RELAXATION BEHAVIOR OF POLYIMIDES BASED ON 2,2'-DISUBSTITUTED BENZIDINES

The mechanical relaxation behavior of polyimides based on a variety of 2,2'-disubstituted benzidines and rigid dianhydrides was investigated . Two transitions were observed in these polyimides. The glass relaxat ion process is relatively weak and occurs at high temperatures due to the linear and rigid nature of these polyimides. The subglass relaxati on is very prominent in these polyimides and is due to main chain rota tional motion localized within the diamine (benzidine) segment. Change s in the dianhydride moiety have little effect on the temperature of t he subglass transition and result in only minor changes in the magnitu de of this relaxation. The presence of 2,2'-CF3 substituents on the be nzidine moiety increases the magnitude and shifts the subglass relaxat ion approximately 150 degrees C to higher temperatures versus Cl or CH 3 in these positions. Incorporating a flexible ether linkage between t he phenyl rings of the benzidine and the CFB side group (e.g., OCF3) s ubstantially reduces the temperature and to some extent the magnitude of the subglass relaxation. Replacement of the 2,2'-disubstituted benz idine unit (two phenyl rings) with one (benzene) or three (terphenyl) unsubstituted phenyl rings results in a substantial decline in both th e temperature and magnitude of the subglass relaxation. Molecular mode ling was used to clarify the nature of the subglass relaxation. Rotati onal energy barriers for the 2,2'-disubstituted benzidines, calculated from both semiempirical and density functional quantum mechanical cal culations, are comparable in magnitude to the experimentally determine d activation energies for the subglass relaxation.