Thermotropic liquid crystal polymers

Abstract

Thermotropic liquid crystal polymers (LCP) are relatively new materials in the marketplace and represent one of the most exciting developments in engineering plastics. The liquid crystalline state is a unique condition in which long-range molecular orientational order persists in the absence of various types of short-range translational order. Liquid crystalline phases can be formed by dissolution of a liquid crystal material in a solvent (lyotropic behaviour) or by melting a liquid crystal material (thermotropic behaviour). These phases are divided into three broad categories: nematic, smectic and cholesteric (Fig. 7.1). The nematic phase consists of parallel molecules with no translational order along the molecular long axis and poorly defined translational order perpendicular to that axis (i.e. the order is one-dimensional). The smectic phase is a layered structure consisting of sheets of molecules with orientational order and translational order between the smectic layers (i.e. the order is two-dimensional). Many different types of smectics exist, displaying varying degrees of order both within an individual smectic layer and between layers. The cholesteric phase is a twisted nematic in which the order parameter (pointed along a molecular axis) exhibits a helical twist with a pitch of between 0.4 and several micrometres.