Jan Jadżyn

Capacitance changes in ferronematic liquid crystals induced by low magnetic fields

The response in capacitance to low external magnetic fields (up to 0.1 T) of suspensions of spherical magnetic nanoparticles, single-wall carbon nanotubes (SWCNT), SWCNT functionalized with carboxyl group (SWCNT-COOH) and SWCNT functionalized with Fe3O4 nanoparticles in a nematic liquid crystal has been studied experimentally. The volume concentration of nanoparticles was φ1 = 10 −4 and φ2 = 10 . Independent of the type and the volume concentration of the nanoparticles, a linear response to low magnetic fields (far below the magnetic Fréederiksz transition threshold) has been observed, which is not present in the undoped nematic. PACS numbers: 61.30.Gd, 77.84Nh, 75.50.Mm, 75.30.Gw ∗ Corresponding author; tothkatona.tibor@wigner.mta.hu

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.

Dipoles–dipole association of mesogenic molecules in solution

Abstract The dielectric polarization has been used to study dipolar association of 4-n-pentyl-4′-cyanobiphenyl in benzene solution. The results have been interpreted with the assumption of a monomer–dimer equilibrium. To explain the relatively high effective dipole moment of the dimers, a new structure has been proposed for them.

Fractional Stokes–Einstein–Debye relation and orientational entropy effects in strongly hydrogen-bonded liquid amides

The impedance spectroscopy studies performed for two strongly hydrogen-bonded liquid amides: N-methylpropionamide (NMP, CH(3)·NH·CO·C(2)H(5)) and N-ethylacetamide (NEA, C(2)H(5)·NH·CO·CH(3)) have shown that the two centers of the peptide linkage, -NH·CO-, active in the C=OH-N hydrogen bonds formation, exhibit quite different sensibilities to the steric screening effects. In contrast to the oxygen atom, a relatively small change (CH(3)- to C(2)H(5)-) in the screening of the hydrogen atom leads to an essential decrease of the degree of the amide self-association. As a consequence, both the static dielectric permittivity and the orientational entropy increment of NEA are essentially lower than those of NMP. However, it was found that the dynamic processes studied are only weakly influenced (in the case of dc conductivity, σ(DC)) or totally not influenced (the dielectric relaxation time, τ(D)) by the different degrees of NMP and NEA self-association. The experiment shows that for both the amides, the logσ(DC) vs. logτ(D) dependence is nonlinear and can be described with the fractional Stokes-Einstein-Debye relation, σ(DC)τ ≅ const, with the exponent s varying from about -0.8 to about -0.6 in the temperature range from 5 °C to 110 °C.

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.

Dimerization of polar mesogenic molecules

Abstract Dielectric polarization method has been used for studies of dipole-dipole self-association of n-pentylcyanobiphenyl (5-CB) and 4-(trans-4′-n-hexylcyclohexyl)isothiocyanatobenzene (6-CHBT) in non-polar solvents. A new approach to the dimer structure has been proposed.

Frequency dependence of the nonlinear dielectric effect in diluted dipolar solutions

Abstract The nonlinear dielectric relaxation in dilute solutions of strongly polar molecules (4,4′-n-hexylcyanobiphenyl) in nonpolar medium (benzene) has been investigated. The nonlinear dielectric increment, induced by a d.c. electric field of high intensity (Eo ≈ 107 V/m), was detected with an a.c. field of small intensity and variable frequency (1 MHz-3 GHz). The relaxational behavior of the increment is discussed in the framework of the theory proposed by Coffey et al.

Conductivity dynamics and static dielectric permittivity of highly conducting molecular liquids studied with impedance spectroscopy. Formamides.

Impedance spectroscopy was used for studies of the frequency dependence of the electric conductivity of two highly conducting liquid amides: N-methylformamide (NMF), strongly self-associated via the hydrogen bonds C=O...N-H, and N,N-dimethylformamide (DMF), which is not able to self-organize by hydrogen bonding. The studies were performed in the frequency range f = 50 kHz-100 MHz and in the temperature range T = 278.15-363.15 K. It was found that, at a given temperature, NMF exhibits static conductivity (sigma(0)) of 2 orders of magnitude higher, and conductivity relaxation time (tau(sigma)) of 2 orders of magnitude shorter, than those of DMF. On the basis on sigma(0) and tau(sigma) values, the static dielectric permittivity (epsilon) was calculated for both liquids studied and the values obtained perfectly agree with those measured with the classical dielectric methods. An empirical equation for epsilon(T) dependence for NMF and DMF is proposed.

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.