Phase structure identification, monotropic phase behavior, crystallization kinetics, and surface effect of discotic liquid crystals

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 (phiR) phase and an orthorhombic crystalline (KI) phase developing sequentially upon cooling can be identified. A crystalline (KII) phase with a higher melting temperature than 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 hearing 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.;To further clarify phase transformation kinetics, systematic analyses using differential scanning calorimetry (DSC), polarized light microscopy (PLM), and wide angle X-ray diffraction (WARD) were carried out. Depending on different isothermal temperatures, the crystallization of the discotic LC HOBT-C8 can be divided into three individual temperature regions. In Region III where the crystallization temperatures are low, the metastable K I phase is dominant before it transforms to the stable KII phase. In Region I where the crystallization temperatures are high, only the stable KII phase develops. Region II is located at temperatures between Regions I and III, where crystallization competition between the K I and KII phases exists and an appearance of the KII phase first followed by the KI phase is observed. Evidences corresponding to the three-region crystallization behavior with emphasis on the competition region obtained from DSC, PLM, and WARD are discussed in detail. Avrami analyses show the primary nucleation type of both the KI and KII phases changes from athermal to thermal as the crystallization temperature increases, which agrees well with the PLM observations.;On a rubbed polyimide alignment layer, the discotic LC HOBT-C8 exhibits uniform homeotropic orientation (flat-on, disk lying down on the surface) in the ND phase, comparing to the normal Schlieren texture on glass surface. Upon the application of an external electric field, HOBT-C8 shows a certain degree of dielectric anisotropy in the ND phase.