Synthesis And Characterization Of Long-chain Polyimides

Polyimides are an important class of high-performance polymers used in such applications as aircraft, automobile, electric, communication fields and so on due to their marked thermal-stability, excellent mechanical, electrical and radiation-resistance properties. However, many of these polymers are insoluable and infusible, rendering them impossible to process by the conventional methods, which has slowed the widespread use of the polymers. That is because there exist the strong inter-molecular interaction, the rigid chain or cross-linking and so on. Therefor, lots of approaches have been launched to synthesize the tractable polyimides that maintain reasonably high strength and thermal stability. The two-step method is widely used for the synthesis of polyimides. It involves reacting a dianhydride and a diamine at ambient condition in a dipolar solvent to yield the corresponding polyamic acid, which is then cyclized into the final polyimide by the chemical or thermal imidization. The thermal imidization is relatively simple. However, some research findings indicated that the performances of the polyimides greatly depend on their thermal curing condition. For example, the surfaces of the parts in the microelectronic areas are multiplicate. The decomposition and crackle will take place, leading to the downtrend of the performances of the polyimides if the polyamic acids films are not cured completely. The average molecular weight,molecular weight distribution of PAAs are also important to the properties of polyimides. Therefore, it's necessary to investigate the condensation kinetics and the thermal cyclization kinetics of the polyamic acids.While the condensation kinetics of the polyamic acids has been extensively studied during the past years, there is a great discrepancy in the literatures regarding the kinetics law that is followed. Besides, the thermal cyclization is a complex process that is still under extensive investigation. The use of different PAAs and techniques has undoubtedly contributed to the general lack of agreement over the thermal cyclization.In this work, the three kinds of long-chain polyimides had been synthesized by the condensations of the long-chain BAPS with PMDA,ODPA and BPADA, respectively. On the one hand, ether and isopropyl are introduced into ODPA and BPADA so as to increase the flexibility of the polymers and improve their solubility and fusibility. Moreover, the density ofthe imide in the backbone is decreased with the increase of the length of repeated unit, which is expected to gain both excellent heat-resistance and convenient process. On the other hand, the investigation on the condensation kinetics and the thermal cyclization kinetics were carried out to support the synthesis of high-performance polyimides.The three kinds of PIs had been synthesized and characterized. The results showed that with the increase of flexibility of dianhydrides, the solubility of PIs was improved gradually, the glass transition temperature decreased, which was 323.1℃,262.9℃ and 228.1℃ respectively.The thermal stability of PIs showed a similar change tendency as that of the glass transition temperature. The initial decomposition temperatures of PIs ranged from 420℃ to 480℃.The condensation kinetics of PAAs was investigated. The change of the relative number-average DP (Xn) of PAA with time was measured at the concentration of 0.025, 0.05 and 0.1 mol/L by the GPC. The results indicated it's an autocatalytic condensation, following the three-order kinetics. The initial rate constants of the condensation of PAAs increased with the increase of the EA of the dianhydride,which are 0.293, 0.464 and 2.371 (m3)2mol-2min-1.The thermal cyclization of the PAA derived from BPADA and BAPS was investigated, especially its cyclization kinetics. FTIR equipped with a hot in-situ cell was applied to monitor the process of the thermal cyclization, which can avoid the error aroused by the different thickness of the film. The results showed that molecular weight of polymers decreased because...