Showing papers by "Jan Jadżyn published in 2010"

Structural Phase Transition in Liquid Crystal Doped with Gold Nanoparticles

Abstract: Structural Phase Transition in Liquid Crystal Doped with Gold Nanoparticles P. Kopcanský, N. Tomasovicova, M. Koneracka, M. Timko, Z. Mitroova, V. Zavisova, N. Eber, K. Fodor-Csorba, T. Toth-Katona, A. Vajda, J. Jadzyn, E. Beaugnon and X. Chaud Institute of Experimental Physics, Slovak Academy of Science, Watsonova 47, 040 01 Kosice, Slovakia Research Institute for Solid State Physics and Optics, Hungarian Academy of Sciences H-1525 Budapest, P.O. Box 49, Hungary Institute of Molecular Physics, Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznan, Poland High Magnetic Field Laboratory, CNRS, 25 Av. des Martyrs, Grenoble, France The influence of the inclusion of the dodecanethiol functionalized gold particles (with diameter 3–5 nm) on the structural transitions was investigated. The studied samples were based on the nematic liquid crystal 4-(trans-4′-n-hexylcyclohexyl)-isothiocyanatobenzene (6CHBT). The volume concentration of the gold particles was φ1 = 2 × 10−4 and φ2 = 10−3. The obtained results showed that the inclusion of the gold particles in the 6CHBT liquid crystal increases the sensitivity of such system on the external magnetic field.

The sensitivity of ferronematics to external magnetic fields

Abstract: The stable colloidal suspensions of nematic liquid crystals with magnetic nanoparticles are called ferronematics. Their behaviour in magnetic field depends on an anchoring energy, volume concentrations of magnetic nanoparticles and sign of anisotropy of diamagnetic susceptibility of liquid crystal as well as on the initial orientation between director (n) of liquid crystal and magnetic moment (m) of magnetic nanoparticles. In this work we present structural changes in ferronematics based on two kinds of liquid crystals: 6CHBT (positive anisotropy of diamagnetic susceptibility) and ZLI1695 (negative anisotropy of diamagnetic susceptibility) exposed to high magnetic fields. In both cases the parallel initial orientation between director and magnetic moments is fulfilled. The density of anchoring energy between liquid crystal molecules and magnetic particles was determinated by Burylov and Raikher's theory. In the case of 6CHBT-based ferronematics the decrease while in the case of ZLI1695-based ferronematics the increase of the critical magnetic field were observed.

Molecular Dynamics and Entropy Effects in Hydrogen-Bonded Supramolecular Polymer N,N′-Di(2-methyl-2-pentylheptyl)urea Dissolved in Nonpolar Medium

Abstract: On the basis of the static dielectric permittivity temperature behavior of the supramolecular polymer formed by N,N′-di(2-methyl-2-pentylheptyl)urea (MPHU) in carbon tetrachloride diluted solution (10% in mole fraction) and that of neat acetonitrile, it was found that two liquids of the same macroscopic polarity (expressed by the same value of the dielectric permittivity ≈ 35) exhibit an essential difference in the field-induced orientational entropy change. A much higher entropy effect is observed for the liquid composed of not too numerous but highly polar molecular entities (MPHU + CCl4) than for the liquid composed of numerous but less polar entities (neat acetonitrile). The analysis of the dielectric relaxation spectra of MPHU + CCl4 solutions was performed with the Davidson−Cole (DC) model. It was found that the DC exponent β changes its value in a quite important range of 0.4−0.8, depending on MPHU concentration and temperature. This reflects an important deviation of the relaxation mechanism occur...

Dielectric and orientational properties of nematogenic 4-n-octyl(4′-cyanophenyl)benzoate

Abstract: Experimental studies of the dielectric properties in the nematic and isotropic phases of 4-cyanophenyl-4′-n-octylbenzoate (8CPB) have been performed in the frequency range from 50 kHz to 100 MHz. The relaxation process related to the rotation around the short molecular axis has been analysed. The results obtained have revealed that in the vicinity of the I-N phase transition, the molecular subdiffusional rotation process occurs. It is connected with the pretransitional effects which are observed both in the static and dynamic dielectric measurements. On the basis of the temperature dependence of the relaxation time in the nematic and isotropic phases, the orientational order parameter 〈P2〉 has been determined. The values of 〈P2〉 obtained in this way have been compared with those evaluated from the measurements of the polarized electronic absorption of the dichroic dye, dissolved as guest probe in the mesogenic host.

Molecular Dynamics and Dipolar Aggregation in Some Fluoro-Substituted Liquid Crystals

Abstract: The article presents the results of the molecular dynamics and dipolar aggregation studies performed with the use of dielectric spectroscopy for two differently fluoro-substituted nematogenic compounds: 4-cyano-3-fluorophenyl 4'-n-octyloxybenzoate [C8H17OPhCOOPh(F)CN] and 4-cyanophenyl 4'-n-octyloxy-3'-fluorobenzoate [C8H17OPh(F)COOPhCN]. The dielectric relaxation spectra recorded in the isotropic and nematic phases of the compounds as well as the activation energies corresponding to the molecular rotation around the short axis and the conductivity are analyzed. The role of the molecular structure in the aggregation ability of the compounds is discussed in relation to the dielectric anisotropy, the most important property of the nematic liquid crystals exploiting in opto-electronic devices.