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 ac id (ANBC-26, where "26" indicates the number of carbon atoms in the alkoxy group) were investigated by differential scanning calorimetric (DSC), polar izing optical microscopic, and X-ray diffraction (XRD) measurements. ANBC-2 6 is a new liquid crystalline material which exhibits successively two type s of three-dimensionally ordered cubic mesophases on heating; the low-tempe rature cubic (CubI) phase has a space group of Im3m, and the high-temperatu re one (CubII phase) is of Ia3d type, identical with the cubic phase of sho rter alkoxy chain homologues such as ANBC-16, and a cubic-cubic phase trans ition occurs at 435 K on DSC, Reversibility of the transitions between them was studied and the structures of those mesophases are discussed. The tran sformation from a one-dimensionally ordered smectic C (SmC) to the CubI pha se and to the CubII phase can be interpreted qualitatively in terms of the increased curvature of the interface. The structured liquid Il phase, an in termediate 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.