Processability of poly(amic acid) gel films

Abstract

During the high-temperature drawing process of polyimide (PI) films, simultaneously changing the physicochemical structure of the films and implementing intricate process control is a complex problem. In this study, the PI precursor poly(amic acid) (PAA) was condensed from pyromellitic dianhydride (PMDA) and 4,4′-diaminodiphenyl ether (ODA) and its transformation from solution to gel and then to the glassy state was investigated using isothermal thermogravimetry, isothermal dynamic mechanical analysis, and differential scanning calorimetry. The experimental results were combined with molecular simulation results to determine the diffusion behaviors and related mechanisms of the solvents and chain segments. After investigating the effects of the solvent system, temperature, and solid content on the processability of PAA gel films, the processing window was identified on a two-dimensional parameter map (temperature versus solid content), which was divided into three regions with distinct processabilities. Both N,N′-dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP) solvent systems exhibited a highly elastic gel state within a broad range of solid contents and temperatures but the NMP system demonstrated better processability, with an elongation rate exceeding 120 % within the 110°C-135 °C range and no chemical structural transformation from PAA to PI. This study successfully realizes a high-elastic gel state of PAA without relying on other chemical reagents. The proposed research idea and technical approach is expected to realize low-temperature drawing processing of PI films.