Study of effective geometry parameter, order parameter, temperature gradients and crossover temperatures of three nematic liquid crystals having highly different clearing temperature

Abstract

The temperature dependent properties of three liquid crystals (LC) namely effective geometry parameter, order parameter and temperature gradients of refractive indices of two fluorinated isothiocyanato nematic liquid crystal (NLC) compounds 4′-pentylcyclohexyl-3,5-difluoro-4-isothiocyanatobiphenyl, 4'−butylcyclohexyl−3, 5− difluoro− 4− isothiocyanato-biphenyl of high clearing temperature (TC) and low TC LC compound 4 (trans-4/-n-hexycyclohexyl) isothiocyanatobenzene (6CHBT) have been studied. In this paper, the effective geometry parameters of the three LC compounds are calculated. The order parameters of three LC compounds are also calculated from the effective geometry parameter. From the available data, it is found that the two fluorinated NLC compounds exhibit moderately high value of birefringence (∼0.24–0.26) and 6CHBT exhibits a lower value of birefringence (∼0.16). From the study, it is found that the order parameter is found to be higher in higher birefringence and higher TC LC compounds. The values of temperature gradient of ordinary refractive index (dno/dT) are found to be showing negative to positive value while increasing temperature which gives the crossover temperature (TCO). The crossover temperature has been calculated from the temperature gradient at the value (dno/dT) = 0. LC compounds with lower TC are found to have larger TCO value which gives narrower temperature between TCO and TC. The TCO value of compound having larger alkyl group is found to be smaller. The Lorentz field factors for both compound are also reported which can be used for order parameter calculation by different aspects.