PHASE STRUCTURES AND TRANSITION BEHAVIORS OF A TRIPHENYLENE DISCOTIC LIQUID CRYSTAL

 The phase behaviors and structures of a triphenylene-derived discotic liquid crystal (LC) hexa-n-octoxyl-triphenylene (C8HET) were studied using the combined techniques of differential scanning calorimetry (DSC), wide angle X-ray diffraction (WAXD), selected area electron diffraction (SAED) and polarized light microscopy (PLM). One-dimensional (1D) powder WAXD results at different temperatures coupled with DSC and PLM observations revealed that the C8HET compound possessed an LC phase and three different crystalline (K₃, K₂ and K₁) phases below the isotropic (I) melt. The I ↔ LC phase transition was thermodynamically reversible and independent of the heating and cooling rates. The development and experimental observation of the three crystalline phases relied on different thermal histories. Among the three crystalline phases in C8HET, the K₃ phase is the most stable phase, while the K₂ and K₁ phases are metastable. Note that the K₁ phase only formed via a quenching process. On the basis of structure sensitive diffraction experiments such as 2D WAXD of oriented samples and SAED of single crystals, detailed structures and molecular packings of these four ordered phases were identified. The LC phase exhibited a hexagonal columnar phase with 2D lattice dimensions of a = b = 2.38 nm and g = 120°. All the three crystalline phases possess monoclinic unit cells, yet the g angle is not 90° in the cases of the K₂ and the K₃ phases, while in the case of the K₁ phase the a angle is not 90°.