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.
The X-ray analyses of the structures of a group of aromatic copolyesters that form liquid-crystalline melts is described. X-ray patterns of melt-spun fibres of wholly aromatic copolymers prepared from p-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show a high degree of axial orientation and there is also evidence for three-dimensional order. The meridional maxima are aperiodic, and it is shown that their positions and intensities are predicted by a model consisting of an assembly of parallel chains of completely random monomer sequences. Analyses of the scattering by aperiodic chains are summarized, first for point monomers and subsequently for an atomic model. From the breadth of some of the meridional maxima it is.possible to estimate the persistence or correlation length for the stiff-chain conformation, which is found to be between 9 and 13 monomer units, depending on the HBA/ HNA ratio. Determination of the three-dimensional structure is initiated via calculation of the cylindrically averaged transform for the random chain, followed by consideration of the interference effects caused by chain packing.
X-ray fiber diagrams of the wholly aromatic copolyesters prepared from 4-hydroxybenzoic acid (HBA) and 2-hydroxy-6-naphthoic acid (HNA) show meridional maxima that are aperiodic and vary in position depending on the monomer ratio. It has been shown that the positions
The sensitivity of X-ray diffraction data to monomer sequence distribution has been investigated for liquid crystalline aromatic copolyesters prepared from 4-hydroxybenzoic acid, 2,6-dihydroxynaphthalene, and terephthalic acid. The meridional intensity maxima in the fiber diagrams are aperiodic, and their positions are reproduced by calculating the theoretical diffraction patterns of copolymer chains, averaged over all possible monomer sequences. The monomer sequence distribution was varied from totally random to highly blocky by variation of the neighbor probabilities in the copolymer chains. For each of three monomer compositions it was found that the best agreement between the observed and calculated meridional data is obtained for completely random sequences and that all but minimal blockiness can be ruled out. Thus it appears that X-ray methods can be used to investigate the monomer sequence distribution of these relatively stiff copolymer chains and hence can provide information not currently available by more usual analytical techniques.
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