Relaxation Behavior of Polyimides Based on 2,2'-Disubstituted Benzidines

Abstract

The mechanical relaxation behavior of polyimides based on a variety of 2,2'-disubstituted benzidines and rigid dianhydides was investigated. Two transitions were observed in these polyimides. The glass relaxation process is relatively weak and occurs at high temperatures due to the linear and rigid nature of these polyimides. The subglass relaxation is very prominent in these polyimides and is due to main-chain rotational motion localized within the diamine (benzidine) sequent. Changes in the dianhydride moiety have little effect on the temperature of the subglass transition and result in only minor changes in the magnitude of this relaxation. The presence of 2,2'-CF 3 substituents on the benzidine moiety increases the magnitude and shifts the subglass relaxation approximately 150 o C to higher temperatures versus Cl or CH 3 in these positions. Incorporating a flexible ether linkage between the phenyl rings of the benzidine and the CF 3 side group (e.g., OCF 3 ) substantially reduces the temperature and to some extent the magnitude of the subglass relaxation. Replacement of the 2,2'-disubstituted benzidine unit (two phenyl rings) with one (benzene) or three (terphenyl) unsubstituted phenyl rings results in a substantial decline in both the temperature and magnitude of the subglass relaxation. Molecular modeling was used to clairify the nature of the subglass relaxation. Rotational energy barriers for the 2,2'-disubstituted benzidines, calculated from both semiempirical and density functional quantum mechanical calculations, are comparable in magnitude to the experimentally determined activation energies for the subglass relaxation