Hindered Rotation as the Cause of Change in Polarizability Tensor Scalar of Mesogenic Molecules

Abstract

In investigations of molecular order parameters S = ½⟨3 cos²θ − 1⟩ for liquid crystal materials, it is usually assumed that the scalar of the polarizability tensor α_‖ + 2α_⊥ = α_s of a molecule is constant. In this work the temperature dependence of the scalar of the polarizability tensor of low molecular weight liquids and of liquid crystal materials in their nematic and isotropic phases was calculated using their densities and refractive index data. The polarizability tensor scalar of mesogenic molecules is a nonlinear function of temperature, suggesting the existence of hindered rotation in the molecules in liquid crystals. The densities and refractive indixes of the low molecular weight liquids were measured, while those for the liquid crystal materials were taken from the literature. A new parameter was introduced, which gives information about the hindered rotation of one part relative to another part of the molecule in a liquid crystal. It was found that there are two causes of change in the polarizability tensor scalar of a liquid crystal molecule.