Showing papers by "Sylwester J. Rzoska published in 2003"

Changes in dynamic crossover with temperature and pressure in glass-forming diethyl phthalate.

Abstract: Dielectric relaxation measurements have been used to study the crossover in dynamics with temperature and pressure, onset of breakdown of the Debye-Stokes-Einstein law, and the relation between the \ensuremath{\alpha} and the \ensuremath{\beta} relaxations in diethyl phthalate. The measurements made over 10 decades in frequency and a broad range of temperature and pressure enable the dc conductivity and the \ensuremath{\alpha}- and the \ensuremath{\beta}-relaxations to be studied altogether. The isobaric data show that the \ensuremath{\alpha}-relaxation time ${\ensuremath{\tau}}_{\ensuremath{\alpha}}$ has temperature dependence that crosses over from one Vogel-Fulcher-Tammann-Hesse form to another at ${T}_{B}\ensuremath{\approx}227\mathrm{K}$ and ${\ensuremath{\tau}}_{\ensuremath{\alpha}}\ensuremath{\approx}{10}^{\ensuremath{-}2}\mathrm{s}.$ The dc conductivity \ensuremath{\sigma} exhibits similar crossover at the same ${T}_{B}.$ At temperatures above ${T}_{B},$ ${\ensuremath{\tau}}_{\ensuremath{\alpha}}$ and \ensuremath{\sigma} have the same temperature dependence, but below ${T}_{B}$ they become different and the Debye-Stokes-Einstein law breaks down. The breadth of the \ensuremath{\alpha} relaxation is nearly constant for $Tl{T}_{B},$ but decreases with increasing temperature for $Tg{T}_{B}.$ The time dependence of ${\ensuremath{\tau}}_{\ensuremath{\beta}}$ is Arrhenius, which when extrapolated to higher temperatures intersects ${\ensuremath{\tau}}_{\ensuremath{\alpha}}$ at ${T}_{\ensuremath{\beta}}$ nearly coincident with ${T}_{B}.$ Isothermal measurements at various applied pressures when compared with isobaric data show that the shape of the \ensuremath{\alpha}-relaxation depends only on ${\ensuremath{\tau}}_{\ensuremath{\alpha}},$ and not on the T and P combinations. At a constant temperature, while ${\ensuremath{\tau}}_{\ensuremath{\alpha}}$ increases rapidly with pressure, the \ensuremath{\beta}-relaxation time ${\ensuremath{\tau}}_{\ensuremath{\beta}}$ is insensitive to applied pressure. This behavior is exactly the same as found in ${1,1}^{\ensuremath{'}}$-bis (p-methoxyphenyl) cyclohexane. The findings are discussed in the framework of the coupling model.

Test of the Einstein-Debye Relation in Supercooled Dibutylphthalate at Pressures up to 1.4 GPa

Abstract: Broadband dielectric measurements were carried out on di-n-butyl phthalate (DBP) under isothermal conditions at hydrostatic pressures up to 1.6 GPa. A comparison of the dielectric relaxation times with the viscosity revealed that no breakdown of the Einstein-Debye relation is induced by high compression. This absence of any decoupling is attributed to the weak intermolecular cooperativity of DBP and its negligible change with pressure. Because of the latter, the dielectric spectra conform to time-pressure superpositioning.

Complex dielectric relaxation in supercooling and superpressing liquid-crystalline chiral isopentylcyanobiphenyl.

Abstract: Results of broadband dielectric studies in glass-forming liquid crystalline chiral isopentylcyanobiphenyl (5(*)CB) are presented. Tests conducted as a function of temperature and pressure revealed the coexistence of glassy and critical properties. The latter are associated with the isotropic-cholesteric phase transition at T(I-Ch) approximately 250 K under atmospheric pressure. Dielectric loss curves in the isotropic liquid and in the cholesteric phase are clearly broadened on cooling and pressuring towards the glass transition. Although in the isotropic phase there is a single stretched loss curve, in the mesophase an additional relaxation process can be distinguished. The evolution of relaxation times is non-Arrhenius and can be portrayed by the Vogel-Fulcher-Tamman relation or its pressure counterpart. The glassy dynamics coexists with the critical-like behavior for the static dielectric permittivity and for the maxima of the dielectric loss curves. Their temperature and pressure dependences are associated with the critical exponent phi=1-alpha approximately 1/2, where alpha approximately 1/2 is the specific heat critical exponent. This behavior is associated with the continuous phase transition placed at DeltaT approximately 1.5 K below the clearing temperature for P=0.1 MPa. It has been found that 5(*)CB shows a unique pressure-temperature phase diagram. Pressure and temperature changes which begin in the isotropic liquid below at ca. T approximately 265 K always result in the transition to the cholesteric phase which can be supercooled or superpressed. For T>265 K the phase transition to another phase, presumably a solid one, always occurs. However, a cholesteric-solid phase border seems to exist only in isothermal pressure tests. It does not appear in the temperature studies.

Critical behavior of dielectric permittivity and electric conductivity in temperature and pressure studies above and below the critical consolute point

Abstract: Results of comprehensive temperature (T) and pressure (P) studies of static dielectric permittivity (e′) and electric conductivity (σ) in the one- and two-phase regions of critical nitrobenzene–dodecane mixture are presented. A strong asymmetry of determined coexistence curves causes a strong violation of the law of rectilinear diameter. The obtained critical anomalies in the homogeneous phase [ehomo′(T),ehomo′(P),σhomo(P)] or the diameter of the binodal [emean′(T),emean′(P),σmean(T),σmean(P)] are associated with the same critical exponent φ=1−α ≈0.88, where α is the specific heat critical exponent. Critical anomalies for the isothermal, pressure path exhibit a set of favorite in comparison with results obtained in σ(T) and e(T) tests. They are: the negligible influence of the critical Maxwell–Wagner effect, the hardly visible appearance of the correction-to-scaling term, a more pronounced manifestation of critical anomalies, and a reduced number of fitted parameters. Particularly noteworthy is the eviden...

Departures from the correlation of time- and temperature-dependences of the α-relaxation in molecular glass-formers

Abstract: There is a well-known correlation between the shape of the relaxation function (nonexponentiality) and the temperature-dependence of the relaxation times (e.g., fragility), with broader relaxations associated with steeper Tg-normalized temperature dependences. Herein, exceptions to this correlation are described. Five molecular glass-formers, all having very similar relaxation functions, are found to exhibit a range of fragilities. We also show for two of these materials that, while pressure does not affect the breadth of the relaxation function, it substantially reduces the fragility.

Apparent exponents for the chain length dependence of the volume fraction in critical polymer solutions

Abstract: The dependence of the critical volume fraction at constant pressure as a function of the chain length of a polymer/solvent system can be described by a power law. The exponent of this power law is investigated based on an equation of state model and experimental data for various chain-molecule solutions here. The results are compared to recent molecular simulation data taken from the literature and analytical models. The theoretical models, simulation, and experimental data show that the exponent depends on the chain length of the dissolved chain molecules. The power law with a constant exponent is therefore not a universal relationship for this dependence. Based on the investigation of the chain length dependence a correlation for the critical volume fraction is proposed here. This function generalizes the Flory and a renormalization group model and is applied to the correlation of the experimental data. This more general relationship includes the power law with the exponent obtained from the Flory theory as limiting behavior. Some additional experimental data for oligomer solutions which are necessary for an investigation of the short chain length limit have been measured.

Effect of Temperature and Pressure on Segmental Relaxation in Polymethylphenylsiloxane

Abstract: Segmental relaxation in a series of polymethylphenylsiloxanes (PMPS) was studied using dielectric spectroscopy. The measurements covered a temperature range of more than 40 deg at pressures from ambient to 115 MPa. The results confirmed that the shape of the loss peak is independent of temperature, pressure and molecular weight. Consequently, the Tg -scaled dependence of the relaxation times was also independent of molecular weight. The pressure dependence of the relaxation times was characterized by means of the activation volume. This quantity changes markedly with pressure at a given temperature. However, the activation volume at the respective glass transition temperatures of the PMPS are essentially invariant to molecular weight. Finally, we measured the dependence of Tg on pressure, with the results well-described by the Andersson equation.

Critical Behavior of the Dielectric Modulus in Nitrobenzene-Dodecane Mixture

Abstract: The dielectric properties of a homogenous critical mixture of nitrobenzene-decane were studied in the range 1 Hz< f 1 Mz. The temperature dependences of the “static” dielectric permittivity e � (1 MHz) and the electric conductivity σ (1 Hz) exhibit pretransitional anomalies which may be associated with the same critical exponent φ = 1 − α ≈ 0.88, where α is the critical exponent of the specific heat. The same data were analyzed using the dielectric modulus representation. They show loss curves for the imaginary part of the modulus M �� ( f ). It was found that the temperature evolutions of the peak frequency τ = 1/2π fp and the peak maximum of M �� = M �� ( f p) also exhibit critical anomalies. Their forms resemble anomalies obtained for the imaginary part of the dielectric permittivity e �� ( f), carried out for 40 MHz < f <1 GHz, in an ethanol-dodecane critical mixture [S. J. Rzoska, K. Orzechowski, and A. Drozd-Rzoska, Phys. Rev. E 65, 042501 (2002)]. – PACS: 64.70.Ja, 77.20.+y , 64.60.Fr