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R Casalini

Publications and source records attributed to R Casalini.

17 recordsLinked to original sources
PubMed2006

Thermodynamic scaling of the viscosity of van der Waals, H-bonded, and ionic liquids.

Viscosities eta and their temperature T and volume V dependences are reported for seven molecular liquids and polymers. In combination with literature viscosity data for five other liquids, we show that the superpositioning of relaxation times for various glass-forming materials when expressed as a function of TV(gamma), where the exponent gamma is a material constant, can be extended to the viscosity. The latter is usually measured to higher temperatures than the corresponding relaxation times, demonstrating the validity of the thermodynamic scaling throughout the supercooled and higher T regimes. The value of gamma for a given liquid principally reflects the magnitude of the intermolecular forces (e.g., steepness of the repulsive potential); thus, we find decreasing gamma in going from van der Waals fluids to ionic liquids. For some strongly H-bonded materials, such as low molecular weight polypropylene glycol and water, the superpositioning fails, due to the nontrivial change of chemical structure (degree of H bonding) with thermodynamic conditions.

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PubMed2006

Thermodynamic interpretation of the scaling of the dynamics of supercooled liquids.

The recently discovered scaling law for the relaxation times, tau(T,upsilon) = I(Tupsilon(gamma)), where T is temperature and upsilon the specific volume, is derived by a revision of the entropy model of the glass transition dynamics originally proposed by Avramov [J. Non-Cryst. Solids 262, 258 (2000)]. In this modification the entropy is calculated by an alternative route. The resulting expression for the variation of the relaxation time with T and upsilon is shown to accurately fit experimental data for several glass-forming liquids and polymers over an extended range encompassing the dynamic crossover. From this analysis, which is valid for any model in which the relaxation time is a function of the entropy, we find that the scaling exponent gamma can be identified with the Gruneisen constant.

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PubMed2006

What can we learn by squeezing a liquid?

Relaxation times tau(T,upsilon) for different temperatures, T, and specific volumes, upsilon, collapse to a master curve vs Tupsilon(gamma), with gamma a material constant. The isochoric fragility, mV, is also a material constant, inversely correlated with gamma. From these experimental facts, we obtain a three-parameter function that accurately fits tau(T,upsilon) data for several glass-formers over the supercooled regime, without any divergence of tau below Tg. Although the values of the three parameters depend on the material, only gamma significantly varies; thus, by normalizing material-specific quantities related to gamma, a universal power law for the dynamics is obtained.

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PubMed2005

Isobaric and isochoric fragilities and the influence of volume on the temperature dependence of local segmental relaxation in polyvinylethylene networks.

The ratio of the isochoric and isobaric activation enthalpies at T(g) is used to quantify the relative contributions of the specific volume and temperature to the local segmental dynamics of polyvinylethylene networks. The dynamics of the networks become more dominated by temperature with increasing cross-link density, consistent with an increasing isobaric fragility. We also find a correlation between the isochoric and isobaric fragilities for the polyvinylethylene networks, in accord with a previously reported relationship between these two quantities for polymers and molecular glass-forming liquids [R. Casalini and C. M. Roland, Phys. Rev. E 72, 031503 (2005)].

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PubMed2005

Do theories of the glass transition, in which the structural relaxation time does not define the dispersion of the structural relaxation, need revision?

Upon decreasing temperature or increasing pressure, a noncrystallizing liquid will vitrify; that is, the structural relaxation time, taualpha, becomes so long that the system cannot attain an equilibrium configuration in the available time. Theories, including the well-known free volume and configurational entropy models, explain the glass transition by invoking a single quantity that governs the structural relaxation time. The dispersion of the structural relaxation (i.e., the structural relaxation function) is either not addressed or is derived as a parallel consequence (or afterthought) and thus is independent of taualpha. In these models the time dependence of the relaxation bears no fundamental relationship to the value of taualpha or other dynamic properties. Such approaches appear to be incompatible with a general experimental fact recently discovered in glass-formers: for a given material at a fixed value of taualpha, the dispersion is constant, independent of thermodynamic conditions (T and P); that is, the shape of the alpha-relaxation function depends only on the relaxation time. If derived independently of taualpha, it is an unlikely result that the dispersion of the structural relaxation would be uniquely defined by taualpha.

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PubMed2005

Why liquids are fragile.

The fragilities (T(g)-normalized temperature dependence of alpha-relaxation times) of 33 glass-forming liquids and polymers are compared for isobaric, mP, and isochoric, mV, conditions. We find that the two quantities are linearly correlated: mP = (37+/-3) + (0.84+/-0.05)mV. This result has obvious and important consequences, since the ratio mV/mP is a measure of the relative degree to which temperature and density control the dynamics. Moreover, we show that the fragility itself is a consequence of the relative interplay of temperature and density effects near T(g). Specifically, strong behavior reflects a substantial contribution from density (jammed dynamics), while the relaxation of fragile liquids is more thermally activated. Drawing on the scaling law log(tau) = I(T upsilon(gamma)), a physical interpretation of this result in terms of the intermolecular potential is offered.

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PubMed2005

Effect of chemical structure on the isobaric and isochoric fragility in polychlorinated biphenyls.

Pressure-volume-temperature data, along with dielectric relaxation measurements, are reported for a series of polychlorinated biphenyls (PCB), differing in the number of chlorine atoms on their phenyl rings. Analysis of the results reveals that with increasing chlorine content, the relaxation times of the PCB become governed to a greater degree by density rho relative to the effect of temperature T. This result is consistent with the respective magnitudes of the scaling exponent gamma yielding superpositioning of the relaxation times measured at various temperatures and pressures, when plotted versus rho(gamma)/T. While at constant (atmospheric) pressure, fragilities for the various PCB are equivalent, the fragility at constant volume varies inversely with chlorine content. Evidently, the presence of bulkier chlorine atoms on the phenyl rings magnifies the effect which the density has on the relaxation dynamics.

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PubMed2004

Comment on: "Disentangling density and temperature effects in the viscous slowing down of glass forming liquids" [J. Chem. Phys. 120, 6135 (2004)].

Recently, Tarjus et al. [G. Tarjus, D. Kivelson, S. Mossa, and C. Alba-Simionesco, J. Chem. Phys. 120, 6135 (2004)] concluded from a review of data for a variety of glass formers that the supercooled dynamics are almost invariably dominated by temperature T, rather than by density rho. By including additional published data into such a compilation, we show that for van der Waals molecular liquids, the dynamics near T(g) are in fact governed as much by density as by temperature. Moreover, relaxation times measured at various temperatures and pressures can be superimposed by plotting as a function rho(gamma)/T. This scaling form can arise from an assumed inverse power law for the intermolecular repulsive potential, with gamma a material constant. (c) 2004 American Institute of Physics.

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PubMed2004

Temperature and volume effects on the change of dynamics in propylene carbonate.

Dielectric relaxation and PVT measurements were carried out on propylene carbonate. From these, we show that thermal energy exerts a stronger influence than volume on the temperature dependence of the dynamic properties. Data obtained at all temperatures and pressures superimpose, when expressed as a function of T-1 V-3.7 . The scaling exponent is consistent with more thermally governed dynamics, and can be interpreted as a reflection of the soft nature of the potential. The change of dynamics observed in the conductivity and relaxation data transpires at a fixed value of either quantity, independent of temperature and pressure.

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PubMed2004

Thermodynamical scaling of the glass transition dynamics.

Classification of glass-forming liquids based on the dramatic change in their properties upon approach to the glassy state is appealing, since this is the most conspicuous and often-studied aspect of the glass transition. Herein, we show that a generalized scaling, log (tau) proportional, variant T-1 V-gamma, where gamma is a material constant, yields superpositioning for ten glass formers, encompassing van der Waals molecules, associated liquids, and polymers. The exponent gamma reflects the degree to which volume governs the temperature and pressure dependence of the relaxation times.

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PubMed2004

Viscosity at the dynamic crossover in o-terphenyl and salol under high pressure.

The viscosities of two prototypical glass formers, o-terphenyl and phenyl salicylate (salol), are shown to exhibit a change in their temperature and pressure dependences at a constant value of the viscosity. This is the first evidence of a dynamic crossover in the viscosity induced by pressure. The characteristic value associated with the change in dynamics is material dependent, but independent of temperature and pressure. These results are in accord with the previous finding, for other glass formers, that the dielectric relaxation time assumes a density-independent value at the dynamic crossover.

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PubMed2004

Adam-Gibbs model for the supercooled dynamics in the ortho-terphenyl ortho-phenylphenol mixture.

Dielectric measurements of the alpha-relaxation time were carried out on a mixture of ortho-terphenyl (OTP) with ortho-phenylphenol, over a range of temperatures at two pressures, 0.1 and 28.8 MPa. These are the same conditions for which heat capacity, thermal expansivity, and compressibility measurements were reported by Takahara et al. [S. Takahara, M. Ishikawa, O. Yamamuro, and T. Matsuo, J. Phys. Chem. B 103, 3288 (1999)] for the same mixture. From the combined dynamic and thermodynamic data, we determine that density and temperature govern to an equivalent degree the variation of the relaxation times with temperature. Over the measured range, the dependence of the relaxation times on configurational entropy is in accord with the Adam-Gibbs model, and this dependence is invariant to pressure. Consistent with the implied connection between relaxation and thermodynamic properties, the kinetic and thermodynamic fragilities are found to have the same pressure independence. In comparing the relaxation properties of the mixture to those of neat OTP, density effects are stronger in the former, perhaps suggestive of less efficient packing.

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PubMed2003

Effect of volume changes on segmental relaxation in siloxane polymers.

From dielectric relaxation and equation-of-state measurements on polymethylphenylsiloxane and polymethyltolylsiloxane, the relative contributions of volume and thermal energy to the temperature dependence of the segmental relaxation times are quantified. In both polymers, volume exerts a substantial effect, being almost as significant as thermal energy. A possible consequence of this prominent role of volume in governing the segmental dynamics is the finding that the relaxation times, measured for a series of temperature at various (fixed) pressures, can be expressed as a single function of the volume normalized by the volume at the glass transition temperature. A similar result is found for the (isothermal) relaxation times measured at various pressures.

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PubMed2003

Cohen-Grest model for the dynamics of supercooled liquids.

Recent experiments have established that, at least for van der Waals glass formers, volume fluctuations contribute significantly to the slowing down of the dynamics near T(g). Accordingly, we use the Cohen-Grest (CG) free-volume model to analyze dielectric relaxation data for six van der Waals liquids. The CG equation accurately describes the structural relaxation times over broader ranges of temperature than the more common Vogel-Fulcher relation. Moreover, the CG equation requires two less adjustable parameters when the data span the Stickel temperature T(B) associated with a change in the dynamics. The characteristic temperature T0 of the CG model can be identified with T(B), suggesting that the crossover reflects onset of percolation of the free volume. The CG parameters used to fit the structural relaxation times allow the free volume per liquidlike molecule to be calculated. These results, however, are at odds with free-volume estimates extracted from pressure-volume-temperature data.

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PubMed2001

Effect of pressure on the dynamics of glass formers.

A description of the pressure dependence of the structural relaxation time has been derived from the Adam-Gibbs theory by writing the configurational entropy in terms of the excess heat capacity and the molar thermal expansion. This new equation was tested successfully on dielectric relaxation data for an epoxy compound over a wide range of temperature and pressure.

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PubMed2001

Pressure dependence of structural relaxation time in terms of the Adam-Gibbs model.

A new equation describing the behavior of the structural relaxation time, tau(T,P), as a function of both pressure and temperature, is discussed. This equation has been derived from the Adam-Gibbs theory by writing the configurational entropy, S(c), in terms of the excess thermal heat capacity and of the molar thermal expansion. Consequently, the parameters introduced in the expression are directly related to specific physical properties of the material, such as the thermal expansion coefficient alpha and the isothermal bulk modulus K0. At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of tau versus temperature with the fragility parameter independent from pressure. The equation for tau(T,P) was successfully tested directly by fitting the dielectric relaxation time data for two isothermal and one isobaric measurements on diglycidyl ether of bisphenol-A, carried out in previous experiments. The parameters estimated by the best fit were in reasonable agreement with the values determined from the known physical properties of the material. Finally, the expression for the change versus pressure of the temperatures at which the same value of tau(max) is obtained (e.g., the change versus pressure of the glass transition temperature) agrees with several expressions previously proposed in the literature to provide a phenomenological description of the observed phenomena.

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