Bulk Viscosity and Its Unstable Behavior upon Storage in Polyimide

Jul 26, 2002 - It has been found that both PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions inherently have poor bulk viscosity stability upon ...
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MATERIALS AND INTERFACES Bulk Viscosity and Its Unstable Behavior upon Storage in Polyimide Precursor Solutions Yuejin Tong,† Tianxi Liu,† Subramanian Veeramani,† and Tai-Shung Chung*,‡ Institute of Materials Research and Engineering, 3 Research Link, Singapore 117602, and Department of Chemical & Environmental Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260

The bulk viscosity behavior of poly(amic acid) and its amine salt solutions, PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine), has been investigated. Both PAA(ODPA/o-tolidine) and PAS(ODPA/ o-tolidine) solutions show strong concentration and molecular weight dependence on bulk viscosity, and display critical values on the concentration dependence of bulk viscosity because of increasing the molecular chain interactions and entanglements. PAA(ODPA/o-tolidine) possesses a higher bulk viscosity than PAS(ODPA/o-tolidine) at the same concentration, indicating a stronger resistance to shear flow. The temperature dependence of bulk viscosity follows the exponential Arrhenius type relation. The activation energies of the fluids depend on the nature, concentration, and molecular weight of the polymers used. It has been found that both PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions inherently have poor bulk viscosity stability upon storage. The bulk viscosity of the polymer solutions decreases dramatically with time upon storage at room temperature while increasing during the storage at -18 °C. A molecular mechanism in terms of a “temporary junction” is suggested to explain the bulk viscosity behavior of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) in concentrated solutions. Introduction Bulk viscosity is an important parameter to be considered in the synthesis of polyimides. In the classic two-step approach of polyimide synthesis, a tetracarboxylic acid dianhydride is usually added to a solution of diamine (dissolved in an aromatic aprotic solvent at 15-75 °C) followed by thermal or chemical imidization to yield a polyimide.1 With the addition of dianhydride, propagation of poly(amic acid) occurs and the bulk viscosity of the solution gradually increases. The change of bulk viscosity is a signal for the formation of a polymer, which has been used as an indicator of molecular weight to monitor the synthesis of poly(amic acid).2-5 The formation mechanism, synthesis conditions, and storage stability of poly(amic acid) can also be investigated by monitoring the bulk viscosity changes with time elapsed. It has been reported that the instability of the bulk viscosity of a poly(amic acid) solution on storage was believed to be related to the molecular weight reduction and chain length equilibrium.2,3,6,7 In these cases, however, only a few works have quantitatively been done dealing with the bulk viscosity of poly(amic acid) solutions.7 On the other hand, polyimides are often processed as precursors because of the intractable properties in full * To whom correspondence should be addressed. Telephone: +65-68746645. Fax: +65-67791936. E-mail: chencts@ nus.edu.sg. † Institute of Materials Research and Engineering. ‡ National University of Singapore.

imidized form. In many applications, such as coating and film casting, moderate bulk viscosity is required for the processing. The solid contents in precursor solutions are limited by their bulk viscosities. In spin coating, the relationship between spin rate and film thickness is mostly determined by the bulk viscosity of the polymer solution.2,6,8 Bulk viscosity also plays an important role in the preparation of membranes.9 Chung et al. used bulk viscosity as an indicator of polymer chain entanglements in doping, which may be one of the requirements to yield air-separation hollow fibers with minimum defects.10 Obviously, a quantitative relation, even an empirical equation for bulk viscosity, is highly desired in practical applications in order to elucidate the mechanism governing bulk viscosity behavior. Poly(amic acid) amine salt, which is the derivative of poly(amic acid) by neutralizing the protons at the carboxylic groups using amines, can also act as a precursor for polyimides. It was found that the solution bulk viscosity greatly increased when adding strong tertiary amines into a poly(amic acid) solution.2,11 The increase of bulk viscosity was attributed to the formation of an expanded conformation caused by ionic groups along the polymer chains. It was also reported that poly(amic acid) amine salts were more stable than its acid form.12 Nowadays, poly(amic acid) amine salts as polyimide precursors are more widely used in practice, for instance, the applications of ionic precursors of photosensitive polyimide in microelectronic fields.13,14 The bulk viscosity affects the processing properties and the performance of the final products. Therefore, the un-

10.1021/ie020159f CCC: $22.00 © 2002 American Chemical Society Published on Web 07/26/2002

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4267 Scheme 1. Preparation of Poly(amic acid) and Its Ionic Salt

Figure 1. Concentration dependence of bulk viscosity for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

derstanding of the bulk viscosity behavior of poly(amic acid) amine salt will facilitate the utility of this precursor. In this study, PAA(ODPA/o-tolidine) has been prepared from 4,4′-oxydiphthalic anhydride (ODPA) and o-tolidine. And an ionic salt, PAS(ODPA/o-tolidine), is formed by PAA(ODPA/o-tolidine) and a tertiary amine, triethylamine (TEA). The dependence of the bulk viscosity behavior on solution concentration and molecular weight for PAA(ODPA/o-tolidine) and PAS(ODPA/otolidine) systems has been investigated and compared. Since the effect of entanglements on the viscosity of polyimide precursor solutions is still largely unexplored, it is one of the topics of this paper. A possible molecular mechanism governing the bulk viscosity behavior is proposed and discussed. Experimental Section Materials. 4,4′-Oxydiphthalic anhydride (ODPA), o-tolidine, and triethylamine (TEA) are commercially available. Before use the dianhydride and diamine were dried under vacuum at 160 and 60 °C, respectively. N-Methyl-2-pyrrolidinone (NMP) was employed as a solvent without further purification. The preparations of poly(amic acid) and its ionic salt were as shown in Scheme 1. ODPA was added in portions to a solution of o-tolidine in NMP, and the mixture was stirred at room temperature for 6 h under nitrogen environment. TEA was then added to the above poly(amic acid) solution on the basis of calculating the mole percent of free carboxylic acid groups in the known weight of poly(amic acid). The PAA(ODPA/o-tolidine) and PAS(ODPA/ o-tolidine) solutions were directly used in the study of the dependence of concentration, temperature, and molecular weight on the bulk viscosity measurements. In the case of monitoring viscosity drifts upon storage, the samples were placed in glass bottles (50 mL) under nitrogen and sealed with airtight caps. Two bottles each for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions were stored at room temperature (25 °C) and freezer temperature (-18 °C), respectively. The samples stored in the freezer were brought to room temperature

before the viscosity measurements. The viscosities of the polymer solutions were monitored once they were synthesized and packaged. Methods. The bulk viscosities of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions were measured using a Brookfield Viscometer DV III with Rheocalc software and a Brookfield water bath TC-200/500. Calibration was done with a certified viscosity standard (B11000, 10630 cPs, 25 °C) before measurements. A CP42 spindle with 5 rpm and 25 °C was adopted in most cases. The temperature dependence of the bulk viscosity was conducted at various temperatures (25, 35, and 45 °C). The sample (1 mL) free of bubbles was put into the viscometer cup. The bulk viscosity reading was taken from the electronic display when the system reached equilibrium after 1 min. Molecular weights of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) were measured using a Waters GPC system: sample concentration, 1 mg/mL; injection volume, 200 µL; flow rate, 1 mL/min; column, Gelpack GLS300MDT-5 × 3; size, 8 mm × 300 mm. Data were acquired from the Waters 2487 UV detector using Millennium 32 software. The relative molecular weight h w, and polydispersity) was calculated against (M h n, M standard polystyrene. The mobile phase was THF/ DMF ) 1:1 by volume containing H3PO4 (0.06 M) and LiBr (0.06 M). Results and Discussion Concentration Dependence of Bulk Viscosity. Figure 1 shows the concentration dependence of bulk viscosity for PAA(ODPA/o-tolidine) and PAS(ODPA/otolidine) solutions. The weight-average molecular weights of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) measured by GPC were 57 000 and 78 000 Da, respectively. It can be seen that both bulk viscosities increase exponentially with increasing concentration and show two regions with quite different slopes, suggesting critical concentrations (c* and c*′) at about 10.7 and 17.0 wt % for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions, respectively. The concentration dependence of bulk viscosity for the PAA(ODPA/o-tolidine)

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system may be expressed by

η ) KcR

(1)

For the case of c < c*, R ) 2.6, and K ) 0.415; and for c > c*, R ) 5.8, and K ) 2.18 × 10-4. Similarly, an empirical equation was derived for the PAS(ODPA/otolidine) system:

η ) KcR

(2)

For the case of c < c*′, R ) 2.9, and K ) 0.043; and for c > c*′, R ) 6.0, and K ) 8.6 × 10-6. Therefore, the effect of concentration on bulk viscosity is quite different near the critical concentration. From eqs 1 and 2, one would note that the concentration dependence on bulk viscosity for the case of c > c* (or c*′) is a factor of 2 higher than that of c < c* (or c*′). Hence, the finding of such a critical concentration indicated that the overlapping of polymer chains becomes more prominent as the concentrations are higher than c*. The interactions and the entanglements between polymer chains probably play important roles in concentrated solutions and govern the bulk viscosity behavior. Maekawa et al.15 have reported that the PAS(BPDA/PDA) in NMP showed a critical concentration at 18 wt %, which differed from those in PAA(ODPA/ o-tolidine) and PAS(ODPA/o-tolidine). This may be due to the difference in structural parameters for different polymer systems. By comparison, here the bulk viscosity of the PAA(ODPA/o-tolidine) solution is much higher than that of the PAS(ODPA/o-tolidine) solution at the same concentration. This indicates that PAA(ODPA/o-tolidine) solutions may withstand larger shearing forces than PAS(ODPA/o-tolidine) solutions at the same concentration. This is in contradiction with some results in the literature,2,11 in which it was observed that the bulk viscosity of poly(amic acid) increased dramatically because of the addition of a tertiary amine. Then, it was concluded that the poly(amic acid) salt has a higher bulk viscosity and stronger resistance to shear than poly(amic acid). However, it was neglected that the nature and molecular weight of the polymer have been changed, and consequently the polymer concentration should be recalculated in a different way after adding a tertiary amine. It is an obvious observation that tertiary amines neutralize the protons at the carboxylic groups of PAA, producing ionic PAS. Usually, the tertiary amine was added to the above PAA solution by calculating the mole percent of free carboxylic acid group in the known weight of the PAA. Since there are two carboxylic acid groups in the repeat unit of PAA, at least two moles of tertiary amine per unit of PAA were need to ensure all the carboxylic groups form ionic bonds. Therefore, the molecular weight of PAS should include PAA and tertiary amine added. At this time, the weight percent of PAS will be higher than that of PAA. It is not suitable to evaluate the viscosity without considering the change in concentration. On the basis of the concentration dependence of PAA and PAS, the increase of bulk viscosity for PAS may be due to the increase of concentration. Although the PASs have a more expanded conformation than PAA in solution, the repulsion due to the electrostatic interaction among ionized groups will weaken the linkages between polymer chains, leading to a lower bulk viscosity.

Figure 2. Temperature dependence of bulk viscosity for PAA(ODPA/o-tolidine) (A) and PAS(ODPA/o-tolidine) (B) at different concentrations.

Temperature Dependence of Bulk Viscosity. Parts A and B of Figure 2 show the temperature dependence of bulk viscosity for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine), respectively. The weightaverage molecular weights of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) measured by GPC were the same as those in the study of concentration dependence. It is not evident from the figures that the bulk viscosity changes more drastically at higher measurement temperatures. The effect of temperature on bulk viscosity follows an Arrhenius-type equation:16,17

η ) AeEa/RT

(3)

where η is the bulk viscosity, A is a pre-exponential factor, Ea is the activation energy, R is the gas constant, and T is absolute temperature. Then, the Ea and A can be obtained from a plot of ln η versus 1/T by the slope and intercept, respectively:

ln η ) ln A + Ea/RT

(4)

The activation energies of shear flows were listed in Table 1. It is well-known that the flow of fluid should

Ind. Eng. Chem. Res., Vol. 41, No. 17, 2002 4269 Table 1. Activation Energies (Ea) of Shear Flows for Various Concentrations and Molecular Weights between 25 and 45 °C polyimide precursor PAA(ODPA/o-tolidine)

PAS(ODPA/o-tolidine)

conc (wt %)

M hw (×104)

Ea (kJ/mol)

R2

19.2 19.2 15.3 11.8 7.5 24.8 24.8 18.6 12.4 8.1

5.7 4.1 5.7 5.7 5.7 7.8 2.9 7.8 7.8 7.8

33.5 27.6 29.7 26.5 21.4 30.4 26.6 28.0 24.3 21.1

0.9997 0.9996 0.9996 0.9977 0.9968 0.9998 0.9988 0.9989 0.9992 0.9981

Figure 4. Molecular weight dependence of bulk viscosities for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

Figure 3. Dependence of activation energies of the flows on the bulk viscosities of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

overcome the energy barrier. For polymer solutions, especially for the concentrated ones, this energy barrier primarily comes from the molecular junctions between polymer chains and/or the formation of “cross-linkinglike” structures (see below). As shown in Table 1, the activation energy increases remarkably with the increase of concentration and molecular weight. The effect of dilution on activation energy was illustrated in Figure 3. Also, it can be observed that the values of Ea for PAA(ODPA/o-tolidine) were higher than those of PAS(ODPA/ o-tolidine) for the concentrated solutions (when >5%), probably because of the “temporary junctions” resulting from the polymer chain interactions and entanglements (see below). Molecular Weight Dependence of the Bulk Viscosity and Bulk Viscosity Drifts during Storage. Figure 4 shows the molecular weight dependence of bulk viscosity for PAA(ODPA/o-tolidine) and PAS(ODPA/otolidine) solutions. For the PAA(ODPA/o-tolidine) system (c ) 19.2 wt % and 25 °C), there is the following experimental equation:

η ) (6.85 × 10-13)M h w3.43

(5)

And for the PAS(ODPA/o-tolidine) system (c ) 24.8 wt % and 25 °C), there is

h w4.96 η ) (3.15 × 10-20)M

(6)

Figure 5. Bulk viscosity drifts with storage time at room temperature for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

where η is the bulk viscosity of the polymer solution and M h w is the weight-average molecular weight. Equations 5 and 6 show that the molecular weights of PAA(ODPA/ o-tolidine) and PAS(ODPA/o-tolidine) strongly affect the bulk viscosity behavior, probably owing to remarkable entanglements among the polymer chains. Higher molecular weight (or larger molecular dimension) results in higher internal friction upon shear deformation, and consequently higher solution bulk viscosity. Miwa and Numata7 reported a similar relationship between weightaverage molecular weight and bulk viscosity for 10% PAA solution at 25 °C (η ) (4.29 × 10-17)M h w3.35). In addition, it is a common observation that the bulk viscosity of the poly(amic acid) solution gradually decreases after synthesis and storage at room temperature or at an elevated temperature. This has been attributed to the fact that poly(amic acid) is unstable over a period of time in the solution form. In fact, poly(amic acid) can undergo many reactions, such as hydrolysis of amide bonds or terminal anhydride groups and imidization. Therefore, to avoid these kinds of side reactions occurring continuously, poly(amic acid) solutions are recommended to be kept at lower temperatures to maintain the properties essential to further processing, when long-term storage is necessary. Figure 5

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Figure 7. Polymer chain entanglements in PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

Figure 6. Bulk viscosity drifts with storage time at -18 °C for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions.

presents the dependence of relative bulk viscosity (η/η0) and molecular weight (M h w) for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions on storage at room temperature. Both of them have shown the dramatic reduction of bulk viscosity upon storage. This can be explained by the fact that the molecular weights of PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) decrease as the storage time elapses. By comparison, PAA(ODPA/o-tolidine) has a more obvious drift than PAS(ODPA/o-tolidine) due mainly to the higher stability of the latter. Therefore, it is expected that the stability of the bulk viscosity of polyimide precursor solutions may probably be achieved by storing at a much lower temperature. This is based on the fact that lower temperature will facilitate the chemical stability of the polymer. Very interestingly, however, in our study, the bulk viscosities of both PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions were found to increase with storage time at -18 °C (Figure 6). Since the molecular weight of polymer does not change much at this low temperature, such an unusual behavior seems very difficult to interpret. Molecular Mechanism Governing the Bulk Viscosity Behavior. The flow property of a dilute solution is quite different from that of a concentrated solution, and the mechanisms governing their viscosities are varied.16,17 In the case of a dilute solution, especially a very dilute solution, compared with the case of the pure solvent, the additional frictional force in solution on the polymer molecules is primarily due to the flow of the solvent. When the solution undergoes deformation (e.g., shear flow), the dissolved polymer molecules will be dragged along the flow direction, but the polymer molecules are so large that they are exposed to different velocity gradients of solvent. The moving of solvent relative to the molecular segments of the polymers exerts external forces on these segments and changes the shape and the average end-to-end distances of polymer molecules. In other words, the rheological behavior is governed by the molecular structure characteristics in the dilute polymer solution. For the concentrated polymer solution, however, the case is very different from that in dilute solution. It is

difficult to give an exact definition of the terms of dilute and concentrated polymer solution because of the different purposes. And it was suggested that a polymer solution might be called a concentrated solution if the solute concentration exceeds 5% by weight.18 However, this assumption does not relate to interactions and overlap of polymer chains. For the purpose of investigating the mechanism, it may be considered as a concentrated solution when the overlap of the polymer chain and entanglements is pronounced. At a given concentration of a polymer solution, polymer chains start to overlap with each other; here we term it as the critical concentration. In the cases of PAA(ODPA/otolidine) and PAS(ODPA/o-tolidine) systems, increasing the solution concentrations leads to a degree of spatial overlapping (due to strong intermolecular interactions) and entanglements of the polymer chains (Figure 7). At a certain concentration (10.7 and 17.0 wt % for PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) solutions, respectively), the polymer chains start to overlap with each other to a large extent and the bulk viscosity behavior increases more dramatically at this point. This critical concentration can be obtained by the cross-point of the tangents of the slopes in the logarithmic plot of the concentration dependence of the bulk viscosity (see Figure 1). Therefore, the bulk viscosity behavior of the concentrated polyimide precursor solutions is primarily governed by the interactions and/or entanglements between polymer molecules, rather than the interactions between the solvent and the polymer molecules. Sometimes these interactions may be so strong that they result in formation of “molecular junctions” or cross-linking-like structures. These junctions do not arise from the formation of covalent bonds but from the polymer chain entanglements and molecular associations (or interactions). Besides the entanglements, there are several other possibilities of forming molecular junctions in PAA(ODPA/o-tolidine) and PAS(ODPA/o-tolidine) systems. For instance, on the basis of the molecular structural features referred to in Scheme 1, the formation of junctions can be driven by electrostatic force and hydrogen bond and intermolecular charge transmit force (Figure 8). And the “cross-linking-like” structure may vary from head to head, head to tail, head to side, and tail to side. Furthermore, the solvent (NMP) may play an important role in the formation of junctions because of its strong tendency to form hydrogen bonds and solvent complexes with polymer. Additionally, the junctions (where the molecular interactions occur) may be moved in response to flow-

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Figure 8. Some temporary junctions in PAA(ODPA/o-tolidine) solution.

induced deformation. Therefore, in any interval of time, some junctions seem to disappear while new ones are being formed at other locations, just like “temporary” cross-linking junctions. It is the formation of all these kinds of temporary cross-linking junctions or a “temporary network” that may resist the shearing and govern the rheological behavior, for example, the bulk viscosity. On the other hand, the junctions may be broken down and slip to permit flow under shear. Finally, the unusual increasing behavior of bulk viscosity with the storage time may be attributed to the reorganization of “temporary junctions” during the freezing process. On cooling, the “temporary junctions” increase in density or strength, since the polymer chains are reorganized into more ordered structures, either by the growth of existing junctions or by the formation of new junctions. The ionic rich regions of the chains provide nucleation sites for the formation of potential junction zones, as the temperature is lowered. Hence, temperature is the driving force for the reorganization and consequently for the increase of bulk viscosity during storage (see Figure 6). Conclusion Both PAA and PAS solutions have inherently poor viscosity stability upon storage. Understanding the mechanism of bulk viscosity instability is of practical importance. The degradation of the polymer chain caused by the hydrolysis reaction can partially explain the viscosity drifts, especially for the dilute solution. Storing precursor solutions at lower temperature after manufacturing cannot thoroughly eliminate the changes

of bulk viscosity up and down. The results obtained in this paper suggest that the bulk viscosity behavior in concentrated solutions is governed by “temporary junctions” originating from polymer chain interactions and entanglements. We may overcome some viscosity problems in stabilizing the viscosity encountered during the storage by paying much attention to the repulsion and attraction effects, the rigidity of polymer chains, the entropy change, the inhibitors that may be used for resisting the viscosity drifts, and so on. Further work on this aspect is still in progress and would be expected to explore a new methodology to overcome the poor viscosity stability without sacrificing any of the characteristics of existing products. Acknowledgment The authors would like to thank Mr. Rhohitkumar Vora for his kind help. Literature Cited (1) Androva, N. A.; Bessonov, M. I.; Laius, L. A.; Rudakov, A. P. Polyimide: A New Class of Heat-Resistant Polymers; Leningard: Nauka, 1968. (2) Bower, G. M.; Frost, L. W. Aromatic Polyimides. J. Polym. Sci. 1963, A1 (10), 3135. (3) Frost, L. W.; Kesse, I. Spontaneous Degradation of Aromatic Poly (pyromellitamic acids). J. Appl. Polym. Sci. 1964, 8 (3), 1039. (4) Dine-Hart, R. A.; Wright, W. W. Preparation and Fabrication of Aromatic Polyimides. J. Appl. Polym. Sci. 1967, A11, 609. (5) Tsimpris, C. W.; Mayhan, K. G. Synthesis and Characterization of Poly(p-phenylene pyromellitamic acid). J. Polym. Sci., Polym. Phys. 1973, 11, 1151.

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(6) Yamada, Y. Siloxane Modified Polyimides for Microelectronics Coating Applications. High Perform. Polym. 1998, 10, 69. (7) Miwa, T.; Numata, S. A. Mechanism Describing Polyamic Acid Solution Viscosity Change on Storage at High Temperature. Polymer 1989, 30, 893. (8) Levinson, W. A.; Amold, A.; Dehodgins, O. Spin Coating Behavior of Polyimide Precursor Solutions. Polym. Eng. Sci. 1993, 33 (15), 980. (9) Ohya, H.; Kudryavtsev, V. V.; Semenova, S. I. Polyimide Membranes: Application, Fabrication, and Properties; Gordon & Breach: Amsterdam, 1996; pp 47-52. (10) Chung, T. S.; Kafchinski, R. E.; Foley, P. Development of Asymmetric Hollowfibers from Polyamides for Air Separation. J. Membr. Sci. 1992, 75, 181. (11) Kreuz, J. A.; Endrey, A. L.; Gay, F. P.; Sroog, C. E. Studies of Thermal Cyclizations of Polyamic Acids and Tertiary Amine Salts. J. Polym. Sci. 1966, A1 (4), 2607. (12) Reynolds, R. J. W.; Seddon, J. D. Amine Salts of Polypyromellitamic acids. J. Polym. Sci. 1968, C23, 45. (13) Yoda, N.; Hiramoto, H. New Photosensitive High-Temperature Polymers for Electronic Applications. J. Macromol. Sci.,

Chem. 1984, A21 (13&14), 1641. (14) Ree, M.; Nunes, T. L.; Chen, K. J. R. Structure and Properties of A Photosensitive Polyimide: Effect of Photosensitive Group. J. Polym. Sci., Polym. Phys. 1995, 33, 453. (15) Maekawa, Y.; Miwa, T.; Horie, K.; Yamashita, T. Solution Properties of Polyamic acids and Their Amine Salts. React. Funct. Polym. 1996, 30, 71. (16) Seven, E. T. Rheology of Polymers; Reinhold: New York, 1967; pp 16-29 and 79-93. (17) Carreau, P. J.; De Kee, D. C. R.; Chhabra, R. P. Rheology of Polymeric Systems: Principles and Applications; Hanser: New York, 1997; pp 21-25 and 35-43. (18) Krevelen, D. W. Van. Properties of Polymers; Elsevier: New York, 1990; p 509.

Received for review February 21, 2002 Revised manuscript received June 3, 2002 Accepted June 6, 2002 IE020159F