Showing papers by "Sylwester J. Rzoska published in 2011"

Enthalpy space analysis of the evolution of the primary relaxation time in ultraslowing systems.

Abstract: For decades the Vogel–Fulcher–Tammann equation has dominated the description of dynamics of the non-Arrhenius behavior in glass forming systems. Recently, this dominance has been questioned. Hecksher et al. [Nat. Phys. 4, 737 (2008)], Elmatad et al. [J. Phys. Chem. B 113, 5563 (2009)], and Mauro et al. [Proc. Natl. Acad. Sci. U.S.A. 106, 19780 (2009)] indicated superiority of several equations showing no divergence at a finite (nonzero) temperature. This paper shows distortion-sensitive and derivative based empirical analysis of the validity of leading equations for portraying the previtreous evolution of primary relaxation time.

Prevalence for the universal distribution of relaxation times near the glass transitions in experimental model systems: rodlike liquid crystals and orientationally disordered crystals.

Abstract: Recently, Nielsen et al. [J. Chem. Phys. 130, 154508 (2009); Philos. Mag. 88, 4101 (2008)] demonstrated a universal pattern for the high frequency wing of the loss curve for primary relaxation time on approaching the glass transition for organic liquids. In this contribution it is presented that a similar universality occurs for glass-forming liquid crystals and orientationally disordered crystals (plastic crystals). Empirical correlations of the found behavior are also briefly discussed.

Universal critical-like scaling of dynamics in plastic crystals

Abstract: Many theoretical models for the glassy dynamics have been proposed so far describing the changes in molecular dynamics along the extraordinary slowing down in the vitrification process of a disordered phase on cooling. Many of these theories share the concept of cooperative rearranging regions firstly proposed by Adam and Gibbs. Among them, the dynamical scaling model (DSM) is based on the random diffusion of free volume which creates random walking clusters formed by cooperatively rearranging entities. Within this framework a critical phenomenon relating a hidden phase transition at TC (below Tg) implies the divergence of the relaxation time (τ) or viscosity (η) τ, η ∝ (T − TC) − ϕ with a universal scaling exponent φ → 9. In this work we apply the DSM model to orientational glasses, obtained from the quenching of orientationally disordered phases (plastic crystals) via the application of the linearized derivative-based transformation of dielectric spectroscopy τ(T) data.

Glassy dynamics in the isotropic phase of a smectogenic liquid crystalline compound.

Abstract: The temperature evolution of the primary relaxation time in the isotropic phase of 4-cyano-4'-tetradecylbiphenyl (14CB) above the isotropic-smectic A (I-SmA) transition is discussed. Based on the enthalpy space and distortion-sensitive analysis, the prevalence of the mode coupling theory (MCT) "critical" and "glassy" dynamics is shown. The obtained singular dependence is related to the MCT critical temperature located approximately 48 K below the clearing (I-SmA) temperature. However, a weak but detectable distortion in the immediate vicinity of the transition occurs. It is also shown that the value of the fragile strength coefficient D(T) is characteristic of a very fragile glassy liquid whereas the steepness index m is typical of a strong one. Both magnitudes anomalously change on approaching the I-SmA phase transition. The static permittivity shows the pretransitional effect linked to the temperature of the hypothetical continuous phase transition located approximately 10.2 K below the I-SmA transition.

Secondary relaxations of orientationally disordered mixed crystals at temperatures lower than the glass transition temperature

Abstract: Low-molecular weight cyclic alcohols as cycloheptanol (C7H14O, hereinafter referred to as cC7-ol) and cyclooctanol (C8H16O, cC8-ol) are prototypical materials displaying OD phases which, under fast cooling give rise to orientational glasses (OG) In addition to the ubiquitous α-relaxation of canonical glasses, several secondary relaxations appear for the mentioned systems (β and γ for cC8-ol and β for cC7-ol) The intramolecular character of these secondary relaxations for these materials as well as their mixed crystals was highlighted at temperatures close but above the glass transition For lower temperatures the low values of dielectric strength makes difficult to account for the relaxation times obtained from the permittivity losses and, thus in this work we present a data analysis based on the Kramers–Kronig relations which connect the real and imaginary parts of dielectric permittivity and shows up a new method to make evident the existence of such secondary relaxations as well as to avoid phenomenological equations for determining the relaxation time

Non-linear dielectric effect in the isotropic phase above the isotropic-cholesteric phase transition

Abstract: Using the Landau-de Gennes theory, the temperature, pressure and frequency dependence of the non-linear effect in the isotropic phase above the isotropic–cholesteric phase transition is calculated. The influence of pressure on the isotropic–cholesteric phase transition is discussed by varying the coupling between the orientational order parameter and the macroscopic polarization of polar cholesterics. Comparing the results of the calculations with existing data, we finally conclude that the model provides a description of the isotropic–cholesteric transition that takes all experimentally known features of the unusual negative and positive pretransitional effect in the isotropic phase of the system into account in a qualitatively correct way.