Showing papers by "Grzegorz Czechowski published in 2000"

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

Abstract: This paper presents the results of measurements of the principal electric permittivities ∊ ∥*(T, ω and ∊⊥*(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.

Static and Dynamic Dielectric Polarization and Viscosity of n-Hexylcyanobiphenyl in the Isotropic and Nematic Phases

Abstract: On the basis of the temperature dependence of the static principal permittivities, and , measured for the nematic phase of n-hexylcyanobiphenyl (C6H13–Ph–Ph–C N, 6CB), a value of the angle between the long molecular axis and the direction of the dipole moment of 6CB, as well as the order parameter S (T), were obtained by use of the Maier-Meier equations. From the dielectric relaxation and viscosity data, the contribution of the nematic potential to the molecular dynamics was estimated. In the isotropic phase the dielectric and viscosity data allow one, in the framework of the Debye model, to estimate the effective length of 6CB molecule.

Viscosity of the Homologous Series of 4-(trans-4-n-Alkylcyclohexyl)isothiocyanatobenzenes

Abstract: This paper presents the results of the viscosity measurements performed for the isotropic phase of the homologous series of mesogenic 4-(trans-4-n-alkylcyclohexyl)isothiocyanatobenzenes (CnH2n+1−CyHxPhNCS, nCHBT) for n = 0 to n = 12.

Dielectric properties of 6-CHBT in isotropic and nematic phases

Abstract: The paper presents the results of measurements of the static and dynamic electric permittivities (epsilon) (parallel)*(T,(omega) ) and (epsilon) (perpendicular)*(T,(omega) ) of 4-(trans-4'-n-hexylcyclohexyl)isothiocyanato- benzene(C6H13-CyHx-Bz-NequalsCequalsS, 6-CHBT). On the basis of the temperature dependence of the static permittivities, using the Maier-Meier equations, the angle (beta) between the dipole moment vector and the long axis of 6-CHBT molecule, the square of the molecular apparent dipole moment (mu) 2app and the nematic order parameter S(T), were determined. The dielectric relaxation spectra were interpreted in the frame of the molecular model proposed recently.© (2000) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Shear Viscosity of the Homologous Series of wCHBT (n = 0 ÷ f 12) in the Isotropic and Nematic Phases

Abstract: Abstract The paper presents results of shear viscosity measurements performed on nematogenic 4-(trans-4'-n-alkylcyclohexyl)isothiocyanatobenzenes (CnH 2n + 1-CyHx-Ph-N=C=S, /nCHBT) in the isotropic (n = 0 ÷ 12) and nematic (« = 4 ÷ 12) phases. The viscosity measured in the nematic phase is, due to the flow alignment phenomenon, close to the Migsowicz T? 2 viscosity coefficient. An odd-even effect in the n dependence of the viscosity-activation energy is observed both in the nematic and isotropic phases of nCHBT.

Molecular model of the dielectric relaxation in nematic liquid crystals

Abstract: The paper presents the results of the measurements of the dielectric relaxation of nematic 6-CHBT performed for differentvalues ofthe angle between the directions of the macroscopic orientation of the sample (director n) and the probing electricfield E. Analysis ofthe evolution ofthe relaxation spectrum from 4(o) (E n) to i(o) (E I n) allowed one to explainthe inconsistency in the molecular interpretation ofthe spectra A model of the molecular dynamics in the oriented nematicsis proposed.Keywords: dielectric relaxation, neniatic, molecular dynamics 1. INTRODUCTION Studies of the dielectric relaxation in nematic liquid crystals are of a special interest and importance. The possibilityof control of the molecular orientation in the whole nematic sample by an electric I magnetic field or by an appropriatepreparation of the electrodes surface, gives an exceptional opportunity for studies of the molecular dynamics in correlation to the directions of the principal axes of the nematogenic molecule. One could expect that in such advantageous

Molecular Dynamics of 4-(trans-4'-n-Heptylcyclohexyl)- Isothiocyanatobenzene Studied with Dielectric Relaxation Spectroscopy

Abstract: Dielectric relaxation study was carried out for the nematic and isotropic phases of 4-(trans-4'-n-heptylcyclohexyl)isothiocyanatobenzene in the frequency range from 100 kHz to 1 GHz. In the nematic phase two relaxation processes were recorded for the electric permittivity component measured parallel to the molecular orientation (director n) and three relaxation processes — for the perpendicular permittivity component. The strength of the nematic potential and the nematic order parameter were estimated. PACS numbers: 64.70.Md, 77.84.Nh, 77.22.Gm