THE ORIGIN OF BETA-RELAXATIONS IN SEGMENTED RIGID-ROD POLYIMIDE AND COPOLYIMIDE FILMS

Most aromatic polyimides and copolyimides show a beta relaxation process below their a alpha relaxation processes in dynamic mechanical and dielectric measurements. The origin of this beta relaxation has been under active discussion. In order to gain a better understanding of the origin of the beta relaxation process, the dynamic mechanical behavior of a series of segmented rigid-rod polyimide and copolyimide films with different dianhydrides and diamines has been systematically studied. For some of the polymers, crystallinity and orientation effects on the beta relaxation process were also investigated. It is found that the beta relaxation process in the unoriented films can mainly be attributed to a relatively noncooperative motion in uncrystallized diamines. However, this noncooperative nature may be gradually lost by increasing the crystallinity and orientation. Molecular modeling and computation have led to a discussion of possible diamine motion in the beta relaxation process.