Orientational Order Parameter of a Binary Liquid Crystal Mixture Showing Induced Smectic Phase

Abstract

Binary mixture of 4 - pentyl 4′ - cyanobiphenyl (5CB) and 4-n-pentyl phenyl 4 - n′ - pentyloxy benzoate (ME50.5) shows the presence of an induced smectic A phase. In the present work we report the density and refractive indices on this system in the entire composition range. The density and refractive indices are used to calculate molecular polarizability values parallel and perpendicular to the molecular long axes using Neugebauer's method. Orientational order parameters (OOP's) as a function of temperature and concentration are determined. The relative change in density at the smectic A-nematic phase transition shows a maximum near equimolar concentration. The orientational order parameter values and calculated packing fraction show a minimum around X ≈ 0.4 at a temperature T=35°C. Similar behaviour is also observed at a temperature T = 0.98TC. in the nematic phase where the induced smectic A phase is the precursor of the nematic phase. Average lateral distance (D) between the molecules is measured from the analysis of x-ray diffraction data. The variation of D with molar concentration in smectic A phase shows a maximum near equimolar concentration.