Characterization and mechanical strength of wholly aromatic liquid crystalline polymers with low melting point

A series of wholly aromatic copolyesters P-IPAx with low melting point (Tm) derived from hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), 4,4'-dihydroxy-biphenyl (BP), terephthalic acid (TPA) and isophthalic acid (IPA) was synthesized by melt polymerization. The copolyesters were characterized by Thermogravimetric Analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), polarized optical microscopy and mechanical strength measurements. The addition of BP/TPA resulted in a decrease in Tm for the copolyester from 280.7 to 218.7 °C compared to the parent copolyester P-HBA73 (HBA/HNA copolymer). As the amount of added IPA was increased, Tm decreased, and when the IPA concentration exceeded 5 mol%, the Tm peak in the DSC was not observed and the glass transition temperature was confirmed. The XRD analysis showed that the crystallinity decreased with increasing IPA concentration, but the ability to form a nematic liquid crystal phase was maintained based on observations by polarizing optical microscopy. In terms of the mechanical properties, there was a decrease in tensile strength but an increase in the modulus of elasticity. The copolymer with 10 mol% IPA could be molded at a relatively low temperature (about 200 °C). The addition of BP/IPA units improved the material processability while maintaining the characteristics typical of wholly aromatic LCPs (especially the modulus). It was also shown that the product could be processed at a lower temperature compared to commercially available wholly aromatic LCPs. Access to a wider range of processing temperatures will contribute to an expansion of applications for wholly aromatic LCPs.