Showing papers by "Sylwester J. Rzoska published in 2008"

Thermodynamic scaling and the characteristic relaxation time at the phase transition of liquid crystals.

Abstract: The longitudinal relaxation time τ of a series of alkyl-isothiocyanato-biphenyls (nBT) liquid crystals in the smectic E phase was measured as a function of temperature T and pressure P using dielectric spectroscopy. This relaxation time was found to become essentially constant, independent of T and P, at both the clearing point and the lower temperature crystalline transition. τ(T,P) could also be superposed as a function of the product TVγ, where V is the specific volume and γ is a material constant. It then follows from the invariance of the relaxation time at the transition that the exponent γ superposing τ(T,V) can be identified with the thermodynamic ratio Γ=−∂log(Tc)∕∂log(Vc), where the subscript c denotes the value at the phase transition. Analysis of literature data on other liquid crystals shows that they likewise exhibit a constant τ at their phase transitions. Thus, there is a surprising relationship between the thermodynamic conditions defining the stability limits of a liquid crystalline phas...

On the pressure evolution of dynamic properties of supercooled liquids

Abstract: A pressure counterpart of the Vogel-Fulcher-Tammann (VFT) equation for representing the evolution of dielectric relaxation times or related dynamic properties is discussed: τ( P) = τ P 0 exp(DPP(P0 − � P)) ,w hereP = P − PSL, P0 is the ideal glass pressure estimation, DP is the pressure fragility strength coefficient, and the prefactor τ P 0 is related to the relaxation time at the stability limit (PSL) in the negative pressure domain. The discussion is extended to the Avramov model (AvM) relation τ( T, P) = τ0 exp(e(Tg(P)/T ) D ), supplemented with a modified Simon-Glatzel-type equation for the pressure dependence of the glass temperature (Tg(P)), enabling an insight into the negative pressure region. A recently postulated (Dyre 2006 Rev. Mod. Phys. 78 953) comparison between the VFT and the AvM-type descriptions is examined, for both the temperature and the pressure paths. Finally, we address the question 'Does fragility depend on pressure?' from the title of Paluch M et al (2001 J. Chem. Phys. 114 8048) and propose a pressure counterpart for the 'Angell plot'.

The liquid-glass and liquid-liquid transitions of TPP at elevated pressure.

Abstract: We studied the polyamorphism of the liquid triphenyl phosphite by means of broadband dielectric spectroscopy at ambient and elevated (p=500 MPa) pressures. The effect of pressure on fragility, liquid-liquid phase transition, and its kinetics is discussed in relation to the two-order-parameter model proposed by Tanaka. The experimental evidence in support of this model is presented.

Universal critical-like scaling of dynamic properties in symmetry-selected glass formers.

Abstract: Evidence for a possible general validity of the critical-like behavior of dielectric relaxation time or viscosity τ,η∝(T−TC)−ϕ with ϕ→9 and TC

Anomalous temperature behavior of nonlinear dielectric effect in supercooled nitrobenzene.

Abstract: Studies of the nonlinear dielectric effect (NDE), describing changes of the dielectric permittivity induced by a strong electric field, in normal and supercooled liquid nitrobenzene are presented. An unusual increase of the stationary NDE, portrayed by the critical-like relation approximately (T-T+)(-1), was obtained. Nitrobenzene samples solidified at TS approximately T+ +2 K , approximately 10 K below the reported melting temperature Tm approximately 278 K . The anomalous increase of the NDE coincided with a slow relaxation process, detected in time-resolved measurements. The results presented offer a reinterpretation of the classical description by Piekara and Piekara [A. Piekara and B. Piekara, C. R. Hebd. Seances Acad. Sci. 203, 852 (1936)] of a positive sign contribution to the NDE in liquids. It is suggested that the obtained anomaly may be associated with the appearance of local quasinematic structures. This is supported by a speculative link to a general model for liquid-liquid transitions [H. Tanaka, Phys. Rev. E 62, 6968 (2000)] and a phenomenological model originally developed for the self-focusing of laser beams [J. Hanus, Phys. Rev. 178, 420 (1969)]. The case of the isotropic-nematic transition in liquid crystalline materials is also recalled. The NDE results reported here are related neither to the glass transition phenomenon nor to the recently developed concept of a second liquid-liquid transition.

Spinodal strength of liquids, solids and glasses

Abstract: With isotropic tri-axial stretching (negative pressure) and/or with heating, the thermodynamic stability limit (spinodal) of condensed matter—like solids, liquids and glasses—can be reached. In this paper, we analyse and compare the spinodal strength (i.e. the negative pressure necessary to reach the spinodal) of liquids, solids and glasses. Some examples with uni-axial stretches are also presented. Moreover, we discuss the possibility to step over the spinodal and to reach the region where the system can exhibit negative compressibility for a finite, nonzero time.

New evidence for a liquid?liquid transition in a one-component liquid

Abstract: Based on specific heat, x-ray diffraction and nonlinear dielectric effect (NDE) measurements the evidence for a liquid–liquid (L–L) transition in trans-1,2-dichloroethylene at TL−L≈Tm+23.6 K≈247 K is shown. The strong pre-transitional anomaly of NDE, similar to the one observed in the isotropic phase of nematic liquid crystals, makes it possible to estimate the weakly discontinuous character of the transition. Based on NDE measurements, the pressure evolution of the L–L transition up to almost 500 MPa was also obtained. Parameterization by the modified Simon–Glatzel-type equation made possible an insight into the negative pressure domain.

Solid–fluid phase transitions under extreme pressures including negative ones

Abstract: Upon heating or isotropic expansion, condensed matters (solids and liquids) can reach their stability limits, where the system must be stabilized by the appearance of a second phase. During isotropic expansion, the pressure can reach negative values. In auxetic materials, volume expansion is an inherent way of behavior, therefore, negative-pressure states can be reached easily during uni-axial stretch. In this paper, a comparison of various stability limits of pure and ordered one-component stretched solids will be mapped, with special focus on auxetic materials.