LIQUID-CRYSTALLINE AROMATIC POLYESTERS DERIVED FROM 2,5-THIOPHENE

We have synthesized new liquid crystalline aromatic polyesters composed of 2,5-thiophenedi-carboxylic acid and (substituted) hydroquinones as part of a continuing investigation of molecular structural effects on liquid crystallinity. Here we examine the effects of monomer (mesogenic core) nonlinearity on mesophase stability in liquid crystalline polymers. To this end, new polyesters containing 2,5-thiophenedi-carboxylic acid are contrasted with corresponding polymers synthesized from terephthalic acid and isophthalic acid. Polyesters containing 2,5-thiophenedicarboxylic acid and substituted hydroquinones form nematic melts; liquid crystallinity is not found in the corresponding isophthalic acid polyesters. The observed melting (softening) temperatures of the polyesters decrease systematically as the core angle-the angle subtended by the exocyclic bonds in the dicarboxylated aromatic rings-changes from 180-degrees (p-phenylene) to 148-degrees (2,5-thiophene) to 120-degrees (m-phenylene). Thermogravimetric analysis shows that the polyesters containing 2,5-thiophenedicarboxylic acid have thermal stabilities similar to those of polyesters containing terephthalic acid and isophthalic acid. These findings indicate that 2,5-thiophenedicarboxylic acid is a viable monomer for inserting nonlinear "defects" into polyesters while maintaining liquid crystallinity, thereby lowering the melting temperature to give tractable materials.