Structure and Properties of Novel Asymmetric Biphenyl Type Polyimides.

Abstract

Asymmetric biphenyl type polyimides (PI) were prepared by thermal imidization of polyamic acids (PAA) 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′- biphenyltetracazboxylic dianhydride (s-BPDA). PI(a-BPDA/PDA) cured at 350°C showed a Tg close to that of PI(s-BPDA/PDA) 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 E′ 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.