Johari-Goldstein Relaxation Research Articles

Discovery of collective nonjumping motions leading to Johari–Goldstein process of stress relaxation in model ionic glass

The slow β, or Johari–Goldstein (JG) relaxation process, has been widely observed in glasses and is known to induce the stress relaxation associated with mechanical properties. So far, jumping motions of only a fraction of the particles were believed to contribute to the JG process in glass. However, there is no direct experimental evidence of the atomic-scale images due to the difficulties in microscopic observation. In this study, atomic motions in the quasi-spherical model ionic-glass-former Ca0.4K0.6(NO3)1.4 were microscopically observed with one-angstrom resolution, the highest resolution to date, using X-ray time-domain interferometry. The microscopic experiment directly indicated that most particles underwent angstrom-scale motions in the time scale of the JG relaxation. This result was further supported by molecular dynamics (MD) simulations. A combined study of experiments and MD simulations revealed that most particles contributed to the JG process through unexpected collective nonjumping motions with angstrom-scale displacement, activated by jumping motions of a fraction of particles. The discovery of nonjumping motions by our atomic-scale dynamic observations has considerably advanced our understanding of the puzzling mechanism of the JG process.

Microscopic investigation of the Johari-Goldstein relaxation in cumene: Insights on the mosaic structure in a van der Waals liquid

The Johari-Goldstein (βJG) relaxation anticipates in time, and is closely connected to, the structural relaxation in deeply supercooled liquids. Probing its microscopic properties is a crucial step for a complete understanding of the glass-transition. We here report the investigation of the van der Waals glass-former cumene using time-domain interferometry, a technique able to probe microscopic density fluctuations at the spatial and temporal scales relevant for the βJG-relaxation. We find that the molecules participating in it undergo a restricted motion, though sufficient to induce local, cage-breaking events at the characteristic time-scale for molecular re-orientations. A detailed characterization of the relaxation strength, i.e. the fraction of molecules involved in the relaxation process, shows that such molecules are connected in a percolating cluster which, above the glass-transition temperature, Tg, is weakly dependent on temperature. Our results confirm thus previous observations of a mosaic structure associated to the βJG-relaxation in the supercooled state, and provide additional information on its temperature evolution above the glass-transition temperature. We conclude that the observed microscopic properties of the βJG-relaxation, and thus of the associated mosaic structure, are generic and independent of the molecular interaction potential. In addition, we show that, while the dynamics within the percolating cluster becomes progressively slower on approaching Tg, the fraction of the molecules involved in cage-breaking events within the βJG-relaxation is not affected by temperature.

Oxygen diffusion in glassy propylene carbonate: Energetics and spatial correlation of jump rates

Quenching of phenanthrene phosphorescence by molecular oxygen was used as a tool to determine the distribution function of the activation free energy of oxygen jumps in glassy propylene carbonate. The distribution width is shown to increase linearly with temperature within the range from 135 to 156 K. The dispersion of the activation free energy is caused by the spatial fluctuations of the activation entropy of oxygen jumps but not the energy barriers; the spatial dispersion of energy barriers is found to be zero. A characteristic length of the spatial correlation of the activation entropy is about 1.3 nm. The apparent activation energy linearly correlates with the macroscopic high-frequency shear modulus. The jump rates of oxygen molecules lie in the range of Johari-Goldstein relaxation rates. We suggest that an oxygen molecule overcomes the barrier between two neighboring interstitial sites due to low-frequency localized vibrational excitations of string-like groups of host molecules which draw the oxygen molecule in the motion.

Fractional Coupling of Primary and Johari-Goldstein Relaxations in a Model Polymer.

A polymer model exhibiting heterogeneous Johari-Goldstein (JG) secondary relaxation is studied by extensive molecular-dynamics simulations of states with different temperature and pressure. Time-temperature-pressure superposition of the primary (segmental) relaxation is evidenced. The time scales of the primary and the JG relaxations are found to be highly correlated according to a power law. The finding agrees with key predictions of the Coupling Model (CM) accounting for the decay in a correlation function due to the relaxation and diffusion of interacting systems. Nonetheless, the exponent of the power law, even if it is found in the range predicted by CM (0<ξ<1), deviates from the expected one. It is suggested that the deviation could depend on the particular relaxation process involved in the correlation function and the heterogeneity of the JG process.

Glass transition, crystallization kinetics, and inter-conformer relaxation dynamics of amorphous mitotane and related compounds

We employ differential scanning calorimetry, broadband dielectric spectroscopy and optical microscopy to investigate the glass transition, molecular relaxation dynamics, and isothermal recrystallization kinetics of amorphous mitotane, the only drug approved for the pharmacological treatment of adrenocortical carcinoma. Amorphous mitotane displays a glass transition at Tg = 243 ± 1 K, characterized by relatively low fragility index of 68 ± 2. Besides the structural and Johari-Goldstein relaxations, amorphous mitotane displays an intramolecular relaxation with activation energy of 25 ± 1 kJ mol−1. The same relaxation process, with virtually the same activation energy and relaxation times, is observed in the closely-related o,p’-dichlorobenzophenone compound, which allows identifying it as the rotation of the chlorobenzene ring with the chlorine closest to the central carbon. Such conformational relaxation is active at human body temperature, and may thus be potentially relevant for the mechanism of action of the drug. Our study shows that the comparative study of the relaxation map of related molecular species is a powerful tool to identify and classify secondary relaxation processes. The amorphous drug is found to be unstable against recrystallization at as well as slightly below room temperature, and to display-two-dimensional growth with only sporadic nucleation, characterized by an Avrami kinetic exponent of 2.05 ± 0.05. The kinetic stability of the amorphous form of mitotane, observed at room temperature in micellar formulations, is therefore limited to the nanoconfined sample and is not observed in the bulk compound.

Drug-Biopolymer Dispersions: Morphology- and Temperature- Dependent (Anti)Plasticizer Effect of the Drug and Component-Specific Johari-Goldstein Relaxations.

Amorphous molecule-macromolecule mixtures are ubiquitous in polymer technology and are one of the most studied routes for the development of amorphous drug formulations. For these applications it is crucial to understand how the preparation method affects the properties of the mixtures. Here, we employ differential scanning calorimetry and broadband dielectric spectroscopy to investigate dispersions of a small-molecule drug (the Nordazepam anxiolytic) in biodegradable polylactide, both in the form of solvent-cast films and electrospun microfibres. We show that the dispersion of the same small-molecule compound can have opposite (plasticizing or antiplasticizing) effects on the segmental mobility of a biopolymer depending on preparation method, temperature, and polymer enantiomerism. We compare two different chiral forms of the polymer, namely, the enantiomeric pure, semicrystalline L-polymer (PLLA), and a random, fully amorphous copolymer containing both L and D monomers (PDLLA), both of which have lower glass transition temperature (Tg) than the drug. While the drug has a weak antiplasticizing effect on the films, consistent with its higher Tg, we find that it actually acts as a plasticizer for the PLLA microfibres, reducing their Tg by as much as 14 K at 30%-weight drug loading, namely, to a value that is lower than the Tg of fully amorphous films. The structural relaxation time of the samples similarly depends on chemical composition and morphology. Most mixtures displayed a single structural relaxation, as expected for homogeneous samples. In the PLLA microfibres, the presence of crystalline domains increases the structural relaxation time of the amorphous fraction, while the presence of the drug lowers the structural relaxation time of the (partially stretched) chains in the microfibres, increasing chain mobility well above that of the fully amorphous polymer matrix. Even fully amorphous homogeneous mixtures exhibit two distinct Johari–Goldstein relaxation processes, one for each chemical component. Our findings have important implications for the interpretation of the Johari–Goldstein process as well as for the physical stability and mechanical properties of microfibres with small-molecule additives.

Characterization of the secondary relaxations in glass-forming liquids. The contribution of the Starkweather line and of the Coupling Model to the rationalization of TSDC results

This work addresses the study by thermostimulated depolarization currents (TSDC) of Johari-Goldstein (JG) β-relaxations in glass forming systems. Here we analyze the differences between the β-relaxation and the fast secondary (γ, δ, ε, …) relaxations on the one hand, and the α-relaxation on the other.It is shown that TSDC is able to characterize JG although this mobility is often close to, or even partially overlapped with, the α-relaxation.It is also shown that the Starkweather line or zero entropy line (ZEL) can be used as a reference element to identify the secondary relaxations: non-compliance with the ZEL is an indicator of cooperativity.We finally showed that the non-cooperativeness of the Johari-Goldstein relaxation is highlighted by the joint consideration of the Coupling Model and the Starkweather line.

Mutual Information in Molecular and Macromolecular Systems.

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari–Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.

Experimental evidence of mosaic structure in strongly supercooled molecular liquids

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

Johari–Goldstein Heterogeneous Dynamics in a Model Polymer

The heterogeneous character of the Johari-Goldstein (JG) relaxation is evidenced by molecular-dynamics simulation of a model polymer system. A double-peaked evolution of dynamic heterogeneity (DH), with maxima located at JG and structural relaxation time scales, is observed and mechanistically explained. {The short-time single-particle displacement during JG relaxation weakly correlates with the long-time one observed during structural relaxation}.

Charge transport and glassy dynamics in a room temperature ionic liquid- [BMPyr][TFSI

In this paper, thermal and spectroscopic investigations on a room temperature ionic liquid, 1-Butyl-1-methyl pyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMPyr][TFSI]) were carried out using differential scanning calorimetry and broadband dielectric spectroscopy over wide temperature to study the molecular mobility, charge transport properties, and conductivity relaxation. The quantum mechanical calculations were used to study its polar nature. The isochronal cooling and heating were measured to get an estimation of the glass transition temperature. The study revealed that [BMPyr][TFSI] is an intermediate fragile system with a fragility index of 96.3 and Tg around 182 K. A strong translational–rotation coupling of the [BMPyr][TFSI] molecules were observed in the glassy state due to intermolecular Johari Goldstein relaxation. It showed an excellent agreement with the coupling model predictions. The conductivity behavior of [BMPyr][TFSI] follows Johncher's power law, and its temperature dependence well fits with the Arrhenius equation having an activation energy of 686.3 kJ/mol. The glass transition temperature obtained from DSC, isochronal cooling data and heating data were in good agreement.

What We Need to Know about Solid-State Isothermal Crystallization of Organic Molecules from the Amorphous State below the Glass Transition Temperature.

In this Perspective, the authors have examined various principles associated with the isothermal crystallization of organic molecules from the amorphous state. The major objective was to better understand the underlying principles influencing long-term crystallization from the glassy state at temperatures sufficiently low enough to prevent crystallization over a period of about 2-3 years; this time frame was chosen based on the requirements for ensuring the physical stability of solid drug products. As such, after considering the general thermodynamic, dynamic (molecular mobility), and structural properties of both supercooled liquids and glasses, current understanding from the literature of overall crystallization, nucleation and growth from glasses, was reviewed. Typically, in attempting to establish the appropriate storage temperature, T, in the glassy state, relative to the glass transition temperature, Tg, i.e., Tg - T, most studies have tended to emphasize the rates of bulk diffusional molecular mobility of molecules at such temperatures and classical crystal nucleation and growth theory. However, a closer analysis of factors affecting crystallization from the glassy state revealed that greater consideration should be given to other contributing factors, including methods of producing the glass, heterogeneous nucleation due to processing conditions, secondary Johari-Goldstein relaxations, nondiffusional crystal growth in the glass (GC-growth), and surface crystallization.

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation.

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Fast secondary dynamics for enhanced charge transport in polymerized ionic liquids.

Segmental dynamics is considered as a major factor governing ionic conductivity of polymerized ionic liquids (PILs), envisioned as potential electrolytes in fuel cells and batteries. Our dielectric studies performed in T-P thermodynamic space on ionene, composed of the positively charged polymer backbone and freely moving anions, indicate that other relaxation modes, completely ignored so far, can affect the charge transport in PILs as well. We found that fast mobility manifested by a secondary β process promotes segmental dynamics and thereby increases ionic conductivity making the studied material a first coupled PIL of superionic properties. The molecular mechanism underlying such a β process has been identified as Johari-Goldstein relaxation giving experimental proof that fast secondary relaxations of intermolecular origin exist also in PILs and thereby reveal a universal character.

Separating β relaxation from α relaxation in fragile metallic glasses based on ultrafast flash differential scanning calorimetry

Despite numerous studies on the glass transition and the related relaxation dynamics, the physical mechanism of activation of multiscale relaxation events under various external stimuli in amorphous materials is still unclear. In this study, by combining traditional differential scanning calorimetry (DSC) and Flash DSC with heating rates spanning over five orders of magnitude, the thermodynamic responses have been systematically studied for several fragile and strong metallic glasses (MGs). A common endothermic event, before the glass transition, is detected when the heating rate increases above a critical value for fragile MGs. This endothermic event is verified to represent the activation of secondary $\ensuremath{\beta}$ relaxation (Johari-Goldstein relaxation), which is commonly found in amorphous materials. For fragile MGs, with an increase in fragility, the critical heating rate to separate $\ensuremath{\beta}$ relaxation from $\ensuremath{\alpha}$ relaxation decreases. In contrast, the $\ensuremath{\beta}$ relaxation does not appear within the current experimental heating rate limit for strong glass systems. Finally, based on the potential energy landscape model and the flow unit model of the heterogeneous structure for MGs, a pathway is proposed for the fragile and strong MGs to understand the physical mechanism for the separation of $\ensuremath{\beta}$ relaxation from $\ensuremath{\alpha}$ relaxation via ultrafast heating. This study clearly demonstrates that the Flash DSC with a wide heating rate range is an effective tool to study the relaxation dynamics in amorphous materials.

Johari-Goldstein relaxation in glass electrets

We investigate the dielectric response in the glass-electret state of two dipolar glass-forming materials. This unusual polar glassy state of matter is produced when a dipolar liquid is supercooled under the influence of a high electric dc field, which leads to partial orientational order of the molecules carrying a dipole moment. Investigation of the prepared glass-electrets by using low-field dielectric spectroscopy reveals a clear modification of their dielectric response in the regime of the Johari-Goldstein beta-relaxation, pointing to a small but significant increase of its relaxation strength compared to the normal glass. We discuss the implications of this finding for the still controversial microscopic interpretation of the Johari-Goldstein relaxation, an inherent property of glassy matter.

A microscopic look at the Johari-Goldstein relaxation in a hydrogen-bonded glass-former

Understanding the glass transition requires getting the picture of the dynamical processes that intervene in it. Glass-forming liquids show a characteristic decoupling of relaxation processes when they are cooled down towards the glassy state. The faster (βJG) process is still under scrutiny, and its full explanation necessitates information at the microscopic scale. To this aim, nuclear γ-resonance time-domain interferometry (TDI) has been utilized to investigate 5-methyl-2-hexanol, a hydrogen-bonded liquid with a pronounced βJG process as measured by dielectric spectroscopy. TDI probes in fact the center-of-mass, molecular dynamics at scattering-vectors corresponding to both inter- and intra-molecular distances. Our measurements demonstrate that, in the undercooled liquid phase, the βJG relaxation can be visualized as a spatially-restricted rearrangement of molecules within the cage of their closest neighbours accompanied by larger excursions which reach out at least the inter-molecular scale and are related to cage-breaking events. In-cage rattling and cage-breaking processes therefore coexist in the βJG relaxation.

Physical Aging of Zr65Cu17.5Ni10Al7.5 Glassy Alloy

Physical aging of Zr65Cu17.5Ni10Al7.5 glassy alloy has been studied after holding its samples with variation of time and temperature of annealing. The results showed that the enthalpy and entropy decreased with time according to a non-exponential kinetics. The Differential Scanning Calorimetry (DSC) heating scan after annealing was analyzed using Tool-Narayanaswamy-Moynihan (TNM) model. The analysis showed that a set of parameters that fitted the heating scan data for one annealing time did not fit the heating scan data for another annealing time. This is explained due to approximations used in the model and the effect of Johari-Goldstein (JG) relaxation to the physical aging.

Stability of lyophilized albumin formulations: Role of excipient crystallinity and molecular mobility.

In freeze-dried protein formulations, the composition governs the physical forms of the excipients and hence their functionality. It is also necessary to understand the effect of composition on the molecular relaxation behavior, a key factor influencing protein stability. Mannitol (bulking agent) - trehalose (lyoprotectant) - bovine serum albumin (BSA) lyophiles with varying trehalose to BSA mass ratios were investigated. The crystalline phases were characterized by X-ray diffractometry. The secondary structure of albumin in lyophiles and reconstituted solutions was evaluated by IR spectroscopy and circular dichroism, respectively. Dielectric spectroscopy was used to obtain the relaxation time of freeze-dried samples. When trehalose to BSA ratio was 0.2, while mannitol crystallized predominantly as the δ-anhydrous polymorph, trehalose remained amorphous. At lower concentrations of BSA, mannitol crystallized in both hemihydrate and anhydrous forms, and trehalose as dihydrate. The extent of dehydration during subsequent drying was dictated by the trehalose to BSA ratio in the formulation. A gradual increase in the Johari-Goldstein relaxation time was observed as the concentration of trehalose increased in the formulation. BSA was more susceptible to stresses from thawing than drying.

Chain Flexibility and the Segmental Dynamics of Polymers.

Using molecular dynamics simulations, we examine the dynamics of a family of model polymers with varying chain length and torsional potential barriers. We focus on features of the dynamics of polymers that are seen experimentally but absent in simulations of freely rotating and freely jointed chains. The reduced effect of volume on the segmental dynamics with increasing chain length, a capacity for pressure densification, and the deviation from constant Johari-Goldstein relaxation time at a constant segmental relaxation time all have a common origin, torsional rigidity, and these effects become increasingly apparent for more rigid chains.