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 (δϵ) for the successive homologues (hexyl to undecyl) shows an alternation similar to that exhibited by the nematic-isotropic transition temperature (T _NI). The dispersion of ϵ_∥ 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-α-methyl-p'-cyanophenyl cinnamates (eighth to eleventh members). For both series the relaxation frequency (f _R) and the frequency of dielectric isotropy (f _o) evaluated at a common relative temperature show an alternation similar to T _NI. For any given compound the Arrhenius plots for f _R and f _o 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.