Influence of measurement frequency on the pretransitional behaviour of the no-linear dielectric effect in the isotropic phase of liquid crystalline materials

Abstract

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 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) up to about C TC50 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, and the measurement time-scale defined by the relaxation time of pretransitional processes (tau),(f1). A quantitative agreement with de Gennes model NDE 1(T) analysis and with the results of time- and frequency-resolved the relation derived from the Landauof 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 TC is discussed. Similiarities between pretransitional behaviour in the isotropic phase of the nematogens and in the homogeneous phase of a critical binary solution are considered. (HCPP) and the