X-ray Studies of Thermotropic Aromatic Copolyesters

Abstract

X-ray diffraction has been used to investigate the solid state structure of aromatic copolyesters that can be spun as high strength fibers from thermotropic melts. Copolymers of 4-hydroxybenzoic acid (HBA), 2,6-dihydroxynaphthalene and terephthalic acid give fiber diagrams that indicate a high degree of axial orientation. Much of the structure has disordered lateral packing but there is some three-dimensional order. The meridional maxima are aperiodic and shift in position with changes in the monomer composition. The positions of these maxima can be reproduced by a model for chains of random monomer sequence, in which the residues are represented by points separated from their neighbors by the appropriate monomer lengths. Extension of these calculations to an atomic model for the chains allows for comparison of the observed and calculated X-ray intensities. We find that refinement of the model by tilting the monomer residues with respect to the chain axis leads to good agreement between the observed intensities and those calculated for a completely random monomer sequence. An indication of the chain packing in the ordered regions comes from study of a second set of copolymers prepared by modification of poly(ethylene terephthalate) by incorporation of HBA. The latter copolymers are found to contain ordered regions with a structure similar to that for the high temperature hexagonal form of homopoly(HBA).