Molecular Order in Condensed States of Semiflexible Poly(Amic Acid) and Polyimide

Abstract

The molecular order in condensed states of the aromatic poly(amic acid) (PAA) poly(4,4'-oxydiphenylenepyromellitamic acid) and the resulting polyimide (PI) poly(4,4'-oxydiphenylenepyromellitimide) obtained by heating PAA films was determined from wide-angle X-ray diffraction studies. PAA chains in films containing solvents of ~30% by weight exhibit parallel alignment and correlated (smectic-like) lateral packing of highly extended chain segments, as shown by the appearance of a diffraction peak due to the intramolecular periodicity of ~14 Å. Upon imidization and removal of solvents by heating at 200 °C, this molecular order is preserved and slightly improved in the resulting PI films. Further improvement in order occurs upon annealing PI films at temperatures above ca. 300 °C, with the details of changes depending strongly on the initial orientation. In all PI samples, however, the high-angle diffraction patterns in the region of the scattering vector s (=(2 sin θ)/λ) > 0.35 Å−1are matched very closely by the theoretical curve calculated for a single repeat unit, thereby indicating the absence of any appreciable long-range crystalline order in the thermally imidized PI films. The ramifications of such molecular order in condensed states of PAA (which cannot crystallize due to the meta-para isomers in the pyromellitamic acid unit) and PI are discussed in light of recent theoretical developments in the statistical thermodynamics of semiflexible polymers as well as some unique characteristics exhibited by this polyimide. © 1984, American Chemical Society. All rights reserved.