Dynamics of Rotational Motion in Liquid Crystalline Systems

Abstract

Liquid crystals combine the anisotropic optical properties, as they are typical for birefringent crystals, with the viscous behaviour of liquids [1, 2]. Application of these materials is based on this specific pattern of properties. Well known, and of great technological importance is the use of nematic liquid crystals in modern displays. Here electrical fields are applied to induce local changes in the orientation of the optic axis. Switching times should be as short as possible and usually are in the order of 10–100 ms. The decisive parameter is the rotational viscosity γ, which describes the ratio between the torque acting on the director (i.e. the optic axis) and its angular velocity. For displays low values of γl are desirable. In a different range of applications nematogenic materials are used to prepare optical components with specific permanent birefringence profiles. Principally this can be achieved by setting up in the nematic phase a specific director field and then fixing it by a quench into the glassy state. This procedure is now gaining special importance in attempts to produce components with nonlinear optical properties. Liquid crystalline polymers, composed of mesogenic groups which are laterally attached to a flexible backbone chain, constitute a class of materials which is convenient for this purpose [3, 4]. These “LC-side group polymers” usually show a glass transition at temperatures above room temperature, so that optical structures prepared in the nematic phase can be fixed by a quenching and then remain stable at ambient temperature.