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, dn_o /dT, and extraordinary refractive index, dn_e /dT, of the liquid crystals are shown. Among the compounds studied, p-n-heptyl–p′-azoxybenzene produced a large value for dn_o /dT, due to its lower clearing temperature.

Keywords:

• order parameter
• molecular polarisability
• birefringence
• effective geometry parameter

Acknowledgements

The authors are grateful for the financial assistance provided by UGC Major Research Project F:34–12\2008 (SR) dated 30 December 2008, and for the UGC DRS Level III programme F.530/1/DRS/2009 (SAP–I), New Delhi, dated 9 February 2009.