Preparation And Properties Of Low-Temperature Cured Polyimides

In order to lower the imidization temperature of polyamic acids(PAA), the catalytic activities of the curing agents p-hydroxybenzoic acid (PHA), quinoline (QL), benzimidazole (BI), benzotriazole (BTA), triethylamine (Et3N) and 1,8-diazabicyclo [5.4.0]undec-7-ene (DBU) were investigated in the process of thermal imidization of PAA. In addition, the effect of these various curing agents on the thermal stabilities and mechanical properties of the resultant polyimide (PI) films was determined. Quinoline was found to be an effective curing accelerator in the use of two-step method for synthesizing PI. Due to its moderate base strength, low steric crowding effect and moderate boiling point, quinoline could not only accelerate PAA to achieve imidization completely at 180℃, but also could maintain the mechanical properties and thermal stability of the ordinary PI film. Any residual quinoline could be removed from PI films with heating at 250℃ for 4h. The influence of catalytic conditions on the catalytic reaction system was further investigated in this article. It was found that just add 1 molecules of quinoline for 1 molecule of repeating structure unit of the PAA showed a great effect. In several kinds of commonly used solvents, the different types of solvents had little effect on the catalytic reaction. The solids content of polyamic acid, by contrast, had obvious effect on the catalytic reaction.p-Hydroxybenzoic acid, which is a principal useful component for accelerating the imidization of polyimide at lower temperature was found to be an effective nucleating agent which could promote the which could promote the crystallization of PI significantly after undergoing high temperature heat treatment. The WAXD results showed that the degree of crystallinity of PI increased from 18.90% to 48.90% as the content of PHA increased from 0wt% to 4wt% after annealing PI films at 360℃ for 2h. The storage modulus, tensile strength, elongation at break and tensile modulus also increased with the content of PHA increasing. When the content of PHA increased to 4wt%, the corresponding values of PI films increased by 273%,69.56%,95.96%,97%, respectively. The TGA, SEM showed that the PHA was still remained in PI films after being annealed at 360℃ for 2h. The DMA results indicated that the residual PHA may cause a "plasticization effect" that provided greater chain mobility in PI films at its molten state. Moreover, the introduction of PHA had little impact on the excellent thermal stability of PI films. This work might propose a generic method to nucleate polyimide for obtaining polyimide films with higher crystallinity; that was, a kind of curing accelerator (PHA) was introduced into PI films which could not only accelerate the curing process ofPAA but also cause a nucleating effect that promote the crystallization of PI at a lower temperature, and then the formed small size spherulites can result in another heterogeneous nucleation during the isothermal annealing at an elevated temperature and thus lead to the high crystallinity of the final PI film. Meanwhile, the residual PHA in PI films may act as a plasticizer to provide greater chain mobility, thereby promoting the proceeding of crystallization.