EFFECT OF ISOTHERMAL AGING ON POST-IMIDIZATION AND GLASS-TRANSITION TEMPERATURE OF LARC-IA POLYIMIDE RESIN

The isothermal aging of partially imidized NASA Langley Research Cente r, 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 o ccurred during aging was determined gravimetrically and by FTIR spectr oscopy, respectively. The imide absorption peak at 1778 cm(-1), increa sed 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 o r equal to 4 wt%, after the expulsion of NMP, was attributed to post-i midization. The imide carbonyl peak absorption at 1721 and 1778 cm(-1) respectively, were broadened after aging at 200 degrees C. The broade ning of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm(-1) and is attributable to post-imidizatio n of the partially imidized polyamic-acid. Dissolution of the polyimid e aged at T greater than or equal to 165 degrees C in dimethyl formami de, DMF, was unsuccessful even after long times of stirring (similar t o 12 h) at elevated temperature (T 85 degrees C). The DSC thermogram f or the LaRC-IA resin showed a series of broad endothermic peaks betwee n 150-180 degrees C and narrow endothermic peaks at 210 degrees C. The low temperature endotherm disappeared after aging at T greater than o r equal to 135 degrees C for t greater than or equal to 1 h. The high temperature endotherm decreased with increased aging temperature and t ime. The glass transition temperature of the polyimide increased with increased aging temperature and time.