Effects of Mesogenic Cycloaliphatic Units on the Microstructure and Properties of Thermotropic Liquid Crystalline Polyesters

We report the synthesis and structure-property relationship of new thermotropic liquid crystalline polyesters (TLCPs) with different mesogenic cycloaliphatic unit content in the mainchain. For this purpose, a series of TLCPs composed of three different repeating units based on 4-hydroxybenzoic acid (HBA, 75.0 mol%), 2-hydroxy-6-naphthoic acid (HNA, 25.0-15.0 mol%), and terephthalic acid/1,4-cyclohexanedimethanol (TPA/CHDM, 0.0-10.0 mol%) were synthesized via two-step bulk polymerization. The structure, thermal and mechanical properties of the TLCPs were investigated by considering the effects of size and content of TPA/CHDM-derived repeating units, in comparison to HNA-based repeating units. Infrared spectroscopic data showed that TPA/CHDM-derived repeating units were successfully incorporated into HBA/HNA-based TLCPs. The polarized optical and electron microscopic images revealed the existence of thermotropic liquid crystalline features and associated fibrillar structures for all TLCPs. The melting temperatures of the TLCPs decreased with increasing the TPA/CHDM-based repeating units, whereas the glass transition temperature increased, which is due to the trade-off effect of the segmental mobility restriction in the amorphous phase and the destruction of the thermotropic liquid crystalline phase by the introduction of TPA/CHDM-derived repeating units in the TLCP mainchain. In addition, the maximum elastic storage modulus was attained for the TLCP with 2.5 mol% TPA/CHDM-derived repeating units.

Automated summary

We synthesized new thermotropic liquid crystalline polyesters (TLCPs) with different mesogenic cycloaliphatic unit content in the mainchain and investigated their structure-property relationship. The TLCPs were composed of three different repeating units and the effects of the size and content of TPA/CHDM-derived repeating units were compared to HNA-based repeating units. The TLCPs showed thermotropic liquid crystalline features and associated fibrillar structures. The melting temperatures of the TLCPs decreased with increasing TPA/CHDM-based repeating units, while the glass transition temperature increased. The TLCP with 2.5 mol% TPA/CHDM-derived repeating units exhibited the maximum elastic storage modulus.