Effect of the ordered network polymer structure of cyclic-siloxane-type liquid crystalline epoxy thermosets on their fracture toughness and thermal conductivity

Liquid crystalline (LC) epoxy thermosets with a cyclic siloxane structure were prepared using two types of aromatic amine curing agents: 4,4’-diaminodiphenylethane (DDE) and p-phenylenediamine (p-PDA). The networked chain structure was investigated in detail via polarized optical microscopy, X-ray diffraction analysis, and transmission electron microscopy. The obtained epoxy thermosets formed an LC domain structure that was approximately 5 μm for the DDE system and 2–5 μm for the p-PDA system. In particular, the LC domains in the p-PDA system were composed of a highly aligned smectic phase. Compared with the DDE system, the p-PDA system had the highest modulus due to the packing structure of the smectic ordered network chains. The effect of the networked polymer structure on the fracture toughness was also investigated. High fracture toughness was observed with the formation of the oriented structures, and a maximum value of 1.15 kJ/m² was attained for the p-PDA system. The introduction of a flexible siloxane structure in the epoxy backbone slightly suppressed the increase in the thermal conductivity. However, the thermal conductivity of the p-PDA system was slightly greater (0.28 W/(m K)) because of the highly oriented structure.