ABSTRACT
We shall demonstrate the important role played by molecular motions in the appearance of a particular phase by exemplifying two cases: one is the successive phase transitions in the solid state arising from conformational disordering of the alkyl-moieties in discotic mesogens and the other is the reverse phase sequences between smectic-C (SmC) and cubic (Cub) mesophases found in cubic mesogens. The existence of solid-to-solid phase transitions with large entropy change seems to play a characteristic role in formation of the discotic and the isotropic mesophase. We found that the chain part of the cubic mesogens plays the same role as solvents do in lyo tropic liquid crystal systems, and that the chain also serves as an entropy reservoir. On the basis of entropy consideration, we conclude that the phase sequence is crucially determined by competing entropy effect between the terminal chain and the core parts of the molecule, and that the CubD and Cub phases would be identical in nature.
