Introduction

Amolecule is said to be mesogenic if it is able to form liquid crystalline phases. However, the actual occurrence of liquid crystallinity depends on other factors as well. In thermotropic systems, the liquid crystalline phase only exists within a particular temperature range. It lies between the crystal melting point, Tm (or in cases where crystallinity is absent, the glass transition temperature, Tg), and the so-called upper transition temperature, T1c→i, where the liquid crystalline phase reverts to an isotropic liquid. When measured using the optical microscope, T1c→i is often referred to as the clearing temperature, due to the optical clarity of the isotropic phase.

This chapter focuses on the relationship between the chemical structure of mesogenic polymer molecules and the temperature range over which they can form liquid crystalline phases. Where appropriate, it also deals with the stability of the different classes of mesophase: nematic, smectic etc., which can occur in polymeric materials. However, a full discussion of the molecular arrangements within the different mesophases is reserved for Chapter 5.

It is helpful to view liquid crystalline polymers (LCPs) as being constructed from the same rigid mesogenic groups as form small molecule liquid crystals. Figure 3.1 (overleaf) illustrates some of the wide range of molecular architectures which can be created through the combination of mesogenic groups and other types of sequence within a polymeric system. The most basic construction, is simply to link the rigid mesogenic groups end to end to form a rigid polymer chain, known as a main-chain LCP.