Showing papers by "Sylwester J. Rzoska published in 2007"

On the glass temperature under extreme pressures.

Abstract: The application of a modified Simon-Glatzel-type relation [Z. Anorg. Allg. Chem. 178, 309 (1929)] for the pressure evolution of the glass temperature is presented, namely, Tg(P)=Tg0[1+ΔP∕(π+Pg0)]1∕bexp[−(ΔP∕c)], where (Tg0,Pg0) are the reference temperature and pressure, ΔP=P−Pg0, −π is the negative pressure asymptote, b is the power exponent, and c is the damping pressure coefficient. The discussion is based on the experimental Tg(P) data for magmatic silicate melt albite, polymeric liquid crystal P8, and glycerol. The latter data are taken from Cook et al. [J. Chem. Phys. 100, 5178 (1994)] and from the authors’ dielectric relaxation time (τ(P)) measurements, which employs the novel pressure counterpart of the Vogel-Fulcher-Tammann equation: τ(P)=τ0Pexp[DPΔP∕(P0−P)], where ΔP=P−PSL (PSL is the stability limit hidden under negative pressure), P0 is the estimation of the ideal glass pressure, and DP is the isothermal fragility strength coefficient. Results obtained suggest the hypothetical maximum of the T...

On the pressure evolution of the melting temperature and the glass transition temperature

Abstract: The evolution of the melting temperature (Tm) and the glass temperature (Tg) from negative pressures up to very high pressures is discussed. It is based on the relation, T g,m ( P ) = T g,m 0 [ 1 + Δ P / ( π g,m + P g,m 0 ) 1 / b ] exp ( Δ P / c ) where ( T g,m 0 , P g,m 0 ) are the reference temperature and pressure, Δ P = P - P g,m 0 , c is the damping pressure coefficient and −πg,m estimates the negative pressure asymptote. Contrary to approximations used so far it is governed solely on pressure invariant coefficients πg,m, b and c. Their reliable estimation is possible basing on experimental data even limited to a moderate range of pressures. Both for Tm(P) and Tg(P) a possible maximum at extreme pressures and a negative pressure asymptote is suggested. The analysis was carried out for sodium, (Ca, Al)(Al, Si)O3 magmatic mixture, liquid crystalline 5CB, germanium, magmatic melt albite, selenium and epoxy resin EPON 828. For 5CB the isotropic–nematic orientational freezing was discussed, including the negative pressures domain. For EPON 828 the supplementary dielectric relaxation time (τ(P)) measurements were carried out. For the latter the analysis of τ(P) evolution is based on the modified Vogel–Fulcher–Tammann (VFT) equation, which makes an insight into the negative pressure domain possible: τ ( P ) = τ 0 P exp [ D P ( P - P S ) / ( P - P 0 ) ] , where P0 is the ideal glass VFT estimation, where DP is the fragility strength coefficient and PS is linked to the absolute stability limit. The obtained dependences enabled to address the question does fragility depends on pressure. For selenium both Tm(P) and Tg(P) behavior were possible to analyze, what yielded the experimental pressure dependence of the Turnbull’s Tg/Tm glass forming ability factor (GFA), linking the glass temperature and the melting temperature.

Anomalous behavior of secondary dielectric relaxation in polypropylene glycols

Abstract: A surprising slow down in the dielectric secondary γ-relaxation with temperature increasing near the glass transition is confirmed for several polypropylene glycols. The peculiar behavior diminishes as the molecular weight grows. The minimal model (Dyre and Olsen 2003 Phys. Rev. Lett. 91 155703) is applied successfully to describe the temperature dependences of the γ-relaxation times. The minimal model parameters are analyzed for different molecular weights. A molecular explanation of the γ-process anomaly for polypropylene glycols is proposed on the basis of the minimal model prediction.

Influence of pressure on the isotropic to smectic-E phase transition

Abstract: A phenomenological approach to the description of the pressure effect on the isotropic to smectic-E phase transition is proposed. The influence of pressure on the isotropic to smectic-E transition is discussed by varying the coupling between order parameters and pressure. The pressure dependence of the non-linear dielectric effect in the isotropic phase above the isotropic to smectic-E transition is calculated. The theoretical predictions are found to be in good agreement with experimental results.

Indirect methods to study liquid-liquid miscibility in binary liquids under negative pressure

Abstract: Liquids below the vapor pressure are metastable. A part of this metastable region is located at negative pressure. In spite of this metastability, these states can be reached experimentally and their lifetime can be long enough for a lot of experimental investigations. Homogeneous solution of partially miscible liquids can exhibit liquid-liquid phase transition by splitting into two liquid phases caused by the change of state parameters such as the pressure. The extensibility of the liquid–liquid critical curve towards negative pressure has been proposed first in the early fifties and finally realized experimentally in the nineties. The reason of this long delay is the experimental difficulty to work with metastable liquids. One has to avoid contamination, mechanical disturbance, etc., in order to suppress sudden boiling caused by cavitations and in order to keep the sample in liquid state. Due to these experimental difficulties, methods which can track liquid–liquid phase equilibrium lines into the metastable region without actually going below zero pressure are very valuable. This tracking is possible by studying the so-called pre-transitional anomalies. These anomalies usually can be seen only in the immediate vicinity of the phase transitions. Fortunately this “immediate vicinity” can be sometimes quite wide. In this paper, several methods mainly dielectric ones will be discussed and compared. Also, we will discuss the danger to get artificial liquid-liquid locus by these methods.

Confined liquid crystaline 5CB in 2D Thermodynamic Space - Preliminary Dielectric Relaxation Study

Abstract: Results of preliminary broadband dielectric spectroscopy studies in a wide range of temperatures and pressures range for a mixture of rod-like liquid crystalline 4–cyano–4–pentylalkylbiphenyl (5CB) and hydrophilic silica spheres (Aerosil 300) are shown. Pretransitional anomaly, observed previously in the bulk 5CB, has been found. Temperature dynamics of the mixture was investigated with via the DC conductivity σ, coupled to the reorientational relaxation. The derivative based analysis of electric conductivity showed a clear non-Arrhenius dynamics and indicated the anomalous increase of the fragility strength coefficient on approaching the isotropic-nematic transition. Pressure investigations of the solidification from the nematic phase showed the increase of the transition temperature on pressuring but with unusual increasing of dT NS /dP coefficient.

Glassy dynamics of rod-like liquid crystals: the influence of molecular structure

Abstract: The evolution of dielectric relaxation times in rod-like liquid crystalline n-octylcyanobiphenyl (8CB) and n-octyloxycyanobiphenyl (8OCB) is discussed. The linearized derivative-based analysis of data made it possible to identify dynamic crossovers linked to different Vogel–Fulcher–Tammann (VFT) equations. It is noteworthy that in the isotropic phase the dynamics can also be well portrayed by the critical-like relation but within the mode-coupling theory (MCT) for 8CB and the dynamical scaling model (DSM) for 8OCB. For both compounds the evolution of relaxation times in the Smectic A phase seems to follow the DSM description. Results obtained clearly indicate the dominant influence of heterogeneities on glassy dynamics in both liquid crystalline compounds.

Nonlinear dielectric spectroscopy near smectic A-smectic C* transition in ferroelectric liquid crystal DOBAMBC

Abstract: Key results associated with nonlinear dielectric effect (NDE) studies in liquids are recalled. Particular attention is paid to advantages and disadvantages of basic experimental implementations. Subsequently, the first ever results for the novel, frequency selective, NDE technique are presented. They are based on measurements in p-(n-decyloxy)benzylidene-p-amino-(2-butyl)cinnamate (abb. DOBAMBC), a ferroelectric liquid crystalline compound, near the SmA - SmC* transition. This effect is compared with the NDE behavior in the isotropic phase on DOBAMBC.

Orientationally disordered glassy phases

Abstract: Broadband dielectric spectroscopy (BDS) studies for a remarkable temperature range (230 K) in a supercooled orientationally disordered phase given rise to an orientational glass have been performed in mixed crystal formed between NPG ((CH3)2C(CH2OH)2) and NPA ((CH3)3C(CH2OH)) compounds. The α-relaxation slows down and the width of the dielectric spectra becomes broader with decreasing temperature without any additional relaxation process, making easy the fitting procedure of the relaxation loss spectra. The existence of three dynamic domains is made evident: For the high temperature region the relaxation time obeys an Arrhenius behavior, while when decreasing temperature two crossover temperatures define the temperature intervals for which two different VFT regimes are clearly visible. The shape parameters α and α·β according to the fit of HN function have been determined. Approaching T g , the common feature of many structural glass forming materials (decrease of the shape parameters with temperature) is found, indicating the strong deviation from the Debye behavior. Both parameters seem to reach the claimed universal value of 0.5.

Influence of Differences in Molecular structure on Behavior of and β Relaxation Processes in Diisooctyl Maleate

Abstract: Dielectric relaxation measurements have been performed to study effect of the molecular structure on the primary and secondary relaxations in two materials with very similar structure: diisooctyl phthalate (DIOP) and diisooctyl maleate (DIOM). The dielectric spectra were measured over ten decades of frequency and a broad range of temperature. The results show that the temperature dependence of the α-relaxation times cannot be described by a single Vogel–Fulcher–Tammann equation and the temperature dependence of the β-relaxation times in the glassy state obey the Arrhenius law with different activation energies, i.e., E A /k = 4231 K and E A /k = 2917 K for DIOP and DIOM, respectively. Moreover, third relaxation process visible as an excess wing of the high frequency part of the α loss peak was identified in dielectric spectra of DIOM.

Ordering effect on dynamics in glass-forming mixture of liquid crystals

Abstract: Relaxation dynamics in glass-forming mixture of liquid crystals (E7) is studied by means of dielectric spectroscopy in the frequency range from 10−2 up to 109Hz and over the temperature range from 200 to 373K. We analyzed both the shape of relaxation spectra and temperature dependencies of behavior is observed in nematic phase. The dominant relaxation process exhibits nearly Debye form that depends on temperature in clearly the non– Arrhenius fashion. The dielectric spectra also show second distingue relaxation process which is related to α-relaxation. Additionally, we carried out the measurements of mechanical relaxation. As a result it was found that mechanical relaxation times correspond to dielectric α-relaxation times.