Effect of Flexible Chains on Thermal Conductivity of Liquid Crystalline Epoxy Resins

Liquid crystalline epoxy resins (LCEs) have attracted scientific attention due to their excellent physical properties, including thermal conductivity. In this study, we report biphenyl epoxy resins (BPERn)─a series of LCEs─and their intermolecular interaction depending on the network structure. Biphenyl derivatives of the liquid crystalline monomers were synthesized by introducing various lengths of alkyl chains and epoxy functional groups at both ends of side monomers and exhibited a mesomorphic orientation in the specific temperature range. The BPERn were prepared by hot compression molding, employing hexamethylene diamine as a curing agent. The curing behavior was monitored by polarizing optical microscopy and differential scanning calorimetry. The formation of an LC orientation was observed during the polymerization process. The thermal properties were optimized by using thermogravimetric analysis, differential scanning calorimetry, dynamic mechanical analysis, and thermal conductivity measurements. The microstructure was optimized by X-ray scattering analysis. The thermal conductivity of the BPERn increased with the flexible chain length, and BPER8 with the longest alkyl chain reached 0.50 W/m·K. The bulk specimens of BPERn exhibited a faster temperature change than conventional commercial samples when observed by an infrared thermal imaging camera. These results suggest improvement in the thermal conductivity of the LCE suitable for application in electronic materials.