Three-dimensional structure of copolymers of p-hydroxybenzoic acid and 2-hydroxy-6-naphthoic acid: A model for diffraction from a nematic structure

Abstract

X-ray methods have been used to investigate the three-dimensional structure of copolymers of 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA). X-ray patterns of melt spun fibres indicate a high degree of axial orientation of the molecules, but with poor lateral order, except in some highly ordered regions. Our previous calculations have shown that the aperiodic meridional maxima are consistent with chains of completely random monomer sequence. Extension of these calculations to the three-dimensional structure has been done by calculating the cylindrically averaged intensity transform for single chains. The results point to a distorted hexagonal array of chains which have extended but otherwise random conformations, i.e. there is a broad distribution of aromatic-carboxyl torsion angles. It is suggested that in some regions the chain packing is more regular, perhaps due to axial register of HBA-rich sequences, giving rise to the off-equatorial reflections characteristic of three-dimensional order.