Kinetics of thermal imidization for long-chain polyamic acid

Thermal imidization of the polyamic acid derived from BPADA and BAPS was investigated mainly by GPC and FTIR equipped with an in-situ hot cell. The non-isothermal imidization was carried out in the temperature range from 100degreesC to 300degreesC with a multi-step heating program. Resalts indicated an initial dissociation of the amide bonds, producing anhydride and amine end groups, and the reduction in molecular weight and the embrittlement of the film. As the temperature was increased further, the above terminal functional groups recombined to increase the chain length and molecular weight, resulting in the toughness recovery of the film. The isothermal imidization included two steps of different reaction rates, a fast step followed by a slow one. That is to say a kinetic interrupt took place. A two-step first-order kinetics model was applied to the isothermal imidization, and the relevant parameters, such as k, E-a and A, were obtained. In addition, as the isothermal imidization was concerned, a new empirical formula was proposed to correlate the degree of imidization at the kinetic interrupt point with the curing temperature, which showed a good agreement with the experimental date. Moreover, the kinetic interrupt was explained based on both the conformation change and the effect of free volume.