Thermal stability of aromatic polyimides: Factors affecting the oxidative stability of poly‐N, N′‐(4,4′‐diphenyl ether) pyromellitimide

Polyimide samples in the form of powders, films and sintered mouldings were prepared from pyromellitic dianhydride and 4,4′‐diaminodiphenyl ether. The effect of environmental variables on the oxidative stability at 400°C of the high molecular weight films was studied, and it was shown that hydrolytic processes could greatly accelerate the degradation. The higher stability of powders was attributed to the presence of low molecular weight fractions which could inhibit these hydrolytic processes. Improvement of the stability of the films by the addition of low molecular weight materials during the casting process and the destabilisation of powders by prolonged solvent extraction gave support to this hypothesis. For comparison of thermal stabilities of polypyromellitimides of differing chemical structure weight loss results should therefore be determined on high molecular weight films under controlled humidity conditions.