Effect of temperature and pressure on the structural (α-) and the true Johari–Goldstein (β-) relaxation in binary mixtures

We investigated the microscopic origin of the excess wing through isothermal and isobaric dielectric relaxation measurements for the Quinaldine/tristyrene mixture. Our results show that the excess wing, characteristic of the high frequency side of the structural loss peak in neat Quinaldine, becomes a well resolved Johari–Goldstein secondary relaxation on mixing with the apolar tristyrene. Analyzing the temperature and pressure behavior of the two processes, a clear correlation has been found between the structural relaxation time, the Johari–Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). These results support the idea that the Johari–Goldstein relaxation acts as a precursor of the structural relaxation and therefore of the glass transition phenomenon.     

Relaxations in polystyrene below the glass transition studied by viscosity measurement

The shear viscosity of organic glass polystyrene has been determined under pure shear deformation mode from room temperature up to the glass transition temperature. A mechanical model of series connection of anelasticity and viscosity was used to determine the viscosity of the material. Relaxation time for the viscous flow was determined as a function of temperature. The relaxation was composed of two thermal-activation type relaxation processes: the high temperature relaxation (HTR) and the low temperature relaxation (LTR). In both relaxations the relaxation time was represented as τ=τ_o exp(E/k_BT), and the values of τ₀ and E were different in specimens treated differently – aged, loaded, and annealed. The observed τ₀ and E were not independent of each other but a compensation effect, a linear decrease of logτ₀ with increased E, was seen. The results were explained using the idea of cooperative relaxations of relaxing elements. HTR and LTR were considered to correspond to the structural and the slow relaxations, respectively, and the relaxing elements could respectively be a single atom or molecule and a segment in molecular chains.     

Recent advances in fundamental understanding of glass transition

Some examples of recent advances in experiment and theory that have impact on solution of the glass transition problem are briefly discussed. The fundamental importance of a special class of secondary relaxations called the Johari–Goldstein β-relaxations becomes clear from the recent advances. The main part of the paper addresses a recent research problem, which is the purported anomalous temperature dependence of the relaxation time of another secondary (γ) relaxation found by fitting dielectric relaxation data in several glass-formers. We show the anomalous T-dependence of this faster γ-relaxation is caused by the influence exerted by the slower Johari–Goldstein β-relaxation in the vicinity.     

Second harmonic generation studies of intrinsic and extrinsic relaxation dynamics in poly(methy1 methacrylate)

Second harmonic generation (SHG) is used to monitor the reorientation a dopant chromophore in slightly entangled poly(methy1 methacrylate) (PMMA). The effect of charge and temperature on both the decay and the much less studied onset modes of SHG signal at temperatures above the glass transition has been examined. At variance with the theoretical predictions, it is shown that the onset and the decay times are not coincident. An isothermal experiment above the glass transition shows a lengthening of relaxation time of the decay mode due to successive poling process, which is ascribed to charge memory effects. In contrast, the latter do not affect the onset characteristic time. The effect of temperature above the glass transition on dopant rotation and polymer relaxations has been also examined. As temperature increases the relaxation times of both the onset and the decay modes decrease. If the surface charge and the charge memory effect are erased, the decay time compares quite well with the structural relaxation time. Differently, the onset time exhibits a partial decoupling.     

Structural relaxation in sputter-deposited silica glass

We investigated structural relaxation below the glass transition temperature in sputter-deposited silica glass. Structural relaxation was obtained from annealing behavior of the IR reflection structural band position. Results were compared with that of bulk silica glass. Results showed the following. (1) The structural relaxation time is 10⁶ times shorter than that of bulk silica glass. (2) The activation energy is close to that of bulk silica glass. (3) Once the structural relaxation reaches a steady state, the structure of silica glass film resembles that of bulk silica glass.     

Does the entropy and volume dependence of the structural α-relaxation originate from the Johari–Goldstein β-relaxation?

Conventional theories of glass transition are focussed on reproducing the thermodynamic and dynamic properties of the structural α-process. However, in the last decade a class of secondary relaxations, called the Johari–Goldstein (JG) β-relaxations, has attracted attention in view of the existence of various correlations and similarities of its properties with the structural α-process. These experimental evidences suggest that the secondary process may play a fundamental role in glass transition, and any theory of glass transition would be incomplete until the inter-relations between the JG β-relaxation and the structural relaxation have been considered. In this paper, some significant experimental results showing several general and nontrivial connections between structural relaxation and JG β-relaxation are reported. In particular, these connections indicate that, like the structural relaxation, the JG β-relaxation is entropy and volume dependent. The experimental facts are consistent with the results of the coupling model.     

Why the glass transition problem remains unsolved?

Glass-forming substances are made of units having nontrivial mutual interactions. Therefore, structural relaxation of glass-formers is a many-body relaxation problem, which unfortunately is still an unsolved problem in statistical mechanics. Conventional theories and models of glass transition bypass solution of this problem, and hence glass transition also remains an unsolved problem. There are plenty of experimental evidences for the many-body relaxation dynamics, and some examples are given. The Coupling Model of the author is not a full solution of the many-body relaxation problem either. Nevertheless, its predictions can explain the properties of structural relaxation originating from many-body relaxation and their relations to its precursor, namely the Johari–Goldstein secondary relaxation. A demonstration of the utility of the Coupling Model is given here by solving two problems in glass transition that involve many-body relaxation. The problems are: (1) the breakdown of Stokes–Einstein and Debye–Stokes–Einstein relations in neat molecular glass-formers and colloidal suspension, and (2) the component dynamics of polymer blends and mixtures of non-polymeric glass-formers.     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

The effect of cooling rate on the glass transition of an amorphous polymer

Cooling down from the equilibrium state at different rates reveals the dynamic behavior of glass forming materials. In particular, the dependence of the glass transition region on the cooling rate, q is commonly agreed to contain information regarding the activation energy of the relaxation time, τ. In this work experimental and theoretical aspects of such a relationship have been highlighted. Experimentally, the glass transition zone of amorphous polystyrene films has been investigated over two decades of cooling rate (0.5–50 K/min) by using refractive index measurements. The shift of the glass transition temperature and the broadening of the transition zone at increased cooling rate have been characterized. Theoretically, the cooling experiments have been simulated within the integral formulation of the Kovacs–Aklonis–Hutchinson–Ramos (KAHR) model using the Vogel temperature dependence for the relaxation time. The Frenkel–Kobeko–Reiner equation, τq = constant, provided the needed relationship between the experiments and the theory, enabling the evaluation of the relevant parameter of the kinetic model, i.e. the Vogel activation energy and the zero configurational entropy temperature, from the shift of the glass transition temperature with cooling rate.     

Volume effects on the glass transition dynamics

The role of jamming (steric constraints) and its relationship to the available volume is addressed by examining the effect that certain modifications of a glass-former have on the ratio of its isochoric and isobaric activation enthalpies. This ratio reflects the relative contribution of volume (density) and temperature (thermal energy) to the temperature-dependence of the relaxation times of liquids and polymers. We find that an increase in the available volume confers a stronger volume-dependence to the relaxation dynamics, a result at odds with free volume interpretations of the glass transition.     

Structural relaxations in disordered solids below Tg: Study by thermal-cycling single-molecule spectroscopy

Structural relaxation processes in disordered solids on the length scale of nanometers have been studied in a broad temperature range, from 4.5 K up to the glass transition, with single-molecule spectroscopy and thermal cycling experiments. Irreversible changes of single-molecule spectra after thermal cycles were observed and attributed to relaxation processes in the local environment of the probe molecules of tetra-tert-butylterrylene in polyisobutylene (PIB) and terrylene in solid ortho-dichlorobenzene. The effects of the relaxation processes on the individual parameters of low-energy matrix excitations (tunneling two-level systems and low-frequency vibrational modes) were analyzed, as well as the temperature dependence of the efficiency of relaxation processes in PIB. The results prove that the local matrix relaxations are characterized by a spatially varying distribution of activation energies.     

An analysis of the suitability of the Lillie number to characterize the glass transition of real glass-forming substances

The Lillie Number, the negative product of the cooling rate and the temperature dependence of the structural relaxation time, characterizes the glass transition for systems with a single relaxation time. In this paper the utility of the concept is examined in “real” glasses, where a spectrum of relaxation times is necessary. While the principle is still useful, the simplicity and elegance of the approach are diminished for the “real” glass.     

Secondary dielectric relaxation in decahydroisoquinoline–cyclohexane mixture

We report dielectric relaxation measurements of the mixture 25% w/w cyclohexane in decahydroisoquinoline. Cyclohexane is a non-polar liquid that acts as an external parameter influencing relaxation dynamics of pure decahydroisoquinoline. Two different secondary relaxation processes dominate the relaxation dynamics below the glass transition temperature, as previously observed in the pure decahydroisoquinoline. Based on the coupling model analysis we identified the fastest secondary process as the genuine secondary Johari Goldstein process, reflecting the motion of the whole molecule. On the other hand, the microscopic origin of the slowest secondary process still remains unknown.     

The secondary relaxation in the dielectric loss of glucose–water mixtures

We studied dielectric relaxation phenomena of glucose–water mixtures in the supercooled and glassy states at frequencies ranging from 0.1 Hz to 10 MHz. We clearly observed that as the water content or temperature decreased, the relaxation time difference between the α and the secondary relaxations increased and the relative relaxation strength between the α and the secondary relaxation decreased. We found that the effects of adding water or increasing temperature on the secondary relaxation are qualitatively similar to that of decreasing the rotation–translation (RT) coupling constant in the schematic mode-coupling theory [W. Götze, M. Sperl, Phys. Rev. Lett. 92 (2004) 105701].     

Relation between the dispersion of α-relaxation and the time scale of β-relaxation at the glass transition

Dielectric spectra of several typical molecular glass-formers, showing one or more secondary processes resolved in the glassy state, have been measured at different temperatures. We found that the genuine Johari–Goldstein β-relaxation is connected to the structural relaxation. In fact, a clear correlation was found between the structural relaxation time, the Johari–Goldstein relaxation time and the dispersion of the structural relaxation (i.e. its Kohlrausch parameter). Moreover, for a group of epoxide oligomers the steepness index is correlated to the broadness of the structural peak and to the time separation between the structural and the Johari–Goldstein relaxation. These results support the idea that the Johari–Goldstein relaxation acts as a precursor of the structural relaxation. A rationale for our results is provided by the coupling model.     

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

The temperature dependence (25–1400 °C) of ²⁷Al NMR spectra and spin–lattice relaxation time constants T₁ have been studied for a calcium aluminosilicate (43.1CaO–12.5Al₂O₃–44.4SiO₂) glass and melt using an in situ high temperature probe, and the glass has been characterized by ambient temperature, high field MAS NMR. The peak positions and the line widths show a consistent behavior as motional averaging of the quadrupolar satellites increases with increasing temperature. The rate of decrease with temperature of T₁ drastically increases near the glass transition temperature T_g, which suggests a change in NMR relaxation process from vibrational to translational motions. Above the T₁ minimum (≈1200 °C), NMR correlation times obtained from T₁ are in good agreement with shear relaxation times estimated from viscosity, suggesting that microscopic nuclear spin relaxation is controlled by the same dynamics as macroscopic structural relaxation, and thus that atomic-scale motion is closely related to macroscopic viscous flow.     

The role of primitive relaxation in the dynamics of aqueous mixtures, nano-confined water and hydrated proteins

The relaxation scenario in aqueous systems, such as mixtures of water with hydrophilic solutes, nano-confined water and hydrated biomolecules, has been shown to exhibit general features, in spite of the huge differences in structure, chemical composition and complexity. Dynamics, in all these systems, invariably shows at least two relaxations: (i) a slower process, related to cooperative and structural motions of water and solute molecules (in the case of mixtures) or related to interfacial processes in the case of confined water and (ii) a faster process, with non-cooperative character originating from water. The latter has properties including timescale and temperature dependence similar or related in all the aqueous systems. This water-specific relaxation can be identified as the primitive relaxation, or the Johari-Goldstein β-relaxation. The primitive process is the precursor of the many-body relaxation process which increases in length-scale with time until the terminal α-relaxation is reached.Using new experimental data (at atmospheric and high pressure) along with a revision of most of the recent literature on the dynamics of confined water and aqueous mixtures, we show that the two abovementioned relaxation processes are inter-related as evidenced by correlations in their properties. For instance, both relaxation time and dielectric strength of the water-specific relaxation exhibit a crossover from a stronger to a weaker dependence with decreasing T, at the temperature where the slow process attains a very long timescale (> 1 ks) and becomes structurally arrested, exactly analogous to that found for β-relaxation in van der Waals liquids. Moreover, the primitive relaxation of water is shown to play a pivotal role in determining the dynamics of hydrated biomolecules in general, including the “dynamic transition” observed by neutron scattering and Mössbauer spectroscopy. We show that the primitive relaxation of the solvent is responsible for the dynamic transition, even in the case that the solvent is not pure water or an aqueous mixture.     

Effects of aluminum impurity on the structural relaxation in silica glass

To elucidate effects of Al impurity on the glass-forming process in silica glass, the structural relaxation in Al-containing silica glass, with alkali ions of only trace levels, was investigated by observing the fictive temperature. The fictive temperature was determined by infrared (IR) absorption analysis. Al, even at concentrations lower than 10 wtppm, increases the relaxation time and the activation energy of the -relaxation. It also suppresses the sub-relaxational process due to OH ions. These results indicated that Al should have other effects on structural relaxation than alkali–aluminate complex formation, as has been thought to be the cause for an increase in the -relaxation time and thus the viscosity of silica glass. Furthermore, the structural relaxation does not merely depend on the number of non-bridging oxygen atoms in the glass network.     

Free energy landscape approach to glass transition

As a Landau-type theory for the glass transition, we present a free energy landscape (FEL) picture which provides a unified understanding of glass transition singularities and show that the FEL can really be calculated by a theoretical and a computational approach. We first give a clear definition of the FEL and argue that there are two kinds of cooperative rearranging region. One is the region defined by the difference between two adjacent basins which could be called simultaneously rearranged region and the other is the atoms involved in the excited state between two adjacent basins. Exploiting the density functional theory, we obtain the FEL for a relaxation process which is characterized by a string motion and determine the size of the cooperatively rearranging region. We also show that the FEL can be determined by the principal component analysis for time dependent configuration obtained by the MD simulation.     

Exponential structural relaxation of a high purity silica glass

While most other glasses exhibit non-exponential structural relaxation characteristics even when the change of fictive temperature is small, a high purity silica glass exhibited exponential structural relaxation. This was demonstrated by showing that the non-exponential exponent or β value of the KWW function of the high purity silica glass approaches unity when the change of the fictive temperature approaches zero both from higher and lower temperature sides of the heat-treatment temperature. The non-exponentiality of the structural relaxation of this glass when fictive temperature change is finite is due to the change of relaxation time during the structural relaxation.