Synthesis and characterization of fully aromatic thermotropic liquid-crystalline copolyesters containing m-hydroxybenzoic acid units

A series of fully aromatic copolyesters based on p-acetoxybenzoic acid (p-ABA), hydroquinone diacetate (HQDA), terephthalic acid (TPA), and m-acetoxybenzoic acid (m-ABA) were prepared by a modified melt-polycondensation reaction. The copolyesters were characterized by DSC, thermogravimetric analysis, H-1 NMR, polarized optical microscopy, X-ray diffraction, and intrinsic viscosity measurements. The copolyesters exhibited nematic liquid-crystalline phases in a broad temperature range of about 150 degreesC, when the content of linear (p-ABA, HQDA, and TPA) units was over 67 mol %. DSC analysis of the anisotropic copolyesters revealed broad endotherms associated with the nematic phases, and the melting or flow temperatures were found to be in the processable region. The flow temperatures and crystal-to-nematic and nematic-to-isotropic transitions depend on the type of linear monomer units, and these transitions increased as the content of the p-ABA units increased, as compared to the HQDA/TPA units. When the content of the p-ABA units increased, as compared to other linear units (HQDA and TPA), the intrinsic viscosity and degree of crystallinity of the copolyesters also increased, implying a higher reactivity for p-ABA in the p-ABA/HQDA/TPA/m-ABA polymer system. The aromatic region in the H-1 NMR spectra of the copolyesters containing equal molar compositions of p-ABA, HQDA, and TPA units were sensitive to the sequence distribution of aromatic rings. (C) 2001 John Wiley & Sons, Inc.