An experimental and theoretical study of liquid crystal phenomena

The work presented in this thesis is concerned with the study of orientational order and mesophase behaviour of thermotropic liquid crystals by means of both theory and experiments. The theory used in this thesis is mainly molecular field theory. The most important experimental technique used in this work is nuclear magnetic resonance although optical microscopy and DSC were employed as well.A comprehensive review of liquid crystals and deuterium NMR is outlined in chapter 1. Chapter 2 deals with the pretransitional and orientational behaviour of nematogenic binary mixtures. The experimental results of a no-equilibrium system have been understood using a layer-structure model. Chapter 3 presents the results of deuterium NMR study of the dramatic odd-even effect of trimeric liquid crystal homologous series. These trimers are novel model compounds used to understand the profound odd-even alternation in the liquid crystalline properties, such as the liquid crystal to isotropic transition temperature and the entropy change of the transition, of main chain liquid crystal polymers. A theoretical exploration of a biaxial nematic phase and its phase transition behaviour is described in chapter 4. A range of different molecules are predicted to show a direct phase transition between a biaxial nematic to an isotropic phase. The corresponding experimental study of 2,3,4-trihexyloxy cinnamic acid which has been claimed to exhibit a biaxial nematic phase is discussed in chapter 5. (DX192697)