Grzegorz Czechowski

On the molecular interpretation of the dielectric relaxation of nematic liquid crystals

This paper presents the results of studies of the dielectric relaxation of nematic 6CHBT obtained for different values of the angle between the directions of the macroscopic orientation of the sample (director n) and the probing electric field E. Analysis of the evolution of the relaxation spectrum from e*‖(ω) (E‖n) to e*⊥(ω) (E⊥n)allows one to explain the hitherto existing inconsistency in the molecular interpretation of the spectra. A model of the molecular dynamics in the oriented nematics is proposed.

Dielectric and viscous properties of 6CHBT in the isotropic and nematic phases

This paper presents the results of measurements of the principal electric permittivities ε ∥*(T, ω and ε⊥*(T, ω) and the viscosity for 4-(trans-4-n-hexylcyclohexyl)isothiocyanatobenzene (C6H13-CyHx-Bz-N=C=S, 6CHBT). In the nematic phase, the Miesowicz η2 viscosity coefficient was measured in a sample oriented due to the flow. On the basis of the temperature dependence of the static permittivities, using the Maier-Meier equations, the angle β between the dipole moment vector and the long axis of the 6CHBT molecule, the square of the molecular apparent dipole moment μ2 app 2 and the nematic order parameter S(T), were determined. From the temperature dependence of the viscosity and the relaxation time corresponding to the molecular rotation around the short axis, the strength of the nematic potential and the effective length of the 6CHBT molecule (in the isotropic phase) were estimated.

The shear viscosity minimum of freely flowing nematic liquid crystals

The analysis of temperature dependence of the shear viscosity of freely flowing nematic liquid crystals shows that the anisotropic fluid adopts such a manner of flow, which corresponds to the minimum of its viscosity at given conditions. We postulate that the principle is of general significance, i.e. it concerns any fluid in the flow.

Dielectric studies of nematic-smectic A-reentrant nematic transition in the 8OCB/6OCB mixtures

Abstract The results of measurement of the anisotropies of the electric permittivity and conductivity for pure 8OCB and 6OCB together with their mixtures are presented. It was found that (i) the dielectric properties of mesophases composed of molecules with a strong tendency to antiparallel association in the nematic phase (8OCB) and molecules which do not exhibit such a tendency (6OCB) undergo a drastic change in mixtures with a relatively low concentration of 6OCB, (ii) the effect of smectic density modulation in the N-SA-NR sequence is very subtle, and (iii) for concentrations higher than a critical value the nematic phase exhibits a smectic-like ordering which vanishes with increasing concentration of 6OCB.

Dielectric relaxation processes in the nematogen 4-cyanophenyl 4-n-heptylbenzoate

Experimental studies of the dielectric relaxation in the nematic and isotropic phases of 4-cyanophenyl 4-n-heptylbenzoate have been performed in the frequency range from 1 kHz to 1 GHz. In the nematic phase, measurements of the complex dielectric permittivity for two orientations of the director n, parallel and perpendicular to the measuring electric field E, have been carried out. It has been found that for E n, as well as in the isotropic phase, the dielectric spectra consist of two elementary domains, described by the Cole-Cole equation, whereas for E n three Cole-Cole-type domains can be distinguished. The possible molecular reorientational movements corresponding to these domains are discussed and comparison with the theoretical predictions is made.

Anomalous rotational diffusion in the vicinity of the isotropic to nematic phase transition

Dielectric relaxation measurements are performed with very high accuracy on a liquid crystalline compound n-octylcyanobiphenyl (8CB) in the isotropic (I), nematic (N) and smectic A (SA) phases. The data obtained display an essential difference in the rotational diffusion process in the vicinity of the I–N phase transition in comparison to that taking place in the vicinity of the N–SA phase transition. Thus, for the I–N transition, anomalously slow diffusion (subdiffusion), characterized by an anomalous coefficient α<1, is observed, while normal Brownian rotational diffusion with is found for the N–SA transition. It is also shown how the fractal parameter α is temperature dependent with an extremely sharp variation at the I–N transition point in the form of a lambda-like profile.

Study of the magnetic Fredericksz transition in ferronematics

Abstract The magnetic Fredericksz transition in a ferronematic (thermotropic nematic liquid crystal MBBA combined with fine magnetic particles of size 10 nm) has been studied as a function of the concentration of magnetic particles by means of simple dielectric measurements. A decrease in the threshold field is observed when the magnetic particle concentration is increased, contrary to the Burylov-Raikher theory of thermotropic ferronematics.

Static and dynamic dielectric properties of aminoalcohols in their pure liquid state and in mixtures with dioxane

Abstract The static permittivities and the dielectric spectra (frequencies ranging between some MHz and 72 GHz) are measured for the following liquids in the pure state and in mixtures with 1,4-dioxane at 20 °C: 2-amino-1-ethanol, 3-amino-1-propanol, 4-amino-1-butanol, 5-amino-1-pentanol, (±)1-amino-2-propanol, (±)2-amino-1-propanol, (±)2-amino-1-butanol, 2-amino-2-methyl-1-propanol. The dielectric loss spectra of these aminoalcohols resemble those of dihydric alcohols but are generally broader. This finding is discussed with respect to the different kinds of hydrogen bonds feasible with the present substances, taking into consideration the relative weight of relaxation contributions ascribable to associates in relation to molecular structure.

Electrical resistivity of Dy3Co11B4 and Er3Co11B4 compounds

Abstract The R 3 Co 11 B 4 (R=Dy and Er) compounds crystallize in the hexagonal Ce 3 Co 11 B 4 -type structure and belong to the R n +1 Co 3 n +5 B 2 n family. The temperature dependence of the electrical resistivity π has been determined by a four-probe measuring technique in the temperature range 4.2–360 K. The ternary compounds exhibit a π α T 2 dependence at low temperatures. These experimental results are discussed on the basis of electron-spin wave scattering in the low-temperature range.

Odd-Even Effects in the Dielectric Properties of Nematics 4-(Trans-4′-N-Alkylcyclohexyl)Isothiocyanatobenzenes

Abstract The anisotropy of the static electric permittivity and conductivity for the homologous series of nematics: 4-(trans-4′-n-alkylcyclohexyl)isothiocyanato-benzenes (n-CHBT) for n = 3–10 have been measured. It is shown that the odd-even effect in the dielectric anisotropy Ac is related to the perpendicular component of electric permittivity. The odd-even effect is found in n-CHBT for both the anisotropy in the conductivity and the activation energy for ionic movement.