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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.

For glass-forming substances, we show that the ratio ${E}_{\ensuremath{\beta}}{/RT}_{g}$ can be predicted quantitatively from the coupling model. Here ${E}_{\ensuremath{\beta}}$ is the glassy state activation enthalpy of the Johari-Goldstein \ensuremath{\beta} relaxation, ${T}_{g}$ is the glass transition temperature of the \ensuremath{\alpha} relaxation, and R is the gas constant. The calculated value is in good agreement with the experimental value in many glass formers. The results locate the origin of this cross correlation between ${E}_{\ensuremath{\beta}}$ of the Johari-Goldstein \ensuremath{\beta} relaxation and ${T}_{g}$ of the \ensuremath{\alpha} relaxation, although there are some notable exceptions to this cross correlation.

Results from a molecular dynamics simulation of a melt of unentangled polymers are presented. The translational motion, the large-scale and the local reorientation processes of the chains, as well as their relations with the so-called ‘‘normal’’ and ‘‘segmental’’ dielectric relaxation modes are thoroughly investigated in wide temperature and pressure ranges. The thermodynamic states are well fitted by the phenomenological Tait equation of state. A global time-temperature-pressure superposition principle of both the translational and the rotational dynamics is evidenced. The scaling is more robust than the usual Rouse model. The latter provides insight but accurate comparison with the simulation calls for modifications to account for both the local chain stiffness and the nonexponential relaxation. The study addresses the issue whether the temperature or the density is a dominant control parameter of the dynamics or the two quantities give rise to comparable effects. By examining the ratio ua tu/a P between the isochronic and isobaric expansivities, one finds that the temperature is dominant when the dynamics is fast. If the relaxation slows down, the fluctuations of the free volume increase their role and become comparable to those of the thermal energy. Detectable cross-correlation between the ‘‘normal-mode’’ and the ‘‘segmental’’ dielectric relaxations is found and contrasted with the usual assumption of independent modes. © 2004 American Institute of Physics. @DOI: 10.1063/1.1630293#

Recent studies relate the slowing down of the structural relaxation time of supercooled liquids to a decrease of the excess entropy ${S}_{\mathrm{exc}}$ of the melt with respect to the crystal. The theoretical basis of such a result was often found in the Adam Gibbs (AG) theory, implicitly assuming a proportionality between ${S}_{\mathrm{exc}}$ and the configurational entropy ${S}_{c}$ of the system. This work presents a direct test of the connection between dynamics and ${S}_{\mathrm{exc}}$ over a wide temperature and pressure interval for three different glass forming systems, o-terphenyl, triphenylchloromethane, and poly(methylmethacrylate). Moreover, a method, based on the AG theory, to estimate the isothermal and the isobaric contribution of ${S}_{\mathrm{exc}}$ to ${S}_{c}$ is proposed. Finally, some comments on the dependence of the fragility on pressure are discussed.

Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)≈10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)≈10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossove...

Starting from the original model of Adam and Gibbs, which gives a relationship between the structural relaxation time τ and the configurational entropy, an equation to describe the temperature and pressure behavior of τ has been recently derived. Herein a test of the validity of this new equation is presented by means of dielectric, calorimetric and volumetric data.

Previous studies have shown that wettability has an effect on the complex permittivity of rock in a wide frequency interval. At low frequencies ( Several experiments carried out on synthetic porous media and reservoir rocks made strongly oil wet using a sylanisation process have been published by the authors [Rev. Inst. Fr. Pet. 53 (1994) 771; Bona, N., Rossi, E., Capaccioli, S., 1999a. Wettability and dielectric constant. Proc. of the 1999 Int. Symp. of the Society of Core Analysts, Golden, Colorado, paper SCA 9925; Bona, N., Rossi, E., Capaccioli, S., 1999b. Electrical measurements in the 100 Hz–10 GHz frequency range for efficient rock wettability evaluation. Proc. of the 1999 SPE Int. Symp. on Oilfield Chemistry, Houston, paper SPE 50720, 207–217; J. Phys. D: Appl. Phys. 33 (2000) 1036]. This paper summarizes the results obtained on “fresh-state” rocks with preserved wettability. Two reservoirs were studied. One is a fractured oil reservoir, with a permeability so low that it cannot be investigated using the conventional wettability test techniques. The other is a carbonate reservoir, with Amott test results available. While the second reservoir was found to be strongly water wet, the first showed the coexistence of water wet and oil wet zones, which were clearly recognised by two distinct relaxation peaks in the dielectric spectra. Air-mercury pore size distribution measurements and CryoSEM analyses were also carried out. All the collected data were used to build a consistent model of the behaviour of the rock.

The dynamics of the epoxy resin phenyl glycidyl ether, a fragile glass-forming liquid, is investigated in the liquid and supercooled phases by time-resolved optical Kerr effect experiment with an heterodyne detection technique. We tested the mode-coupling theory and found that the predicted dynamic scenario allows to reproduce properly the measured signal, for t1 ps, in the whole temperature interval investigated. Furthermore, the values of T(c) and lambda, obtained from the analysis of three different and independent dynamic regimes (alpha regime, von Schweidler, beta regime), are in remarkable agreement. Moreover, relaxation times obtained from optical Kerr effect and dielectric spectroscopy measurements are compared. The two time scales differ only for a constant factor in the whole temperature interval investigated.

We present a comparative analysis of dielectric spectroscopy and optical Kerr effect measurements of relaxation processes in glass-forming phenyl glycidyl ether. In particular we focus our attention on the structural α-relaxation process and the most important finding is the existence of a constant scaling factor between the characteristic times obtained by the two experiments in the high-temperature region.

A comparative study is reported on the dynamics of a glass-forming system when the glass transition is approached through different paths: cooling, compression, and polymerization. In particular, the influence of temperature, pressure and connectivity on the dynamics has been investigated by dielectric spectroscopy. A phenomenological description of the behavior of the dielectric response shows the existence of both differences and deep similarities between the change of some dielectric parameters during a polymerization reaction and during the temperature or pressure changes. A unified reading of our experimental results can be given in the framework of the Adam-Gibbs theory.

The dynamics of the epoxy compound polyphenyl-glycidyl-ether-coformaldehyde was investigated by wideband dielectric spectroscopy (10/sup -2/ to 10/sup 10/ Hz) by varying either pressure or temperature. The measurements were undertaken in two different conditions: under isobaric condition (atmospheric pressure) varying temperature down to the supercooled and glassy phases and under isothermal conditions by changing the pressure from 0.1 to 270 MPa. The analysis of the structural relaxation time behavior either varying the temperature or the pressure was found to agree well with a previously derived expression for /spl tau/(T, P) based on a cooperative model. In particular it was found that with a unique set of parameters it is possible to describe the 2-dimensional surface /spl tau/(T, P) over a wide range of temperature and pressure. This result agrees well with the observed independence of the fragility in both isothermal and isobaric conditions.

Summary This paper summarizes an experimental and theoretical study conducted to investigate the effect of rock wettability on the electrical response of water- and oil-saturated core samples in a wide frequency band. The aim of the study was to evaluate a technique not previously applied to wettability determination as a practical alternative to conventional tests, and to ascertain the consequent economic and technical benefits in reservoir appraisal.

The dynamics of the glass-forming tri-epoxy triphenylolmethane triglycidyl ether (TPMTGE) was investigated in the supercooled and glassy state by broadband dielectric spectroscopy. Measurements were performed in a frequency range between 10-2 Hz and 1010 Hz for temperatures in the range between 120 and 350 K. The dielectric response revealed the existence of two relaxation processes: the lower frequency one (α-relaxation) slows down very rapidly on cooling the system and leaves the experimental window on approaching the glass transition temperature Tg; the higher frequency one (β-relaxation) is observable both above and below Tg. The β-relaxation is comparatively quite fast, thus a large separation between the main and the secondary peaks is observed, and no complete merging of the α- and the β-process is found within the experimental window even at the highest temperatures. Besides, a third relaxation (β'), very weak indeed, was observed in the glassy state. The detailed analysis performed on the experimental data indicated that a single Vogel-Fulcher-Tamman equation well describes the change of the α-relaxation time in the entire temperature interval. In the same temperature range, the conductivity and the α-relaxation time are related to each other by a fractional Debye-Stokes-Einstein law. Moreover, our results confirmed the proportionality between the β-activation energy and the glass-transition temperature, which was found in other systems. As observed in previous experiments, both the shape parameters and the relaxation strength of β-relaxation are strongly affected by the transition from liquid to glassy state.

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.

A new equation describing the behavior of the structural relaxation time, $\ensuremath{\tau}(T,P),$ 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 \ensuremath{\alpha} and the isothermal bulk modulus ${K}_{0}.$ At a fixed pressure, for low pressures, the found equation reduces to a Vogel-Fulcher-Tammann equation of \ensuremath{\tau} versus temperature with the fragility parameter independent from pressure. The equation for $\ensuremath{\tau}(T,P)$ 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 ${\ensuremath{\tau}}_{\mathrm{max}}$ 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.

The permittivity of different microporous systems, including sintered glass filters, Berea sandstones and reservoir rocks, was measured in the frequency range 102 -1010 Hz. For each system, the natural pore wettability was modified to get two different sets of samples: those strongly oil-wet and those water-wet. The samples were saturated with a hydrocarbon oil and water at different degrees of water content. Dielectric spectra showed a conductivity contribution due to ionic carriers in addition to a Maxwell-Wagner-Sillars relaxation, active up to high frequencies. Because of the dominant role that wettability plays in the electrical response of partially saturated microporous solids, water-wet and oil-wet samples showed remarkable differences in the loss tangent spectra, as well as different times and strengths of the Maxwell-Wagner-Sillars relaxation. The dielectric spectra were analysed by using some standard models for extracting valuable information about the pore wettability. Moreover, the use of impedance measurements for in situ determination of the wettability of reservoir rocks was evaluated.

This paper summarises the experimental and theoretical study conducted in ENI/Agip to investigate the effect of rock wettability on the dielectric response of water and oil saturated core samples. The aim of the study was to evaluate a dielectric technique not previously applied to wettability determination as a practical alternative to conventional tests, and to ascertain the consequent economic and technical benefits in reservoir appraisal.

The dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below to above the glass transition temperature, Tg, has been investigated through the temperature behavior of relaxation times, strengths, and conductivity, determined in a wide frequency range (102–2×1010 Hz). In all systems the main and secondary relaxations define a splitting temperature TS∼1.3×Tg; moreover, a crossover temperature TB∼TS is recognized, marking the separation between two different Vogel–Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribution of the overall energy between the relaxation processes and no peculiar behavior is revealed at TS. A strong increase characterizes the strength of the secondary relaxation on crossing the glass transition from the lower temperatures. Conductivity data have been analyzed to test the dynamics in terms of the Debye–Stokes–Einstein (DSE) diffusion law. The prediction of the DSE model is well verified for mono- and diepoxide up to the high viscosity...

The conducting polymer poly(3n-decylprrole) (P3DP) has a promising chemical stability and processability. The long alkylic chains makes P3DP soluble in common organic solvents even if they partially hinder a regular molecular arrangement. On account of structural disorder, the materials has a moderate conductivity despite the high doping level. The d.c. conductivity and the dielectric response of P3DP films with different dopants and synthesis conditions were measured at temperatures between 80 and 300 K. The d.c. conductivity of all the samples was well described by the variable range hopping model; the hopping parameters were found to be much affected by the synthesis and doping conditions. The electrical response exhibited a well-defined relaxation peak, visible only after deducting the d.c. conductivity contribution from the loss factor. The temperature behavior of the loss peak frequency parallelled that of the d.c. conductivity. The Barton-Nakajima-Namikawa equation, relating d.c. conductivity, relaxation time and relaxation strength, was verified. The relaxation strength, too large for being connected with a dipolar relaxation, was ascribed to the displacement of hoping charges. The conclusion of the analysis has been that the electrical response of the system was dominated by the hopping charge transport.

The dynamics of monoepoxide (CGE) and diepoxide (DGEBA) compounds, investigated by wideband (102–1010 Hz) dielectric spectroscopy from below to above the glass transition temperature in the range 130–360 K, shows a bimodal relaxation and the presence of a dc conductivity contribution. The fitting of the data by the Havriliak–Negami function provided the temperature behaviour of conductivity, relaxation times, dielectric strengths and shape parameters. The glass transition phenomenon was detected by the changes of both the relaxation strength and the low frequency slope of the secondary relaxation, and a transition temperature TB > Tg between two different relaxation regimes, coincident with the splitting temperature of the systems, was also recognised. The comparative analysis of the results draws an overall picture which improves our information on the dynamics of the systems and also supports some recent ideas on the splitting between main and secondary relaxations.

A novel approach to the dielectric analysis of relaxation phenomena in glass-forming liquids is discussed. Instead of cooling, the system is polymerized so that the glass transition is approached by a chemical vitrification process. Measurements were carried out in a wide frequency interval covering 7 decades, from 103 to 1010 Hz, on different polymerizing systems, namely two epoxy systems, the butyl acrylate and an unsaturated polyester. The relaxation function adopted for extracting the dielectric parameters is validated through a careful verification of the chemical and physical meaning of the changes observed in the dielectric susceptibility as the reaction goes on. The evolution of the shape of the relaxation function is discussed within the percolation theories developed or undercooled liquids and a satisfactory agreement between experimental results and theoretical predictions was found. The scaling behavior vs. conversion of relaxation times of both main and secondary processes closely parallels those observed in many glass-formers vs. temperature and it can well be represented by Vogel Fulcher-like Arrhenius-like equations, respectively.© (1997) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

By now it is well established that the structural α-relaxation time, τ(α), of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, τ(α) is a function Φ of the product variable, ρ(γ)/T, where ρ is the density and T the temperature. The constant γ as well as the function, τ(α) = Φ(ρ(γ)/T), is material dependent. Actually this dependence of τ(α) on ρ(γ)/T originates from the dependence on the same product variable of the Johari-Goldstein β-relaxation time, τ(β), or the primitive relaxation time, τ(0), of the coupling model. To support this assertion, we give evidences from various sources itemized as follows. (1) The invariance of the relation between τ(α) and τ(β) or τ(0) to widely different combinations of pressure and temperature. (2) Experimental dielectric and viscosity data of glass-forming van der Waals liquids and polymer. (3) Molecular dynamics simulations of binary Lennard-Jones (LJ) models, the Lewis-Wahnström model of ortho-terphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO(3))(2)·3KNO(3) (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ρ(γ)/T with the same γ. (5) In polymers, the chain normal mode relaxation time, τ(N), is another function of ρ(γ)/T with the same γ as segmental relaxation time τ(α). (6) While the data of τ(α) from simulations for the full LJ binary mixture obey very well the thermodynamic scaling, it is strongly violated when the LJ interaction potential is truncated beyond typical inter-particle distance, although in both cases the repulsive pair potentials coincide for some distances.

Local dielectric spectroscopy is performed to study how relaxation dynamics of a poly-vinyl-acetate ultra-thin film is influenced by inorganic nano-inclusions of a layered silicate (montmorillonite). Dielectric loss spectra are measured by electrostatic force microscopy in the frequency-modulation mode in ambient air. Spectral changes in both shape and relaxation time are evidenced across the boundary between pure polymer and montmorillonite sheets. Dielectric loss imaging is also performed, evidencing spatial variations of dielectric properties near to nanostructures with nanometer scale resolution., Comment: 21 pages, 4 figures, 1 table

The alpha-relaxation dynamics of 1-cyano-adamantane (CNA) and its mixtures with 1-chloro-adamantane (ClA) has been studied by means of broadband dielectric spectroscopy. The existence of orientationally disordered (OD) face centered cubic mixed crystals (ClA(1-X)CNA(X)) for 0.5or = Xor = 1 has been put in evidence by thermodynamics and structural analyses. In addition to the OD phase of CNA, mixed crystals with compositions higher than the equimolar one exhibit a freezing of the orientational degrees of freedom into a glassy state, which involves also a strong increase of the antiferroelectric order at temperatures higher than the dielectric glass transition temperature. This experimental evidence is revealed by a stairlike effect in the variation of the Kirkwood factor with the temperature as a consequence of a twin effect in the dielectric strength without any anomaly in the temperature-density curves. The characteristic relaxation times are analyzed as a function of temperature and mole fraction. By setting a common temporal origin ("isochronal origin") at tau(T(g)) = 100 s for each mole fraction, it emerges that the substitution of ClA molecules by those of CNA (diminution of X) gives rise to a slow down in the dynamics, despite that the molecular volume of ClA molecules are smaller than those of CNA. This fact goes along and is accompanied by a diminution of the lattice packing with the decrease of composition. It is also shown that the heterogeneities produced by the concentration fluctuations due to the chemical disorder are the main contribution to the non-exponential character of the alpha-relaxation peaks.

Relaxation in glass-forming substances is necessarily a many-body problem because of intermolecular interactions and constraints. Results from molecular dynamics simulations and experiments are used to reveal the critical elements and general effects originating from many-body relaxation, but not dealt with in conventional theories of the glass transition. Although many-body relaxation is still an unsolved problem in statistical mechanics, these critical elements and general effects will serve as guides to the construction of a satisfactory theory of the glass transition. This effort is aided by concepts drawn from the coupling model, whose predictions have been shown to be consistent with experimental facts.

The mechanisms governing mechanical dissipation in amorphous tantala are studied at microscopic scale via Molecular Dynamics simulations, namely by mechanical spectroscopy in a wide range of temperature and frequency. We find that dissipation is associated with irreversible atomic rearrangements with a sharp cooperative character, involving tens to hundreds of atoms arranged in spatially extended clusters of polyhedra. Remarkably, at low temperature we observe an excess of plastically rearranging oxygen atoms which correlates with the experimental peak in the macroscopic mechanical losses. A detailed structural analysis reveals preferential connections of the irreversibly rearranging polyhedra, corresponding to edge and face sharing. These results might lead to microscopically informed design rules for reducing mechanical losses in relevant materials for structural, optical, and sensing applications.

Bituminous concrete were irradiated with intense electromagnetic fields at 2.45GHz in a laboratory microwave oven The observed heating rate was similar to that of an equal volume of water put in th...

The dynamics of a fragile glass-former (diglycidyl ether of bisphenol-A) has been studied by wideband dielectric spectroscopy down to the temperature-induced and the pressure-induced glass transition. A full check of the temperature and pressure dependence of the structural relaxation time, predicted within an extended free volume model by the generalised Cohen–Grest function, is reported. From our investigation it is evident that this function suitably describes the structural relaxation time at ambient pressure, but on the other hand, it strikingly fails in describing the isothermal data under variable pressure.

When a liquid is cooled to produce a glass its dynamics, dominated by the structural relaxation, become very slow, and at the glass-transition temperature Tg its characteristic relaxation time is about 100 s. At slightly elevated temperatures (~1.2 Tg) however, a second process known as the Johari-Goldstein relaxation, βJG, decouples from the structural one and remains much faster than it down to Tg. While it is known that the βJG-process is strongly coupled to the structural relaxation, its dedicated role in the glass-transition remains under debate. Here we use an experimental technique that permits us to investigate the spatial and temporal properties of the βJG relaxation, and give evidence that the molecules participating in it are highly mobile and spatially connected in a system-spanning, percolating cluster. This correlation of structural and dynamical properties provides strong experimental support for a picture, drawn from theoretical studies, of an intermittent mosaic structure in the deeply supercooled liquid phase., The Johari-Goldstein relaxation is a precursor of the glass transition in liquids. Caporaletti et al. use time-dependent interferometry data to substantiate its suggested structural appearance as a globally percolating, fluctuating mosaic.