CHARACTERIZATION AND ANALYSIS OF THE PHASE-BEHAVIOR OF POLY(1,4-PHENYLENE 2,5-DI-N-ALKOXYTEREPHTHALATE)S

The synthesis and characterization of two poly(1,4-phenylene 2,5-di-n-alkoxyterephthalate)s (n = 6 and 16) with defined end groups is described. These materials represent stiff-chain polyesters with flexible side chains which exhibit sufficient solubility in common organic solvents and a melting point far below thermal decomposition. Analysis of the intrinsic viscosities as a function of molecular weight shows that these polyesters are semiflexible with a Kuhn length of the order of 10(2) angstrom. The various solid and liquid crystalline phases have been investigated by thermal analysis and wide-angle X-ray scattering. Polyesters with short side chains (n = 6) exhibit a glass transition, asmectic-like, layered mesophase, and a nematic mesophase which transforms to an isotropic phase. The material with long (n = 16) side chains exhibits a side chain melting, a smectic-like mesophase, and a transition to the isotropic phase. The occurrence of a layered mesophase can be explained by a microphase separation of the main and side chains. It is shown that the transition temperatures of the liquid crystalline phases reach plateau values with increasing molecular weight as expected for the semiflexible main chain.