Showing papers by "Jan Jadżyn published in 2006"

Determination of the nonlinear dielectric increment in the Cole-Davidson model.

Abstract: The problem of the nonlinear dielectric relaxation of complex liquids is tackled in the context of the Cole-Davidson [J. Chem. Phys. 19, 1484 (1951)] model. By using an appropriate time derivative of noninteger order, an infinite hierarchy of differential-recurrence relations for the moments (expectation values of the Legendre polynomials) is obtained. The solution is established for the stationary regime of an ensemble of polar and symmetric-top molecules acted on by a strong dc bias electric field superimposed on a weak ac electric field. The results for the first three nonlinear harmonic components of the electric susceptibility are analytically established and illustrated with the help of Argand diagrams for the nonlinear dielectric increment and three-dimensional dispersion and absorption spectra for the second and the third harmonic components as a function of the anomalous exponent β⩽1, the value of which gives rise to skewed arcs (Argand plots) and asymmetric shapes (loss spectra) in the high-freq...

Temperature dependence of the critical magnetic field of the structural transition in MBBA-based ferronematics

Abstract: We studied the structural transitions in ferronematics based on the thermotropic nematic liquid crystal MBBA (4′ -methoxybenzylidene-4-n-butylaniline) having a nematic-to-isotropic transition temperature T N–I = 48.0○C and in MBBA-based ferronematics doped with a magnetic suspension consisting of Fe3O4 particles (10 nm in diameter) coated with oleic acid as a surfactant. The ferronematic samples were prepared with different volume concentrations of magnetic particles φ =,1× 10−4, 2× 10−4 and 5×10−4. The temperature dependences of the critical magnetic fields in a bias electric field under strong applied magnetic fields are presented. We calculated the surface density of anchoring energy W at the nematic–magnetic particle boundary. Scaling of the structural transition in the MBBA and MBBA-based ferronematics with the temperature of the nematic-to-isotropic transition was observed.

Dielectric and viscous properties of mesogenic n-nonylcyanobiphenyl (9CB)

Abstract: Static and dynamic linear dielectric properties and shear viscosity of freely flowing 9CB in the isotropic (I), nematic (N) and smectic A (SA) phases were investigated. Pretransitional effects observed in the vicinity of the I to N phase transition manifest themselves as anomalies in the temperature dependence of the static permittivity, the rotational diffusion exponent and the activation energy of molecules rotating around their short axis. As a presmectic effect, a jump in the temperature dependence of the shear viscosity of freely flowing nematic 9CB is observed at about 0.5 °C before the transition to the SA phase.

The study of structural transitions in liquid crystal droplets doped with magnetic particles

Abstract: The magnetically active ferronematic droplets were prepared in solutions of nematic liquid crystal 6CHBT with fme magnetic particles, dissolved in phenyl isocyanate. The phase diagram of the transitions between isotropic, droplet and nematic phases in prepared sample was found. The structural transitions in 6CHBT-based ferronematic and in 6CHBT ferronematic droplets, exposed to external fields, were investigated. The possibility to use the observations of structural transitions for the determination of the type of anchoring of nematic molecules on magnetic particles surfaces was demonstrated.

Dielectric Relaxation in Hydrogen Bonded Urea-Based Supramolecular Polymer N,N'-di(2,2-dipentylheptyl)urea

Abstract: Results of the dielectric relaxation studies, performed for supramolecular polymer formed by N, N ′-di(2,2-dipentylheptyl)urea dissolved in carbon tetrachloride, are presented. The measurements were done for N, N ′-di(2,2-dipentylheptyl)urea concentration up to about 7% (in mole fraction) in the frequency region from 100 kHz to 100 MHz and at the temperatures from 5◦C to 50◦C. The analysis of the experimental data were performed with the Havriliak–Negami equation. In the studied range of N, N ′-di(2,2-dipentylheptyl)urea concentration and temperature, the obtained values of exponents α and β of the Havriliak–Negami equation are equal to 0.9± 0.1 and 0.7 ± 0.1, respectively, showing an anomaly in the dielectric relaxation behavior close to the Davidson–Cole type. Two examples of the modeling of dielectric properties of the supramolecular polymer solutions were presented.

Viscous Behavior of Binary Mixtures of Mesogenic Solvent/Non-Mesogenic Solute

Abstract: The paper presents results of the shear viscosity measurements performed on diluted binary mixtures of mesogenic solvent n-hexylcyanobiphenyl (C6H13PhPhC≡N, 6CB) and two non-mesogenic admixtures: (i) n-heptylcyanophenyl (C7H15PhC≡N, 7CP), composed of the molecules of the same polarity as the solvent molecules but of a slightly shorter length, and (ii) 4-n-propylcyclohexyl-4’-n-pentylphenyl (C3H7CyHxPhC5H11, 3CyP5), composed of the non-polar molecules but of a length very close to that of the mesomorphic solvent molecules. The experiment showed that the concentrational depression of the clearing temperature and the temperature extent of the isotropic + nematic (I + N) two-phase region in the mixtures are significantly smaller, i.e. the nematic phase is more thermodynamically stable, when the admixture molecular length is compatible to that of the mesogenic solvent, regardless of the polarity of the admixture molecules. The activation energy for freely flowing mixtures in the isotropic, nematic, and two-phase I + N regions was determined and discussed.