Temperature dependences of dielectric dispersions in the homogenous and homeotropic alignment cells of 2-chloro-4-(4-pentylbenzoyloxy)benzoate (PCPB) were measured in the nematic phase in the frequency range between 100 Hz and 1 MHz. The obtained dielectric dispersions, ε$(ω) and ε$$(ω), could be explained using the Cole-Cole formula accounting for DC conduction. The parameter values in this formula were determined by the least-squares method. The frequency dependence of the dielectric anisotropy, including the dual-frequency characteristic, was well reproduced with our parameter values. The activation energy E a for molecular reorientation was estimated as approximately 9 ' 10 2 meV. The crossover frequency f c was approximately proportional to the relaxation frequency f r in PCPB.
Dielectric dispersions in the homogeneous (
) and the homeotropic (
) aligned cells in the nematic phase of PCPB/MBBA mixtures have been measured in the frequency range between 100 Hz and 1 MHz. The Cole–Cole formula accounting for DC conduction has been found to reproduce dielectric dispersions well in all of the mixtures. The crossover frequency
monotonously increases with increasing the MBBA concentration. It has been also found that the strengths of dielectric anisotropy both in the high and the low frequency regions are enhanced by adding of MBBA at least below 50 mol%. The dielectric anisotropy in the present system has been discussed qualitatively on the basis of the Landau-type free energy density.
The Freedericksz transition was investigated in a dual-frequency liquid crystal consisting of a mixture of 2-chloro-4-(4-pentylbenzoyloxy)benzoate and N-(4-methoxybenzylidene)-4-butylaniline, in which dielectric anisotropy strongly depends on the frequency of the applied ac electric field. Because the threshold voltage for the transition from an undistorted state of orientation to a distorted one is a function of the dielectric anisotropy and of the Frank elastic constants, the threshold voltage was expected to depend on the frequency. The threshold voltage increased steeply as the frequency approached the crossover frequency at which the dielectric anisotropy vanished and the threshold voltage eventually diverged. Furthermore, we examined the Freedericksz transition under ac voltages of two different frequencies.
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