Dielectric studies on liquid crystals of strong positive dielectric anisotropy

Abstract

The static dielectric constants of the following compounds have been measured as functions of temperature in their nematic and isotropic phases: (i) 4'-n-alkyl-4-cyanobiphyenyls, (ii) trans-p-n-alkoxy-agr-methyl pacute-cyanophenyl cinnamates and (iii) CBOOA. All these exhibit a very large positive dielectric anisotropy because of the CequivN end group. For all the compounds the average dielectric constant b.epsi evaluated from the measured values of b.epsi par and b.epsipar is, throughout the nematic phase, less than the extrapolated isotropic value. Further, b.epsi decreases with decreasing temperature, the effect being greter in compounds whose nematic phase is preceded by smectic phase. These facts are in conformity wiht the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. For the two homologous series (i) and (ii), the dielectric anistropy does not show an alternation with increasing chain length, but decreases continuously.