ORIENTATIONAL ORDER IN LIQUID-CRYSTALS - EXPERIMENT AND THEORY - PLENARY LECTURE

The long range orientational order of a liquid crystal is one of its prime characteristics. Before the extent of this order can be measured, however, it is necessary to define what is meant by the orientational order. This can be achieved in a variety of ways the most pragmatic of which is via the order parameters; these vanish in the isotropic phase and are one for systems with complete order. The techniques available for the determination of the order parameters are described, with particular attention paid to deuterium NMR spectroscopy. The results obtained for rod-like mesogenic molecules are considered and the use of model systems, as studied by computer simulation, to understand them is discussed. Of particular significance is the relative role of anisotropic attractive and repulsive forces in determining liquid crystal behaviour. In recent years chemists have developed a new generation of liquid-crystalline materials in which the fundamental building blocks of rods, discs and chains are linked together in many ways. As an example the transitional behaviour of liquid crystal dimers is considered in detail. The orientational order for dimers with both odd and even spacers has been studied using NMR. The results of such investigations have prompted the development of a novel model for the molecular organisation in the nematic phase. Finally, the synergy of order and conformation for mesogens composed of the simplest flexible liquid crystal dimers is explored using computer simulation techniques.