The variation of refractive indices with temperature of three pure nematogenic compounds 5OCB, CPHB and 6CHBT were studied experimentally using thin prism technique. The refractive indices no and ne were measured using 633 nm lines from a HeNe laser. A modied four-parameter model was taken which is based on the Vuks equation describing the eect of temperature on the refractive indices of liquid crystals. In this paper we report the variation of refractive indices of these three liquid crystals with temperature, using the thin prism method. The variation in birefringence was determined experimentally. The variation in refractive indices and birefringence were also tted theoretically using the modied Vuks equation. On comparison, the theoretically tted values show close agreement with the experimental values.

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Two fluorinated isothiocyanato nematic liquid crystalline compounds, 4 0 -butylcyclohexyl-3, 5-difluoro-4-isothiocyanatobiphenyl and 4 0 -pentylcyclohexyl-3, 5-difluoro-4-isothiocynatobiphenyl are studied in detail to obtain their different physical parameters. Optical polarizing microscopy, differential scanning calorimetry, density and dielectric studies have been carried out for the two samples. Both the samples were found to have high clearing temperature ([100°C) and exhibit small enthalpy of transition. The two samples exhibit high optical birefringence (Dn [ 0.2). The values of order parameters for the two samples were obtained using different approaches, namely, Vuks', Neugebauer's, modified Vuks' and direct extrapolation method from birefringence data. Experimentally obtained values of order parameters have also been compared with theoretical Maier-Saupe values. The parallel and perpendicular components of dielectric permittivity values of the two compounds were also calculated and their anisotropy values were found to be small. The effect of temperature on the molecular dipole moment l and the angle of inclination b of the dipole axis with the director have also been investigated in this work.

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