Dielectric Relaxation Experiments Research Articles

Dielectric Study of n-Propanol during Physical Vapor Deposition: NoSurface Mobility and No Kinetic Stability.

Dielectric relaxation experiments have been performed on n-propanol (NPOH) films during physical vapor deposition at temperatures above and below its glass transition, Tg = 97 K. The results for NPOH are compared with those of analogous experiments on methyl-m-toluate (MMT) and 2-methyltetrahydrofuran (MTHF), with all three deposited at the same reduced temperature, 0.82Tg. While MMT and MTHF display clear signs of a highly mobile surface layer, no such feature is observed for NPOH. The existence of this in situ observed mobile surface layer correlates perfectly with the material's ability to form kinetically stable glasses, as NPOH differs from MMT and MTHF by not displaying kinetic stability.

Soft pinning: Experimental validation of static correlations in supercooled molecular glass-forming liquids.

Enormous enhancement in the viscosity of a liquid near its glass transition is a hallmark of glass transition. Within a class of theoretical frameworks, it is connected to growing many-body static correlations near the transition, often called "amorphous ordering." At the same time, some theories do not invoke the existence of such a static length scale in the problem. Thus, proving the existence and possible estimation of the static length scales of amorphous order in different glass-forming liquids is very important to validate or falsify the predictions of these theories and unravel the true physics of glass formation. Experiments on molecular glass-forming liquids become pivotal in this scenario as the viscosity grows several folds (), and simulations or colloidal glass experiments fail to access these required long-time scales. Here we design an experiment to extract the static length scales in molecular liquids using dilute amounts of another large molecule as a pinning site. Results from dielectric relaxation experiments on supercooled Glycerol with different pinning concentrations of Sorbitol and Glucose, as well as the simulations on a few model glass-forming liquids with pinning sites, indicate the versatility of the proposed method, opening possible new avenues to study the physics of glass transition in other molecular liquids.

Intra- vs Intermolecular Cross-Links in Poly(methyl methacrylate) Networks Containing Enamine Bonds

The molecular dynamics of a copolymer composed of methyl methacrylate (MMA) and (2-acetoacetoxy)ethyl methacrylate (AEMA) monomers and the influence on it of intra- to intermolecular cross-links of AEMA units with ethylenediamine (EDA) was studied by combining dielectric relaxation experiments and thermal investigations. The dielectric spectra of the non-cross-linked copolymer show three dynamical processes: a slow relaxation (α) and a faster (β), both dominated by the MMA dynamics, and an even faster secondary relaxation (γ) reflecting the AEMA dynamics. Already for low cross-linking densities, the γ process is very much affected and eventually disappears, increasing the cross-linking density. The secondary β relaxation however was nearly unaffected by cross-linking. The effect of cross-linking on the α relaxation was very pronounced with an important increasing of the glass transition temperature Tg. There was also an increase of the dynamic heterogeneity and the relaxation intensity when increasing the cross-linking density (up to the maximum explored, 9 mol % EDA). The quality of the average time scale and Tg value have similarities in behavior for intra- and intermolecular cross-linking, but clear differences in the dynamic heterogeneities where observed. These differences can be interpreted in connection with the sparse internal structure of the collapsed single chains obtained by intramolecular cross-linking.

Substituted Thiols in Dynamic Thiol–Thioester Reactions

The thiol–thioester reaction has emerged as a promising method for developing covalent adaptable networks (CANs) due to its ability to exchange rapidly under low temperature conditions in a number of solvents, orthogonality among other functional groups, and tunability. Here, the effects of thiol substitution (i.e., primary vs secondary) were assessed with respect to their reactivity in two dynamic thioester reactions: the thiol–thioester exchange and the reversible thiol–anhydride addition. Model NMR experiments were conducted using small-molecule compounds to observe how polymers of similar components would behave in thiol–thioester exchange. It was determined that the Keq was near unity for mixtures of primary thiols and secondary thioesters, and vice versa, in both a polar solvent, DMSO-d6, and at most slightly favors primary thioesters in a relatively nonpolar solvent, CDCl3. Dielectric spectroscopy and stress relaxation experiments were used to determine the relaxation times and activation energies of the two thioester-containing networks: Thiol-ene networks, which undergo thioester exchange, displayed activation energies of 73 and 71 kJ/mol from dielectric measurements and 36 and 53 kJ/mol from stress relaxation for the primary and secondary thiols, respectively. Thiol–anhydride-ene networks, which undergo both thioester exchange and reversible thiol–anhydride addition, displayed activation energies of 94 and 114 kJ/mol from dielectric and 111 and 139 kJ/mol from stress relaxation for primary and secondary thiols, respectively. In both types of networks, the secondary thioester-based networks demonstrated slower dynamics as compared to the same primary network by at least one order of magnitude. In the anhydride network, the secondary thiol also biased the dynamics toward reversible addition.

Structural Relaxation and Recovery: A Dielectric Approach.

We compare structural relaxation and structural recovery dynamics for molecular glass-formers, both measured by dielectric techniques in the regime of linear responses. It is emphasized that structural recovery restores ergodicity, whereas structural relaxation or α-processes characterize fluctuations of the system in equilibrium (and thus do not involve a change of structure within experimental resolution). Evidence is provided that structural recovery is linked to rate exchange and thus is distinct from structural relaxation dynamics, even in the limit of small perturbations. As a consequence, structural recovery is somewhat slower and more exponential than the equilibrium dynamics as derived, for instance, from low field dielectric relaxation experiments. This contrasts the standard assumption inherent in models of physical aging, which assume the identity of both responses if measured in the limit of a small perturbation. Typical experiments associated with physical aging and scanning calorimetry involve nonlinear responses and are thus even more complex.

Dielectric response of light, heavy and heavy-oxygen water: isotope effects on the hydrogen-bonding network's collective relaxation dynamics.

Isotopic substitutions largely affect the dielectric relaxation dynamics of hydrogen-bonded liquid water; yet, the role of the altered molecular masses and nuclear quantum effects has not been fully established. To disentangle these two effects we study the dielectric relaxation of light (H216O), heavy (D216O) and heavy-oxygen (H218O) water at temperatures ranging from 278 to 338 K. Upon 16O/18O exchange, we find that the relaxation time of the collective orientational relaxation mode of water increases by 4-5%, in quantitative agreement with the enhancement of viscosity. Despite the rotational character of dielectric relaxation, the increase is consistent with a translational mass factor. For H/D substitution, the slow-down of the relaxation time is more pronounced and also shows a strong temperature dependence. In addition to the classical mass factor, the enhancement of the relaxation time for D216O can be described by an apparent temperature shift of 7.2 K relative to H216O, which is higher than the 6.5 K shift reported for viscosity. As this shift accounts for altered zero-point energies, the comparison suggests that the underlying thermally populated states relevant to the activation of viscous flow and dielectric relaxation differ.

Invariance of the relation between α relaxation and β relaxation in metallic glasses to variations of pressure and temperature

Dielectric relaxation experiments performed at ambient and elevated pressures P in molecular, ionic, and polymeric glass formers have established that the relation of the Johari-Goldstein (JG) \ensuremath{\beta}-relaxation time ${\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)$ to the \ensuremath{\alpha}-relaxation time ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)$ is invariant to changes of T and P while the latter is kept constant. This property of the JG \ensuremath{\beta} relaxation is remarkable despite the fact that the invariance of the ratio ${\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)/{\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)$ from experiment is sometimes approximate because the \ensuremath{\beta} relaxation is composed of a distribution of processes, and the ${\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)$ determined can be arbitrary. The property indicates the fundamental importance of the JG \ensuremath{\beta} relaxation and it cannot be neglected whenever the \ensuremath{\alpha} relaxation is considered. Notwithstanding, the property has not been checked on whether it applies to metallic glasses. Conventional experiment techniques cannot fulfill the task, and the alternative is molecular dynamics simulations. In this paper we report the results of molecular dynamics simulations of dynamical mechanical spectroscopy performed on two very different metallic glasses, $\mathrm{Z}{\mathrm{r}}_{50}\mathrm{C}{\mathrm{u}}_{50}$ and $\mathrm{N}{\mathrm{i}}_{80}{\mathrm{P}}_{20}$, at different pressures P. The JG \ensuremath{\beta} relaxation appears as an excess wing on the low-temperature side of the \ensuremath{\alpha} loss peak at ${T}_{\ensuremath{\alpha},P}$ in the isochronal loss modulus spectra ${{E}_{P}}^{\ensuremath{''}}(T)$. On the other hand, the isochronal non-Gaussian parameter $\ensuremath{\alpha}{2}_{P}(T)$ peaks at the temperature ${T}_{\ensuremath{\alpha}2,P}$ different from ${T}_{\ensuremath{\alpha},P}$ of ${{E}^{\ensuremath{''}}}_{P}(T)$. From the fact that ${T}_{\ensuremath{\alpha}2,P}$ is significantly lower than ${T}_{\ensuremath{\alpha},P}$, we identified the peak temperature ${T}_{\ensuremath{\alpha}2,P}$ of $\ensuremath{\alpha}{2}_{P}(T)$ with the JG \ensuremath{\beta} relaxation, and hence the JG \ensuremath{\beta} relaxation is fully resolved by studying the isochronal non-Gaussian parameter $\ensuremath{\alpha}{2}_{P}(T)$. After scaling temperature by ${T}_{\ensuremath{\alpha},P}$, the normalized ${{E}_{P}}^{\ensuremath{''}}(T/{T}_{\ensuremath{\alpha},P})$ and $\ensuremath{\alpha}{2}_{P}(T/{T}_{\ensuremath{\alpha},P})$ both show superposition of data taken at various pressures for all $T/{T}_{\ensuremath{\alpha},P}$ covering the JG \ensuremath{\beta} relaxation and the \ensuremath{\alpha} relaxation. Moreover the ratio ${T}_{\ensuremath{\alpha}2,P}/{T}_{\ensuremath{\alpha},P}$ is invariant to changes of T and P while ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)$ is maintained constant. Thus we have verified for two different metallic glasses, $\mathrm{Z}{\mathrm{r}}_{50}\mathrm{C}{\mathrm{u}}_{50}$ and $\mathrm{N}{\mathrm{i}}_{80}{\mathrm{P}}_{20}$, that ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)/{\ensuremath{\tau}}_{\ensuremath{\beta}}(T,P)$ is invariant to changes of T and P at constant ${\ensuremath{\tau}}_{\ensuremath{\alpha}}(T,P)$, as found in soft matter.

Modeling the high frequency mechanical relaxation of simplified industrial polymer mixtures using dielectric relaxation results

The mechanical characterization of rubber materials at the frequencies relevant for the tire function during braking (about 1 MHz) is extremely challenging. On the other hand, dielectric relaxation experiments provide a suitable tool for the analysis of relaxation phenomena over a broad frequency range with high sensitivity. Here we show that by analyzing dielectric relaxation experiments in the framework of a simple model the mechanical relaxation of simplified industrial systems based on mixtures of a styrene butadiene rubber (SBR) with a polystyrene oligomer can be anticipated, at least for the SBR rich mixtures that are more relevant for development of tire polymers. The model needs the description of the dielectric and mechanical relaxation processes of the single components of the mixture and uses concepts and ideas introduced in the past for describing the segmental dynamics of athermal polymer blends. The model description of dielectric relaxation experiments on the mixtures provides a quantification of the thermal fluctuations of concentration in the blends. With this input, the mechanical relaxation of the mixtures are calculated using well established equations for the mechanical properties of composite materials. The present approach should be helpful in a rational design of the rubber material depending on the specific requirements.

Dielectric Relaxation and Hydration Interactions for Protic and Aprotic Ionic Liquids using Time Domain Reflectometry.

H-Bonding abilities of ionic liquids (ILs) along with hydrophobicity and cooperativity effects increases their hydration numbers making them capable for dissolving sparingly soluble organic molecules in aqueous or polar nonaqueous media, and hence ILs are potential candidates in pharmaceutical and medicinal sciences besides the different technological and academic interests. In this work, dielectric spectra were measured and analyzed for diethylammonium-based protic ionic liquids (PILs), imidazolium-based aprotic ionic liquids (APILs), and their aqueous solutions (∼0.02 to ∼0.8 mol·dm-3) over a frequency range from 0.01 to 50 GHz using time domain reflectometry at 298.15 K. The Cole-Cole (CC) model for neat ILs and a combination of the Debye and Cole-Cole (D+CC) models for their aqueous solutions best describes the experimental dielectric relaxation spectra. Higher values of static permittivity and relaxation time were observed for less viscous PILs compared to more viscous APILs due to the existence of hydrogen bonding in PILs, ionic translational motion, and the existence of transient, short-lived proton transfer responsible for solvent polarization. For aqueous solutions of ionic liquids, the fast collective relaxation of solvent (bulk water) observed at higher frequencies (∼20 GHz) and slow relaxation is detected at lower frequency (∼5 to ∼10 GHz) due to hydrophobic hydration with or without cooperative H-bonding effect. The apparent concentrations of bulk water, cbwap, and slow water, cswap, were used to obtain effective hydration numbers to understand the ion solvation. Hydration numbers revealed that imidazolium-based APILs are weakly hydrated than the diethylammonium-based PILs. Static permittivity and relaxation time of pure ILs and of aqueous solutions of studied ILs are discussed in terms of effect on alkyl chain length of cation/anion, H-bonding abilities of ions, dipole moments of ions, viscosity, hydrophobic effects, etc.

Molecular Insights into Dipole Relaxation Processes in Water-Lysine Mixtures.

Dielectric spectroscopy is a robust method to investigate relaxations of molecular dipoles. It is particularly useful for studies of biological solutions because of the potential of this method to cover a broad range of dynamical time scales typical for such systems. However, this technique does not provide any information about the nature of the molecular motions, which leads to a certain underemployment of dielectric spectroscopy for gaining microscopic understanding of material properties. For such detailed understanding, computer simulations are valuable tools because they can provide information about the nature of molecular motions observed by, for example, dielectric spectroscopy and to further complement them with structural information. In this work, we acquire information about the nature of dipole relaxation, in n-lysine solutions by means of molecular dynamics simulations. Our results indicate that the experimentally observed main relaxation process of n-lysine has different origins for the single monomer and the polypeptide chains. The relaxation of 1-lysine is due to the motions of whole molecules, whereas the experimentally observed relaxation of 3-lysine and 4-lysine is due to the motions of the residues, which, in turn, are promoted by water relaxation. Furthermore, we propose a new structural model of the lysine amino acids, which can quantitatively account for the experimental dielectric relaxation data. Hydrogen bonding and the structure of water are also discussed in terms of their influence on relaxation processes.

Exploring Aqueous Solution Dynamics of an Amphiphilic Diblock Copolymer: Dielectric Relaxation and Time-Resolved Fluorescence Measurements.

We explore in this work, after synthesizing and appropriately characterizing an amphiphilic diblock copolymer, its interaction with water molecules and the subsequent aqueous solution dynamics by employing time-resolved fluorescence measurements (TRF) and megahertz-gigahertz dielectric relaxation (DR) experiments. The synthesized amphiphilic diblock copolymer is poly(2-(((tert-butoxycarbonyl)alanyl)oxy)ethyl methacrylate)-b-poly(polyethylene glycol monomethyl ether methacrylate) (P(Boc-l-Ala-HEMA)-b-PPEGMA). Dynamic light scattering measurements of aqueous solutions indicate formation of 14-20 nm particles from a balance between the chain lengths of the hydrophobic (P(Boc-l-Ala-HEMA) and hydrophilic (PPEGMA) segments. Field-emission scanning electron microscopy, on the other hand, suggests a spherical shape for the dried micelles. The critical micelle concentration of the P(Boc-l-Ala-HEMA)-b-PPEGMA block copolymer at different block lengths in aqueous media, determined via steady-state fluorescence measurements, is very low (∼4-8 mg/L), and the resultant micellar size has been found to be insensitive to the polymer concentration. Interfacial and bulk aqueous dynamics have been investigated by tracking the solution frictional resistance on rotational motion of dissolved hydrophobic and hydrophilic dipolar solute probes of comparable sizes. TRF anisotropy measurements reflect the biphasic temporal profile for the frictional resistance. Interestingly, the hydrophobic probe, because of its preferential location at the micellar interface, experiences greater frictional resistance than the hydrophilic counterpart, although the latter reports stronger polymer concentration dependence of the frictional retardation than the former. DR measurements at the highest of the polymer concentrations considered suggest presence of aqueous dynamics slower than that for neat bulk water, although evidence for such "slow" dynamics at lower concentrations has not been detected in the present DR measurements.

Theoretical Verification of the Formula of Charge Function in Time of Capacitor (q = c*v) for Few Cases of Excitation Voltage

We have a developed and derived a formula for capacitor i.e. charge as a function of time, which is convolution operation of time varying capacity function and time-varying voltage function. This is different to the usual and conventional way of writing capacitance multiplied by voltage to get charge stored in a capacitor. This new deliberation with convolution operation works well for classical capacitors (i.e. ideal loss less capacitors), that is of a constant capacity at all frequencies, and also for a time varying capacity function given by decaying power-law: that gives the formation of a fractional capacitor. In this paper, we use this developed new charge storage expression and apply to various types of inputs excitation voltage-sinusoidal, step, ramp voltage and then analyze and interpret the results for charge stored, the current expressions, the loss-tangent and the memory effects. With this new formulation, we also evaluate impedance function of a classical capacitor as well as a fractional capacitor, and also elaborate on the Nyquist’s diagram, that is employed to study various dielectric materials via impedance spectroscopy. This new approach of charge storage concept is yet to be practically as well as theoretically applied-though some initial work has started. This paper gives a theoretical validity test i.e. analytically obtained in several applications for this new formulation, of charge storage formula. This paper will be useful in various super-capacitor studies, dielectric relaxation experiments, and impedance spectroscopy for various material developments for electrical energy storage missions; however, this concept is yet to be used to its full potential.

The effect of alkyl-chain branching on the hydration and ion association of tetrabutylammonium cations

Association constants and single-ion conductivities of the bromide salts of five isomeric tetrabutylammonium ions were determined with dilute-solution conductivity measurements for aqueous solutions at 298.15K. In addition, for two of the isomers dielectric relaxation experiments were performed. It was found that, despite similar size of all cations, the hydrodynamic radii of the branched isomers were significantly smaller than that of the un-branched Bu4N+ species. It appears that the more compact branched cations exert only a minor perturbation on the H-bond network of the surrounding solvent and easily strip off their hydration shell. This effect, and the —on average— closer approach between anions and cations in ion pairs of Bu2isoBu2NBr and isoBu4NBr probably explain the rather large association constants of the latter salts.

Dynamic Kerr effect in a strong uniform AC electric field for interacting polar and polarizable molecules in the mean field approximation.

Analytical formulas for the electric birefringence response of interacting polar and anisotropically polarizable molecules due to a uniform alternating electric field are derived using Berne's forced rotational diffusion model [B. J. Berne, J. Chem. Phys. 62, 1154 (1975)] in the nonlinear version described by Warchol and Vaughan [J. Chem. Phys. 71, 502 (1979)]. It is found for noninteracting molecules that the signal consists of a frequency-dependent DC component superimposed on an oscillatory part with a frequency twice that of the AC driving field. However, unlike noninteracting molecules, the AC part strongly deviates from its dilute counterpart. This suggests a possible way of motivating new experimental studies of intermolecular interactions involving electro-optical methods and complementary nonlinear dielectric relaxation experiments.

The Role of Concentration Dependent Static Permittivity of Electrolyte Solutions in the Debye-Hückel Theory.

The Debye-Hückel theory has been extended to allow for arbitrary concentration dependence of the electrolyte solution static permittivity. The theory follows the lines advanced by Erich Hückel ( Hückel, E. Phys. Z. 1925, 26, 93) but gives rise to more general and lucid results. New theoretical expressions have been obtained for the excess free energy of solution, activity coefficient of water and mean ionic activity coefficient. The thermodynamic functions contain two terms representing interionic interactions and ion-water (solvation) interactions. The theory has been applied to calculate the activity coefficients of components in the aqueous solutions of alkali metal chlorides from LiCl to CsCl at ambient conditions making use of permittivities taken from experimental dielectric relaxation studies. Calculations without parameter adjustment have demonstrated a semiquantitative agreement with experimental data, reproducing both the nonmonotonic concentration dependence of the activity coefficients and the ordering of activity coefficients for the salts with different cations. A good agreement with experimental data is obtained for the aqueous solutions of LiCl in the concentration range up to 10 mol/kg. The nonmonotonic concentration dependence of activity coefficients is explained as a result of a balance between the effect of interionic interactions and the solvation contribution which appears quite naturally in the framework of the Debye-Hückel approach after incorporation of variable permittivity of solution.

Ion Distribution around Synthetic Vesicles of the Cat-Anionic Type

Aqueous alkyltrimethylammonium bromides, or dialkyldimethylammonium ones, were mixed with sodium alkyl sulfates and dialkanesulfonates. Depending on the overall surfactant concentration, charge and/or mole ratios, cat-anionic vesicles were formed by mixing nonstoichiometric amounts of oppositely charged species. The resulting vesicles are thermodynamically and kinetically stable. ζ-potential and dynamic light scattering characterized the systems. As a rule, cat-anionic vesicles have sizes in the 10(2)-10(3) nm range and bear significant amounts of surface charges. At fixed surfactant concentration, the vesicle surface charge density scales with mole ratios and tends to zero as the latter approach unity. Conversely, the hydrodynamic radius diverges when the cationic/anionic mole ratio is close to 1. The double-layer thickness and surface charge density are controlled by mole ratios and addition of NaBr, which plays a role in vesicle stability. The salt screens the surface charge density and modulates both vesicle size and double-layer thickness. Slightly higher concentrations of NaBr induce the transition from vesicles toward lamellar phases. The electrokinetic properties of cat-anionic dispersions were analyzed by dielectric relaxation experiments. The measured properties are sensitive to vesicle size distributions. In fact, the relaxation frequency shifts in proportion to vesicle polydispersity. Model calculations proposed on that purpose supported the experimental findings.

Dielectric relaxation of poly (trimethylene terephthalate) in a broad range of crystallinity

Here we present dielectric relaxation experiments carried out in semicrystalline poly (trimethylene terephthalate) (PTT) samples covering a broad range of crystallinity values. Special attention has been devoted to characterize the two extremes of low and high crystallinity. In particular a high temperature relaxation in the dielectric spectra of highly crystalline PTT, attributed to a Maxwell–Wagner–Sillars process, has been revealed. Moreover, the existence of ordering phenomena during the induction period prior to crystallization has been characterized by dielectric spectroscopy.

Approximations of the discrete slip-link model and their effect on nonlinear rheology predictions

The discrete slip-link model (DSM) was developed to describe the dynamics of flexible polymer melts. The model is able to predict linear viscoelasticity of monodisperse linear, polydisperse linear, and branched systems. The model also shows good agreement with dielectric relaxation experiments, except for the single data set available for bidisperse linear systems with a small volume fraction of long chains. In this work, both shear and elongational flow predictions obtained using the DSM without parameter adjustment are shown. Model predictions for shear flow agree very well with experimental results. The DSM is able to capture the transient response as well as the steady-state viscosity. However, for elongational flow, agreement is unsatisfactory at large strains. The DSM captures the onset of strain hardening, but after a Hencky strain between 2 and 3, it predicts transient strain softening, whereas experiments show only monotonic growth. We explore a number of assumptions and approximations of the model and their effect on flow predictions. The approximations are related to the neglect of these phenomena, which are expected to be more sensitive in elongational flow: finite extensibility, convective constraint release, and deformation of dangling ends. We indeed find that shear flow predictions are insensitive to these approximations, but elongational flow is affected. However, none of these effects is able to bring prediction in line with experiments. We conclude that the currently accepted view of entanglement dynamics is incomplete.

On the dynamics of liquids in their viscous regime approaching the glass transition

Recently, Mallamace et al. (Eur. Phys. J. E 34, 94 (2011)) proposed a crossover temperature, T(×), and claimed that the dynamics of many supercooled liquids follow an Arrhenius-type temperature dependence between T(×) and the glass transition temperature T(g). The opposite, namely super-Arrhenius behavior in this viscous regime, has been demonstrated repeatedly for molecular glass-former, for polymers, and for the majority of the exhaustively studied inorganic glasses of technological interest. Therefore, we subject the molecular systems of the Mallamace et al. study to a "residuals" analysis and include not only viscosity data but also the more precise data available from dielectric relaxation experiments over the same temperature range. Although many viscosity data sets are inconclusive due to their noise level, we find that Arrhenius behavior is not a general feature of viscosity in the T(g) to T(×) range. Moreover, the residuals of dielectric relaxation times with respect to an Arrhenius law clearly reveal systematic curvature consistent with super-Arrhenius behavior being an endemic feature of transport properties in this viscous regime. We also observe a common pattern of how dielectric relaxation times decouple slightly from viscosity.

The role of the isothermal bulk modulus in the molecular dynamics of super-cooled liquids

Elastic models imply that the energy expended for a flow event in ultra-viscous matter coincides with the elastic work required for deforming and re-arranging the environment of the moving entity. This is quite promising for explaining the strong non-Arrhenius behavior of dynamic quantities of fragile super-cooled liquids. We argue that the activation volume obtained from dielectric relaxation and light-scattering experiments for super-cooled liquids should scale with the Gibbs free energy of activation, with a proportionality constant determined by the isothermal bulk modulus and its pressure derivative, as described by an earlier thermodynamic elastic model. For certain super-cooled liquids the bulk compression transpiring in the local environment, as governed by the isothermal bulk modulus, play a significant role in the reorientational dynamics, with far-field density fluctuations and volume changes avoided by shear deformation.