ABSTRACT:The liquid crystal behavior of the thermotropic polyester derived from (4,4'-terephthaloyldioxy)dibenzoic acid and tetraethylene glycol was studied by differential scanning calorimetry, polarizing microscopy, and X-ray diffraction. The polymer sample exhibits the phase sequence isotropic-nematic-smectic A-smectic E-crystal, with decreasing temperature. Attention was devoted to the optical textures of the mesophases, as compared to those of low molar mass mesogens, and unusual high strength singularities were detected in the schlieren texture of the nematic phase.
KEY WORDSLiquid-Crystalline Polymers/ Thermotropic Polyesters/ Liquid-Crystalline Polymorphism/ Smectic Mesophases / Schlieren Textures/ glycol ethers of different lengths:The thermotropic liquid-crystalline behavior of main chain polymers comprising alternating mesogenic units and flexible spacers is well established and attracts ever increasing interest. The great majority of such polymers presents only one mesophase, typically nematic ( or cholesteric ), but smectic phases have also been observed, including ordered smectic B 1 , E 2 , G 3 or H 4 phases. A few examples are known of mesophase polymorphism5, this feature being in marked contrast with what is most commonly observed in low molecular weight mesogens.In a previous paper 6 some of us reported on the synthesis and characterization of the thermotropic properties of a class of polyesters derived from 4,4' -(terephthaloyldioxy)dibenzoic acid (HTH) and ethylene (or propylene) Polymer samples having n = 2, 3, or 4 displayed two different enantiotropic liquidcrystalline phases, the lower temperature mesophase being a provisionally unidentified smectic phase and the higher temperature mesophase being recognized as nematic. 6 In the same context, unusual textural behaviors were observed in such polymers, especially at the mesophase-mesophase transition. All these features have stimulated further investigation and accordingly the polyester sample of that class with n = 4, here designated as HTH/TEG, was freshly prepared from HTH diacid chloride and tetraethylene glycol (TEG) and t To whom correspondence should be addressed.