Liquid Crystalline and Monotropic Phase Behaviors of 2,3,6,7,10,11-Hexa(4‘-octyloxybenzoyloxy)triphenylene Discotic Molecules

Abstract

Multiple phase transitions and monotropic phase behavior are observed in a discotic liquid crystal, 2,3,6,7,10,11-hexa(4‘-octyloxybenzoyloxy)triphenylene (HOBT-C8). Detailed phase structural analyses using wide-angle X-ray diffraction and electron diffraction show that, in addition to a discotic nematic (ND) phase at high temperatures, a rectangular columnar (ΦR) phase and an orthorhombic crystalline (KI) phase developing sequentially upon cooling can be identified. A crystalline (KII) phase with a higher melting temperature than that of the KI phase forms during heating. Therefore, the KI phase represents a metastable phase with respect to the KII phase, and the more stable KII phase can be bypassed during cooling at relatively fast cooling rates. The KI phase is thus monotropic with respect to the KII phase at relatively slow heating rates. The optical texture of each phase can be distinguished from polarized light microscopy. A relationship between Gibbs free energy and temperature for different phases can be constructed, which thermodynamically and kinetically represents these multiple phase transitions and the monotropic phase behavior of the KI phase.