Showing papers by "Sylwester J. Rzoska published in 2001"

Fluidlike behavior of dielectric permittivity in a wide range of temperature above and below the nematic-isotropic transition.

Abstract: Singular behavior of the static dielectric permittivity of n-alkyloxycyanobiphenyls $({\mathrm{C}}_{n}{\mathrm{H}}_{2n+1}\mathrm{O}\ensuremath{-}\mathrm{P}\mathrm{h}\ensuremath{-}\mathrm{P}\mathrm{h}\ensuremath{-}\mathrm{C}\ensuremath{\equiv}N,$ $n=6,$ 7) was studied above and below the nematic clearing point ${(T}_{I\ensuremath{-}N}).$ On approaching the clearing point, the evolution of principal components of the nematic permittivity tensor, ${\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}$ and ${\ensuremath{\varepsilon}}_{\ensuremath{\perp}},$ is described by the order parameter exponent $\ensuremath{\beta}\ensuremath{\approx}0.25.$ The mean value of the nematic permittivity ${\ensuremath{\varepsilon}}_{\mathrm{mean}}=({\ensuremath{\varepsilon}}_{\ensuremath{\parallel}}+2{\ensuremath{\varepsilon}}_{\ensuremath{\perp}})/3$ exhibits a singular behavior similar to that observed in the isotropic phase and that for the diameter of the coexistence curve in binary mixtures. The derivative of experimental data $d{\ensuremath{\varepsilon}}_{\mathrm{iso}}(T)/dT$ and $d{\ensuremath{\varepsilon}}_{\mathrm{mean}}(T)/dT$ shows the specific-heat-like behavior with universal exponents $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\approx}0.5.$ Results obtained confirm the hypothesis of the fluidlike, pseudospinodal, and tricritical behavior of the isotropic to nematic phase transition. [A. Drozd-Rzoska, Phys. Rev. E 59, 5556 (1999)].

Pretransitional behavior of dielectric permittivity on approaching a clearing point in a mixture of nematogens with antagonistic configurations of dipoles.

Abstract: The results of studies of static dielectric permittivity epsilon(T) in the isotropic phase of 4-cyano-4-pentylalkylbiphenyl and n-p-methoxybenzylidene-p'-butylaniline (5CB-MBBA) mixtures are presented. 5CB and MBBA are nematogens with antagonistic permanent dipole moments configurations. An increase in MBBA concentration strongly decreases the pretransitional effect. However, a derivative analysis detected the existence of a pretransitional anomaly even if its weakness made a straightforward epsilon(T) fit impossible. The obtained anomalies can be well portrayed by fluidlike equations, isomorphic to that applied in critical binary mixtures. Every time the same value of the specific heat critical exponent alpha approximately 0.5 was obtained. A preliminary discussion of the influence of the permanent dipole configuration on the pretransitional behavior of dielectric permittivity was also possible.

Quasicritical behavior of the low-frequency dielectric permittivity in the isotropic phase of liquid crystalline materials.

Abstract: Results presented give evidence of the existence of quasicritical, fluidlike behavior in the isotropic phase of 4-cyano-4-pentyl-biphenyl (5CB) for frequencies ranging from the static to the ionic-dominated [low-frequency (LF)] region. Despite the boost of dielectric permittivity on lowering the frequency below 1 kHz, values of the isotropic-nematic transition discontinuity (approximately 1.1 K) and the critical exponent alpha (approximately 0.5) remain constant. It is shown that the contribution from residual ionic impurities is a linear function of temperature in the critical, prenematic fluctuation-dominated region. The validity of the fluidlike and critical behavior for LF dielectric permittivity confirmed results of a derivative analysis of the experimental data: d(epsilon)/dT proportional to (T-T*)(-alpha), originally proposed for critical mixtures. Results of a preliminary test in the isotropic phase of 4-decyl-4'-isothiocyanatobiphenyl (10BT), on approaching the smectic-E phase, may indicate a general validity of results obtained.

Critical anomaly of dielectric permittivity for the temperature and pressure paths on approaching the critical consolute point.

Abstract: The experimental results of isothermal pressure dielectric permittivity « studies in a critical mixture characterized by a negative shift of critical temperature induced by pressure ( dTC / dP ,0) are presented. The critical effect is portrayed by the same relation as in previous «(T) and «( P) studies, with the critical exponent a50.1260.03. The advantage of pressure studies is the negligible influence of the correction-to-scaling term and the low-frequency Maxwell-Wagner effect. This conclusion is supported by the distortion-sensitive derivative analysis of the experimental data. In contrast to previous «( P) studies, carried out in mixtures with dTC / dP .0, the critical effect manifests by the bending-up behavior near the critical point. It is suggested that signs of the critical amplitudes of «( P) and «(T) anomalies may be related to the excess volume V E and the excess enthalpy H E , respectively.

Temperature and pressure studies of the isotropic—mesophase transition discontinuity by static nonlinear dielectric effect

Abstract: Results of studies of static nonlinear dielectric effect (NDE) in the isotropic phase in the homologous series of nDBT: (5-n-alkyl-2-(4-isothio-cyanatophenyl)-1,3-dioxane) for n = 4, 6 and 8 exhibiting the I-SmA transition are presented. The strong pretransitional anomaly in the isotropic phase is similar to the one observed in nematogens. It has been found that the discontinuity of the I-N transition increases from 10.8 K to 18.8 K when moving from 4DBT to 8DBT. It has been also shown that pressure reduces the discontinuity of the I-SmA transitions. Results obtained are compared with similar studies in two other homologous series (Phys. Rev. E61 (2000) 5349 and Phys. Rev. E61 (2000) 5355).

Temperature Studies of Dielectric Permittivity and Mass Density Pretransitional Anomalies in Binary Mixtures

Abstract: Results of temperature studies of dielectric permittivity and mass density in binary mixtures are presented. The anomaly of dielectric permittivity as a function of temperature in nitrobenzene-n-alkanes binary mixtures has been studied. Molar dielectric permittivity which takes into account the anomaly of dielectric permittivity and the mass density anomaly has been introduced.

Universal Scaling of Alpha Relaxation in Complex Liquids

Abstract: A plot is given, showing the result of a scaling analysis of dielectric loss curves containing, apart from low molecular glass former data (glycerol, dibutyl phtalate), also loss curves of the following liquid crystallinematerials, mostly in the isotropic phase: 4-(2-methylbutyl)-4'cyanobiphenyl (5*CB, supercooled isotropic phase), 4-cyano-4-n-alkyl biphenyls (nematogens 5CB and 8CB, isotropic phase), 4-(4-cyano-4-butylcyclohexyl)-4'-octylbiphenyl (laterally substituted nematogen, isotropic phase), and 4-n-alkyl-4'-isothiocyanatobiphenyl (5 and 10 BT, isotropic and SmE phases). The plot applies the scaling formula originally proposed forglassforming, supercooled liquids [Dendzik et al. 7 ]. The result supports the recent suggestion that dielectric relaxation in the isotropic phase of nematogens may show some features typical for "glassy" materials.