Synthesis and properties of thermotropic liquid crystalline copolyesters containing p-hydroxyphenylacetic acid and m-hydroxybenzoic acid units

A series of copolyesters based on p-acetoxybenzoic acid (p-ABA), m-acetoxybenzoic acid (m-ABA), and p-acetoxyphenylacetic acid (p-APAA) were prepared by a modified melt polycondensation reaction. The effect of linear p-ABA structure, kink m-ABA and flexible p-APAA moiety on the properties of copolyesters were systematically investigated. The copolyesters were characterized by DSC, thermogravimetric analysis, 1 H-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 110 degrees C, when the content of linear p-ABA units was over 55 mol%. DSC analysis of the anisotropic copolyesters revealed broad and weak endotherms associated with the nematic phases, and the melting temperatures were found to be in the processable region. The degree of crystallinity, thermal decomposition temperatures, crystal-to-nematic and nematic-to-isotropic transitions depend on the type of monomer units, and these transitions increased as the content of the rigid-linear p-ABA units increased, or as the content of the flexible/kink p-APAA/m-ABA units decreased