X-Ray Studies of the Structure of HBA/HNA Copolyesters

X-ray diffraction patterns of fibers spun from liquid crystalline melts of ρ-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation. Several meridional maxima are detected which are aperiodic and also change in position and number with the monomer composition. The positions of these maxima can be predicted by calculating the theoretical scattering of random copolymer chains, in which the residues are represented by points separated by the monomer lengths. Both peak positions and intensities are reproduced when intraresidue interferences are allowed for in an atomic model for the random chains. This procedure also allows determination of the stiff-chain persistence (or correlation) length from the breadth of the maximum at d =2.1A which increases from 9 to 13 residues as the HBA content is increased from 25 to 75%.