Structure and properties of novel asymmetric biphenyl type polyimides

Abstract

Asymmetric biphenyl type polyimides (PI) were prepared by thermal imidization of polyamic acids (PA A) derived from 2,3,3', 4'biphenyltetracarboxylic dianhydride (a-BPDA) and p-phenylenediamine (PDA) or 4,4'-oxydianiline (ODA). The degrees of molecular orientation, film densities, and dynamic mechanical properties of these Pis were compared with those of familiar Pis based on symmetric 3,4,3',4'biphenyltetracarboxylic dianhydride (s-BPDA). PI(a-BPDA/PDA) cured at 350°C showed a Tg close to that of PI(s-BPDATPDA) prepared under the same condition in spite of the bent chain structure of the former. Comparison of the Pis cured at 400°C manifested that PI(a-BPDA/PDA) has a Tg at 410°C (abrupt decrease in E') whereas PI(s-BPDA/PDA) show no distinct Tg. Similar result was also observed for the ODA systems. The unexpectedly high Tgs of a-BPDA-based Pis were explained in terms of the restricted conformational change around the phenyl-phenyl bond in the a-BPDA unit. The difference of the E1 decrement at the Tg for both type of Pis are probably attributed to the difference of the intensity of intermolecular interactions. In PI(a-BPDA/ PDA) thin film cured on a substrate, the segments unexpectedly align slightly to the film plane. © 1996 TAPJ.