Diffraction measurements of crystalline morphology in a thermotropic random copolymer

Abstract

The crystal structure and morphology has been examined in a series of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) thermotropic random copolymers having different compositions and molecular weights. For rapidly cooled HBA-rich samples, the structure is pseudo-hexagonal, while for the more HNA-rich it is slightly distorted to orthorhombic, a phase designated O″. On annealing, the HBA-rich samples undergo a partial phase transformation to a denser orthorhombic phase with a greater distortion from the hexagonal than the O″. This new phase is designated O′. Annealing of the samples richer in HNA leads to a greater distortion of the O″ cell away from hexagonal, but no increase in density. Discrete small-angle diffraction maxima corresponding to a long period of ∼ 300 A ̊ appear only in those samples containing the denser O′ phase, with the implication that both pseudo-hexagonal and O″ crystals have densities very close to that of the non-crystalline nematic matrix. Anneals above 230°C lead to a marked increase in long period and there was some evidence of a specific doubling mechanism. Precise measurement of crystallinity gave values between 17.5% and 25.5%, the lower values being in the samples closer to the 1:1 composition and in those annealed.