V. F. Shabanov

Dielectric Properties of Liquid Crystals of the Cyano Derivative Compounds with Different Fragments in the Molecular Core

This paper reports on the results of investigations into the influence of variations in the chemical composition of the aromatic core of cyano-containing molecules of liquid crystals on their dielectric properties in the frequency range 1–2000 MHz. It is shown that the dispersion of the longitudinal permittivity is adequately described by the sum of two Debye processes with different weighting factors and relaxation times. The frequency dependence of the transverse permittivity is well approximated by the Debye process with a continuous distribution of relaxation times in a specified range. It is established that the replacement of one benzene ring in the biphenyl core of the 5CB liquid-crystal molecule by a cyclohexane (or bicyclooctane) fragment leads to a considerable decrease in both relaxation times for the longitudinal permittivity, a change in the low-frequency limit of the relaxation time range for the transverse permittivity, and the evolution of the frequency dependence of the dielectric anisotropy.

Dielectric anisotropy of 5CB liquid crystal in a decimeter wavelength range

Parallel and perpendicular components of the dielectric constant for 5CB nematic liquid crystal are measured in a relaxation region. Two frequencies f1≈30 and f2≈350 MHz are determined at which the inversion of the dielectric anisotropy sign is observed. It is shown that the observed sizable high-frequency shift of f2 with respect to the value calculated according the Debye theory is caused by the dielectric resonance near the relaxation region.

Temperature dependence of the dielectric characteristics of a 5CB liquid crystal within the relaxation region

The temperature dependence of the real part of the permittivity of a 4-n-pentyl-4-cyanobiphenyl liquid crystal was studied within a broad frequency range of 30–5000 MHz and in the temperature interval 20–60°C in the vicinity of the nematic-isotropic-liquid phase transition. It was established that the dispersion of the longitudinal component of the permittivity can be fitted well by a sum of two Debye terms with strongly differing relaxation times. The temperature and frequency dependence of the shorter relaxation time was determined for which the best fit between calculations and experiment was achieved.

DIELECTRIC ANISOTROPY OF NEMATIC 4-PENTIL-4′-CYANOBIPHENYL

Abstract The dielectric anisotropy and dispersion of the real part of permittivity of 4-pentil-4′-cyano-biphenyl (5CB) were investigated in the relaxation frequency range from 10 MHz to 500 MHz. Two frequencies f 1 = 30 MHz and f 2 = 350 MHz characterizing the change of a sign of the dielectric anisotropy were obtained. The Debye relaxation fitting procedure showed that a quality of the fits was satisfactory in defining the frequency f 1, but there were discrepancies between the experimental and fitting results for the frequency f 2. It is due to the appearance of singularities on the dielectric spectrum around 300 MHz. The dielectric spectra are interpreted as a superposition of dielectric relaxation and resonance contribution. It was shown that isotropic phase and diluted solution of 5CB in benzene display the resonance dispersion region too.

High-frequency dielectric spectra from liquid crystals of series nCB and nOCB

The design of a resonant frequency-tunable high-sensitivity microstrip sensor is suggested. The permittivity dispersion of liquid crystals of two homologic series, alkylcyanobiphenyls (7CB and 8CB) and alkyloxycyanobiphenyls (7OCB and 8OCB), is studied at frequencies of 100–900 MHz. The dielectric spectra are shown to be the sum of the Debye relaxation and dielectric resonances observed at f≈160, 280, 360, 450, 550, and 650 MHz. The dielectric resonances are present in the spectra of all the samples in both the nematic and isotropic phase. The substitution of an oxygen atom (series nOCB) for a carbon atom (series nCB) in liquid crystal molecules has a minor effect on the dielectric resonance frequencies but changes the resonance intensities and splits some of the resonance lines.

Dielectric properties of liquid crystals in polycapillary matrices

This paper reports on the results of investigations into the dielectric properties of liquid crystals embedded in polycapillary matrices and describes a technique for their measurement. It has been revealed that the chemical structure of the rigid core and the length of mobile alkyl groups of liquid-crystal molecules of the alkylcyanobiphenyl group substantially affect the equilibrium configuration of the liquid-crystal director in capillaries. The reorientation of liquid-crystal molecules embedded in capillaries in the nematic phase under the influence of an external magnetic field has been investigated.

Resonance Sensors for Measuring Dielectric Spectra of Liquid Crystals in a Wide Frequency Range

Designs of resonance sensors covering a wide frequency range of 1–104 MHz and intended for measuring the dispersion of the permittivity of liquid crystals are proposed. Using an example of a 4-methoxybenzyliden-4′-buthylanilin (MMBA) liquid crystal, it is shown that these sensors are characterized by a high sensitivity and allow measurements of temperature and field dependences of dielectric spectra of samples with volumes of ∼1 mm3. The behavior of dispersion of the MMBA permittivity observed experimentally is described by the Debye equation with a continuous spectrum of relaxation times within a certain range. A numerical method is proposed that allows reconstruction of the relaxation-time distribution function from the measured spectra of the real component of the permittivity. This distribution function represents the features of both the motion of molecules and the processes of intramolecular vibrations of movable fragments.

Specific features of the approximation of the dielectric spectra of alkylcyanobiphenyl liquid crystals

The frequency dependences of the longitudinal and transverse permittivities of oriented nematic liquid crystals belonging to the alkylcyanobiphenyl group nCB (n=5–8) are measured in the relaxation region in the meter and decimeter wavelength ranges. It is established that the dispersion of the longitudinal permittivity is well approximated by the sum of two Debye processes with different relaxation times. The frequency dependence of the transverse permittivity is represented by the dispersion relation with a continuous distribution of relaxation times in a specified time range. It is demonstrated that, in the high-frequency range (f>200 MHz), in which the dielectric spectra exhibit a number of weakly pronounced dispersion features, the total dispersion of the permittivity is adequately described by the sum of relaxation and resonance processes.

Reconstruction of the distribution function of relaxation times for 7CB and 7OCB liquid crystals from dielectric spectra

A technique for reconstructing the distribution function of relaxation times from the dielectric spectra measured over a wide range of frequencies is proposed and tested using 7CB and 7OCB liquid crystals as examples. The objective function, which is the sum of the squares of the differences between the calculated and measured permittivities, is minimized with the Mathcad program package. It is demonstrated that, in the case of parallel orientation of the molecular director with respect to the polarization of the ac electric field, the distribution function of relaxation times is consistent with the Debye relaxation model; however, there are insignificant deviations in the short-time interval corresponding to the high-frequency portion of the dielectric spectrum. For perpendicular orientation of the director, the distribution function of relaxation times exhibits specific features that are most pronounced for the 7OCB liquid crystal and probably reflect intramolecular motions of the rigid core and fragments of the alkyl chain.

Distribution function of relaxation times for a 4-n-pentyl-4′-cyanobiphenyl liquid crystal

The distribution function of relaxation times is reconstructed from the dielectric loss spectrum measured over a wide range of frequencies for a 4-n-pentyl-4′-cyanobiphenyl (5CB) liquid crystal. It is demonstrated that the distribution function for the isotropic and nematic phases is asymmetric in shape. Comparison shows that the reconstructed distribution function is in qualitative agreement with similar functions that are analytically derived from the Cole-Davidson and Havriliak-Negami empirical equations. The specific features observed in the behavior of the distribution function with a variation in the angle between the direction of polarization of a microwave electric field and the director of the liquid-crystal molecules are analyzed. A complex dependence of the permittivity for the liquid crystal in the range of ultrahigh frequencies is explained in terms of additional relaxation mechanisms associated with different motions of molecular fragments of alkyl chains.