Aromatic Liquid Crystalline Copolyesters with Low T_m and High T_g: Synthesis, Characterization, and Properties

摘要

In this study, a series of aromatic copolyesters P-BPAx with lower melting temperature and higher glass transition temperature derived from hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), bisphenol A (BPA) and terephthalic acid (TA) were synthesized via melt polymerization. The copolyesters were characterized by FTIR, solid state 13C NMR, DSC, TGA, polarized optical microscopy, X-ray diffraction, and rheometry measurements. With addition of BPA, the resulting copolyester's melting temperature decreased from 260 to 221℃ and its glass transition temperature increased from 70 to 135℃, compared with the parent copolyester P-HBA70 (HBA/HNA copolymer). With exception of copolyester P-BPA5.0 (225–280℃), the copolyesters could maintain liquid crystalline behavior in a broad temperature range from 230℃ to higher than 410℃. The ability to form nematic liquid crystalline phase disappeared when BPA concentration became higher than 15 mol %. X-ray diffraction analysis showed crystallinity decreased as the BPA content increased. A slightly distorted O" and a substantially distorted O′ orthorhombic phase was observed for P-BPA2.5. Upon annealing at 220℃, the O" phase disappeared and the O′ phase became stronger gradually. Rheology study data showed the ability to process the copolyesters improved in those compositions containing <2.5 mol % BPA. Continuing to increase concentrations of BPA, they became intractable.