Pretransitional behaviour in the vicinity of the isotropic–nematic transition of strongly polarcompounds

Abstract

The isotropic–nematic transition, being weakly first order, exhibits pretransitional effectssignifying the appearance of the nematic-like regions in the isotropic phase. In the isotropicphase, strongly polar liquid crystals, such as the popular alkyl and alkoxy cyano biphenylbehave in a non-standard fashion: whereas far away from the transition the dielectric constantεiso hasa 1/T dependence (a feature also commonly seen in polar liquids), on approachingthe nematic phase the trend reverses resulting in a maximum inεiso, at a temperature slightly above the transition, an effect explained on the basisof short-range correlations with an antiparallel association of the neighbouringmolecules. Recently, there has been a revival in studies on this behaviour to possiblyassociate it with the order of transition. Here we report dielectric measurementscarried in the vicinity of this transition for a number of compounds having differentmolecular structures including a bent core system, but with a common featurethat the molecules possess a strong terminal polar group, nitro in one case andcyano in the rest. Surprisingly, the convex shape of the thermal variation ofεiso wasmore an exception than the rule. In materials that exhibit such an anomaly we find a linear correlationbetween δε = (εpeak−εIN)/εIN and δT = Tpeak−TIN,where εpeak is the maximum value of the dielectric constant in the isotropic phase,εIN the value at thetransition, and Tpeak and TIN the corresponding temperatures.