Introduction

Friedel (1922) classified liquid crystalline phases as nematic, cholesteric and smectic, and the structural basis for each of these classes has been set out for small molecule mesophases in Chapter 2 (see page 30). The nematic and cholesteric classes are characterised by intermolecular orientational correlations which are long range compared with molecular dimensions. The smectic class, which has many sub-divisions, allows for various types of positional correlation in addition to the long-range orientational correlations which are the basis of all types of liquid crystallinity.

The orientational order in liquid crystalline polymers (LCPs) is sufficiently similar to that seen in small molecule materials for the Friedelian classification to remain appropriate. However, there are several features peculiar to polymeric mesophases which raise new issues in the application of Friedel's scheme.

This chapter will consider the evidence for different types of liquid crystalline order in polymeric systems, and show how they can be classified within the conventional framework.

Nematic polymers

Principles of formation

As with small molecule liquid crystals the most simple polymeric mesophase is the nematic phase. The long axes of the mesogenic units mutually align in relation to a single director, but there is no long-range positional order which might contribute to a crystal lattice.

The simple rod model of the nematic phase introduced for small molecules (Fig. 5.1(a)) can be extended to main-chain polymers by using longer rods as shown in Fig. 5.1(b). The diagrams suggest that the distribution of orientations about the director will be more restricted in the polymer case so that the quality of the alignment will be better.