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 Im 3 m, and the high-temperature one (CubII phase) is of Ia 3 d 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 I 1 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.