Showing papers by "Sylwester J. Rzoska published in 2005"
TL;DR: This paper recalls the recent discussions on the glassy dynamics of a "hard-ellipsoid" liquid and the possible relationship between the glass transition, critical phenomena, and isotropic-nematic transition.
Abstract: Linear and nonlinear dielectric measurements of liquid crystalline chiral isopentylcyanobiphenyl $(5*\mathrm{CB})$ and $n$-pentylcyanobiphenyl (5CB), combined with viscosity $\ensuremath{\eta}(T)$ data, are presented. The $5*\mathrm{CB}$ compound glassifies on cooling in the cholesteric phase whereas 5CB crystallizes in the nematic phase. In both compounds the temperature evolution of dielectric relaxation times, the dc conductivity, and the viscosity are well described by the ``critical-like'' description from mode coupling theory (MCT). However, for $5*\mathrm{CB}$ a unique coincidence of the MCT ``critical'' temperature and extrapolated temperature of the hypothetical continuous isotropic-cholesteric $(T*)$ phase transition was found. The temperature dependence of the strong electric-field-induced changes of the dielectric permittivity exhibits a strong anomaly in the direction of negative values on approaching $T*$, not observed up to now. The anomaly is described by the susceptibility-related critical exponent $\ensuremath{\gamma}=1$. The divergence of the ``nonlinear'' dielectric relaxation follows a power dependence described by the exponent $y=1$. This paper recalls the recent discussions on the glassy dynamics of a ``hard-ellipsoid'' liquid and the possible relationship between the glass transition, critical phenomena, and isotropic-nematic transition.
31 citations
TL;DR: In this article, the authors show that properties of the relaxation measured below Tg are sensitive to the structure of the glass; that is, the thermodynamic path from the equilibrium liquid strongly affects the relaxation times, their distribution, and the activation energy quantifying their temperature dependence.
Abstract: The relaxation is a principal source of information about the dynamics in the glassy state; however, the nature of this process remains a controversial issue. In this paper, we show that properties of the relaxation measured below Tg are sensitive to the structure of the glass; that is, the thermodynamic path from the equilibrium liquid strongly affects the relaxation times, their distribution, and the activation energy quantifying their temperature dependence. These results support the idea that the Johari-Goldstein process is the precursor to the structural relaxation transpiring at longer times. We discuss the experimental findings in light of the heterogeneous and homogeneous scenarios for the process.
29 citations
TL;DR: In this article, broadband dielectric spectroscopy (BDS) studies, for a remarkable temperature range (130 K), in supercooled rodlike liquid crystalline n-butylcyanobiphenyl (4CB) are shown.
Abstract: Results of broadband dielectric spectroscopy (BDS) studies, for a remarkable temperature range (130 K), in supercooled rodlike liquid crystalline n-butylcyanobiphenyl (4CB) are shown. They are supplemented by static nonlinear dielectric effect (NDE) measurements in 4CB and 3CB (n-propylcyanobiphenyl), giving unequivocal estimations of the hypothetical isotropic-nematic (I-N) continuous phase transition temperature T* and of the discontinuity of this transition deltaT. The distribution of relaxation times becomes strongly non-Debye on approaching the I-N transition and follows the Debye pattern for T > T(I-N) + 75 K. The temperature evolution of relaxation times [tau(T)] and dc conductivity [sigma(T)] remain non-Arrhenius in the tested temperature range. Their descriptions via the Vogel-Fulcher-Tammann (VFT), Cohen-Grest (CG), and mode coupling theory (MCT) based relations are discussed. The derivative analysis of tau(T) and sigma(T) experimental data enabled the detection of dynamic crossovers and the insight into the behavior of the apparent activation energy which follows the relations E(A)(tau),E(A)(sigma) (sigma) is proportional to (T/T-T0)2, with a small distortion in the immediate vicinity of the I-N transition. The glassy dynamics coexist with the critical-like behavior of temperature dependences of the static dielectric permittivity, maximum of the loss curves and strong pretransitional NDE pretransitional anomaly. These results indicate the significant role of prenematic fluctuations-heterogeneities in the isotropic, fluidlike, surrounding regarding both static and dynamic properties.
25 citations