Aromatic liquid crystalline copolyesters with low T m and high T g : Synthesis, characterization, and properties

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 13 C 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°C and its glass transition temperature increased from 70 to 135°C, compared with the parent copolyester P‐HBA70 (HBA/HNA copolymer). With exception of copolyester P‐BPA5.0 (225–280°C), the copolyesters could maintain liquid crystalline behavior in a broad temperature range from 230°C to higher than 410°C. 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°C, 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. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131 , 40487.