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-A BA), hydroquinone diacetate (HQDA), terephthalic acid (TPA), and m-acetoxyb enzoic acid (m-ABA) were prepared by a modified melt-polycondensation react ion. The copolyesters were characterized by DSC, thermogravimetric analysis , H-1 NMR, polarized optical microscopy, X-ray diffraction, and intrinsic v iscosity measurements. The copolyesters exhibited nematic liquid-crystallin e phases in a broad temperature range of about 150 degreesC, when the conte nt of linear (p-ABA, HQDA, and TPA) units was over 67 mol %. DSC analysis o f the anisotropic copolyesters revealed broad endotherms associated with th e nematic phases, and the melting or flow temperatures were found to be in the processable region. The flow temperatures and crystal-to-nematic and ne matic-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 in creased, as compared to other linear units (HQDA and TPA), the intrinsic vi scosity and degree of crystallinity of the copolyesters also increased, imp lying a higher reactivity for p-ABA in the p-ABA/HQDA/TPA/m-ABA polymer sys tem. The aromatic region in the H-1 NMR spectra of the copolyesters contain ing equal molar compositions of p-ABA, HQDA, and TPA units were sensitive t o the sequence distribution of aromatic rings. (C) 2001 John Wiley & Sons, Inc.