The Thermodynamic Behaviour and Miscibility of Discotic Liquid Crystals

This thesis is concerned with the self-organization of molecules that have disc-like shapes. The disc-like molecules may have relatively rigid structures as in polyaromatic systems, or they may have amphiphilic structures with polyaromatics at their cores, and soft outer shells made up of aliphatic chains. This research seeks to explore molecular compatibility and the ensuing structures formed by such disc- like systems through the study of phase diagrams. Thus, the thermodynamic behaviour and miscibilities of discotic liquid crystal materials were investigated by the formation of Gibbs phase diagrams and calculations using the Schröder-van Laar equation for liquid crystals that have structural common features.
Polyaromatics were reviewed and investigated as they are hard discs, and are as the central cores of the molecules that form discotic liquid crystals. Existing discotic materials, such as triphenylene hexa-esters, phenyl hexa-esters, phenyl hexa-alkynes, and rod-like compounds such as benzoate esters were studied and analysed using polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction where appropriate, for the preparations of mixture studies.
Mixture series made by triphenylene derivatives with each other, and with rod-like molecules or star-shape molecules were prepared and examined in order to investigate the potential co-miscibilities for both discotic nematic phase and hexagonal columnar phases. Mixture series made by triphenylene derivatives with polyaromatics were also prepared, and examined by POM and DSC to explore the virtual N-I phase transition temperatures for polyaromatics. These studies appear to show that nematic discotic materials are liquid crystals, whereas columnar materials exhibit 2D crystallinic soft solid phases. In addition, a novel new phase was found to form in the isotropic liquid of mixtures of polyaromatic materials, suggesting the possibility of the cubatic nematic phase being present.