Investigations into effective order geometry in a series of liquid crystals

Abstract

The effect of temperature variations on refractive index and molecular polarisability have been studied in liquid crystals based either on p,p′-alkoxyazoxybenzene derivatives, in which the alkyl group is n-pentyl or n-heptyl, or on p-alkoxybenzoic acids, in which it is n-hexyl or n-nonyl. The order parameter has been calculated using the Vuks, Neugebauer and Wu-modified equations, and also from birefringence measurements. In addition, an order parameter equation involving the effective geometry parameter has been derived and its influence on the deflection of light by the liquid crystalline compounds studied. At low temperatures, and at high values of order parameter and birefringence, the effective geometry parameter was low. In the light of the results obtained, a comparison of the order parameter derived by various methods is presented. The temperature variation of temperature gradient of the ordinary refractive index, dno /dT, and extraordinary refractive index, dne /dT, of the liquid crystals are shown. Among the compounds studied, p-n-heptyl–p′-azoxybenzene produced a large value for dno /dT, due to its lower clearing temperature.