Role of Polymorphous Metastability in Crystal Formation Kinetics of 2,3,6,7,10,11-Hexa(4‘-Octyloxybenzoyloxy)-Triphenylene Discotic Molecules

Abstract

Phase transition behaviors of a discotic liquid crystal 2,3,6,7,10,11-hexa(4‘-octyloxybenzoyloxy)-triphenylene (HOBT-C8) were investigated. Four phases having different orders and symmetries existed in this system: a discotic nematic phase, a rectangular columnar (ColR) phase, an orthorhombic crystalline (CrI) phase developed upon cooling, and a monoclinic crystalline (CrII) phase formed during heating (Tang, B. Y., et al. Mater. Chem. 2001, 13, 78). The CrI phase represented a metastable phase compared to the CrII phase. On the basis of the results of differential scanning calorimetry, wide-angle X-ray diffraction, and polarized light microscopy (PLM) experiments, three crystallization-temperature (Tc) regions were identified to help study the crystallization kinetics of the CrI and CrII phases. In the large Tc region below 114 °C (region III), the CrI phase formed first despite its metastable nature (monotropic) due to the faster crystallization kinetics of the ColR → CrI phase in this region. The CrI → CrII phase transformation then followed. However, when the Tc ≥ 115 °C in region I, which was close to the melting temperature (Tm) of the CrI phase (Tm = 121 °C), only the CrII phase formed directly from the ColR phase. In the narrow Tc region II where 114 °C ≤ Tc < 115 °C, primary nucleation and overall crystallization rates of CrI and CrII phases were reversed. Linear crystal growth rate measurements in PLM revealed that a minimum in the CrII phase growth rate was found in region II. This indicated that as long as the growth rates of both phases were similar and on the same growth front, the CrII phase formation could be hampered by the growth of the metastable CrI phase.