Showing papers by "Grzegorz Czechowski published in 2008"
TL;DR: The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4'-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematics region.
Abstract: The linear and nonlinear dielectric relaxation spectra, recorded in the isotropic phase of mesogenic 4-(trans-4‘-n-hexylcyclohexyl)isothiocyanatobenzene (6CHBT), reveal an equilibrium between the molecules that are involved and non-involved in the pseudo-nematic domains, which are spontaneously formed in the prenematic region. A perturbation of the equilibrium by a static electric field of high strength (E0 ≈ 107 V/m) is followed by a nonlinear dielectric relaxation process that, measured with the probing electric field E(ω) of small amplitude and variable high frequency (up to 100 MHz), reflects the rate of the domains formation. The fraction of the mesogenic molecules involved in the prenematic domains at different temperatures is estimated.
19 citations
TL;DR: The singularities in dependence of the entropy increment on the temperature and on the mixtures composition are discussed in terms of the prenematic molecular self-organization extent in mesogenic liquids of different density of dipoles.
Abstract: The paper concerns two aspects of the entropy in mesogenic systems: (i) the entropy jump (ΔS0NI) at the phase transition from the isotropic liquid (I) to the nematic liquid crystalline state (N), and (ii) the entropy increment (ΔS) caused by the ordering action of the probing electric field applied to the dipolar system. The system studied are the mixtures of strongly polar mesogenic solvent n-hexylcyanobiphenyl (C6H13PhPhCN, 6CB) and the nonpolar nonmesogenic admixture 4-ethylcyclohexyl-4′-n-nonylphenyl (C2H5CyHxPhC9H19, 2CyPh9). The entropy jump at the I−N phase transition in pure 6CB [ΔS0NI= 1.52 J/(mol K)] was evaluated from the analysis of the phase diagram of the mixture 6CB + 2CyPh9 with use of the Landau−Lifshitz theory; the resulting value of the transition enthalpy (ΔH0NI = TNIΔS0NI = 0.50 kJ/mol) agrees well to that obtained with the calorimetric methods. The field-induced entropy increment (ΔS) was calculated, at the given temperature, from the static dielectric permittivity derivative value (...
7 citations
TL;DR: In this article, a review of recent results on static dielectric behavior in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices is presented.
Abstract: The paper presents a review of recent results on static dielectric behaviour in vicinity of weakly first order transition from isotropic liquid to a liquid crystalline nematic phase of compounds used in opto-electronic devices. Temperature behaviour of static permittivity and its derivative are interpreted in terms of a basic thermodynamic quantities increment (internal energy, entropy and Helmholtz free energy) induced by a probing electric field.
7 citations