X-Ray Analysis of Liquid Crystalline Copolyesters and Copolyamides

Abstract

Abstract The results of X-ray analyses are presented for two main chain liquid crystalline polymer systems: the copolyesters prepared from hydroxybenzoic acid (HBA), terephthalic acid (TPA) and biphenol (BP) (which is the basis of of XYDAR resins produced by Dartco) and the copolyamide prepared from TPA, p-phenylene diamine and 3,4'-diphenylene ether diamine (HM-50 fibers produced by Teijin). X-ray fiber diagrams of both copolymers contain a series of aperiodic intensity maxima along the meridian (fiber axis). These are predicted accurately by a nematic array of copolymer chains that have completely random monomer sequences and highly extended conformations. We have investigated these conformations based on the line profile of the meridional maximums at d = 2.1 Å (2θ = 43°), which is seen for both copolymer systems. This width is a measure of the correlation length for the stiff chain conformation in the solid state. The continuous intensity distribution is predicted by treatment of the copolymer chain as a linear para- crystal with multiraodal coordination statistics. An idealized linear chain with constant axial advance for each monomer type leads to a very sharp peak at 2.1 Å; the width at half height predicted for an infinite chain of 50/25/25 copoly(HBA/TPA/BP) is 0.006° compared to the observed width of 1.4°. Significant nonlinearity is inherent in any real chain, and this is incorporated in the calculations in the form of histograms of the axial monomer lengths. The observed line widths are matched by relaxing the conditions for fully extended conformations by progressively increasing the allowed range of torsion angles and the diameter of the cylinder occupied by a single chain. Results for these copolymers are compared with those for other copolyesters in order to correlate chain extension with chemical structure.