Liquid‐crystalline copolyesters based on poly(p‐oxybenzoate) and poly(p,p‐biphenylene terephthalate)

Two copolymers composed of p‐hydroxybenzoate (PHB) and biphenylene terephthalate (BPT) with PHB/BPT ratios of 1/2 and 2/1 were characterized with respect to their tendency to exhibit liquid‐crystalline behavior in the melt phase. The BPT ‐rich copolyester, PHB/BPT = 1/2, displayed a birefringent melt phase of very high viscosity and no tendency to crystallize on cooling. The resulting fused material exhibited what apeared to be a second‐order transition at 170°C. The PHB‐rich composition, PHB/BPT = 2/1, also exhibited a highly birefringent melt phase of high viscosity which was quite shear sensitive. This polymer melt had little tendency to crystallize on cooling; however, on reheating no apparent second‐order transition could be detected. The observed phase changes were characterized by differential scanning calorimetry, hot‐stage microscopy, and wide‐angle x‐ray diffraction techniques. Additional data, pertaining to the compositional nature and apparent sequence distribution, were obtained by 13C‐NMR spectroscopy of the solid materials through magic‐angle spinning, dipolar decoupling, and cross‐polarization techniques.