MOLECULAR MODELING OF THE STRUCTURE OF THE COPOLYESTER PREPARED FROM P-HYDROXYBENZOIC ACID, BIPHENOL AND TEREPHTHALIC ACID

The X-ray data for melt-spun fibres of the 33/33/33 copolyester prepar ed from p-hydroxybenzoic acid, biphenyl and terephthalic acid are char acteristic of a completely random microstructure. Nevertheless, these copolymers adopt three-dimensionally ordered structures in the solid s tate, in which the chains are packed on hexagonal or orthorhombic poly morphic lattices. We have used molecular mechanics modelling to optimi ze the packing of random sequences in the higher-density orthorhombic form. The models consisted of 48 non-identical chains of nine monomers each. The random sequences were restricted to three monomers of each type, so that they had approximately the same length, making it possib le to apply a periodic boundary condition. The initial model had the e xtended chains in register, i.e. their central ester oxygens were in a plane perpendicular to the chain axis direction. This structure had h igh potential energy due to overlap between the non-identical sequence s. Energy minimization eliminated these bad contacts, at a cost of onl y similar to 1.8 kcal mol(-1) per monomer, by relatively small changes in the torsion angles at the phenylene-ester linkage bonds. These sma ll arrays also predict Bragg maxima on the equator and layer lines tha t match those observed. We conclude that non-identical chains can be p acked in a stereochemically acceptable manner according to the geometr y defined by the X-ray data. Copyright (C) 1996 Elsevier Science Ltd.