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

Most aromatic polyimides and copolyimides show a beta relaxation proce ss below their a alpha relaxation processes in dynamic mechanical and dielectric measurements. The origin of this beta relaxation has been u nder active discussion. In order to gain a better understanding of the origin of the beta relaxation process, the dynamic mechanical behavio r of a series of segmented rigid-rod polyimide and copolyimide films w ith different dianhydrides and diamines has been systematically studie d. For some of the polymers, crystallinity and orientation effects on the beta relaxation process were also investigated. It is found that t he beta relaxation process in the unoriented films can mainly be attri buted to a relatively noncooperative motion in uncrystallized diamines . However, this noncooperative nature may be gradually lost by increas ing the crystallinity and orientation. Molecular modeling and computat ion have led to a discussion of possible diamine motion in the beta re laxation process.