Thermosetting process and thermal degradation mechanism of high-performance polyimide

Transcription

1 Journal of Analytical and Applied Pyrolysis 64 (2002) Thermosetting process and thermal degradation mechanism of high-performance polyimide Xigao Jin a, *, Liyan Huang b, Yi Shi a, Shiyong Yang c, Aijun Hu c a State Key Laboratory of Polymer Physics and Chemistry, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing , People s Republic of China b Department of Chemistry, Beijing Normal Uni ersity, Beijing , People s Republic of China c State Key Laboratory of Engineering Plastics, Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing , People s Republic of China Received 1 August 2001; accepted 17 January 2002 Abstract The thermosetting process of norbornene-ended polyimide, PMR II, was investigated by high resolution pyrolysis-gas chromatography-mass spectrometry (HR PyGC-MS) in a temperature range of C. The cross-linking reactions of PMR II under the temperature range of C for 0 20 h were discussed based on the composition and distribution of the pyrolyzates related to the end groups of the polymer. Then the thermosetting pathway and thermal decomposition mechanism of PMR-II polyimide were suggested Elsevier Science B.V. All rights reserved. Keywords: Polyimide; Thermosetting process; Thermal degradation; High resolution pyrolysis-gas chromatography-mass spectrometry This article is in honor of Professor Shin Tsuge for his extensive efforts and excellent contributions in analytical pyrolysis of synthetic and natural polymeric materials and instrumentation of PyGC as well as the world communication. * Corresponding author. Tel.: ; fax: address: jinxg@infoc3.icas.ac.cn (X. Jin) /02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S (02)

2 396 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Introduction In-situ polymerization of monomer reactant (PMR) is one of the significant methods especially to prepare high performance polyimide materials[1 8]. The first generation of these materials is PMR-15, which was reported in the early 1970s. At present, PMR-15 is one of the most widely used matrix resins of polyimide composites considering of its low cost and comprehensive properties. Despite being the leading system available, PMR-15 has a number of limitations affecting its widespread use. For example, PMR-15/carbon fiber laminates have a continuous use temperature of about C. Extending the use temperature is extremely desirable. Second generation PMR polyimides that can withstand engine operating temperatures up to 425 C (800 F) have been developed. PMR-II based upon 4,4 -(hexafluoroisopropylidene)diphthalic anhydride diethyl ester diacid (HFDE) exhibits improved thermo-oxidative stability over PMR-15 resin. The flexible strength of PMR-II-50/T40-R composite at 371 C (700 F) is 320 MPa. PMR-II shows considerable promise for long term service at temperatures up to 371 C. The chemical preparation including the thermosetting of PMR-II has been an area of intensive research activity for many years, but the image of the thermosetting process and thermal degradation mechanism is still not clear due to lack of direct experimentally evidence. Because of its novel heat resistant properties, the details of the thermal behavior of the polymer are important for the development and application [9,10]. In this work, the thermal degradation behavior of the norbornene end-capped polyimide with various thermosetting conditions was investigated by HR PyGC- MS in the pyrolysis temperature range of C. The cross-linking and degradation of PMR-II were discussed based on the composition and distribution of the pyrolyzates. 2. Experimental 2.1. Materials PMR-II was prepared from 4,4 -(Hexafluoroisopropylidene)diphthalic anhydride diethyl ester diacid (HFDE), p-phenylene diamine (p-pda), and monoethyl-5-norbornene-2,3-dicarboxylate (NE) with a molar ratio of 9:10:2 in ethanol solution with a concentration of 50 wt.% [8]. Scheme 1 shows the two major preparation steps. The first step yields a PMR-II prepolymer (code A in Table 1) and occurred in the temperature range of C. The second one carried out at the elevated temperatures of 330, 350, 370 and 390 C, respectively, for various periods (0 20 h), and then gave out thermosetting PMR-II polyimides with various cross-linking levels (code B K in Table 1).

3 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) HR PyGC-MS studies The samples with the weight range of 3 5 mg were pyrolyzed in a JHP-3 Curie Point Pyrolyzer (Japan Anal. Ind. Corp.) at the pyrolysis temperatures of 310, 386, 485, 590 C with a pyrolysis period of 5 s. The separation and identification of the fragments generated during pyrolysis period were carried out by an on-line QP GC-MS (Shimadzu). The fused silica capillary column, CBP5 (0.25 mm i.d. 25 m), was held at 30 C for 6 min then temperature-programmed to 250 C Scheme 1. Preparation of PMR-II polyimide. Table 1 Thermosetting condition for PMR-II polyimide Code A B C D E F G H I J K Thermosetting temperature and period Prepolymer 330 C, 2 h 330 C, 2 h+350 C, 2 h 330 C, 2 h+350 C, 2 h+370 C, 1 h 330 C, 2 h+350 C, 2 h+370 C, 2 h 330 C, 2 h+350 C, 2 h+370 C, 2.5 h 330 C, 2 h+350 C, 2 h+370 C, 3 h 330 C, 2 h+350 C, 2 h+370 C, 2 h+390 C, 2 h 330 C, 2 h+350 C, 2 h+370 C, 2 h+390 C, 5 h 330 C, 2 h+350 C, 2 h+370 C, 2 h+390 C, 10 h 330 C, 2 h+350 C, 2 h+370 C, 2 h+390 C, 20 h

4 398 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) at 5 C min 1. Helium was used as the carrier gas, the total flowwas13mlmin 1 with a split ratio of 10:1. The EI source in MS was operated at 70 ev and 250 C. 3. Result and discussion 3.1. Effect of the thermosetting condition for samples A G on cross-linking reaction PMR polyimide is the reaction product of the three monomers: p-pda, HFDE and NE. As shown in Scheme 1, the first step, condensation polymerization, involves heating the monomers at C, which causes imidization to yield a PMR-II prepolymer (A). The second step occurred at elevated temperatures ( C) allows the norbornene endcapes to cross-link. The final additional cross-linking is very complex. During this stage, a reverse Diels Alder reaction of the norbornene endcaps takes place and results in cyclopentadiene and maleimide groups on the end of the macromolecules. The resulted cyclopentadiene with the double bonds then proceed to polymerization with the maleimide end groups or/and unreacted norbornene end groups to produce a cross-linked structure, as shown in Scheme 2. Therefore we can reasonably suppose that there are still uncross-linked norbornene end groups in the polymer if cyclopentadiene could be identified after preliminary thermosetting at an elevated temperature. In other words, none cyclopentadiene would be released and the norbornene end groups disappear for the completely cross-linked polymer. It means the cross-linking process can be dynamically analyzed based on the concentration alteration of the end groups in the polymer and released cyclopentadiene in the system. The PMR-II samples cured for various temperatures and periods were subjected to pyrolysis in the temperature range of C. The partial TIC pyrograms of samples A G with different thermosetting stages pyrolyzed at the temperature of 485 C for 5 s are shown in Fig. 1. Obviously, the major pyrolyzate of sample A, cyclopentadiene, is the product of a reverse Diels Alder reaction of the norbornene endcaps of the prepolymer without cross-linking. On the other hand, the pyrolyzates of cyclopentene and small amount of norbornene (does not show in Fig. 1) together with cyclopentadiene were observed in the pyrograms for samples B G, which underwent the thermosetting up to 370 C. These additional pyrolyzates formed from the thermal scission via carbon carbon and carbon carbonyl bond scissions on the polymer ends, as shown in Scheme 3. But for the samples H, I, J and K cured finally at 390 C, both cyclopentadiene and cyclopentene were not detected in their pyrograms. The yields of cyclopentene gradually increased (A E in Fig. 1) to a maximum, then decreased (F and G in Fig. 1) and finally disappeared when the thermosetting temperature and period increased. As mentioned above, the pyrolysis of PMR-II prepolymer (A) produced a significant amount of cyclopentadiene, but no cyclopentene was detected. This fact indicated that cyclopentene is not the secondary reaction product of cyclopentadiene. It is confirmed by presence

5 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Scheme 2. Cross-linking illustration of PMR-II polyimide. of cyclopentene segments in the cross-linking bridges after their preliminary cure, i.e. cyclopentene is the degradation product of the corresponding cross-linking bridges for the preliminary cross-linked polymer, as shown in Schemes 2 and 3. Therefore, the yield of cyclopentene also reflects the relative cross-linking level in one side. The pyrolysis products of the samples with various curing stages were identified at different pyrolysis temperatures of 315, 386, 485 and 590 C, respectively. As mentioned previously, some characteristic pyrolyzates, such as cyclopentadiene and norbornane related to the norbornene end-capped groups were identified in the pyrograms of samples B G, but these pyrolyzets did not show in the pyrograms of sample H K. The tendency was independed of pyrolysis temperature and it reconfirmed that the samples cured at C contain some uncross-linked norbornene segments, contrarily, the samples of H K were cross-linked completely.

6 400 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Fig. 1. The partial TIC pyrograms of samples A G pyrolyzed at 485 C. (1) Cyclopentadiene; (2) cyclopentene. Scheme 3. Thermal breakdown of the end groups for PMR-II polyimide.

7 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Fig. 2. The plot of pyrolyzete yield versus thermosetting period at 370 C, pyrolyzed at 485 C. (a) Cyclopentadiene; (b) cyclopentadiene and norbornane; (c) cyclopentene. In order to further understand the thermosetting process for the samples cured at different conditions, the yields (peak area of the pyrolyzate produced from 1 mg sample) of the two characteristic pyrolyzates, cyclopentadiene and norbornane,

8 402 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) related to the scission of end groups, were selected to characterize the cross-linking level of the samples. For example, the yields of cyclopentadiene for samples A, B and C were , and , respectively. In other words, when the prepolymer (A) was cured at 330 C for 2 h (B), or first at 330 C for 2 h and then at 350 C for 2 h (C), the yields of cyclopentadiene were 1/2.83 and 1/7.15 times smaller than that of the prepolymer A. The effect of cured period on the composition and distribution of the pyrolyzates was especially investigated. Fig. 2a denoted that the relative yields of cyclopentadiene for samples from C to G decreased gradually at the thermosetting temperature of 370 C. The yields of norbornane also followed the same trend. As a result, the total relative yields of cyclopentadiene and norbornane decreased as the thermosetting period increased at 370 C, as shown in Fig. 2b. That is to say, the Fig. 3. The TIC pyrogram of PMR-II polyimide (sample E) pyrolyzed at 485 C.

9 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Fig. 4. The TIC pyrogram of PMR-II polyimide (sample E) pyrolyzed at 590 C. cross-linking level from sample B to G increased gradually and it confirmed that the two pyrolyzates are related to the cross-linking of PMR-II polyimide. Moreover, it was found that the relative yield of cyclopentene increased with increasing the curing period and reached a maximum in h, as shown in Fig. 2c Analysis of pyrolytic products The total ion current pyrograms of sample E shown in Figs. 3 and 4 were obtained at the pyrolysis temperatures of 485 and 590 C for 5 s, respectively. The primary pyrolysis products were separated by a capillary column and identified by on-line EI-MS. It is noted that there are three kinds of pyrolyzates as summarized in Table 2. The first group includes low boiling point products such as trifluoromethane and tetrafluorosilicane originated from the scissions of fluoro-content side groups ( CF 3 ) and its related compound reacted with SiO 2 on the silica column surface. The second includes some characteristic pyrolyzates of the polymer, such as cyclopentadiene, cyclopentene and norbornane, related to the end groups of the prepolymer and partially cross-linked polymers. The third, includes 2, 2-diphenyl-hexafluoropropane, carbonitrile-2, 2-diphenyl-hexafluoropropane, 2- phenyl-2-phththalimide-hexafluoro-propane, 2-phenyl-2-(N-phenyl) and phththalimide-hexafluoropropane that reflect the characteristic structure of the polymer backbone Effect of pyrolysis temperature and the degradation mechanism of PMR-II Considering the composition and distribution of the major pyrolyzates of the polymer, it can be concluded that all of the uncross-linked and partially crosslinked samples (samples A G) pyrolyzed at a lower temperature (315 and 386 C) produced some end-group related compounds such as cyclopentadiene and norbor-

10 404 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) nane, and small amount of the main chain decomposition products, 2,2-diphenylhexafluoropropane, at the pyrolysis temperature of 485 C. At a higher pyrolysis temperature ( 590 C), besides the scission of the norbornene end-capped groups, the scissions of the carbon nitrogen bonds and carbon carbon bonds in the imide Table 2 Summary of pyrolyzates of PMR-II polyimide (sample E) identified by on-line MS

11 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) Scheme 4. Thermal degradation mechanism of PMR-II polyimide. rings as well as the carbon carbon bonds along the main chain of the polymer took place. These backbone cleavages gave rise to some characteristic pyrolyzates such as 2,2-diphenyl-hexafluoropropane, 2-phenyl-2-phththalimide-hexafluoropropane, and 2-phenyl-2-(N-phenyl)phththalimide-hexafluoro-propane, reflecting the structural feature of the polymer backbone. As a result, the thermal degradation mechanism of PMR-II polyimide is suggested as shown in Scheme Conclusion We have employed HR PyGC-MS to investigate the thermosetting process and thermal degradation mechanism of norbornene-ended polyimide (PMR-II). The pyrolysis products of the PMR-II samples with various curing stages were identified. Several significant conclusions can be drawn from the results obtained.

12 406 X. Jin et al. / J. Anal. Appl. Pyrolysis 64 (2002) The pyrolyzate distribution of PMR-II polyimides depends significantly on the cross-linking level and pyrolysis temperature. Several pyrolyzates are related to the cross-linking of PMR-II. Cyclopentadiene and norbornane were selected to characterize the cross-linking level of the samples. All of the uncross-linked and preliminary cross-linked samples pyrolyzed at a lower temperature (315 and 386 C) produced some end-group related compounds such as cyclopentadiene and norbornane. At a higher pyrolysis temperature ( 590 C), besides the scission of norbornene end-capped groups, the scissions of backbone in the polymer took place. These backbone cleavages gave rise to some characteristic pyrolyzates which reflecting the structure feature of the polymer backbone. The thermosetting pathway and thermal degradation mechanism of PMR-II polyimide were suggested. Acknowledgements The authors would like to express their thanks to the State Key Laboratory of Polymer Physics and Chemistry, China for the financial support (99-B-15) that made the work possible. References [1] T.T. Serafini, P. Delvigs, G.R. Lightsey, J. Appl. Polym. Sci. 16 (1972) 905. [2] R.W. Lauver, J. Polym. Sci. Polym. Chem. 17 (1979) [3] A.C. Wong, M.M. Ritchey, Macromolecules 14 (1981) [4] J.C. Johnston, M.A.B. Meador, W.B. Alston, J. Polym. Sci. Polym. Chem. 25 (1987) [5] G. Gacia, T.T. Serafini, J. Polym. Sci. Polym. Phys. 25 (1980) [6] J.P. Daynes, R.M. Panos, C.L. Hamermesh, J. Appl. Polym. Sci. 25 (1980) [7] J.K. Sutter, J.M. Jobe, E.A. Crane, J. Appl. Polym. Sci. 57 (1995) [8] D. Wilson, Br. Polym. J. 20 (1998) 405. [9] F. Li, L. Huang, Y. Shi, X. Jin, Z. Wu, Z. Shen, K. Chuang, R.E. Lyon, F.W. Harris, S.Z.D. Cheng, J. Macromol. Sci. Phys. B38 (1999) 107. [10] L. Huang, Y. Shi, X. Jin, Z. Wu, F. Li, S.Z.D. Cheng, Sci. China B42 (1999) 316.