The X-ray diffraction data for stiff-chain liquid crystalline aromatic copolyesters have been interpreted by calculation of the scattering characteristics of random copolymer chains. The meridional region of fiber diagrams of copolymers of p-hydroxybenzoic acid (HBA), 2,6-dihydroxynaphthalene (DHN), and terephthalic acid (TPA) and copolymers of HBA and 2-hydroxy-6-naphthoic acid (HNA) contains maxima that are aperiodic and shift in their positions depending on the monomer ratio. We have shown that excellent agreement is obtained between the positions of these observed maxima and those predicted for an aperiodic array of points where each point represents a monomer in a random chain, separated from adjacent points by the appropriate monomer lengths. Extension of these calculations to atomic models for the copolymer chains shows that only small changes occur in the positions of the predicted maxima, but there is now reasonably good agreement between the observed and calculated intensities.
X-ray methods are used to investigate the structure of melt-spun fibers of p-hydroxybenzoate/ethylene terephthalate copolymers, by analogy with the electron diffraction patterns obtained from single crystals of homopoly(p-hydroxybenzoate) [systematic name: poly(l,4-oxybenzoyl)]. The fiber diagrams of copolymers containing 60-80% p-hydroxybenzoate are very similar to the electron diffraction patterns of homopolymer single crystals that have been heat treated at 360 °C. These observations indicate that the fibers contain oriented, ordered regions with the same basic structure as the high-temperature form of the homopolymer. These regions probably consist of copolymer sequences rich in hydroxybenzoate, in which some ethylene terephthalate units are present as defects. The data also define the orientation of the chains with respect to the homopolymer crystal morphology. Both the X-ray and electron diffraction data show that the poly(p-hydroxybenzoate) chain has a stiff, extended 2X helical conformation, with two monomer units repeating in ~12.4 ± 0.2 A, for which a stereochemically acceptable model is presented.
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