Synthesis and structure-property relationship of thermotropic liquid crystalline polyesters containing thiophene-aromatic and cycloaliphatic moieties

We report the structure and property characterization of a series of thermotropic liquid crystalline polyesters (TLCPs) composed of three different repeating units based on 1,4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), and 2,5-thiophenedicarboxylic acid/1,4-cyclohexanedimethanol (TDCA/CHDM), which are synthesized through two-step melt polymerization. The effect of the TDCA/CHDM-based repeating unit (0.0-10.0 mol%) on the structure, thermal and mechanical properties of the TLCPs with 75.0 mol% HBA and 25.0-15.0 mol% HNA were investigated. The polarized optical and electron microscopic images confirmed the presence of thermotropic liquid crystalline and fibrillar structures for all synthesized TLCPs. The glass transition and melting temperatures as well as the crystallinity index of the TLCPs were found to increase with increment of the TDCA/CHDM unit up to 5.0 mol% due to the improved interchain packing, but they decreased at higher amounts of 7.5-10.0 mol% TDCA/CHDM units owing to the enhanced flexibility by the cyclohexylene ring and the mismatching in the length between HNA and TCDA/CHDM units. In addition, there was a crystal lattice transition of TLCPs from pseudo-hexagonal to orthorhombic phases. Although the thermal decomposition temperature and elastic storage moduli of the TLCPs decreased with the amount of the TDCA/CHDM unit, all TLCPs remained stable up to 300 degrees C and had excellent elastic moduli above 4.1 GPa at 50 degrees C.

Automated summary

This study reports the structure and property characterization of a series of thermotropic liquid crystalline polyesters (TLCPs) composed of different repeating units, and investigates the effect of TDCA/CHDM-based repeating unit on the structure, thermal and mechanical properties of the TLCPs. It is found that the TDCA/CHDM unit affects the glass transition temperature, melting temperature, and crystallinity of the TLCPs, but beyond a certain amount, these properties decrease. Additionally, TLCPs exhibit a crystal lattice transition and remain stable at high temperatures with good elastic moduli.