone carbonyl concentration decrease. These results con®rmed that the above-mentioned hypothetical mechanism is reasonable. As plotted in Fig. 48, the oxygen inhibition effect was unexpectedly small in contrast to the fact that triplet BP is effectively quenched by oxygen both in solutions and polymeric solids, although this is an important advantage in practical lithography. The quantum yield of crosslinking, F cr, is derived from the equation [163]:

Fcr 2:303meMw0 Dg 21

13

where Mw0 is the initial weight-average molecular weight, m represents the molarity of the photoreactive group in the ®lm, and e denotes its molar extinction coef®cient at wavelength of irradiation (310 nm). Using m 1:67 mol l21 ; e 5:41 £ 103 M21 cm21 ; Mw0 26; 000; and Dg 26:0 mJ cm22 6:73 £ 1028 einstein cm22 ; they obtained Fcr 0:027 in air in good agreement with the value by Pfeifer et al. (20 mJ cm 22 at 310 nm). They also determined the quantum yield for the benzophenone carbonyl group disappearance as a function of the benzophenone conversion, F (x), by monitoring the intensity of the benzophenone carbonyl band at 1680 cm 21. After prolonged exposure, the 55% benzophenone carbonyl remained unreacted, indicating that this system includes 45% of the reactive site where the benzophenone units and hydrogen donors are located in a spatially favorable arrangement for photoreduction in the frozen solid state. A good curve ®tting for the plot of F (x) was obtained assuming that the simple bimodal distribution composed of the 55% unreactive and 45% reactive sites when the ®tting parameter of 0.03 as the reaction ef®ciency was used [159]. In our opinion, the good agreement of this value with the F cr calculated from Dg may suggest that the hydrogen-abstracted benzophenone carbonyls are subjected to the reverse reaction or successive crosslinking without other major possible photoreduction pathways such as disproportionation. Lin et al. speculated that the low F cr value is probably due to the presence of the reverse reaction. For an analogous negative working soluble PI derived from BTDA and bis(4-amino-3-ethylphenyl)methane (DEDPM), Higuchi et al. [164] determined F cr from a weight-average molecular weight (Mw) change monitored by GPC measurements during exposure at room temperature, according to David's


309

Fig. 50. Arrhenius plot of the quantum yield for photocrosslinking in BTDA±DEDPM.

expression [165]: Mw Mw0 =1 2 2uw0 x

14

where uw0 is the initial weight-average degree of polymerization and x denotes the crosslinking density (the number of crosslinks per the repeating unit). As shown in Fig. 49, the Mw increased with photoirradiation time. The decrease in Mw after 30 min means that a certain fraction of the photoproduced gel is ®ltered off for the GPC measurements. The slope in the linear plot of x vs exposure time provided the values of Fcr 2:0 £ 1023 in vacuum and 1:2 £ 1023 in air), which were comparable to the value derived from Dg Fcr 3:5 £ 1023 under N2) for the same system [164]. However, these F cr values, which are one order lower than the value obtained by Lin et al. are not explained only by the difference of the fewer numbers of the ethyl substituents in the repeating unit. Higuchi et al. also examined the temperature dependence of photocrosslinking ef®ciency to investigate the effect of molecular mobility. Fig. 50 shows the Arrhenius plot for F cr. Here, thermo-photoreaction is rather complex above 2008C, since thermal crosslinking also occurs simultaneously. The slope of the linear plot gave an apparent activation energy ( 2.4 kcal mol 21), which is much lower than those for other aromatic polymers [166]. No in¯ection was observed in the Arrhenius plot in contrast to the fact that photoreaction is signi®cantly affected by the abrupt changes at Tg and sub-Tg as shown in poly(ether sulfone) [166]. Elevated temperature in general tends to contribute to a decrease in photoreactivity owing to an increased deactivation of the excited state. Nonetheless, the increased F cr at higher temperatures (below 2008C) means that local molecular motions in the PI solid signi®cantly promoted photocrosslinking. They also stated that the quite low F cr for BTDA±DEDPM may be related to the CT interactions in wholly aromatic PIs. On the same procedure, Jin et al. [167] determined F cr for BTDA-hydrogenated DEDPM (DMDHM) semi-aromatic PI to be 4 £ 1023 both in vacuum and air. Note that this PI is CT-inhibiting. Since the values of F cr for PIs from BTDA and alkyl-substituted diamines were incompatible between different research groups, in comparison of the data within Horie's group, alicyclic diamine-derived BTDA± DMDHM showed about two (vacuum) to four times (air) higher F cr than aromatic BTDA±DEDPM. The higher crosslinking ef®ciency for this semi-aromatic PI is most likely attributed to its CT-inhibiting character. However, the photosensitivity improvement was not so prominent compared to that initially

Model compounds

Air-saturated

Degassed

Air-saturated

Degassed

1.1

0.3 a

0.29 a

0.88 a

0.2

0.4

0.56

0.56

0.15 b

0.14

0.02

0.066

0.001 c

0.002 c

0.13 c

0.06 d a b c d

In ethylbenzene (EB), in THF for others. Independent of the hydrogen donor solvents (THF, CH2Cl2, and EB). Quantum yield for crosslinking. Quantum yield for decomposition.


Polyimides

310

Table 7 Quantum yields for photocrosslinking in diazo- and benzophenone-containing PIs and their model compounds


311

Fig. 51. Chemical structures of the 6FDA-derived PIs used.

expected. A possible explanation is the presence of effective disproportionation pathways, which forms no crosslinks, after a tertiary or secondary hydrogen on the DMDHM residue was abstracted by the benzophenone carbonyl triplet [167]. Yamashita et al. [168] prepared a new type of diazo-containing photosensitive PI through a chemical modi®cation of the benzophenone carbonyl group in BTDA±DEDPM to diazo group. The photoirradiation of this PI ®lm at 400 nm formed a long-lived active intermediate, carbene, by N2 elimination via the singlet excited state and successively caused crosslinking with an about two orders higher Fcr 0:13 in air than that of the corresponding BTDA±DEDPM. A model experiment suggested that the photoreactivity of the diazo-containing PI is not signi®cantly affected by the presence of the CT interactions. The results for F cr of the above-described PI systems were summarized in Table 7. 4.2. Photodegradation There are many reports dealing with the effects of photoirradiation and UV laser etching on some properties of Kapton and other type of PI. In this section, we pick up only one example for 6FDA-type PIs. Hoyle et al. [169±171] reported the photodegradation of 6FDA±ODA and 6FDA±MDA (see Fig. 51 for their structures) in solution and in solid state. GPC and viscometric measurements showed that the Mws of these ¯uorinated PIs decrease with time of Pyrex-®ltered exposure (.300 nm) with a medium pressure mercury lamp in air. This is due to effective chain cleavage. Un®ltered deep UV irradiation in air caused photo-oxidation, and consequently, photoablation, as con®rmed by a weight loss of the PI ®lms and uniform reduction over the whole infrared absorption bands with exposure time. On the other hand, when photolysis was conducted in the absence of oxygen, even prolonged exposure results in little chain cleavage and no photo-oxidative ablation. From the comparisons with the results of other PIs such as ODPA±ODA and much more photostable PMDA±ODA, they stated that CT interactions may participate in the photodegradation of these PIs. To understand the primary photochemistry of 6F±PIs in air, Hoyle et al. [172±174] used Nphenylphthalimide as a model compound, since this model probably re¯ects the structural units in 6F±PIs where conjugation between the diimide groups is prevented by the C(CF3)3 bridge. Upon irradiation with a Pyrex ®ltered medium pressure mercury lamp, this model in an air-saturated acetonitrile solution rapidly decomposes to form phthalic anhydride (quantum yield F 3 £ 1024 ) as major product, phthalimide F 2 £ 1025 ; and a trace amount of nitrobenzene (this originates probably from more unstable nitrosobenzene). On the other hand, no photodecomposition proceeded essentially in the absence of oxygen, as for the polymer systems. The proposed primary photooxidative reaction in this

312


Table 8 Effect of solvent polarity on the quantum yields of photoreaction and intersystem crossing and ¯uorescence peak wavelength of N-phenylphthalimide. Reproduced with permission from Macromolecules 1992;25:6651. q 1992 American Chemical Society [173] Solvent

F loss a

F PhAnh a

F isc b

l max c (nm)

CH3CN CH2Cl2 Cyclohexane

6.3 £ 10 24 8.2 £ 10 24 9.3 £ 10 23

2.8 £ 10 24 3.2 £ 10 24 5.0 £ 10 23

0.03 0.04 0.17

540 530 505

a b c

^ 20%, lex 313 nm: ^ 20% relative to benzophenone Fisc 1:0: lex 325 nm:

model is an a-cleavage of the N±CO bond to give a diradical which can successively react with oxygen [165]. However, no appreciable anhydride formation was observed for actual 6FDA±ODA polyimide. They also examined the effects of solvent polarity and p-substituent on the photodecomposition quantum yields for N-(4-substituted phenyl)phthalimide [174]. The results are listed in Table 8. With increasing solvent polarity, the quantum yields of the model disappearance (F loss) and the phthalic anhydride formation (F PhAn) decreased both signi®cantly in accordance with a concomitant red-shift and an intensity decrease of the broad and long-wavelength ¯uorescences. From the results, Hoyle et al. proposed a hypothesis that the intramolecular CT interaction participates in the photodecomposition of the model in solution. Table 9 lists the effect of p-substituents (R). It is obvious that both F loss and F PhAn decrease in going from R ±CN (electron withdrawing group) to ±Ph±O±Ph (electron donating group). This result supported strongly the CT hypothesis mentioned above. In accordance with the prediction from these model compound results, the photostability of the PIs decreased in order of 6FDA±ODA . 6FDA±6H . 6FDA±6F, parallel to CT character (see Fig. 51 for their structures) [174]. The addition of a triplet quencher with a lower ET into the N-phenylphthalimide solution Table 9 Effect of p-substituent on the quantum yields of photoreaction and intersystem crossing and ¯uorescence peak wavelength for N-(p-substituted phenyl)phthalimide in solution. Reproduced with permission from Macromolecules 1992;25:6651. q 1992 American Chemical Society [173] Phthalimides

F loss a

F PhAnh a

F isc b

l max c (nm)

PA±A PA±POA PA±ClA PA±CAN

8.2 £ 10 24 8.8 £ 10 25 8.3 £ 10 24 1.2 £ 10 22

5.2 £ 10 24 1.4 £ 10 25 2.8 £ 10 24 5.5 £ 10 23

0.04 0.02 ± 0.16

505 550 525 470

a b c

^ 20%, in CH2Cl2, lex 313 nm: ^ 20% in CH2Cl2 relative to benzophenone Fisc 1:0: In cyclohexane, lex 325 nm:


313

Fig. 52. Phosphorescence spectra of model compound and benzophenone at 77 K in an ethanol±methanol glass, taken upon excitation at 320 nm. Reproduced with permission from J Photochem Photobiol, A 1988;44:99. q 1988 Elsevier Science [175].

suppressed the photodecomposition; on the other hand, the addition of a sensitizer enhanced the photoreactivity. These results suggested that the photooxidative decomposition proceeds from the triplet state. This hypothesis is supported by the result that F loss or F PhAn varied parallel to F isc (Table 8). The criteria obtained in the model systems were applicable to the actual 6FDA±PI systems. 4.3. Model compounds approach for photochemistry of benzophenone-containing polyimides The photochemical mechanism of the BP-containing PI was discussed, but it is not still well understood. Scaiano et al. [175] were the earliest to study the photochemical mechanisms of a model compound in solution using conventional and laser time-resolved techniques. In particular, it is important to learn whether the benzophenonediimide behaves as BP or as N±arylphthalimides. They used a model prepared from BTDA and 2-isopropylaniline. This model compound might be subjected to intramolecular photocyclization at the imide carbonyls as reported in the literature [162]. But no

Fig. 53. (1) Transient absorption and (2) phosphorescence spectra taken with a delay of 0.2 ms after excitation at 308 nm and triplet decay curve (inserted) for the model compound (see Fig. 52) in acetonitrile. Reproduced with permission from J Photochem Photobiol, A 1988;44:99. q 1988 Elsevier Science [175].

314


Table 10 Photochemical kinetic parameters for several model compounds and benzophenone Model

i-PA±BTDA±i-PA 2,4-DMA±BTDA±2,4DMA n-BA±BTDA±n-BA b n-BA±BTDA±n-BA b o-EA±BTDA±o-EA c MCHA±BTDA±MCHA 2,6-DEA±BTDA±2,6-DEA 2,6-DEA±BTDA±2,6-DEA 2,6-DEA±BTDA±2,6-DEA (by transient) 2,4-DMA±BTDA±2,4DMA 3-EA±BTDA±3-EA BP BP BP BP a b c

208C

2 1968C

Solvent

F M (vac)

F M (air)

F isc

kabs (M 21 s 21)

2-PrOH EtB a

± 0.066

± 0.025

± 1.0

2.5 £ 10 7 9.3 £ 10 4

± ±

THF MTHF MTHF CH2Cl2 CH2Cl2 THF THF

0.56 ± ± 0.38 0.41 ± ±

0.56 0.34 0.054 ± ± ± ±

± ± ± 1.0 0.87 ± ±

± ± ± 1.3 £ 10 5 3.4 £ 10 5 4.0 £ 10 6 1.0 £ 10 7

CH2Cl2

0.22

±

0.35

CH2Cl2 2-PrOH 2-PrOH EtB CH2Cl2

0.096 ± ± 0.88 0.40

± ± ± 0.29 ±

0.19 1.0 1.0 1.0 1.0

t p (ms)

Fp

Ref. no

± ±

[175] [164]

± ± ± 15.6 7.8 ± ±

± ± ± 0.71 0.50 ± ±

[167] [167] [167] [178] [178] [178] [178]

4.6 £ 10 5

9.9

0.12

[178]

2.3 £ 10 5 3.2 £ 10 6 2.0 £ 10 6 1.0 £ 10 6 3.0 £ 10 5

6.6 ± ± ± 2.6

0.051 ± ± ± 0.82

[178] [175] [177] [164] [178]

EtB ethylbenzene. n-BA n-butylamine. o-EA o-ethylaniline.

photocyclized product was obtained. On the contrary, in NMP, UV-irradiation of the model almost quantitatively formed the photoreduced benzhydrol product, similar to the chemically reduced one. Fig. 52 displays the phosphorescence spectra of BP and the model at 77 K in an ethanol±methanol rigid glass. The model provided a structured phosphorescence similar to that of BP but with a slight redshift. The triplet energies calculated from the 0±0 bands were 65.1 kcal mol 21 for the model and 69.2 kcal mol 21 for BP. The phosphorescence lifetime of the model at 77 K was estimated to be 20 ms. Transient absorption and phosphorescence spectra of the model in acetonitrile (non-hydrogen donor) at room temperature is shown in Fig. 53. This transient spectrum of the model in CH3CN was very similar to that of PI derived from BTDA and 2,3,5,6-tetramethylphenylelediamine (TMPDA) in a dichloromethane solution [176]. The transient absorption lifetime (4±6 ms) agreed well with that of the phosphorescence, supporting the assignment that the transient spectrum is based on T±T absorption. This assumption was also con®rmed from the fact that the Stern±Volmer plot for the transient absorption lifetime of the model vs the concentration of 2,5-dimethyl-2,4-hexadiene as a typical triplet quencher ET 58:7 kcal mol21 led to an approximately diffusion-controlled quenching rate constant kq 1:0 £ 1010 M21 s21 : A long lived ketyl radical was observed for the model in the presence of 2propanol (hydrogen donor) [175], as for BTDA±TMPDA polyimide in a tetrahydrofuran solution [176]. The T±T absorption spectrum of the model differed in shape from that of the ketyl radial formed via hydrogen abstraction, but these were somewhat similar to each other. To avoid overlap between them, they preferred to monitor the phosphorescence rather than the transient absorption for determining the


315

Fig. 54. Chemical structures of the series of BTDA-derived model compounds used.

hydrogen abstraction rate constant kabs. A modi®ed Stern±Volmer plot led to kabs 2:5 £ 107 M21 s21 for 2-propanol in CH3CN at room temperature. Note that this value is about one order of magnitude higher than the data for BP in 2-propanol±CH3CN (kabs 3:2 £ 106 M21 s21 from their own measurements and 2 £ 10 6 M 21 by another group) [177]. Scaiano et al. [175] explained the enhanced reactivity of the model in terms of the electron-withdrawing effect of the diimide groups. For the model derived from BTDA and 2,4-dimethylaniline, Higuchi et al. [164] determined kabs using ethylbenzene as a hydrogen donor on the basis of the quantum yields of the model consumption both in the air-saturated and degassed conditions. The photochemical parameters obtained are summarized in Table 10; for comparison refer to the data by Scaiano et al. [175]. The quantum yield of intersystem crossing, F isc, was determined as unity, as in BP. It should be noted that, in the ethylbenzene hydrogen donor system, kabs of the model was approximately one order lower than that of BP. This result con¯icts with Scaiano's result for the 2-propanol system described above. Higuchi et al. [164] speculated that the decreased reactivity for the model may be related to the nature of the lowest triplet state T1; it might be a mixed state of 3(n, p p) and 3CT con®gurations, in contrast to the fact that BP always has a pure T1 (n, p p) state responsible for photoinduced hydrogen abstraction. They also stated that the more effective triplet deactivation process in the model might originate in intermolecular CT interactions with ethylbenzene added as a hydrogen donor. On the basis of the conventional Stern±Volmer plots both in the absence and in the presence of a typical triplet quencher (naphthalene), Hasegawa et al. [178] determined the values of kabs and F isc for a

316


Fig. 55. UV±vis absorption spectra of benzophenonediimides in dichloromethane: (a) c 5 £ 1025 M; and (b) 5 £ 1023 M: The numbering corresponds to the model compounds depicted in Fig. 54.

series of BTDA-derived model compounds with different CT characters (see Fig. 54 for the model structure) in dichloromethane, which has a certain degree of hydrogen-donating ability and high solubility towards all the models used here. Fig. 55 exhibits the UV±vis absorption spectra of the models in dichloromethane. One can see that the absorption tails extend toward longer wavelength with an increase in the N±Ar conjugation associated to their dihedral angles, whereas the peak positions around 300 nm are insensitive to the amine structures. From this result, the models can be arranged in the order of decreasing intramolecular CT character as follows: 3-EA±BTDA±3-EA . 2,4-DMA±BTDA±2,4DMA . 2,6-DEA±BTDA±2,6-DEA . MCHA±BTDA±MCHA. This order agrees with the prediction

Fig. 56. Schematic energy diagram for BTDA-derived model compounds.


317

from ª¯uorescence criteriaº discussed previously for biphenyldiimides. The kinetic parameters at room temperature are listed in Table 10. It is clearly shown that the quantum yields of model compound consumption, F M, decreased parallel to F isc with increasing CT character. Additionally, the change in F isc at room temperature also corresponds well to the phosphorescence yield at 77 K, F p, whereas no correlation of F M with kabs and with kd was observed. The data means that the triplet state of all the models used have the same level of hydrogen abstraction ability independent of their CT characters. These results led to a reasonable photophysical mechanism depicted schematically in Fig. 56. After photoexcitation to the S1 (n, p p) state, the intersystem crossing competes with the CT process; as a result, the hydrogen abstraction ef®ciency is dominated by F isc rather than by kabs and kd. This mechanism is very likely if one take into account that the rate of the intramolecular CT process (kCT . 5 £ 1011 s21 for s-BPDA±PI models [49]) is comparable to that of the intersystem crossing (kisc . 2 £ 1011 s21 for BP [179]). An increase in CT character of the models should result in an increase in kCT, and consequently, in a decrease in F isc associated to F M. The kabs values unaffected by the N±aryl structure as listed in Table 10 probably mean that T1 remains the pure (n, p p) state independent of the CT characters of the models. This hypothesis was evidenced by the phosphorescence spectra of the models at 77 K, which are very similar in shape and position independent of the intramolecular CT character. This conclusion con¯icts with Higuchi's speculation that T1 of the model prepared from BTDA and 2,4-DMA might not be in the pure (n, p p) but probably in a (n, p p)/CT mixed state. The time-resolved emission spectra of the model also supported the proposed energy scheme in Fig. 56 [178]. The model compound results revealed by Hasegawa et al. [178] emphasizes that the presence of alkyl substituents at the ortho position contributes to an increased photoreactivity owing to the N±Ar molecular plane distortion, which reduces the intramolecular CT character. This is the reason why BTDA±PIs derived from the ortho-alkyl substituted diamine are highly reactive but that from the meta-substituted one is less reactive, as stated by Pfeifer et al. [155]. 5. Optical properties of polyimides The color of PI ®lms, which is affected by CT interactions and processing conditions (cure temperature, monomer purity, kind of solvents, and atmosphere in thermal cure), were already discussed in Section 2.2.1. In the present section, we mainly describe the molecular design in controlling optically transparency, refractive indices of PIs for some practical applications, and non-linear optical properties of dye-doped PI ®lms. 5.1. Optically transparent polyimides Strong absorption of wholly aromatic PIs over the ultraviolet to visible range sometimes becomes a serious obstacle in practical cases. For example, visible absorption of PI ®lms can be a trigger for overheating of the ®lms, which is unfavorable in applications on space components such as multilayer insulation blankets, solar cell, and thermal control coating systems. The transparency of PIs as matrix polymers is also a critical factor for applications to photosensitive PI materials since photoreactive groups are masked by the matrix polymers themselves if their absorption regions overlap. This situation limits the choice of photoreactive groups. To conduct exposure at the stage of more transparent polyamic acids than corresponding PIs is an approach to avoid such a masking effect by matrix polymers.

318


Table 11 Cut-off wavelengths, reduced viscosities (0.5 wt%) of PAAs in DMAc at 358C, Tg's, and ®lm color of various transparent PI ®lms. Reproduced with permission from SAMPE J 1985;21:28. q 1985 SAMPE [181] Polyimide

h inh (dl g 21) (12.5 mm thick)

Tg (8C) (5 mm thick)

Film color

Cut-off (nm)

6FDA±APB 6FDA±3,3 0 -ODA 6FDA±BDAF 6FDA±3,3 0 -DDS 6FDA±3,3 0 -6F 6FDA±4,4 0 -ODA 6FDA±DABP 6FDA±MDA

1.20 1.00 0.92 0.47 0.60 ± ± ±

206 244 263 279 260 ± ± ±

Pale to colorless Pale to colorless Pale yellow Colorless Colorless Light yellow Light yellow Yellow

336 330 328 312 316 ± ± ±

ODPA±APB ODPA±3,3 0 ±ODA ODPA±BDAF ODPA±3,3 0 -DDS

0.87 1.09 1.08 0.51

187 192 241 258

Pale Pale Pale Pale

yellow to colorless yellow yellow

349 352 346 352

BDSDA±APB BDSDA±BDAF BDSDA±3,3 0 -DDS

0.41 1.10 0.43

167 210 213

Pale to colorless Pale to colorless Pale yellow

368 353 365

BTDA±APB BTDA±BDAF BTDA±3,3 0 -DDS

0.80 0.94 0.42

202 249 257

Yellow Yellow Pale orange

378 388 365

PMDA±APB PMDA±BDAF PMDA±3,3 0 -DDS

0.44 0.47 0.27

221 305 338

Yellow Yellow Pale orange

366 340 360

However, wholly aromatic polyimide precursors such as PMDA±ODA polyamic acid keep a certain degree of electron donor±acceptor character; therefore, the absorption in the 300±400 nm range still remains strong. Optically transparent PIs are also of special importance for optoelectronic devices such as optical waveguides for communication interconnects and optical half-waveplates for planar lightwave circuits. 5.1.1. Control of CT character in the PI chain sequence A strategy for obtaining transparent PIs is to use lower electron-acceptability of dianhydrides and lower electron-donatability of diamines as monomers for weakening both intra- and intermolecular CT


319

Fig. 57. (a) Relationship between the yellowness index and the extent of whiteness of a variety of PI ®lms: (A) PMDA; (W) s-BPDA; (K) and BTDA systems (50 mm thick ®lms); and (b) (S) ODPA and (L) 6FDA (35 mm thick ®lms).

interactions. St. Clair et al. [180,181] investigated the structure±coloration relationship systematically. The results are summarized in Table 11. According to the above-mentioned criteria for monomer design, they revealed that the combinations of ODPA, BDSDA, and 6FDA as dianhydrides with 3,3 0 -diaminodiphenylsulfone (3,3 0 -DDS) and 2,2 0 -bis(3-aminophenyl)hexa¯uoropropane (3,3 0 -6F) as diamines can give rise to more transparent PI ®lms with a high solubility in aprotic organic solvents such as DMAc and CH3Cl. Particularly, 6FDA±3,3 0 -DDS is a promising candidate as a transparent thermostable PI, although the use of less reactive 3,3 0 -DDS simultaneously brings about a dilemma with a great dif®culty of obtaining high molecular weight PAAs (see Table 11). The effect of para/meta isomerism was also demonstrated. Replacing 4,4 0 -ODA diamine by 3,3 0 -ODA in 6FDA-derived PIs varied the ®lm color from light yellow to pale yellow/colorless. The decreased conjugation on the meta-linkage in 3,3 0 -ODA is responsible for the improved transparency. Noda et al. [182] also essentially reached the same

320


conclusion. They evaluated the coloration of PI ®lms using the yellowness index. The plot of the yellowness index vs the extent of whiteness in Fig. 57 again listed up the combination of 6FDA, ODPA, and s-BPDA with 3,3 0 -DDS and 3,3 0 -ODA as transparent PI candidates. Ando et al. [183] described that 15N NMR chemical shift of the amino nitrogen in diamine monomers, which represent electron-donating ability, is a useful indicator for predicting the color intensity of the resulting PI ®lms with a ®xed dianhydride component, together with the 13C NMR chemical shift of carbonyl carbon in dianhydrides. The extent of through-bond conjugation can be dramatically altered by not only the para/meta isomerism but also by the torsional (dihedral) angle between molecular planes in PI chains. The model compounds experiments described in Sections 2 and 3 revealed that the absorption tail extending to the visible region can be shortened owing to the effect of sterically prohibited conjugation when the benzimide±N-phenyl molecular planes are highly distorted by the presence of bulky ortho-substituents. In addition, distortion of the biphenyl planes in s-BPDA-type polyimides should result in a pronounced decrease in the absorption intensity in the ultraviolet region. This prediction is based on the fact that ortho-substituted 2,2 0 ,6,6 0 -tetramethylbiphenyl exhibits a much weaker absorption band than substituents-free biphenyl, almost identical in intensity with the twofold spectrum of the corresponding monomeric unit [184]. Hasegawa et al. [81] reported that a-BPDA-derived PIs exhibit higher transparency than the s-BPDA-counterparts. This is attributed to both its bent chain structure and the highly distorted biphenyl conformation originating from the serious orthohydrogen±2-carbonyl steric hindrance, which no longer allows dense chain stacking, and in turn, interchain CTC formation. In fact, a-BPDA±PDA and s-BPDA±DMPDA (DMPDA 2,5-dimethyl-p-phenylenediamine), both possessing highly distorted conformations, showed practically no annealing effects for ®lm color, density, and CT ¯uorescence intensity, in contrast to a pronounced annealing effect observed in coplanarization-allowed s-BPDA±PDA (Hasegawa, unpublished results). 5.1.2. Use of ¯uorinated monomers Rogers [185] reported earlier that a PI derived from 6FDA and a ¯uorine-containing diamine results in an optically transparent/colorless ®lm. In 6FDA, the electron-withdrawing ±CF3 groups linked to the quaternary carbon atom should contribute somewhat toward increasing the overall electron af®nity of the dianhydride, although its effect is predicted to be much lower than the case of their direct bonding onto the dianhydride aromatic rings. This prediction agrees with the actual high reactivity of 6FDA (high molecular weights of resultant PAAs) as in highly reactive PMDA and BTDA. In fact, the electron af®nity of 6FDA calculated using the MNDO-PM3 semi-empirical MO approximation was arranged between the values of PMDA and BTDA [183]. Nonetheless, the resultant PI ®lms derived from 6FDA were highly transparent/colorless, most likely meaning that the bulky ±CF3 groups hinder chain stacking, and consequently, interchain CTC formation. This argument points out that intermolecular CT interactions contribute to the PI ®lm color in no small way. Although 6FDA±3,3 0 -DDS and 6FDA±3,3 0 -6F provide a highly transparent ®lm, their Tg's are unfortunately not so high (2798C for the former, 2608C for the latter). This is due to the high conformational ¯exibility of these diamine residues. The current manufacturing processes for integrated circuits and multichip modules demand higher soldering resistance Tg . 2708C and short-term thermal stability at temperatures up to 4008C. Matsuura et al. [186] reported that the combination of 6FDA and 2,2 0 bis(tri¯uoromethyl)-4,4 0 -diaminobiphenyl (TFDB) gives rise to a high Tg 3358C: This PI also


321

Table 12 Properties of ¯uorinated PIs. Reproduced with permission from Macromolecules 1991;24:5001. q 1991 American Chemical Society [186] Properties

6FDA±TFDB

PMDA±TFDB

Fluorine content (%) h inh of PAA (dl g 21) 10% weight loss (in N2) (8C) Tg (by DSC) (8C) e 0 at 1 KHz (dry) e 0 at 1 KHz (wet) Refractive index (at 589.6 nm) Water uptake (%) CTE (in 50±3008C) (ppm K 21)

31.3 1.00 569 335 2.8 3.0 1.556 0.2 48

23.0 1.79 610 . 400 3.2 3.6 1.647 0.7 3.0

realizes considerably high optical transparency, concurrently with a low dielectric constant (e 0 ), a low refractive index, a low water uptake, and a high PAA molecular weight hinh 1:0 dl g21 : Table 12 lists the properties of this PI with those of PMDA±TFDB for comparison. In spite of the presence of electron-withdrawing ±CF3 substituents, the maintained reactivity of TFDB is most likely based on the meta-substitution onto benzidine. If the ortho-substituted diamine counterpart was used, it must be dif®cult to obtain high molecular weight PAA in the conventional way because of its expected much lower reactivity. The transmission spectra of a series of TFDB-based PIs in Fig. 58 indicate how the 6FDA±TFDB polyimide ®lm is optically transparent. A secondary positive effect of the ±CF3 substituents in TFDB on the ®lm transparency is the weakened intermolecular cohesive force due to lower polarizability of the C±F linkage. This functions negatively to interchain CTC formation. A drawback of 6FDA±TFDB is, however, a comparatively high linear CTE (48 ppm K 21). This seems to be inevitable as long as 6FDA is used as a dianhydride component, since the kink point in 6FDA lowers both the chain linearity and intimate interchain stacking necessary for causing high PI chain orientation along the substrate plane. The relationship between CTE and chain orientation will be described later. Fluorinated polyimides were also investigated for applying to a long-distance optical communication

Fig. 58. Transmission spectra of 20 mm thick PI ®lms: (1) 6FDA±TFDB; (2) PMDA±TFDB; and (3) PMDA±o-TOL (otolidine). Reproduced with permission from Macromolecules 1991;24:5001. q 1991 American Chemical Society [186].

322


Fig. 59. Near-infrared absorption spectra of PIs in acetone-d6 solution: (1) 10FEDA±4FMPD; and (2) 6FDA±TFDB. Reproduced with permission from Macromolecules 1992;25:5858. q 1992 American Chemical Society [188].

component [187]. For this purpose, the transparency in the near infrared region (1.0±1.7 mm) are required, especially at 1.3 and 1.55 mm, rather than that in the visible range. Fig. 59 displays the near-infrared absorption spectrum of 6FDA±TFDB. Fortunately, these communication wavelengths are located just at the ªwindowº of the absorption bands originating mainly from the harmonics and from the coupling of C±H stretching vibrations. However, the unfavorable C±H bonds still remain in partially ¯uorinated 6FDA±TFDB. Further efforts to completely erase the optical loss have been done. An attempt was to synthesize a per¯uorinated PI from di(tri¯uoromethyl)-substituted PMDA (P6FDA) and wholly ¯uorinated m-phenylenediamine (4FMPD) with the highest reactivity among several per¯uorinated diamines examined (see Fig. 60 for their structures). Unfortunately, the combination of these monomers provided only a brittle ®lm, probably owing to its very stiff polymer chain (insuf®cient entanglement) [188]. Newly developed 10FEDA overcame this problem. The most striking feature of the 10FEDA±4FMPD per¯uorinated PI ®lm is the extreme transparency over the entire optical communication wavelengths as illustrated in Fig. 59, although the PI ®lm has a visible absorption (pale yellow like PMDA±ODA). This PI exhibited a glass transition at 3098C, suf®ciently higher than soldering temperature and a low e 0 of 2.8 at 1 kHz [188]. Synthesis and dielectric constants of a variety of ¯uorinated PIs were reviewed in the literature [189].

Fig. 60. Chemical formula of 6PFDA±4FMPD.


323

5.1.3. Use of alicyclic monomers According to the intramolecular CT criteria discussed above, semi-aromatic PIs must be optically transparent if common dianhydrides are used. Moreover, wholly aliphatic PIs are supposed to show a higher transparency in the ultraviolet region. However, the limited numbers of commercially available aliphatic dianhydrides often force us to derive semi-aromatic PIs from aromatic dianhydrides and aliphatic diamines. Then, non-cyclic aliphatic monomers such as n-alkylenediamine are avoided in practical use because of very low thermooxidative stability of resultant PIs. As mentioned previously, even alicyclic diamine-based s-BPDA±t-CHDA possesses much lower thermal stability than the corresponding wholly aromatic s-BPDA±PDA, although this semi-aromatic PI has short-term heat resistance up to 4008C. Matsumoto [190±193] reported that the introduction of a bicycloaliphatic structure leads to a high Tg and a high thermal decomposition (5% weight loss) temperature which results from a reduced possibility of thermal main chain scission. Such a high Tg and a high short-term thermostability overcoming soldering process are advantageous for microelectronics applications. However, note that even bicycloaliphatic moieties are still thermally labile during the prolonged high temperature exposure compared to aromatic units. A serious obstacle in the use of aliphatic diamines is the dif®culty of obtaining high molecular weight PAAs. Owing to the high basicity of aliphatic diamines, the reaction mixtures are always subjected to salt formation between unreacted amino groups and carboxyl groups formed by primary acylation. Hasegawa et al. [194] investigated the polymerization conditions for obtaining high molecular weight semi-aromatic PAAs from aromatic dianhydrides with t-CHDA and showed that there are effective ways several: to drop the diamine solid into the dianhydride solution in contrast to the common procedure, the use of non-amide solvents such as dimethylsulfoxide, to elevate the reaction temperature, and the addition of a salt like LiBr, although the optimum conditions depend on the systems. On the other hand, PAA polymerization of aliphatic dianhydrides and aromatic diamines readily proceeds in many cases if there is no problem of monomer purity. A typical aliphatic dianhydride with a high reactivity is cyclobutanetetracarboxylic dianhydride (CBDA) [150,195±197]. For example, the reaction of this monomer with 4,4 0 -ODA forms a high molecular weight PAA (hred 1:5 dl g21 in DMAc at 258C) [150]. Polyimides prepared from another alicyclic dianhydride, 2,3,5-tricarboxycyclopentylacetic acid dianhydride, developed by Japan synthetic Rubber Co. for the LCD-alignment layer [198], also expectedly show high transparency [199]. 5.2. Control of refractive indices in polyimides Recently, research and development of functional polyimides have been extending to optoelectronic applications [200]. This ®eld demands extremely precise control of refractive indices and its anisotropy (birefringence). Fluorinated PIs were successfully applied to an optical waveguide material [201±203] because of their high transparency at the transmitting wavelength as described in the preceding section, high thermal stability resistant to soldering and short-term heating process (,4008C) for optoelectronic interconnection between LSI chips, and high humidity resistance. Conventional waveguide polymeric materials such as poly(methyl methacrylate) Tg 1008C and polycarbonate Tg 1508C are not suitable for the current manufacturing processes. Another important requirement is to suppress the optical loss due to scattering. In general, the magnitude of optical loss tends to increase with increasing cure temperature owing to scattering from microvoids, surface roughness, and density ¯uctuation induced by local ordering (crystallization and

324


Fig. 61. Refractive indices at a light wavelength of 1.3 mm, birefringence, linear coef®cient of thermal expansion, and dielectric constant of 6FDA;PMDA±TFDB copolymer as a function of the 6FDA content. Reproduced with permission from Macromolecules 1993;26:419 and 1994;27:6665. q 1993/1994 American Chemical Society [204,205].

CTC formation) and decomposition. 6FDA-derived PIs, which possess the highly bent/bulky hexa¯uoroisopropylidene group, hinder ordering as a primary origin for scattering. Thus, ¯uorinated PIs such as 6FDA±TFDB are lighted up for the present purpose because of the combination of its noncrystalizability, higher Tg and thermooxidative stability, and transparency. The NTT research group [201,202] fabricated planar-type single mode waveguides consisting of a higher refractive index ¯uorinated PI core/a lower refractive index cladding (core-embedding type). The condition for the single mode slab waveguiding is given by d , ncore 2 nclad l=2

15

where d is core thickness, l is light wavelength, and ncore and nclad are the refractive indices of core and cladding layers, respectively. They obtained a low optical loss (,0.3 dB cm 21) waveguide by precisely controlling the refractive indices (^0.001) of core and cladding. The copolymerization technique is very useful for refractive index control, as demonstrated in the 6FDA;PMDA±TFDB copolymer system where the refractive index (also CTE and e 0 ) is controllable over a wide range by varying the 6FDA


325

Fig. 62. Chemical formula of PMDA±3F.

content without a signi®cant decrease in the near-infrared transparency [204,205]. Fig. 61 shows the refractive index, CTE, and e 0 as function of the 6FDA content in this copolymer. PI ®lms deposited on a metal or glass substrate usually show birefringence, expressed by (nin 2 nout) where nin and nout are refractive indices for the in-plane and out of-plane directions, respectively, as the results of chain orientation along the ®lm plane and residual stress in the PI ®lms. The birefringence in the core, caused inevitably in the planar-type waveguide fabrication, is desirable to be as low as possible. Higher temperature annealing after cure is often effective in erasing the birefringence if the PI has a suf®cient molecular mobility above the Tg. For example, in 10FEDA±4FMPD-per¯uorinated PI, the birefringence of 7.6 £ 10 23 at 1.3 mm for the on-substrate ®lm was reduced signi®cantly to nearly zero (1 £ 10 24) by subsequent annealing of the ®lm on the substrate at 3508C/1 h [206]. Other optical communication components such as optical interference ®lter [187] and optical waveplate [207] have also been developed by precisely controlling the degree of uniaxial chain orientation and ®lm thickness of ¯uorinated PIs. The control of refractive index of PIs also means the control of the dielectric constant (e 0 ) according to Maxwell's relation e 0 n2 (e 0 1:1n2 practically for the directly measured e 0 at around 1 kHz frequency). When PI ®lms are used as interlayer dielectrics in microelectronic packaging, the pulse propagation delay is proportional to e 0 of the PI ®lms used. Accordingly, the reduction of e 0 allows faster machine time concurrently with lowering crosstalk noise. This is the reason why low e 0 polyimides are so eagerly desired. Fluorination of PIs is also effective in reducing e 0 . Another unique approach toward low e 0 is to disperse ®ne foams in PI ®lms, since the e 0 of air is unity. This technique developed by Hedrick et al. [208] typically involves the preparation of PS±PAA±PS (PS: polystyrene) triblock copolymer, imidization, and ®nally higher temperature annealing where thermally labile PS block undergoes thermolysis (depolymerization) to form submicron pores. They utilized a variety of other thermally unstable block such as poly(a-methylstyrene), poly(propylene oxide), PMMA, poly(e-caprolactone), and aliphatic polyesters and examined the effects of chemical structure, fraction, and molecular weight of the block on the resultant morphology (pore size, shape, porosity) and dielectric and thermal, and mechanical properties. In this case, the resulting porous structure depends on the initial microphase separation domain structure of the thermally labile triblock. For example, nanofoamed PI (19% porosity) prepared from triblock consisting of PMDA±3F [3F 1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-tri¯uoroethane] (see Fig. 62 for its structure) and poly(propylene oxide) showed a considerably lower e 0 ( 2.3) than that of the non-porous homo PMDA±3F e 0 2:9 [209]. 5.3. In-plane orientation responsible for birefringence in polyimide ®lms PI ®lms, prepared through thermal imidization of PAA ®lms cast on a metal substrate, unexceptionally generate birefringence [74,210,211]. This is based on the actual chain orientation along the substrate plane. Such an anisotropic structure Ð in-plane orientation (IPO) Ð is closely related to the linear CTE

326


of the ®lm plane direction. In some PI systems, it was demonstrated that the degree of IPO is almost inversely proportional to the linear CTE [74,210]. Thus, to learn how IPO takes place in PI systems is indispensable for CTE control. The CTE of PI ®lms is a critical factor for residual stress generation in PI±metal laminates. Large CTE mismatch between PI ®lms and substrates, generated during cooling to room temperature after cure, can be a trigger of delamination, curling, and cracking which affect adversely the reliability of microelectronic devices. Unfortunately, many PIs possess a comparatively high CTE in the range 40± 60 ppm K 21 whereas the CTEs of metal substrates are much lower, e.g. 17 ppm K 21 for copper and 2.3± 2.8 ppm K 21 for silicon [200]. Therefore, much effort has been made to decrease the CTE of PI ®lms by molecular design [74,200]. Numata et al. [212] investigated the structure±CTE relationship for a variety of PIs and concluded that the chain linearity is a primary factor for obtaining lower CTEs. Hasegawa et al. [210] utilized a reactive dichroic dye, diamino-PEDI (see Fig. 34) to evaluate the degree of IPO for PAA and PI chains by measuring the dichroic absorption ratio on a tilt angle con®guration. They showed that the polymer chains in as-cast PAA ®lms align slightly along the substrate plane, independent of the PAA chemistry, whereas a typical ¯exible polymer, poly(vinyl chloride), has nearly zero IPO. However, the degree of IPO after thermal cure depended strongly on the PI chemistry: for example, rod-like PMDA±PDA and semi-rigid s-BPDA±PDA displayed extremely high IPO but ¯exible s-BPDA±ODA caused orientational relaxation during cure. The degrees of IPO are affected not only by the PI chemistry but also by the on/off-substrate, thermal conditions (®nal cure temperature, heating rate), kinds and content of residual solvent, and side chain substituents. However, the mechanism of the cure-induced spontaneous IPO behavior observed in rigid PI systems still remains far from being fully understood. 5.4. NLO properties in polyimide ®lms NLO materials are currently of interest to a large number of research groups as they have potential applications in areas such as telecommunications, optical information processing, and data storage. Many reports have been published on the developments of polymers with NLO chromophores [213,214]. In order to provide second-order NLO properties, e.g. second harmonic generation and electro-optical (EO) effects in polymer ®lms, non-centrosymmetry is required to be made, which is usually achieved by a poling technique by applying an electric ®eld. However, the instability of dipole orientation of NLO chromophores tending to long-term relaxation back into the isotropic state is one of the major dif®culties of NLO polymer materials. Therefore, aromatic PIs have been selected as promising candidates [215] to overcome this problem because their high thermal stability is effective in restraining the relaxation of the NLO chromophore alignment induced by poling. In 1991, the ®rst PI-based guest±host electro-optical (EO) systems were described [216,217] and the example of fabrication of an all-PI Mach-Zehnder modulator of a vertical triple-stack with the cladding PI±EO-core PI±cladding PI sandwiched between electrodes using a commercial laser dye DCM [4-(dicyano-methylene)-2-methyl-6-(p-dimethylaminostyryl)-4H pyran] is given in a monograph [215] in order to show the development pathway of a prototype device in the use of polyimides for EO devices. The high processing temperature of PI ®lms requires dopant NLO molecules to have high thermal decomposition temperature and not to sublime. Some examples of NLO chromophores [218±221] with thermal stability exceeding 3008C are shown in Fig. 63. SY177 [220] and DADC [221] exhibit


327

Fig. 63. Examples of some NLO chromophores stable to .3008C: (a) triarylimidazole; (b) triaryloxazole; (c) SY177; (d) DADC; and (e) DR1.

Fig. 64. Synthetic scheme of diamine monomers bearing NLO chromophore.

328


Fig. 65. Chemical structures of PI-1, PI-2, and PI-3 as NLO polyimides.

hyperpolarizabilities comparable to the standard NLO chromophores such as DR1 (Disperse Red 1) with greatly increased thermal stability. Though high Tg is desirable for suppressing the relaxation of dipole alignment, poling at high temperatures also becomes necessary in high Tg polymers for ef®cient dipole alignment and high NLO ef®ciencies. Attempts to optimize NLO ef®ciency and thermal stability were carried out for dye-doped systems by using fully imidized, organic soluble, thermally and photochemically crosslinkable, guest±host PI with DADC [221] and by real-time poling vapor codeposition of PMDA, ODA, and DR1 [222]. In these past several years, work has begun to attach NLO chromophores covalently to PI. Yu et al. [223,224] proposed a general approach to synthesize NLO polyimides by preparing diamine monomers bearing NLO chromophores through a route shown in Fig. 64 and by successive PAA formation and imidization. A polyimide made of 6FDA and DR1-substituted PDA showed a high Tg (,2308C), longterm NLO stability at elevated temperatures such as 1508C, and a low optical loss (2 dB cm 21 at 632 nm). A polyimide with a benzothiazolyl-diazo chromophore prepared through a route similar to Fig. 64 showed the second harmonic coef®cient, d33, of 138 pm V 21 at 1.064 mm with Tg of 1808C [225]. Polyimides in which the electron-donor part of the NLO chromophore is imbedded in the polymer backbones (Fig. 65, PI-1 and PI-2) have exceptional chemical stability at elevated temperatures (3508C) and excellent poled order stability at extremely high temperatures as is evaluated in the temperature dependence of relaxation time, t , (the point at which the electrooptic coef®cient has decayed to 1=e of its initial value) in Fig. 66 [226]. Several PIs with side-chain NLO azo chromophores


329

Fig. 66. The second-order NLO stability evaluated as the relaxation time, t , as a function of temperature for PI-1, PI-2, and PI-3. Reproduced with permission from Science 1995;268:1604. q 1995 American Association for the Advancement of Science [226].

[227±230] are also reported together with the analysis [227,228] of the temperature dependence of relaxation processes of EO properties. The orientational changes in side-chain NLO azo chromophores of PAA alkyl amine salt monolayers before and after the imidization process were studied by optical surface second-harmonic generation (SHB) [231]. The decrease in NLO coef®cients after imidization of the LB ®lm was due to the change in the polar angle of side chain units. An attempt to produce lightinduced orientation of an NLO azo chromophore attached to the side chain of a PI was reported at room temperature [232]. The effects of a number of methods: (a) irradiation of polarized light at room temperature; (b) application of polarized light and electric ®elds at temperatures ranging from room temperature to the polymer Tg; and (c) application of electric ®elds at Tg on poling and depoling ability were compared [233], showing large dependence of the results on the way of connecting NLO chromophore into the polymer backbone for the samples shown in Fig. 65. The NLO properties in the above-mentioned PIs are attributed to the NLO chromophores which are molecularly dispersed in or covalently bonded to thermally stable PI chains. Aromatic polyisoimides (PiI), isomers of PI, have conjugated structure along their main chains, and hence are supposed to show intrinsic third-order NLO properties. The third-order non-linear susceptibility, x (3), of several PiI ®lms was estimated to be in the order of 10 212 esu by third harmonic generation measurements (Table 13) with Table 13 The chemical structure of synthesized polyisoimides and their absorption peak, l max, absorption edge, x (3), and isoimide contents Polyisoimide

Isoimide content (%)

l max (nm)

Cut-off (nm)

x (3) (10 212 esu)

PMDA±PDA PMDA±PDA;DEDPM s-BPDA±PDA BTDA±PDA BTDA±MPDA a

85 94 97 98 98

432 420 297/388 395 360

520 505 470 470 460

6.7 5.0 3.2 1.3 1.2

0

276/330

350

1.4

PMDA±PDA polyimide a

MPDA 5-methyl-1,3-phenylenediamine.

330


the highest values of 6.7 £ 10 212 esu for PiI(PMDA±PDA) [8,234]. The value for the PI from PMDA± PDA was about one ®fth the value for the corresponding PiI. 6. Conclusions Photophysics, photochemistry, and linear as well as NLO phenomena of aromatic and alicyclic PIs and their interrelations have been discussed in the present article. The excellent PI properties, including outstanding thermal stability, are correlated with both the chain stiffness (rigidity) and strong intermolecular interactions. From a large numbers of experimental evidence, both intra- and interchain CT interactions were proved to exist in aromatic PI solids, although quantitative approaches for the CT interactions still remain insuf®cient. The CT processes in aromatic PIs have been clearly shown to be a key phenomenon for controlling thermal, mechanical, photophysical and optical properties, and photochemical reactivity. This review also reveals how ¯uorescence spectroscopy is a powerful tool in studying intermolecular interactions and local ordered structures in PIs. References [1] Mittal KL, editor. Polyimides: synthesis, characterization, and applications. New York: Plenum Press, 1984. [2] Bessonov MI, Koton MM, Kudryavtsev VV, Laius LA, editors. Polyimides: thermally stable polymers. New York: Consultants Bureau (Plenum Press), 1987. [3] Feger C, Khojasteh MM, MacGrath JM, editors. Polyimides: materials, chemistry, and characterization. Amsterdam: Elsevier, 1989. [4] Bessonov MI, Zubkov VA, editors. Polyamic acids and polyimides: synthesis, transformations, and structure. Boca Raton, FL: CRC Press, 1993. [5] Feger C, Khojasteh MM, Molis SE, editors. Polyimides: trends in materials and applications. New York: Society of Plastic Engineers (Mid Hudson section), 1996. [6] Ghosh MK, Mittal KL, editors. Polyimides: fundamentals and applications. New York: Marcel Dekker, 1996. [7] Horie K, Yamashita T, editors. Photosensitive polyimides: fundamentals and applications. Lancaster, PA: Technomic, 1995. [8] Horie K. Polymeric materials for microelectronic applications. In: Ito H, Tagawa S, Horie K, editors. ACS Symposium Series 579. Washington, DC: American Chemical Society, 1994. p. 2. [9] Bessonov MI, Koton MM, Kudryavtsev VV, Laius LA, editors. Polyimides: thermally stable polymers. New York: Consultants Bureau (Plenum Press), 1987. p. 14. [10] Sep WJ, Verhoeven JW, deBoer TJ. Tetrahedron 1975;31:1065. [11] Price WC. Chem Rev 1947;41:257. [12] Watanabe K. J Chem Phys 1957;26:542. [13] Merri®eld RE, Phillips WD. J Am Chem Soc 1958;80:2778. [14] Hatano M, Tamura N, Kambara S. Kogyokagaku Zasshi 1967;70:2012 (in Japanese). [15] Handbook of chemistry. Japan: Chemical Society of Japan, 1994. p II-622. [16] Ferstandig LL, Toland WG, Heaton CD. J Am Chem Soc 1961;83:1511. [17] Chowdhury M. J Phys Chem 1962;66:353. [18] Nakayama Y, Ichikawa Y, Matsuo T. Bull Chem Soc Jpn 1965;38:1674. [19] Matsuo T. Bull Chem Soc Jpn 1965;38:2110. [20] Rosenberg HM, Eimutis EC. J Phys Chem 1966;70:3494. [21] Short GD, Parker CA. Spectrochim Acta A 1967;23:2487. [22] Ilmet I, Rashba PM. J Phys Chem 1967;71:1140. [23] Iwata S, Tanaka J, Nagakura S. J Chem Phys 1967;47:2203. [24] Prochorow J, Siegoczynski R. Chem Phys Lett 1969;3:635.

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