Polarization Field and Molecular Order in Nematic Liquid Crystals

Abstract

In a nematic medium, as a result of the anisotropic distribution of the molecules, the average local electric field is of the form E+ _γP, where E is the applied field, P the polarization, and γ is a factor which is different for the directions parallel and perpendicular to the optic axis. For the nematic medium, the Neugebauer's relation is valid. Here, n_s and n_o are the principal refractive indices, N the number of molecules per unit volume and α_e and α_o are respectively the effective average polarizabilities along the optic axis and perpendicular to it. For the crystalline phase wherein the long axes of the molecules are parallel, a similar relation is there connecting the principal indices with α_∥ and α_⊥ the polarizabilities along the long axis and perpendicular to it. Using these relations along with the Lorenz-Lorentz relation for the liquid phase α_e, α_o, α_∥, α_⊥ and the γ and S-factors are calculated for PAA and PAP for different wavelengths and temperatures from the latest refractive index data of Germain. The values of γ and S are consistent. The S-factors are in good agreement with the values reported previously by Chandrasekhar and Madhusudana. The anisotropic molecular distribution in these compounds, as deduced from the values of γ, is discussed.