Pretransitional behaviour in the vicinity of the isotropic-nematic transition of strongly polar compounds

The isotropic-nematic transition, being weakly first order, exhibits pretransitional effects signifying the appearance of the nematic-like regions in the isotropic phase. In the isotropic phase, strongly polar liquid crystals, such as the popular alkyl and alkoxy cyano biphenyl behave in a non-standard fashion: whereas far away from the transition the dielectric constant ε(iso) has a 1/T dependence (a feature also commonly seen in polar liquids), on approaching the nematic phase the trend reverses resulting in a maximum in ε(iso), at a temperature slightly above the transition, an effect explained on the basis of short-range correlations with an antiparallel association of the neighbouring molecules. Recently, there has been a revival in studies on this behaviour to possibly associate it with the order of transition. Here we report dielectric measurements carried in the vicinity of this transition for a number of compounds having different molecular structures including a bent core system, but with a common feature that the molecules possess a strong terminal polar group, nitro in one case and cyano in the rest. Surprisingly, the convex shape of the thermal variation of ε(iso) was more an exception than the rule. In materials that exhibit such an anomaly we find a linear correlation between δε = (ε(peak)-ε(IN))/ε(IN) and δT = T(peak)-T(IN), where ε(peak) is the maximum value of the dielectric constant in the isotropic phase, ε(IN) the value at the transition, and T(peak) and T(IN) the corresponding temperatures.