Thermotropic copolyesters of 4-hydroxybenzoic acid

Copolyesters of 4-hydroxybenzoic acid were prepared by thermal polycondensation of 4-acetoxybenzoic acid with various acetylated comonomers, such as 4-mercaptobenzoic acid, 3-hydroxybenzoic acid, N-(4-carboxyphenyl)4-hydroxyphthalimide or hydroquinone in combination with 1,12-dodecane bistrimellitimide. The copolyesters were characterized by elemental analyses, inherent viscosities, ¹H- or ¹³C NMR spectroscopy, DSC-measurements, WAXS-measurements at temperatures between 25 and 425°C, microscopic observation, thermomechanical and thermogravimetrical analyses. Copolyesters containing 4-mercaptobenzoic acid or N-(4-carboxyphenyl)4-hydroxyphthalimide possess a reversible first order phase transition which represent a change of modifications and not a melting process. Both classes of copolyesters adopt the same kind of high temperature modification as poly(4-hydroxybenzoate), namely a pseudo-hexagonal chain packing. In contrast, copolyesters of 3-hydroxybenzoic acid or copoly(ester imide)s with aliphatic spacer containing more than 50% 4-hydroxybenzoic acid form a nematic phase over a broad temperature range. However, in the case of copolyesters derived from 3- and 4-hydroxybenzoic acid the homogeneous nematic melt is thermodynamically unstable and gradually turns into a heterogeneous more or less crystalline material with blocks of 4-hydroxybenzoate units.