Dielectric properties of some nematics of positive dielectric anisotropy

Abstract

The static dielectric constants of biphenyl-4-p-n-alkoxybenzoates have been measured as functions of temperature. The dielectric anisotropy (b.deltab.epsi) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (TNI). The dispersion of b.epsipar in the frequency range of 0.1-15 MHz has been studied for four successive members of biphenyl-4-p-n-alkoxybenzoates (heptyl to decyl derivatives) as well as of trans-p-n-alkoxy-agr-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (fR) and the frequency of dielectric isotropy (fo) evaluated at a common relative temperature show an alternation similar to TNI. For any given compound the Arrhenius plots for fR and fo give the same value of the activation energy (W). In the case of the biphenyl esters the vaue of W is the same for the seventh and eighth members, but increases for the higher members which exhibit a smectic phase at lower temperatures. On the contrary for cinnamic acid esters W is found to be the same for all homologues irrespective of whether the nematic phase is preceded by a smectic phase or not.