Slow Structural Relaxation Research Articles

Variations in the impurity composition and microhardness of surface layers in silicon crystals caused by a magnetic field

Variations in microhardness and impurity composition of surface layers in silicon crystals caused by a magnetic field are observed. It is assumed that the mechanism of slow structural relaxation in silicon crystals is related to an increase in the adsorption function of silicon as a result of the effect of a magnetic field; this mechanism underlies the magnetomechanical effect (the effect of variation in microhardness).

Anomalous diffusion, structural relaxation and shear thinning in glassy hard spherefluids

The stochastic nonlinear Langevin equation theory of the single-particle dynamics ofglassy hard sphere fluids and suspensions has been applied to address severalissues raised by recent experimental and simulation studies. The theoreticallypredicted degree of non-Fickian behaviour at intermediate times, and the slowstructural relaxation component of the small wavevector incoherent dynamic structurefactor, are compared quantitatively with recent measurements and reveal goodagreement. A roughly power law growth, with a common exponent, of the classic andalternative non-Gaussian parameter amplitudes with mean alpha relaxation timeis predicted, and qualitative similarities with a multi-point susceptibility thatquantifies dynamic heterogeneity effects are noted. The nonlinear rheology version ofthe theory has been quantitatively applied to explain recent measurements ofshear-induced acceleration of the single-particle relaxation time on the local cage scale.The resulting no adjustable parameter calculations are in remarkable agreementwith the experimental observations for the absolute magnitude, volume fractiondependence and fractional power law scaling aspects of the shear thinning phenomenon.

Anomalously Slow Solvent Structural Relaxation Accompanying High-Energy Rotational Relaxation

Experimental and theoretical work on the relaxation of rapidly rotating solutes in liquids have pointed out a number of striking features. Even in rapidly relaxing solvents, the relaxation proceeds quite slowly, exhibiting a manifestly nonlinear response that depends explicitly on the initial rotational energy. In this paper, we show how the long-time behavior, in particular, stems from a strong coupling of solute orientation to local solvent geometry. This coupling creates a rotational friction that decreases sharply with rotational energy, allowing for the protracted survival of not only high-angular-momentum rotational states but the cavity-like low-friction solvent geometries. We show, further, that the slow dynamics is dynamically heterogeneous. The distribution of excited rotors is marked by a distinct population of slowly relaxing hot rotational states. This population can be traced directly to the small subset of liquid configurations that happen to have low rotational friction values at the instant at which the rapid rotation started, indicating an unusual failure of the normally chaotic environment of a liquid to randomize initial conditions.

Comparative study of microstructural evolution during melting and crystallization

Molecular dynamics simulations, with the interaction between atoms described by a modified analytic embedded atom method, have been performed to obtain the atomic-scale details of isothermal melting in nanocrystalline Ag and crystallization from supercooled liquid. The radial distribution function and common neighbor analysis provide a visible scenario of structural evolution in the process of phase transition. The results indicate that melting at a fixed temperature in nanocrystalline materials is a continuous process, which originates from the grain boundary network. With the melting developing, the characteristic bond pairs (555), (433), and (544), existing in liquid or liquidlike phase, increase approximately linearly till completely melted. The crystallization from supercooled liquid is characterized by three characteristic stages: nucleation, rapid growth of nucleus, and slow structural relaxation. The homogeneous nucleation occurs at a larger supercooling temperature, which has an important effect on the process of crystallization and the subsequent crystalline texture. The kinetics of transition from liquid to solid is well described by the Johnson-Mehl-Avrami equation.

Evolution of vibrational properties during a macromolecule’s growth

The elastic constants and vibrational contributions to thermal properties of three polymerizing liquids were investigated by using the available hypersonic velocity measured by Brillouin light scattering in real time. During the addition polymerization to a molecular network structure, Poisson's ratio upsilon(Poisson) decreases approximately according to exp[-(kt(polym))]n, where both k and n are composition dependent. The Debye frequency increases and the corresponding heat capacity, energy, and entropy approaching a limiting value. upsilon(Poisson) of the vitrified polymer continues to decrease but much more slowly, indicating its continued slow polymerization and structural relaxation with time. In the potential energy landscape interpretation, a polymerizing liquid's state point continuously shifts to another landscape's more curved, deeper minima.

Unified Description of Aging and Rate Effects in Yield of Glassy Solids

The competing effects of slow structural relaxations (aging) and deformation at constant strain rate on the shear yield stress tau(y) of simple model glasses are examined using molecular simulations. At long times, aging leads to a logarithmic increase in density and tau(y). The yield stress also rises logarithmically with rate but shows a sharp transition in slope at a rate that decreases with increasing age. We present a simple phenomenological model that includes both intrinsic rate dependence and the change in properties with the total age of the system at yield. As predicted by the model, all data for each temperature collapse onto a universal curve.

Structural Relaxation in a Binary Mixture

We study the dynamic feedback mechanism of the mode-coupling model in a binary liquid in the context of structural Glass transition. The dependence of the dynamic transition point on the mass ratio of the constituents in the mixture is studied from the model equations. The effects of slow structural relaxation on the single particle dynamics are also analyzed. The feedback effects on the tagged particle dynamics from coupling of density fluctuations show that the extrapolated self-diffusion coefficient approaches zero at a low temperature in agreement with simulation results.

Glass Transitions in Peat: Their Relevance and the Impact of Water

This contribution aims to expand the macromolecular view of fractionated natural organic matter (NOM)to organic matter in whole soils. It focuses on glass transition behavior of whole soil organic matter (SOM) and its interrelation with water through use of differential scanning calorimetry (DSC) and thermomechanical analysis (TMA). Three processes of structural relaxation related to macromolecular mobility were distinguished. Process I occurs in thermally pretreated and very low water-content samples and corresponds to classic glass transition behavior. Process II occurs in water-containing samples, where water is believed to act as an antiplasticizing agent in the peat at water contents below 12%, causing decreased macromolecular mobility and increased glass transition temperature. We suggestthe formation of hydrogen bond-based cross-links being responsible for this antiplasticizing effect. Process III represents a slow swelling process induced by water uptake with a time constant of swelling in the order of days, with water acting as a plasticizing agent. Results from this work are of particular importance for environmental systems as changes in environmental conditions (e.g., water content, temperature) may induce slow structural relaxation processes in NOM over periods of time ranging from days to weeks. These influences on NOM macromolecular mobility lead to continuous changes in physicochemical properties that may greatly influence sorbate-sorbent interactions in surface and subsurface environments.

Mesoscopic simulation of slow structural relaxation in fluid mercury near themetal–nonmetal transition

The Langevin diffusion equation approach is proposed for studying the metal–nonmetal(M–NM) transitions in expanded fluid mercury from a mesoscopic and dynamicalviewpoint. In this theory, time evolutions of coarse-grained atomic densities are calculatedin accordance with the free energy functionals that incorporate changes in the electronicstates across the M–NM transitions. Because of the intrinsically discontinuous nature ofthe M–NM transitions, irregular mixing of high-density metallic domains andlow-density nonmetallic domains is predicted in the M–NM transition range. Thermalfluctuations cause local transformation between metallic and nonmetallic domains.The timescale of structural relaxation involving such a local M–NM transition isremarkably slow, which is reflected in the strikingly slow time decay of the dynamicaldensity correlation functions in the M–NM transition range. This result stronglysupports the experimental evidence of slow structural relaxation found throughanomalous sound wave attenuation. An increase in isothermal compressibilities and amodest enhancement of long-wavelength static structure factors are predictedacross the transition. Discontinuous density jumps are smeared out by structuraldisorder, leading to a continuous M–NM transition in agreement with observations.

Thin films in wetting and spreading

Thin films differ from bulk phases both from a thermodynamic point of view, i.e. the chemical potential of a molecule in a film depends on the film thickness, and in their dynamical response, because relaxation times may become large in these confined media. Examples of slow structural relaxation in thin films of liquid crystals, and their consequences on the wetting properties of the systems are presented first. Then, we illustrate the thermodynamic specificity of thin films when evaporation is considered. The consequences on the wetting dynamics of macroscopic evaporating droplets are presented in the last part of the paper.

Slow structural relaxation in liquid Te–Se mixtures

We have measured the sound attenuation, α, in liquid Te 50Se 50 at 20, 32 and 43 MHz and Te 70Se 30 at 32 MHz in a wide temperature and pressure range and have found three distinct absorption mechanisms. Near the melting point, α decreases rapidly with increasing temperature, which may be due to conformational changes within the chain molecules. At intermediate temperatures a sharp absorption peak appears and its position shifts to higher temperature with increasing frequency, while at high temperatures around 820 °C a broad peak appears irrespective of the frequency. We discuss the origin of these absorption peaks in connection with the semiconductor–metal transition.

Modeling of solute release from polymeric monoliths subject to linear structural relaxation

AbstractA previously developed model of solute release from a swellable polymeric matrix induced by solvent uptake that obeys Fickian kinetics is here extended to cover the case of non‐Fickian solvent uptake caused by slow structural relaxation of the swelling polymer. For this purpose, we have adopted a model description of the absorption process (also previously developed in our laboratory), which has been shown to be capable of realistically simulating a wide range of non‐Fickian kinetics (including the well‐known two‐stage sorption and case II regimes). The aforementioned structural relaxation phenomena constitute important characteristics of glassy polymeric matrices. Accordingly, the predictions of the resulting combined model have been investigated in some detail in this series of articles. This particular article is concerned with the simplest version of the model, involving constant uptake kinetic parameters (diffusion coefficient and relaxation frequency) for both osmotically inactive and osmotically active solutes. Emphasis is put on possibilities of achieving and sustaining nearly constant release rates, and it is shown that such possibilities are considerably more numerous here than under conditions of Fickian solvent uptake kinetics. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1171–1188, 2002

Slow-structural relaxation in the metal–nonmetal transition range of liquid mercury: I. Experimental evidence

The sound absorption coefficient, α, of expanded liquid mercury has been measured by means of the ultrasonic pulse-echo method at 20, 32 and 44 MHz in the temperature and pressure range up to 1600 °C and 210 MPa. Besides the critical attenuation, we have observed the secondary maximum in the density dependence of α/f2 around 9 g cm−3, where the metal–nonmetal (M–NM) transition occurs. When the frequency increases, the secondary maximum of α/f2 tends to be smaller, which suggests that some kind of relaxation process takes place in the M–NM transition range of liquid mercury. We have separated the observed sound attenuation into the critical attenuation and the anomalous attenuation in the M–NM transition region utilizing the difference of the frequency dependence between the two components. Assuming a Debye-type relaxation model for the relaxation process due to the M–NM transition, we have estimated the relaxation time, τ, and the relative strength of the relaxation, βr/β0 ≡ (β0 − β∞)/β0, where β0 and β∞ are the adiabatic compressibility in the low-frequency and the high-frequency limit, respectively. The resultant τ is about 2 ns and almost independent of density. On the other hand, βr/β0 depends on density and has a broad maximum (∼4%) around 8.5 g cm−3.

`Covalent' effects in `ionic' liquids

Many binary systems (and their mixtures) which might be expected to be `ionic', from electronegativity considerations, are found to exhibit pronounced `covalent effects' in their condensed-phase structure and dynamical properties. Recent work, involving both electronic structure calculations and computer simulation, has suggested that the interactions arise and are describable within the ionic model - provided that many-body effects, whose origin is the change in an ion's properties caused by interaction with its environment, are included. In systems where they are substantial, the many-body effects promote remarkably rich changes in the intermediate-range structure of an ionic liquid. AlCl3 becomes molecular, BeCl2 a `living polymer' of extended chains, and the distinctive intermediate-range order (IRO) of the three-dimensional-network, glass-forming systems ZnCl2, BeF2 and SiO2 is reproduced. The structural changes have considerable dynamical consequences: for ZnCl2 the slow structural relaxation, leading to the glass transition, may be traced back to the relaxation of the IRO. On shorter timescales (higher frequency) these liquids exhibit spectroscopic bands usually assigned to quasi-molecular units. The formation and dissociation of these units is crucial in ionic conduction, and other transport properties.

Atomistic simulation of aging and rejuvenation in glasses

Slow structural relaxation ("aging") observed in many atomic, molecular, and polymeric glasses substantially alters their stress-strain relations and can produce a distinctive yield point. Using Monte Carlo simulation for a binary Lennard-Jones mixture, we have observed these phenomena and their cooling-rate dependences for the first time in an atomistic model system. We also observe that aging effects can be reversed by plastic deformation ("rejuvenation"), whereby the system is expelled from the vicinity of deep minima in its potential energy surface.

Slow structural relaxations of glass-forming Maltitol by modulated DSC calorimetry

The possibilities offered by the modulated differential scanning calorimetry technique to investigate the glass transition are analyzed and discussed on Maltitol. The technique provides specific tools to follow the structural relaxation in the frequency domain above Tg and in the time domain below Tg. Evidence is obtained that the complex heat capacity can be measured in the course of a routine differential scanning calorimetry (DSC) scan. The enthalpic relaxation can be analyzed which allows us to discuss the fragility degree of the glass former. The smearing effect of the underlying ramp is specifically investigated. It is shown that the method provides estimates both of the Vogel and of the Kauzmann temperatures. It thus offers an attractive way to check for a correlation between dynamics and thermodynamics. The concept of the “nonreversing” component is used and discussed in order to study the slow sub-Tg relaxations.

Fast structural relaxation of polyvinyl alcohol below the glass-transition temperature

In order to obtain information about structural relaxations of polymers within a time window of several nanoseconds, the absorption, site-selective steady-state fluorescence and time-resolved fluorescence spectra have been measured for polyvinyl alcohol doped with rhodamine 640 in the 150–300 K temperature range. The temperature dependence of the absorption and fluorescence spectra has been analyzed on the basis of one- and two-dimensional configuration-coordinate models. In spite of the measurement below the glass-transition temperature of the matrix, the existence of a fast relaxation process which is completed within a few hundred ps has been clarified. The magnitude of this relaxation increases with increasing temperature, while the relaxation mechanism cannot be ascribed to the thermal crossing of static energy barriers. It has been found that the experimental results are not explained by the two-dimensional configuration coordinate model in which the fast and slow structural relaxations are assumed to occur independently along the two axes. A relaxation process triggered by temperature-dependent release from the constraint preventing the structural change is shown to account for the experimental results well using a one-dimensional configuration coordinate model.

Interpretation of the dynamic heat capacity observed in glass-forming liquids

Slow structural relaxations can complicate the interpretation of thermodynamic measurements on glass-forming liquids. Here we demonstrate using model calculations that structural recovery can lead to an apparent frequency-dependent heat capacity in ac calorimetry experiments. The model is shown to describe the complex heat capacity data reported in the literature for glycerol and poly(vinyl acetate). Importantly, the model does not invoke a complex heat capacity; rather, only static heat capacities are used. The analysis further suggests that ac calorimetry should provide a powerful way of testing models of structural recovery.

Molecular-dynamics study of incoherent quasielastic neutron-scattering spectra of supercooled water

A molecular-dynamics simulation of extended simple point charge water in a time interval of 1 fs to 50 ns has been performed to study the single-particle dynamics of water at supercooled temperatures. In spite of the fact that upon supercooling water progressively evolves into a more open structure locally, the single-particle dynamics is nevertheless shown to be dominated by the so-called cage effect experienced by the test particle. The slow structural relaxation of the cage at low temperatures leads to the phenomenon of slow dynamics that can only be completely studied by following the trajectories into the nanosecond range. The objective of this paper is twofold. First, we test the accuracy of various approximations used in previous analyses of spectra from incoherent quasielastic neutron-scattering experiments. Second, we explore the possibility of an alternative method of analysis of high-resolution quasielastic neutron-scattering spectra, taking into account the slow dynamics in supercooled water. The approximations tested are the decoupling of the self-intermediate scattering function into a product of rotational and translational components, the physical interpretation of the origin of the experimentally observed Debye-Waller factor, the rotational-diffusion approximation of the rotational intermediate scattering function, and the random jump diffusion approximation of the translational intermediate scattering function. Various approximations used previously for the component intermediate scattering functions are not sufficiently accurate. The reason for this is that at supercooled temperatures, due to the dominant cage effect, the conventional picture of the stochastic single-particle diffusion loses its validity. The diffusion process is then progressively controlled by the structural relaxation of the cage. @S1063-651X~97!10310-5#

Single-particle motion in liquid sodium: thermal crossover between two dynamical regimes

Several features of single-particle dynamics in liquid Na were investigated by both computer simulation and inelastic neutron scattering at two temperatures: 380 K (close to the melting point) and 900 K (not far from the boiling point). In both cases the system is a rather dense liquid, but the dynamics probed by the self intermediate scattering function are found to reflect entirely different phenomena at the two temperatures, indicating a gradual, thermally-induced crossover from a regime dominated by cage effects and slow structural relaxation to another one characterized by the onset of a vortex pattern around the tagged particle.