Low temperature relaxation in polypyromellitimide

Abstract

AbstractThe effect of heat treatment at temperatures above 300°C on the low temperature relaxation of poly(4,4′‐oxydiphenylenepyromellitimide) (Kapton H‐film) was studied by wide‐line nuclear magnetic resonance (NMR), mechanical, and dielectric measurements. In the NMR line spectrum of the as‐received film, a narrow component above −60°C and a broad component which begins to narrow at about −100°C were observed. It is proposed that the narrow component is associated with absorbed water, because it disappeared in the heat‐treated film at 300°C in N2. On the other hand, the behavior of the broad component was not influenced by heating to 300°C in N2. Mechanical and dielectric loss peaks were observed at −75°C (11 Hz) and −65°C (1 kHz), respectively, in the as‐received film. These loss peaks did not change in intensity with heating to 300°C in N2. It is proposed that the mechanical and dielectric loss peaks corresponding to the narrowing of the NMR broad component are associated with the local‐mode motion of the diphenylether portion of the polypyromellitimide chain. It was found that crosslinks are formed by heating to 374°C in air through coupling of the diphenylether portions of the molecular chains. With the formation of crosslinks the dielectric loss peak shifted toward higher temperature and the intensity decreased through restriction of the local‐mode motion of the diphenylether portion of the molecular chain.