X-ray analysis of the structure of liquid-crystalline copolyesters

Abstract

The X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts is described. X-ray patterns of melt-spun fibres of wholly aromatic copolymers prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation and there is also evidence for three-dimensional order. The meridional maxima are aperiodic, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences. Analyses of the scattering by aperiodic chains are summarized, first for point monomers and subsequently for an atomic model. From the breadth of some of the meridional maxima it is possible to estimate the persistence or correlation length for the stiff-chain conformation, which is found to be between 9 and 13 monomer units, depending on the HBA/HNA ratio. Determination of the three-dimensional structure is initiated via calculation of the cylindrically averaged transform for the random chain, followed by consideration of the interference effects caused by chain packing.