High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass

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1 High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass C. Gouri, C.P. Reghunadhan Nair* and R. Ramaswamy Polymers and Special Chemicals Division, Vikram Sarabhai Space Centre, Thiruvananthapuram , India Received: 26 June 2002 Accepted: 6 November 2002 ABSTRACT Diallyl bisphenol A formaldehyde novolac (ABPF) resin was cured with four structurally different bismaleimides (BMIs) at high temperatures through an Alder-ene reaction which resulted in thermally stable network polymers. The adhesive characteristics of the different BMI-ABPF systems were evaluated in terms of the lap shear strength (LSS) on aluminium substrates at varying temperatures up to 250 C. The LSS properties were not significantly affected by the structure of the BMI. Although the LSS of BMI-ABPF systems per se were not particularly high due to the brittle nature of the cross-linked structures, all the systems exhibited remarkably good retention of LSS at high temperatures. Replacing ABPF with its monomeric analogue i.e. o,o - diallyl bisphenol A (DABA) resulted in better adhesion, but in a poorer thermo-adhesive profile. Comparison of DMA and thermo-adhesive profiles implied that in the majority of the cases molecular relaxations at higher temperature are conducive to matrix toughening which results in enhanced adhesion properties. INTRODUCTION Polymers based on heterocyclic-aromatic, resonancestabilised structures such as polyimides, polybenzimidazoles, polyphenylquinoxalines, polybenzoxazoles, polypyrrones, etc. having good thermo-oxidative stability are noted high-temperature resistant structural adhesives 1-3. To be used successfully as an adhesive, the thermally stable polymer must be processable under moderate conditions with no volatile evolution, possess good shelf life and tackiness, and be compatible with the adherends 4. Only a few high temperature polymeric adhesives are commercially available because of an elusive market and an unfavourable combination of price, processability and performance. Such polymers are often made and used for specialized applications and are generally not available commercially. Among the numerous high-temperature resistant polymers, the polyimides (PI) have achieved by far * Author for correspondence the greatest commercial success, due to their ability to maintain acceptable mechanical properties at elevated temperatures together with a measure of thermal stability for long-term use at temperatures around 300 C 5,6. In addition to the condensation and thermoplastic family of polyimides, the third type based on imide pre-polymers also has gained considerable importance recently because of their ease of processability. In the case of imide prepolymers, the oligomers having the stable cyclic imide structure are pre-formed and can be further cured via the unsaturated end groups through addition reactions. Three main types belonging to this class which showed potential as high-temperature adhesives are those based on norbornene 7, acetylene 8 and maleimide 9 functionalities. Among these, the bismaleimides (BMIs) have been studied extensively, mainly because they offer the greatest benefits in terms of cost-effectiveness and enhanced processability. Although BMIs are excellent high-temperature resistant matrices in high-performance composites, they have rarely been studied for their adhesive Polymers & Polymer Composites, Vol. 11, No. 4,

2 C. Gouri, C.P. Reghunadhan Nair and R. Ramaswamy properties. The major reason is their brittleness. Of the several approaches reported for BMI matrix toughening, co-reaction with allylphenol is now gaining wide acceptance 10. Several resin formulations based on the reaction blends of BMI and allylphenol derivatives are known, e.g. Allyl Xylok (product of M/s. Midland Silicones Ltd, USA) - BMI blend and Matrimid 5292 A/B (product of M/s. Ciba Geigy) 11,12. The co-reaction between the two, occurring via the Alder-ene reaction, results in network polymers with high fracture toughness, particularly in composites. The majority of the published reports concern the applicability of BMI-allylphenol resins as matrices in advanced composites 11,13,14. Their adhesive properties have rarely been addressed in the literature, except a few reports that deal with the processability and adhesive performance of some commercially available polyimide systems based on BMIallylphenol reactive blends 15. Since BMI-allylphenol systems have been proven to be tough matrices in composites with good fibre-wetting and thermal characteristics, it is expected that they can be prospective high-temperature adhesives. However, when these systems are used in the neat form, the toughness is not substantial due to their high crosslink densities. In this perspective, we investigated the adhesive characteristics of the reactive blend of diallyl bisphenol A novolac and BMIP. The synthesis of an allyl functional phenol novolac resin by reaction of diallyl bisphenol A and formaldehyde (ABPF) and its curing with BMIs through an Alder-ene reaction has been studied separately 16. The adhesive characteristics of the BMI-ABPF blend, studied using a typical BMI i.e. BMIP has shown that the system per se can serve as a good high-temperature adhesive with excellent retention of LSS properties at elevated temperatures 17. The properties could be improved tremendously by matrix modification with advanced thermoplastics, with only a marginal penalty over high temperature performance 17. The main objective of the present work is to investigate the influence of the BMI structure on the adhesive and thermo-mechanical characteristics of BMI-ABPF reactive blends. The effect of replacing the novolac with its monomeric analogue, i.e. o,o -diallyl bisphenol A (DABA) has also been studied. EXPERIMENTAL Materials Maleic anhydride (CDH, India) was purified by sublimation. 4,4 -diamino diphenylmethane, 4,4 - diamino diphenyl ether and 4,4 -diamino diphenyl sulfone were the products of E-Merck, Germany and were used as received. Solvents such as acetone, methyl ethyl ketone (MEK), hexane and dichloromethane were AR grade and were distilled before use. Silane-treated, plain weave, E-glass of mm thickness (Unnathi Corporation, India) was used as received for making laminates. Diallyl bisphenol A-formaldehyde (ABPF) resin was synthesised as detailed previously 16. The resin was characterised by FTIR, NMR and GPC analyses. The polymer had a molecular weight of M n , M w and dispersity 3.3, with an average degree of polymerisation (DP) of 4.3. The three BMIs, viz. bis (4- maleimido phenyl) methane (BMIM), bis (4- maleimido phenyl) ether (BMIE), and bis (4-maleimido phenyl) sulfone (BMIS), were prepared by reacting the respective diamines with maleic anhydride by known methods 18. The BMIs were purified by dissolving them in acetone and passing through a basic alumina column. The eluted solution was precipitated into hexane and the precipitate was filtered and dried in vacuum. The products were characterised by FTIR and acid value (the acid value was nil, implying complete imidisation). 2,2-bis 4- [(4-maleimido phenoxy) phenyl] propane (BMIP) was supplied by Mr. T.M. Vijayan of PSCD, VSSC, who synthesised it by a standard procedure, and it was used as such. The chemical structures of DABA, ABPF resin and the four BMIs used in the study are given in Figure 1. METHODS Dynamic Mechanical Analysis (DMA) The thermo-mechanical properties were evaluated by DMA as per ASTM D-4092 at a frequency of 1 Hz. For DMA, a DuPont Instrument DMA 983 model with a DuPont 9900 thermal analyser was used at a heating rate of 5 C/min under nitrogen atmosphere. Due to the non-availability of a hot-melt technique for casting the resin into the required shape for carrying out the thermal analyses, the laminate route was adopted. The laminates were made using silane-treated E-glass fabric as the reinforcement and prepregging was done by a solution impregnation technique using MEK as solvent. The prepregs were dried overnight, 6-8 plies were stacked together and then moulded under a pressure of 0.5 MPa in a hot air oven to get the laminates. The cure schedule adopted was stepwise heating at 160 C and 200 C for 30 min each and at 250 C for 2 h. The cured laminates were then cut to 312 Polymers & Polymer Composites, Vol. 11, No. 4, 2003

3 High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass Figure 1 Chemical Structures of: (a) DABA; (b) ABPF; (c) bis (4-maleimido phenyl) methane (BMIM); (d) bis (4- maleimido phenyl) ether (BMIE); (e) 2,2-bis 4-[(4-maleimido phenoxy) phenyl] propane (BMIP); and (f) bis (4- maleimido phenyl) sulfone (BMIS) specimens of required size of 60 mm x 12 mm x 2-3 mm using a diamond wheel cutter. Evaluation of Adhesive Properties The adhesive testing method employed was the lap shear strength test (LSS), as per ASTM D-1002, using sodium dichromate-sulphuric acid etched, B-51-SWP aluminium as the substrate. A 25% solution in MEK of the blend of ABPF and BMI was uniformly applied as a thin layer over the aluminium substrate. The solvent was allowed to evaporate by keeping the specimens in a hot air oven at C for about 2 h. They were then allowed to cool to room temperature Polymers & Polymer Composites, Vol. 11, No. 4,

4 C. Gouri, C.P. Reghunadhan Nair and R. Ramaswamy (RT) and assembled together for testing the adhesive properties. Curing at the desired temperature was carried out in a hot air oven and a pressure of nearly 0.5 MPa was applied over the bonded specimens using the lever-press assembly. Since the co-reaction and eventual gelation of the resin sets in at sufficiently lower temperature (~100 C), the application of pressure did not lead to any resin bleeding from the bonded surfaces. The bonded specimens were tested in an Instron UTM Model 4202, at a cross-head speed of 10 mm/min. For determining the LSS at higher temperatures, the specimens were soaked for 10 min at the desired temperature and then tested at that temperature. Five specimens were tested in each lot and the average value and standard deviation were calculated and reported. After testing, the nature of the failure was visually observed. RESULTS AND DISCUSSION Cure Optimisation of BMI-ABPF Blend Maleimides are known to react with allylphenols through the Alder-ene reaction (via an intermediate Wagner-Jauregg reaction) to give rise to cyclic network structures 14,19,20. BMIP was selected as the representative BMI for the cure optimisation studies. A recent DSC kinetic study of Alder-ene reaction has shown that the structure of the BMI does not have much influence on the reaction kinetics 18 and therefore the results obtained from BMIP can be extrapolated to other maleimides without serious error. The possible reactions between ABPF and BMIP at high temperatures leading to the crosslinked network structure are depicted in Scheme 1. The resultant structure would be highly dependent on the reactant stoichiometry, cure conditions and reaction kinetics. Thus, at a lower maleimide stoichiometry, structures (3) and (4) would be predominant, whereas at a higher maleimide ratio, the more crosslinked structures (5), (6) and (7) would be formed. So also, the cure temperature and mode of heating can have pronounced effects on the resulting network structure. Slow and step-wise cure, as in the present work, ensures the dominance of (3) and (4) type structures in the network 18. The details of the cure optimisation studies of the blend for obtaining the optimum adhesive properties have been reported earlier 16. The cure schedule of BMIP-ABPF system was optimised by DMA, DSC and FTIR, together with the actual evaluation of adhesive properties of the system cured under different conditions. Based on these studies, it was concluded that following a stepwise cure of 30 min each at 160 C and 200 C followed by a final cure at 250 C for 2 h results in moderate cross-link density leading to the optimum adhesive properties. However, for attaining complete cure, higher temperatures and more time may be required. The focus of the present study was the attainment of maximum adhesive properties; so higher temperatures and longer periods of time might not be attractive because of the possibility of increased brittleness in such structures. Thus, the cure condition selected for further studies was stepwise heating up to 200 C and then final cure at 250 C for 2h. Optimisation of BMI-ABPF Stoichiometry It is well known that the ideal Alder-ene reaction requires an maleimide-allylphenol stoichiometric ratio of 3:1. This could produce the optimum cyclic network structure with maximum cross-linking. However, dense cross-linking may not be conducive to optimum adhesion 16. Hence, the optimum stoichiometry was ascertained indirectly from the adhesive property evaluation of varying ratios of the blend of BMIP with ABPF, cured under the optimised conditions of stepwise cure up to 200 C and final cure at 250 C for 2 h 17. It was observed that the LSS properties and the retention of LSS at 150 C of the system are optimised at the 1:1 ratio. As discussed earlier, under these conditions, the network structure would be dominated by structures (3) and (4) with a moderate cross-link density. This ratio exhibited LSS values of 4.1 MPa at RT and 4.8 MPa at 150 C, with a retention of 117% at 150 C for BMIP-ABPF system 17. Dependency of the Adhesive Properties on BMI Structure The dependency of the adhesive characteristics of the network system on the BMI structure was studied in terms of the LSS of the BMI-ABPF system taken on 1:1 stoichiometry, using 4 different BMIs, whose structures and abbreviations are shown in Figure 1. Their high-temperature capabilities were assessed from the LSS properties determined at various test temperatures. The average LSS values together with the standard deviations obtained at each test temperature are compiled in Table 1. The general trend in LSS properties observed was BMIS> BMIE>BMIP>BMIM. LSS was practically unaltered at the test temperatures for BMIM and BMIE, whereas it increased with test temperature for BMIS and BMIP. The nature of the failures observed was cohesive in all cases. This indicated the attainment of a sufficiently strong metal-adhesive interface in the joint for all systems. Hence, the variation in LSS 314 Polymers & Polymer Composites, Vol. 11, No. 4, 2003

5 High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass Scheme 1 Alder-ene reaction of ABPF and BMIP at high temperatures Polymers & Polymer Composites, Vol. 11, No. 4,

6 C. Gouri, C.P. Reghunadhan Nair and R. Ramaswamy Table 1 LSS properties (in MPa) of different BMI-ABPF and BMI-DABA systems ( 1:1 stoichiometry; cured at 160 C /30 min C /30 min C/2 h) System At RT A t 150 C A t 200 C A t 250 C BMIP-ABPF 4. 1 ± ± ± ± 0. 3 BMIM-ABPF 4. 2 ± ± ± ± 0. 5 BMIE-ABPF 5. 4 ± ± ± ± 0. 4 BMIS-ABPF 5. 9 ± ± ± ± 0. 4 BMIP-DABA 7. 6 ± ± ± BMIM-DABA 7. 3 ± ± ± could be due to the difference in cohesive strength of the cured network structures of the different BMI- ABPF systems. The polar groups such as sulfone and ether in BMIS and BMIE respectively would contribute to enhanced cohesive strength of the polymer, thereby leading to better joint strength. Thus, these systems perform better than the other two, particularly at RT. Although BMIP also possesses the polar ether groups, the adhesive properties at RT were inferior to those of BMIE. This could be due to the presence of the bulky, diphenyl propane moiety in the backbone, which might reduce the overall cohesive strength. BMIM showed the lowest LSS properties because of the absence of polar groups and the shortest distance between cross-links in the cured structure, leading to higher brittleness. The increase in LSS at higher temperatures can be attributed to secondary relaxations, which can add to the toughness of the system. The retention of LSS at different test temperatures, compared to the RT value (calculated based on the average values), is shown in Figure 2. Both BMIP and BMIS systems showed retention of LSS higher than 100% at temperatures up to 250 C, whereas the other two systems had almost a constant retention of 100% at all temperatures. For BMIP, retention of LSS increased with temperature due to the enhanced relaxation exhibited by the diphenyl propane groups. In the case of BMIS, the increase in LSS with temperature can be attributed to the presence of thermally stable, rigid and polar sulfone groups, which become less rigid at higher temperatures, contributing to the toughness of the system. Although BMIE exhibited higher LSS at RT than BMIM, the retention of LSS was not enhanced at higher temperatures. This arises from the inherent flexibility of the system derived from the ether linkages, which is in operation even at room temperature. BMIM showed poor performance when compared to the other three BMIs. An overall comparison of LSS values shows that all the systems behaved in a brittle manner and the variation of LSS among the different systems were not particularly high (the maximum variation was only of the order of 3 MPa). Although the absolute values of LSS of these systems were not very high (compared to 8.7 MPa for the Matrimid 5292 A/B 15 system and the usual requirement of minimum LSS of 7 MPa for a structural adhesive), all the systems exhibited excellent property retention even at 250 C. Therefore, they are potential high-temperature adhesives. Figure 2 Dependency of relative retention of LSS of different BMI-ABPF systems on temperature (1:1 stoichiometry; cured at 160 C/30 min C/30 min C/2 h) 316 Polymers & Polymer Composites, Vol. 11, No. 4, 2003

7 High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass Effect of Molar Mass of Novolac DABA, the precursor diphenol for ABPF was blended with BMIP and BMIM respectively at 1:1 stoichiometry and cured under identical conditions as the BMI- ABPF system. The LSS at temperatures of RT, 150 C and 200 C obtained for the two systems are presented in Table 1. The % retention of LSS at these temperatures, calculated from the average values, are given in Figure 3. As observed in the case of polymerbased systems, cohesive failure of the joint was observed, showing that the metal-adhesive interaction was sufficiently strong. As with the polymer-based systems, the adhesive properties were better for the BMIP than for the BMIM system. The absolute values of LSS for both the BMI-DABA systems were better than those of the corresponding BMI-ABPF systems. Cured under identical conditions, the monomeric version is less cross-linked and thus could be a tougher system than the polymer, accounting for the enhanced adhesive property of the former. Due to the reduced cross-linking, the monomeric systems showed a rather poor thermo-adhesive profile in comparison to their polymeric counterparts (compare Figure 3 with the corresponding curves for the polymeric systems given in Figure 2). Thus, the% retention of LSS properties of the monomeric systems, shown in Figure 3, demonstrates a continuous decrease with test temperature. The polymeric (BMI- ABPF) system is therefore recommended for high temperature applications vis-à-vis the monomeric blend formulation. Dynamic Mechanical Behaviour With a view to understanding the dynamic mechanical properties of the different BMI-ABPF systems under study and to derive a possible correlation with their thermo-adhesive profile, a dynamic DMA of the cured systems was performed. Data on the dynamic storage modulus (E ), loss modulus (E ) and tan δ versus temperature for the systems were recorded. Since the resin content in the laminates for the various systems could not be maintained exactly the same, comparative analyses were made between the temperature profiles for the % retention of E. Glass transition temperature (T g ) data for the systems are compiled in Table 2. T g has been calculated based on two approaches: (1) The temperature corresponding to the inflection point in E or % retention of the E curve (Tg 1 ); (2) The temperature corresponding to the maximum of the tan δ curve (Tg 2 ). Table 2 T data for different cured systems from g DMA studies System Molar ratio Tg 1 * Tg 2 ** BMIP-ABPF 1: Figure 3 Dependency of relative retention of LSS of BMIP-DABA and BMIM-DABA systems on temperature (1:1 stoichiometry; cured at 160 C/30min C/30 min C/2 h) # BMIM-ABPF 1: 1 > >350 >350 > # BMIE-ABPF 1: 1 ~ 380 ~380 BMIS-ABPF 1: BMIP-DABA 1: , 325 * based on the E c urve; ** corresponding to tan δ peak temperature # For BMIM-ABPF system, T data could not be g r ecorded beyond 350 C. However, the DMA curve showed a steep rise in tan δ a t 350 C indicating that the T was very much higher than this g temperature Effect of BMI Structure The Tg 1 and the Tg 2 values for the systems (Table 2) were in the order: BMIM ~ BMIE > BMIS ~ BMIP. For BMIM system T g data could not be recorded beyond 350 C. The DMA curve showed a steep rise in tan δ at 350 C, indicating that the T g was higher than this temperature. It could well be beyond 380 C. For BMIE, the Tg 2 was around 380 C (the tan δ curve tends to flatten out here). The highest T g observed for Polymers & Polymer Composites, Vol. 11, No. 4,

8 C. Gouri, C.P. Reghunadhan Nair and R. Ramaswamy the BMIM-ABPF system can be attributed to the higher crosslink density arising from the shorter methylene spacings. The slightly lowered T g of BMIE could be due to the presence of the more flexible ether group in place of the -CH 2 - group in BMIM. When compared to BMIM, the presence of a phenoxy phenyl isopropyl group in the network of BMIP leads to larger spacings between the crosslinks, thus reducing the rigidity of the structure, leading to a decreased T g. Although the polar sulfone group was expected to lead to a higher T g for the resultant network, the free rotation possible for this group may be compensating this, thereby leading to a T g almost the same as that for BMIP. Figure 4 presents the comparative tan δ and % retention of E for the different BMI systems blended with ABPF in 1:1 stoichiometry. The figure shows that the % retention of E for BMIP system is better than that of the other three up to about 325 C. The behaviors of BMIM and BMIE, with the shortest distance between crosslinks, were nearly identical, BMIE performing slightly better at the high temperature end. Although the absolute value of E for the BMIS system was very high compared to all other systems (nearly 8 times the value for the BMIP system), its retention was the lowest, showing a steady decrease throughout the test temperature regime. The very high E value might be a consequence of the presence of polar sulfone groups contributing Figure 4 Variation in retention of E and tan δ with temperature for the different BMI-ABPF systems to intermolecular attractive forces and enhanced cohesive energy for the system. However, as the temperature is increased, the intermolecular forces are broken down at a faster rate, resulting in a drastic reduction in retention of E. The better performance of BMIP system is not understood well. On comparing the thermo-adhesive and DMA profiles of these systems, it seems that a direct correlation between the two does not exist in all cases. This is so, because the molecular characteristics deciding the mechanical properties may not be the most relevant factor in deciding the adhesive performance. Whereas the secondary molecular relaxations could lead to a decrease in modulus properties, such phenomena are conducive to stress relaxation during shear when subjected to adhesive testing. An example is the BMIS system, which showed a poor DMA profile, but exhibited a good LSS and good retention at higher temperatures. Effect of Molar Mass of Novolac The comparative DMA analysis for BMIP-ABPF and BMIP-DABA systems is given in Figure 5. The figure clearly demonstrates the superior thermo-mechanical performance of the polymer-based system. There is a difference of around 75 C in Tg 1 and around 30 C in Tg 2 values between the two systems (Table 2). For the DABA- system, in addition to the transition at 325 C, a lower temperature transition at around 285 C is also observed in the tan δ curve. It is inferred that the Figure 5 Variation in retention of E and tan δ with temperature for BMIP-ABPF and BMIP-DABA systems 318 Polymers & Polymer Composites, Vol. 11, No. 4, 2003

9 High-Temperature Adhesives Based on Alder-ene Reaction of Diallyl Bisphenol A Novolac and Bismaleimide: Effect of BMI Structure and Novolac Molar Mass additional cross-links through the methylene groups in the polymer-based system suppress this low temperature transition almost completely. Also, the tan δ curve for DABA-based system shows a sharp rise beyond 350 C, which could be due to the initiation of thermal degradation. In contrast, ABPF-based system is stable in this temperature region. The thermo-adhesive profile of the two systems compared earlier has also shown the superiority of the polymer-based system. Since a one to one correlation between thermo-adhesive profile and dynamic mechanical properties is found to exist for this case, it is inferred that the matrix cohesive properties have a say in the adhesive properties of these two systems. CONCLUSIONS Although the absolute adhesive properties of the Alder-ene adduct of BMI-ABPF blend were not remarkably high, they exhibited excellent high temperature retention up to 250 C. The LSS properties of the systems were not significantly affected by the structure of the BMI. The high temperature LSS retention was better for BMIP and BMIS systems, probably due to the stress relaxation possible through the phenoxy phenyl propane and sulfone groups, respectively. Replacing ABPF with its monomeric analogue (DABA) resulted in better adhesive properties at RT; but was not conducive to hightemperature performance. The polymeric version is advisable for high-temperature adhesive applications. The thermal properties of various systems evaluated by dynamic DMA showed the superiority of the BMIM and BMIE systems, both with higher crosslink densities manifesting a better thermo-mechanical profile and higher T g. BMIP showed a relatively good DMA profile in line with its thermo-adhesive profile, whereas BMIS exhibited a rather continuous decline in mechanical properties with temperature. The DMA of the BMIP-cured DABA system, when compared with the BMIP-ABPF blend, clearly demonstrated the superior thermo-mechanical performance of the latter system. Comparison of DMA and thermoadhesive profiles implied that in the majority of the cases molecular relaxations at higher temperature are conducive to matrix toughening, which results in better adhesion. ACKNOWLEDGEMENTS Thanks are due to our colleagues in the Analytical and Spectroscopy Division of VSSC for support in the various analyses. The permission granted by the Director, VSSC, to publish this article is also acknowledged. REFERENCES 1. Hergenrother P.M., in Adhesive Chemistry: Developments and Trends, Plenum Press, New York, (1984) Chang C., in Adhesives, Sealants and Coatings for Space and Harsh Environments, Plenum Press, New York, (1988) Michaud P., and Camberlin Y., in Polyimides and Other High-Temperature Polymers, Elsevier, NewYork, (1991) Subrahmanian K.P., in Structural Adhesives: Chemistry and Technology, Plenum Press, New York, (1986) Shaw S.J., in Handbook of Adhesion, Longman Scientific and Technical, England, (1992) St. Clair A.K. and St. Clair T.L., in Polyimides: Synthesis, Characterisation and Applications : Proceedings of a Technical Conference, Vol.2, Plenum Press, New York, (1982) St.Clair T.L. and Progar D.J., 24 th Natl. SAMPE Symp. Proc. 2, (1979) Landis A.L., Bilow N., Boschom R.H., Lawrence R.E. and Aponyi T.J., Polym. Prepr., American Chemical Society., Div. Polym. Chem., 15, (1974) Darmory F.P., in New Industrial Polymers, American Chemical Society Symposium Series, 29, (1974) Liang G., Lan L. and Gu A., ACTO of Composites, 12 (1), (1995) King J.J, Chaudhari M. and Zahir S., Int.Natl. SAMPE Symp., 29, (1984) Stenzenberger H.D., Herzog M., Romer W., Scheiblieh R., Pierce J., Canning M. and Lear K., Int.Natl. SAMPE Symp., 30, (1985) Liang G. and Gu A., Polymer Journal, 29, 7, (1997) 553 Polymers & Polymer Composites, Vol. 11, No. 4,

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