Static dielectric properties of two nematogenic compounds and their binary mixtures showing induced smectic Ad and re-entrant nematic phases

Abstract

Static dielectric permittivity measurements on a binary system of 4-cyanophenyl [4′ (4″-n-heptylphenyl)] benzoate (referred to as 7CPB) + 4-cyanophenyl 4-nonylbenzoate (referred to as 9.CN) showing both induced smectic Ad and re-entrant nematic phases in a certain concentration range (0.4<x9.CN<0.87) are being reported. The temperature variation of permittivities at the nematic–smectic Ad and smectic Ad–re-entrant nematic transition for all the mixtures are found to be continuous. In the case of the mixture with x=0.662, the parallel component of dielectric permittivity shows a pronounced decrease in the smectic Ad phase due to dipole–dipole correlation. A similar effect is seen in other mixtures but on a reduced scale. Mesogenic molecular dipole moments μ and their inclination angles β with the molecular long axis have also been determined following the Maier and Meier equations. The effective values of the dipole moments (μeff) in the mixtures are found to be less than those observed in the pure compounds. Moreover, μeff in the smectic Ad phase is seen to be slightly smaller than that obtained by extrapolation from either the nematic or the re-entrant nematic phases. The lower effective dipole moment in the smectic Ad phase indicates the development of additional anti-parallel ordering of the molecular dipoles in the induced smectic phase.