Thermodynamic symptom of coexistence of two aggregation modes in the Im3m cubic phase formed in thermotropic mesogen, ANBC(n)

The heat capacities of two members of a representative cubic mesogen, 4′-alkoxy-3′-nitrobiphenyl-4-carboxylic acid, were measured precisely. In accordance with the tendency reported previously for other members, the heat capacity in the cubic phase is smaller than that in the neighboring smectic C phase. The chain-length dependence of the reduction in the heat capacity upon the phase transition showed opposing trend depending on symmetry of the cubic phase (Ia3d or Im3m). This finding is rationalized by assuming the existence of two aggregation modes (rod and shell) in the Im3m phase in contrast to only the former mode in the Ia3d phase.