Introduction

In the previous chapter, the design of mesogenic molecules was examined in some detail with a view to understanding the relationship between chemical structure and liquid crystalline stability. However, just as it is important to relate properties to the chemical detail, it is of equal significance to be able to determine the stability of a mesophase on the basis of simple schematic representations of the molecules – the simpler the model, the more likely it is to be attractive to the theorist and it will have the great advantage of exposing the essence of any situation. For, in finding just those few features which are really necessary to make the model work, one is at the same time identifying them as the key elements.

The theories described in this chapter focus on one issue: namely whether a given type of molecule will lead to an isotropic or a liquid crystalline state. The transition between the two states is examined in detail as a function of temperature and/or solvent content. Theory to predict either the crystalline melting point or the glass transition temperature is not included, for the main reason that it has yet to be developed much beyond the empirical stage. In most cases, the anisotropic phase formed will be nematic. However, there is little difference in energetic terms, between a nematic phase and a cholesteric phase, so most of the theories can equally be applied to the cholesteric–isotropic phase transition.