Effect of the Polymerization Conditions on the Microstructure of a Liquid Crystalline Copolyester

The melt polycondensation of mixtures of sebacic acid (S), 4,4'-diacetoxybiphenyl (B), and 4-acetoxybenzoic acid (H), carried out for the synthesis of semiflexible liquid-crystalline copolyesters referred to as SBH 1 : 1 :x, has been studied with the aim of clarifying the effect of the reaction conditions on the microstructure and the thermal properties of the products. It has been shown that the segregation of a liquid-crystalline phase within the polymerizing mixture, coupled with the thermody namic tendency of the two phases to undergo compositional differentiation as polymerization proceeds, is responsible for the formation of blocky, rather than ideally random, copolyesters with poor processibility, when the mole ratio of H to the other two monomers is higher than x approximate to 1.90. The results of this study have shown that this unwanted effect can be considerably limited by carrying out the polycondensation at a relatively high temperature from the very beginning, rather than by the standard technique involving progressive heating of the reaction mixture, thus allowing the production of SBH copolyesters with a higher degree of aromaticity. The results are discussed in terms of the relative rates of the condensation reactions, which are responsible for chain growth, and of the concurrent acidolysis and esterolysis reactions leading to copolyester sequence reorganization. (C) 2000 John Wiley & Sons, Inc.