Effect of isothermal aging on post-imidization and glass transition temperature of LaRC-IA polyimide resin

摘要

The isothermal aging of partially imidized NASA Langley Research Center, LaRC-IA polyimide resin containing 70 wt% N-methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200 degrees C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm(-1), increased in intensity as the aging temperature was increased from 65 to 200 degrees C. The expulsion of NMP (70 wt%) was completed after similar to 2.5 h of aging at 135 degrees C. Additional weight loss less than or equal to 4 wt%, after the expulsion of NMP, was attributed to post-imidization. The imide carbonyl peak absorption at 1721 and 1778 cm(-1) respectively, were broadened after aging at 200 degrees C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm(-1) and is attributable to post-imidization of the partially imidized polyamic-acid. Dissolution of the polyimide aged at T greater than or equal to 165 degrees C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (similar to 12 h) at elevated temperature (T 85 degrees C). The DSC thermogram for the LaRC-IA resin showed a series of broad endothermic peaks between 150-180 degrees C and narrow endothermic peaks at 210 degrees C. The low temperature endotherm disappeared after aging at T greater than or equal to 135 degrees C for t greater than or equal to 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.