Showing papers by "Grzegorz Czechowski published in 2007"

Prenematic behavior of the electric-field-induced increment of the basic thermodynamic quantities of isotropic mesogenic liquids of different polarity.

Abstract: Temperature dependences of the static dielectric permittivity and its derivative, obtained for isotropic mesogenic liquids composed of the molecules of different polarity in relation to the basic thermodynamic quantities (internal energy, entropy, and Helmholtz free energy), are analyzed. A role of the molecular polarity in the dielectric behavior of the liquids in the vicinity of the isotropic to nematic phase transition is discussed.

Contribution to understanding of the molecular dynamics in liquids.

Abstract: The dielectric relaxation spectroscopy is used for studying the orientational molecular dynamics in the isotropic (I) and nematic (N) phases of two mesogenic liquids composed of the molecules of similar structure and length, but of an essentially different polarity: n-heptylcyanobiphenyl, C7H15PhPhCN, 7CB (molecular dipole moment μ ≈ 5D) and 4-(trans-4‘-n-hexylcyclohexyl)isothiocyanatobenzene, C6H13CyHxPhNCS, 6CHBT (μ ≈ 2.5D); advantageously, the temperatures of the I−N phase transition for the two compounds are very close to each other (TNI = 316.6 ± 0.2 K). It is shown that regardless of the differences in polarity of 7CB and 6CHBT molecules and their abilities in dipolar aggregation, the values and temperature dependences of the relaxation time (corresponding to the rotational diffusion of the molecules around their short axis) are very close to each other, in both the isotropic and nematic phases of the liquids studied. Therefore, the data show that the dielectric relaxation processes occurring in di...

Singular Pretransitional Behavior of the Electric Field-Dependent Part of the Thermodynamic Quantities of Strongly Polar Mesogenic Liquids in the Isotropic Phase

Abstract: A new interpretation of the pretransitional temperature behavior of the static dielectric permittivity and its derivative in terms of the basic thermodynamic quantities: the internal energy, the entropy, and the Helmholtz free energy is presented. It was shown that in the case of strongly polar mesogenic compounds (as the homologous series of alkylcyanobiphenyls) the electric field-induced increments of both the internal energy and the entropy exhibit a critical-like temperature dependence in the vicinity of transition from the isotropic liquid to the nematic phase. As a most important pretransitional effect, it is shown that at a temperature of about 10 degrees before the phase transition, an electric field applied to the isotropic liquids induces an increase in the entropy (∆S > 0) instead of its decrease, as observed far from the phase transition or for the non-mesogenic dipolar liquids.

Broadband dielectric investigations on a compound consisting of bent‐shaped molecules

Abstract: Dielectric measurements on the antiferroelectric smectic C (SmCPA) and isotropic (I) phases of a substance formed by bent‐shaped molecules were performed in the frequency range from 0.1 Hz to 0.1 GHz. Three absorption ranges, corresponding to a slow collective process, the reorientation of the molecular core around the long axes and the fast dynamics of the terminal groups, were detected in the SmCPA phase, whereas only two ranges, related to the two last processes, appear in the isotropic state. At the I–SmCPA phase transition the process of molecular reorientation around the long axes changes stepwise with respect to the absorption band intensity and the relaxation time, while the activation energy remains practically unchanged. The activation energy for the dynamics of the terminal groups increases at the I–SmCPA transition.

Static and Dynamic Dielectric Properties of Mesogenic n-Nonyloxycyanobiphenyl (9OCB)

Abstract: The dielectric properties of n-nonyloxycyanobiphenyl in the isotropic (I), nematic (N) and smectic A (SA) phases were investigated. The dielectric relaxation spectra, recorded in the frequency range 50 kHz‐100 MHz, were analyzed with use of the Cole-Cole equation. An anomalous temperature behavior of the static permittivity, the rotational diffusion exponent and the activation energy of mesogenic molecules rotating around their short axis, observed in the vicinity of the phase transitions, are discussed.