Showing papers by "Grzegorz Czechowski published in 2006"

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.

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.