Principal Polarizability and Orientational Order Parameter Study of some Pure Cholesteric Liquid Crystals and their Homogeneous Mixtures: Phase Transition Behaviour

Abstract

Objectives: To study the behavior of the molecular polarizabilities (α e , α o ) and order parameter (S) in various phases of Cholesteryl Propionate, Cholesteryl Benzoate and their homogeneous mixtures of weight fractions 0.25, 0.50 and 0.75. Methods: The measurement of extraordinary refractive index (n e ) and ordinary refractive index (n o ) was carried out with the help of Abbe's refractometer and modified wedge method, in the temperature range of 85 0 C-180 0 C. The anisotropic internal field model is used to calculate the internal field factor (a) and order parameter (S). Findings: The decrease in the ratio of the principal polarizabilities (α o /α e )with increase in temperature has been observed. At the transition points of solid to cholesteric (S-Ch transition) and cholesteric to isotropic (Ch-I transition), the decrease in the ratio of principal polarizabilities is much sharper for both Cholesteryl Benzoate and Cholesteryl Propionate and also for their mixtures. The value of the order parameter (S) remains constant in solid phase and sharp decrease occurs in its values for pure samples as well as their mixtures, when materials go from solid to cholesteric phase (S-Ch transition). After the S-Ch transition, the order parameter (S) goes on decreasing with further increase in temperature until the cholesteric to isotropic transition (Ch-I transition) occurs. Novelty: Some researchers have calculated the principal polarizabilities and order parameter of optically positive nematic liquid crystals and aligned cholesteric liquid crystals by using isotropic internal field model such as Vuk's approach. This itself raises the question of how can an anisotropic environment give rise to isotropic field. The present study is based on the anisotropic field model suggested by Maier and Saupe in which the internal field acting on a molecule is considered to be anisotropic. This approach used to calculate principal polarizabilities and order parameter is more realistic and gives more accurate results.