Influence of measurement frequency on the pretransitional behaviour of the non-linear dielectric effect in the isotropic phase of liquid crystalline materials

Results are presented of the non-linear dielectric effect (NDE) as a function of temperature (T) in the isotropic phase on approaching the nematic phase for the liquid crystalline materials 4'-cyano-4-hexylbiphenyl (6CB), 5-heptyl-2-(4'-cyanophenyl)pyrimidine (HCPP) and the smectic A phase of 4'-cyano-4-decylbiphenyl (10CB). For low measurement frequencies (f) the NDE-1(T) exhibits linear behaviour from the clearing temperature (T-C) up to about T-C+50 K. Increase of f causes an increased deviation from this dependence near T-C. The influence of the measurement frequency can be associated with the link between the system time-scale, defined by the relaxation time of pretransitional processes(t tau), and the measurement time-scale (f(-1)). A quantitative agreement with the relation derived from the Landau-de Gennes model-NDE-1(T) analysis-and with the results of time-and frequency-resolved Kerr effect studies is shown. A possible influence of the intensity of the weak measurement field on the pretransitional effect in the immediate vicinity of T-C is discussed. Similarities between pretransitional behaviour in the isotropic phase of the nematogens and in the homogeneous phase of a critical binary solution are considered.