Synthesis and characterization of a nematic fully aromatic polyester based on biphenyl 3,4′-dicarboxylic acid

Abstract

Melt acidolysis polymerization of hydroquinone with a kinked monomer, biphenyl 3,4′-bibenzoate, afforded a novel liquid crystalline polymer (LCP), poly(p-phenylene 3,4′-bibenzoate) (poly(HQ-3,4′BB)). Selection of hydroquinone diacetate (HQ_a) or hydroquinone dipivilate (HQ_p) facilitated either a tan or white final polymer, respectively. ¹H NMR spectroscopy confirmed consistent polymer backbone structure for polymers synthesized with either derivative of hydroquinone. Poly(HQ-3,4′BB) exhibited the onset of weight loss at about 480 °C, similar to commercially available Vectra® LCP. Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) revealed a glass transition temperature (T_g) of 190 °C and an isotropic temperature (T_i) near 330 °C. The observation of a melting temperature (T_m) depended upon the thermal history of the polymer. Wide-angle X-ray scattering (WAXS) and polarized optical microscopy (POM) confirmed the formation of a nematic glass morphology after quench-cooling from the isotropic state. Subsequent annealing at 280 °C or mechanical deformation induced crystallization of the polymer. Rheological studies demonstrated similar shear thinning behavior for poly(HQ-3,4′BB) and Vectra® RD501 in the power law region at 340 °C. Zero-shear viscosity measurements indicated that HQ_a afforded higher melt viscosities after identical polymerization conditions relative to HQ_p, suggesting higher molecular weights.