Elastic properties of the nematic phase influenced by molecular properties

Abstract

The intrinsic splay, twist, and bend deformations of nematic molecules are discussed. The intrinsic twist deformation leads to the cholesteric phase. On this molecular basis, the helical twisting power of cholesteric mixtures is discussed. The intrinsic splay and bend deformation cannot induce spontaneous deformation of a nematic liquid crystal but do influence the splay and bend elastic constants. Analogy is made between the change of the elastic constants and the electric orientation polarization produced by an electric dipole. The frequency dependence of the elastic constants is therefore similar to those of a dipole in a nematic surrounding. The splay elastic constant k11 has a low frequency relaxation (kHz-MHz), and the bend elastic constant k33 has a high frequency relaxation (MHz-GHz). The change of the elastic constants can be estimated for a realistic nematic molecule to be 10−6 dyn. The elastic constants can be described by two terms. The first one has some relation to the elasticity of ideal metals and the second one to rubber where the elasticity is controlled by the entropy.