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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.

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