Dielectric Studies on Liquid Crystals of Strong Positive Dielectric Anisotropy

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-cyanobiphenyls, (ii) tras-p-n-alkoxy-α-methyl p'-cyanophenyl cinnamates and (iii) CBOOA. All these exhibit a very large positive dielectric anisotropy because of the C≡N end group. For all the compounds the average dielectric constant ε evaluated from the measured values of ε and is, throughout ε nematic phase, less than the extrapolated isotropic value. Further, ε decreases with deceasing temperature, the effect being greater in compounds whose nematic phase is preceded by a smectic phase. These facts are in conformity with the model of antiferroelectric short range order proposed by Madhusudana and Chandrasekhar. For the two homologous series (i) and (ii), the dielectric anisotropy does not show an alternation with increasing chain length, decreases continuously.