Thermotropic behavior of aromatic copolyesters based on 3,4′‐substituted diphenyl ether

Abstract

AbstractRecently, flexible rod‐like monomers based on conformational isomerism have been used to synthesize fully aromatic thermotropic polymers of reduced processing temperature and good thermal stability. 3,4′‐Dihydroxy and 3,4′‐dicarboxydiphenyl ethers HE and DE, obtained by modified Ullmann condensations between proper hydroxy and bromo derivatives, are investigated as basic components for homo‐ and copolyesters with the above‐mentioned characteristics. Homopolymers from HE and terephthaloyl chloride or DE chloride and 2‐methyl (or phenyl) hydroquinone, synthesized by low temperature solution polycondensation, melt to an isotropic fluid; the chain packing is frustrated in the latter by asymmetrical substitution on the aromatic rings, which causes a sequence randomization as well as an increased chain separation. The incorporation of linear rigid units, by partial substitution of the flexible monomers with 1,4 aromatic moieties, leads to thermotropicity, but the critical content of 1,4 units varies with the steric hindrance of the diphenol. Variation of chain rigidity, arising from the chemical structure, composition and sequence distribution, can account for the thermal behavior of the samples and, in particular, for their different abilities to give liquid crystalline behavior.