Molecular dissipation phenomena of nanoscopic friction in the heterogeneous relaxation regime of a glass former

Nanoscale sliding friction involving a polystyrene melt near its glass transition temperature Tg (373 K) exhibited dissipation phenomena that provide insight into the underlying molecular relaxation processes. A dissipative length scale that shows significant parallelism with the size of cooperatively rearranging regions (CRRs) could be experimentally deduced from friction-velocity isotherms, combined with dielectric loss analysis. Upon cooling to approximately 10 K above Tg, the dissipation length Xd grew from a segmental scale of approximately 3 A to 2.1 nm, following a power-law relationship with the reduced temperature Xd approximately TR-phi. The resulting phi=1.89+/-0.08 is consistent with growth predictions for the length scale of CRRs in the heterogeneous regime of fragile glass formers. Deviations from the power-law behavior closer to Tg suggest that long-range processes, e.g., the normal mode or ultraslow Fischer modes, may couple with the alpha relaxation, leading to energy dissipation in domains of tens of nanometers.     

Changes of relaxation dynamics of a hydrogen-bonded glass former after removal of the hydrogen bonds

Dielectric relaxation spectra of two closely related glass formers, dipropylene glycol [H-(C3H6O)2-OH] and dipropylene glycol dimethyl ether [CH3-O-(C3H6O)2-CH3], were measured at ambient and elevated pressures in the supercooled and the glassy states are presented. Hydrogen bonds formed in dipropylene glycol are removed when its ends are replaced by two methyl groups to become dipropylene glycol dimethyl ether. In the process, the primary relaxation, the excess wing, and the resolved secondary relaxation of dipropylene glycol are all modified when the structure is transformed to become dipropylene glycol dimethyl ether. The modifications include the pressure and temperature dependences of these relaxation processes and their interrelations. Thus, by comparing the dielectric spectra of these two closely related glass formers at ambient and elevated pressures, the differences in the relaxation dynamics and properties in the presence and absence of hydrogen bonding are identified.     

Structural relaxation and glass transition behavior of binary hard-ellipse mixtures

Structural relaxation and glass transition in binary hard-spherical particle mixtures have been reported to exhibit unusual features depending on the size disparity and composition. However, the mechanism by which the mixing effects lead to these features and whether these features are universal for particles with anisotropic geometries remains unclear. Here, we employ event-driven molecular dynamics simulation to investigate the dynamical and structural properties of binary two-dimensional hard-ellipse mixtures. We find that the relaxation dynamics for translational degrees of freedom exhibit equivalent trends as those observed in binary hard-spherical mixtures. However, the glass transition densities for translational and rotational degrees of freedom present different dependencies on size disparity and composition. Furthermore, we propose a mechanism based on structural properties that explain the observed mixing effects and decoupling behavior between translational and rotational motions in binary hard-ellipse systems.     

The glass transition and dielectric secondary relaxation of fructose-water mixtures

Broad-band dielectric measurements for fructose-water mixtures with fructose concentrations between 70.0 and 94.6 wt% were carried out in the frequency range of 2 mHz to 20 GHz in the temperature range of -70 to 45 degrees C. Two relaxation processes, the alpha process at lower frequency and the secondary beta process at higher frequency, were observed. The dielectric relaxation time of the alpha process was 100 s at the glass transition temperature, T(g), determined by differential scanning calorimetry (DSC). The relaxation time and strength of the beta process changed from weaker temperature dependences of below T(g) to a stronger one above T(g). These changes in behaviors of the beta process in fructose-water mixtures upon crossing the T(g) of the mixtures is the same as that found for the secondary process of water in various other aqueous mixtures with hydrogen-bonding molecular liquids, polymers, and nanoporous systems. These results lead to the conclusion that the primary alpha process of fructose-water mixtures results from the cooperative motion of water and fructose molecules, and the secondary beta process is the Johari-Goldstein process of water in the mixture. At temperatures near and above T(g) where both the alpha and the beta processes were observed and their relaxation times, tau(alpha) and tau(beta), were determined in some mixtures, the ratio tau(alpha)/tau(beta) is in accord with that predicted by the coupling model. Fixing tau(alpha) at 100 s, the ratio tau(alpha)/tau(beta) decreases with decreasing concentration of fructose in the mixtures. This trend is also consistent with that expected by the coupling model from the decrease of the intermolecular coupling parameter upon decreasing fructose concentration.     

Secondary relaxation behavior in a strong glass

In the present paper we study the enthalpy relaxation behavior of the hyperquenched GeO(2) (HQGeO(2)) glass, one of the strongest glass systems. By applying the hyperquenching-annealing-calorimetry approach, we have found that unlike fragile glasses the strong HQGeO(2) glass relaxes in a manner that all the secondary relaxation units contribute to the primary relaxation. By analyzing dynamic properties of the secondary relaxation, we have identified two typical features of the Johari-Goldstein relaxation in the HQGeO(2) glass. First, the quantitative relationship observed here between E(beta) and T(g) agrees well with the empirical relation of the JG relaxation. Second, the characteristic relaxation time of the GeO(2) glass at T(g) is found to be about 10 s, larger than that of relatively fragile glasses. These results verify that the JG peak in strong glasses is hidden by the alpha peak in the dielectric loss curves.     

Light scattering study on the glass former o-terphenyl

Combining double monochromator, tandem Fabry-Pérot interferometry (TFPI), and photon correlation spectroscopy (PCS), the full range of the dynamic susceptibility of the glass former o-terphenyl is covered (240-440 K). A time resolution of 6 ns is reached by the PCS setup, and the measuring gap between PCS and TFPI has significantly shrunk. Combined time domain data are presented, which cover 0.01 ps-100 s. Regarding the primary α-process, no recognizable change in shape and amplitude is observed, i.e., frequency-temperature superposition holds over the entire temperature range investigated. The only change in the susceptibility is the emergence of the excess wing contribution, leading to a flattening of the susceptibility minimum close to T(g). Separating relaxation contribution from fast and slow dynamics, we determine the relaxation strength 1-f(rel)(T) of the fast dynamics, which shows a cusplike temperature dependence with a crossover temperature T(c)=316±10 K. Due to relative small changes of 1-f(rel)(T) the effect is difficult to identify in f(rel)(T) itself. The crossover temperature is larger than previously reported as no extrapolation scheme is applied.     

Evidence for a simple monatomic ideal glass former: the thermodynamic glass transition from a stable liquid phase

Under cooling, a liquid can undergo a transition to the glassy state either as a result of a continuous slowing down or by a first-order polyamorphous phase transition. The second scenario has so far always been observed in a metastable liquid domain below the melting point where crystalline nucleation interfered with the glass formation. We report the first observation of the liquid-glass transition by a first-order polyamorphous phase transition from the equilibrium stable liquid phase. The observation was made in a molecular dynamics simulation of a one-component system with a model metallic pair potential. In this way, the model, demonstrating the thermodynamic glass transition from a stable liquid phase, may be regarded as a candidate for a simple monatomic ideal glass former. This observation is of conceptual importance in the context of continuing attempts to resolve the long-standing Kauzmann paradox. The possibility of a thermodynamic glass transition from an equilibrium melt in a metallic system also indicates a new strategy for the development of bulk metallic glass-forming alloys.     

Dependence of the fragility of a glass former on the softness of interparticle interactions

We study the influence of the softness of the interparticle interactions on the fragility of a glass former by considering three model binary mixture glass formers. The interaction potential between particles is a modified Lennard-Jones type potential, with the repulsive part of the potential varying with an inverse power q of the interparticle distance, and the attractive part varying with an inverse power p. We consider the combinations (12,11) (model I), (12,6) (model II), and (8,5) (model III) for (q,p) such that the interaction potential becomes softer from model I to III. We evaluate the kinetic fragilities from the temperature variation of diffusion coefficients and relaxation times, and a thermodynamic fragility from the temperature variation of the configurational entropy. We find that the kinetic fragility increases with increasing softness of the potential, consistent with previous results for these model systems, but at variance with the thermodynamic fragility, which decreases with increasing softness of the interactions, as well as expectations from earlier results. We rationalize our results by considering the full form of the Adam-Gibbs relation, which requires, in addition to the temperature dependence of the configurational entropy, knowledge of the high temperature activation energies in order to determine fragility. We show that consideration of the scaling of the high temperature activation energy with the liquid density, analyzed in recent studies, provides a partial rationalization of the observed behavior.     

On the nature of the high-frequency relaxation in a molecular glass former: a joint study of glycerol by field cycling NMR, dielectric spectroscopy, and light scattering

Fast field cycling (1)H NMR relaxometry is applied to determine the dispersion of spin-lattice relaxation time T(1)(omega) of the glass former glycerol in broad temperature (75-360 K) and frequency (10 kHz-30 MHz) ranges. The relaxation data are analyzed in terms of a susceptibility chi(")(omega) proportional, variantomegaT(1)(omega), related to the second rank (l=2) molecular orientational correlation function. Broadband dielectric spectroscopic results suggest the validity of frequency temperature superposition above the glass transition temperature T(g). This allows to combine NMR data of different temperatures into a single master curve chi(")(omegatau(alpha)) that extends over 15 decades in reduced frequency omegatau(alpha), where tau(alpha) is the structural alpha-relaxation time. This master curve is compared with the corresponding ones from dielectric spectroscopy (l=1) and depolarized light scattering (l=2). At omegatau(alpha)<1, NMR susceptibility is significantly different from both the dielectric and light scattering results. At omegatau(alpha)>1, there rather appears a difference between the susceptibilities of rank l=1 and l=2. Specifically, at omegatau(alpha)>1, where the susceptibility is dominated by the so-called excess wing, the NMR and light scattering spectra (both l=2) rather coincide with each other and are about three times more intense than the dielectric (l=1) spectrum. This is explained by assuming that the high frequency dynamics correspond to only small-angle excursions. Below T(g), dielectric and NMR susceptibility compare well and exhibit an exponential temperature dependence.     

Inertial suppression of protein dynamics in a binary glycerol-trehalose glass

The traditional approach used to predict the ability of a glassy matrix to maximally preserve the activity of a protein solute is the glass transition temperature (T(g)) of the glass. Recently it has been shown that the addition of a low T(g) diluent (glycerol) can rigidify the structure of a high T(g) glassy matrix in binary glycerol-trehalose glasses. The optimal density of glycerol in trehalose minimizes the average mean square displacements of non-exchangeable protons in the glass samples. The amount of glycerol added to a trehalose glass coincides with the maximal recovery of biological activity in a separate study using similar binary glass samples. In this study, we use molecular dynamics (MD) simulations to investigate the dynamics of a hydrated protein encased in glycerol, unary trehalose and binary glycerol-trehalose glasses. We have found that we are able to reproduce the rigidification of the glycerol-trehalose glassy matrix and that there is a direct correlation between bulk glass dynamics and the extent of atomic fluctuation of protein atoms. The detailed microscopic picture that emerges is that protein dynamics are suppressed mainly by inertia of the bulk glass and to a lesser extent specific interactions at the protein-solvent interface. Thus, the inertia of the glassy matrix may be an influential factor in the determination of pharmaceutically relevant formulations.     

A study of the static yield stress in a binary Lennard-Jones glass

The stress-strain relations and the yield behavior of a model glass (a 80:20 binary Lennard-Jones mixture) is studied by means of molecular dynamics simulations. In a previous paper it was shown that, at temperatures below the glass transition temperature, Tg, the model exhibits shear banding under imposed shear. It was also suggested that this behavior is closely related to the existence of a (static) yield stress (under applied stress, the system does not flow until the stress sigma exceeds a threshold value sigmay). A thorough analysis of the static yield stress is presented via simulations under imposed stress. Furthermore, using steady shear simulations, the effect of physical aging, shear rate and temperature on the stress-strain relation is investigated. In particular, we find that the stress at the yield point (the "peak"-value of the stress-strain curve) exhibits a logarithmic dependence both on the imposed shear rate and on the "age" of the system in qualitative agreement with experiments on amorphous polymers, and on metallic glasses. In addition to the very observation of the yield stress which is an important feature seen in experiments on complex systems like pastes, dense colloidal suspensions and foams, further links between our model and soft glassy materials are found. An example is the existence of hysteresis loops in the system response to a varying imposed stress. Finally, we measure the static yield stress for our model and study its dependence on temperature. We find that for temperatures far below the mode coupling critical temperature of the model (Tc = 0.435 in Lennard-Jones units), sigmay decreases slowly upon heating followed by a stronger decrease as Tc is approached. We discuss the reliability of results on the static yield stress and give a criterion for its validity in terms of the time scales relevant to the problem.     

Equilibrium density of states and thermodynamic properties of a model glass former

This paper presents an analysis of the thermodynamics of a model glass former. We have performed equilibrium sampling of a popular binary Lennard-Jones model, employing parallel tempering Monte Carlo to cover the crystalline, amorphous, and liquid regions of configuration space. Disconnectivity graphs are used to visualize the potential energy landscape in the vicinity of a crystalline geometry and in an amorphous region of configuration space. The crystalline global minimum is separated from the bulk of the minima by a large potential energy gap, leading to broken ergodicity in conventional simulations. Our sampling reveals crystalline global minima that are lower in potential energy than some of the previous candidates. We present equilibrium thermodynamic properties based on parallel tempering simulations, including heat capacities and free energy profiles, which depend explicitly on the crystal structure. We also report equilibrium melting temperatures.     

Light scattering from a binary mixture with internal relaxation

The hydrodynamic modes of a binary liquid mixture are investigated for two cases: (1) the bulk viscosity contains a frequency dependent part, and (2) an additional internal variable is introduced. The consequences for the light scat- tering spectrum are discussed and comparisons are made with previous investigations by other authors.     

Single particle jumps in a binary Lennard-Jones system below the glass transition

We study a binary Lennard-Jones system below the glass transition with molecular dynamics simulations. To investigate the dynamics we focus on events (jumps) where a particle escapes the cage formed by its neighbors. Using single particle trajectories we define a jump by comparing for each particle its fluctuations with its changes in average position. We find two kinds of jumps: "reversible jumps," where a particle jumps back and forth between two or more average positions, and "irreversible jumps," where a particle does not return to any of its former average positions, i.e., successfully escapes its cage. For all investigated temperatures both kinds of particles jump and both irreversible and reversible jumps occur. With increasing temperature, relaxation is enhanced by an increasing number of jumps and growing jump lengths in position and potential energy. However, the waiting time between two successive jumps is independent of temperature. This temperature independence might be due to aging, which is present in our system. We therefore also present a comparison of simulation data with three different histories. The ratio of irreversible to reversible jumps is also increasing with increasing temperature, which we interpret as a consequence of the increased likelihood of changes in the cages, i.e., a blocking of the "entrance" back into the previous cage. In accordance with this interpretation, the fluctuations both in position and energy are increasing with increasing temperature. A comparison of the fluctuations of jumping particles and nonjumping particles indicates that jumping particles are more mobile even when not jumping. The jumps in energy normalized by their fluctuations are decreasing with increasing temperature, which is consistent with relaxation being increasingly driven by thermal fluctuations. In accordance with subdiffusive behavior are the distributions of waiting times and jump lengths in position.     

Shear softening and structure in a simulated three-dimensional binary glass

Three-dimensional model binary glasses produced by quenching from a range of liquid temperatures were tested in shear over a range of strain rates using molecular-dynamics techniques. Tests were performed under constant volume and constant pressure constraints. The simulations revealed a systematic change in short-range order as a function of the thermal and strain history of the glass. While subtle signs of differences in short-range order were evident in the pair distribution function, three-body correlations were observed to be markedly more sensitive to the changes in structure. One particular structural parameter, the number of aligned three-atom clusters, was analyzed as a function of the degree of supercooling, the strain and the strain rate. The glasses quenched from the supercooled liquid regime were observed to contain an initially higher number of such clusters, and this number decreased under shear. Those quenched from high-temperature equilibrium liquids contained lower numbers of such clusters and these increased or remained constant under shear. The glasses quenched from the supercooled liquid regime showed higher strength, more marked shear softening, and an increased propensity toward shear localization. The evolution of this structural parameter depended both on its initial value and on the imposed shear rate. These results were observed to hold for simulations performed under both constant density and constant pressure boundary conditions.     

On the relaxation theory of glass transition in liquids

A method for calculating the constant in the main equation of glass transition (which relates the relxation time and the cooling rate near the glass transition temperature) with consideration given to the temperature dependence of the activation energy in this region is proposed. A modification of the main glass transition equation is considered. Application of this equation to the relaxation spectrometry of amorphous polymers and inorganic glasses is discussed.     

Relationship between the nonexponentiality of relaxation and relaxation time in the problem of glass transition

By analyzing the experimental data for various glass-forming liquids and polymers, we find that the nonexponentiality, beta, and the relaxation time, tau, are commonly related: log(tau) is an approximately linear function of 1/beta, followed in most cases by a crossover to a higher linear slope. We rationalize this relationship in the recently developed elastic approach to the glass transition. The key to the observed common relationship between beta and tau is that the two quantities are governed by the same parameter, the liquid elasticity length, d el. The increase of d el on lowering temperature increases tau and decreases beta, resulting in the observed common relationship between beta and tau. In this picture, we also discuss the crossovers of beta and tau at low temperature.     

Enthalpic relaxation and the glass transition in polymer blends

The kinetics of enthalpic relaxation are reviewed and applied to the ageing of a range of blends made from polyether imide and polyether ether ketone. DSC has been used to follow the development of enthalpic relaxation and a Williams-Watt stretched exponential equation relating the extent of relaxation, ϕ(t), to the ageing time t and an average relaxation time, t́_a', has been used to quantify the ageing process. where β' is inversely related to the breadth of the relaxation spectrum such that 0<β>1.0. The relationship was modified to incorporate non-linearity in the relaxation behaviour. ϕ(t) was measured directly from the enthalpy change observed in the endotherms on heating aged specimens through the glass transition in the DSC. The PEI/PEEK blends were compatible over the full composition range in that they exhibited a single glass transition with a temperature that varied almost linearly with composition between those of the homopolymers. Enthalpic relaxation was found to be a useful technique for probing the molecular relaxations of polymer blends and confirming the degree of compatibility of the system. The β' values changed systematically with the blend composition between those of the homopolymers suggesting that the breadth of the relaxation spectra were similar in the blends to that in the homopolymers. Physical ageing was observed to embrittle the blends, and there was a close correlation between the extent of enthalpic ageing and the change in mechanical and impact behaviour. The yield stress increased and the elongation to break decreased progressively with ϕ(t) in addition to a reduction in impact strength. The model of enthalpic relaxation and the kinetic relationships, outlined above, have been used to determine the onset of the glass transition temperature and subsequent progress of enthalpic relaxation at fixed ageing temperatures, for direct comparison with the change in specific heat observed in DSC experiments. Good agreement was observed between experiment and calculated glass transitions and the effect of variables, such as activation enthalpies, pre-exponential factors, non-linear factors such as X and β' and fictive temperature on the observed glass transition temperatures and the temperature range over which the glass transition occurred determined. Modifications to the model for the enthalpic relaxation have been suggested.