Phase transitions of 4′-n-hexacosyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-26):: Two types of thermotropic cubic phases

The phase transitions of 4'-n-hexacosyloxy-3'-nitrobiphenyl-4-carboxylic acid (ANBC-26, where "26" indicates the number of carbon atoms in the alkoxy group) were investigated by differential scanning calorimetric (DSC), polarizing optical microscopic, and X-ray diffraction (XRD) measurements. ANBC-26 is a new liquid crystalline material which exhibits successively two types of three-dimensionally ordered cubic mesophases on heating; the low-temperature cubic (CubI) phase has a space group of Im3m, and the high-temperature one (CubII phase) is of Ia3d type, identical with the cubic phase of shorter alkoxy chain homologues such as ANBC-16, and a cubic-cubic phase transition occurs at 435 K on DSC, Reversibility of the transitions between them was studied and the structures of those mesophases are discussed. The transformation from a one-dimensionally ordered smectic C (SmC) to the CubI phase and to the CubII phase can be interpreted qualitatively in terms of the increased curvature of the interface. The structured liquid Il phase, an intermediate phase between the CubII and a true isotropic liquid I-2 phases, has been also studied, indicating that this phase could be characterized by Ornstein-Zernike type short-range fluctuation, which means the presence of molecular clusters consisting of several ANBC-26 dimers in the temperature region.