Origin of ργ/T scaling of primary and secondary conductivity relaxation times in mixture of water with protic ionic liquid.

Murali et al. [J. Phys. Chem. Lett., 2024, 15, 3376-3382] made ambient and high pressure dielectric measurements of a supercooled aqueous mixture of an acidic ionic liquid to find the presence of the primary (σ) conductivity relaxation together with the secondary (ν) conductivity relaxation originating from the water clusters confined by the cations and anions with relaxation times τσ and τν respectively. From the isothermal and isobaric conductivity relaxation data found on varying thermodynamic conditions (i.e. T and P) at constant τσ are the invariance of (i) the frequency dispersion or the Kohlrausch function exponent (1 - n) of the primary conductivity relaxation, and (ii) the ratio of the primary and secondary conductivity times, τσ/τν. This co-invariance of τσ, τν, and (1 - n) at constant τσ was observed before in non-aqueous ionic liquids, but it is found for the first time in aqueous ionic liquids. The new data together with PVT measurements enable Murali et al. to show additionally that both τσ and τν are functions of ργ/T with a single exponent γ = 0.58. The Coupling model is the only theory predicting the co-invariance of τσ, τν, and (1 - n) as well as the ργ/T scaling of both τσ and τν. It is applied herein to address and explain the data of the ionic liquid-water mixture.

The origin of the faster mechanism of partial enthalpy recovery deep in the glassy state of polymers.

A novel finding made by Cangialosi and coworkers in the physical aging of several polymers way below the glass transition temperature Tg is that equilibrium recovery occurs by reaching a plateau in the enthalpy with partial enthalpy recovery. This observation points to the existence of a much faster mechanism capable of partial equilibrium recovery deep in the glassy state. A similar phenomenon was found in different glassy materials. The generality of the phenomenon indicates that the faster mechanism of equilibrium recovery is universal and fundamental. In this paper the faster mechanism is identified to be the universal JG ß-relaxation having dynamic and thermodynamic properties analogous to the α-relaxation, and thus capable of effecting enthalpy and volume recovery far below Tg in several high-Tg polymers. The JG ß-relaxation is also the mechanism responsible for the first step of two steps in the approach to equilibrium found in another polymer with much lower Tg. The Coupling Model is used to explain why the first step transpires far below Tg in some polymers but much closer to Tg in another polymer.

Why the Brillouin Light Scattering Relaxation Times of Molten Zinc Chloride Are Shorter and Weaker in Temperature Dependence than the Structural Relaxation Times from Depolarized Light and Neutron Spin Echo Spectroscopy.

A longstanding problem in the Brillouin light scattering (BLS) study of polymers is the relaxation times τBLS(T) being more than an order of magnitude shorter than the α-relaxation times τα(T) determined by dielectric, depolarized light scattering (DLS), and molecular dynamics simulations. In tackling the problem, τBLS(T) was identified with the relaxation time τ0(T) of the primitive relaxation in the coupling model, which can be calculated from τα(T) and the stretch exponent ßK of the Kohlrausch correlation function for the α-relaxation.. The problem was solved by finding that indeed τ0(T) is in good agreement with τBLS(T). A recent work performed the neutron spin echo study of the structural α-relaxation of the network ionic liquid ZnCl2 and found the same anomaly as polymers. The α-relaxation time τNSE(T) from neutron spin echo (NSE) as well as the α-relaxation time τDLS(T) from DLS of ZnCl2 are much longer than τBLS(T) from BLS obtained before by several research groups. The finding of the same anomaly in ZnCl2 and polymers with very different chemical and physical structures offers an opportunity to critically test the explanation given before. The test was carried out by calculating the primitive relaxation times τ0,DLS(T) and τ0,NSE(T) from τDLS(T) and τNSE(T), respectively, in zinc chloride. Good agreements of τBLS(T) from BLS with τ0,DLS(T) and τ0,NSE(T) were found and thus the explanation given for polymers remains valid for ZnCl2. The test was extended to glycerol by comparing τBLS(T) with τ0,ICNS(T) and τ0,CNS(T) calculated from the α-relaxation time τICNS(T) and τCNS(T) from incoherent and coherent neutron scattering, respectively. There is good agreement between τBLS(T) and τ0,ICNS(T) in glycerol. There is also semiquantitative agreement of τBLS(T) with τ0,DS(T) from dielectric spectroscopy as well as τ0,CNS(T). Thus, the explanation for polymers is verified in the two very different glass formers, ZnCl2 and glycerol, and it is an advancement in the application of BLS to study the dynamics of glass formers.

Isochronal superpositioning of the caged dynamics, the α, and the Johari-Goldstein ß relaxations in metallic glasses.

The superposition of the frequency dispersions of the structural α relaxation determined at different combinations of temperature T and pressure P while maintaining its relaxation time τα(T, P) constant (i.e., isochronal superpositioning) has been well established in molecular and polymeric glass-formers. Not known is whether the frequency dispersion or time dependence of the faster processes including the caged molecule dynamics and the Johari-Goldstein (JG) ß relaxation possesses the same property. Experimental investigation of this issue is hindered by the lack of an instrument that can cover all three processes. Herein, we report the results from the study of the problem utilizing molecular dynamics simulations of two different glass-forming metallic alloys. The mean square displacement 〈Δr2t〉, the non-Gaussian parameter α2t, and the self-intermediate scattering function Fsq,t at various combinations of T and P were obtained over broad time range covering the three processes. Isochronal superpositioning of 〈Δr2t〉, α2t, and Fsq,t was observed over the entire time range, verifying that the property holds not only for the α relaxation but also for the caged dynamics and the JG ß relaxation. Moreover, we successfully performed density ρ scaling of the time τα2,maxT,P at the peak of α2t and the diffusion coefficient D(T, P) to show both are functions of ργ/T with the same γ. It follows that the JG ß relaxation time τß(T, P) is also a function of ργ/T since τα2,maxT,P corresponds to τß(T, P).

Predicting the α-relaxation time of glycerol confined in 1.16 nm pores of zeolitic imidazolate frameworks.

Uhl et al. [J. Chem. Phys., 2019, 150, 024504] studied the molecular dynamics of glycerol confined in a microporous zeolitic imidazolate framework (ZIF-8) with well-defined pore diameters of 1.16 nm by broadband dielectric spectroscopy. Of interest is a fast process in the central part of the pores identified as the α-relaxation of the confined supercooled glycerol with relaxation times τα,conf(T) reduced from τα,bulk(T) of bulk glycerol and having a temperature dependence different from the super-Arrhenius temperature of the latter. The focus of Uhl et al. was relating the confined molecular dynamics to the cooperativity length scales Lcorr(T) of molecular motion above the glass transition, and deducing the limiting high-temperature value of the correlation length of about 1.22 nm. Not yet considered by anyone are the observed values of τα,conf(T) and temperature dependence. Since the cooperativity length scales Lcorr(T) were found to be larger than the pore size of ZIF-8 over the temperature range studied and the density of the glycerol in the pore is possibly lower than the bulk, the cooperativity of the α-relaxation of glycerol confined in ZIF-8 is drastically reduced. Thus, within the framework of the Coupling Model (CM), τα,conf(T) should be nearly the same as the primitive relaxation time τ0(T) for glycerol when devoid of intermolecular coupling and cooperativity. Consistent with the absence of cooperativity of the glycerol confined in ZIF-8, we find the calculated τα,conf(T) are either the same or slightly longer than the calculated values of τ0(T). The quantitative prediction of the CM is verified. At this time we know of no other theory that can make such a quantitative prediction.

The structural α-relaxation times of prilocaine confined in 1 nm pores of molecular sieves: quantitative explanation by the coupling model.

The molecular glass-former and pharmaceutical, prilocaine, distinguishes itself by exhibiting seven general and fundamental dynamic and thermodynamic properties [Z. Wojnarowska, et al., J. Phys. Chem. B, 2015, 39, 12699.], all of which have been explained using the coupling model. What has not been studied before are the changes in properties of the structural α-relaxation of prilocaine when subjected to extreme nano-confinement in spaces with a size of about 1 nm. Recently, Ruis et al. [G. N. Ruiz, et al., Phys. Chem. Chem. Phys., 2019, 21, 15576.] measured the α-relaxation times, τα,conf(T), of prilocaine confined in 1 nm pores of molecular sieves. They found that τα,conf(T) are significantly reduced from those of bulk prilocaine, τα,bulk(T), and assume a weaker temperature dependence. The data in toto pose a challenge for any theory of glass transition to explain quantitatively. The coupling model (CM) was applied to this problem to predict the α-relaxation times of prilocaine when cooperativity is removed, which is expected because only a few prilocaine molecules can fit into the 1 nm pores. The results from the CM are in quantitative agreement with the experimental values of τα,conf(T) and the temperature dependence. The success is nontrivial because no other extant theory can do the same to the best of our knowledge.

Critical structural fluctuations of proteins upon thermal unfolding challenge the Lindemann criterion.

Internal subnanosecond timescale motions are key for the function of proteins, and are coupled to the surrounding solvent environment. These fast fluctuations guide protein conformational changes, yet their role for protein stability, and for unfolding, remains elusive. Here, in analogy with the Lindemann criterion for the melting of solids, we demonstrate a common scaling of structural fluctuations of lysozyme protein embedded in different environments as the thermal unfolding transition is approached. By combining elastic incoherent neutron scattering and advanced molecular simulations, we show that, although different solvents modify the protein melting temperature, a unique dynamical regime is attained in proximity of thermal unfolding in all solvents that we tested. This solvation shell-independent dynamical regime arises from an equivalent sampling of the energy landscape at the respective melting temperatures. Thus, we propose that a threshold for the conformational entropy provided by structural fluctuations of proteins exists, beyond which thermal unfolding is triggered.

The JG ß-relaxation in water and impact on the dynamics of aqueous mixtures and hydrated biomolecules.

Although by now the glass transition temperature of uncrystallized bulk water is generally accepted to manifest at temperature Tg near 136 K, not much known are the spectral dispersion of the structural α-relaxation and the temperature dependence of its relaxation time τα,bulk(T). Whether bulk water has the supposedly ubiquitous Johari-Goldstein (JG) ß-relaxation is a question that has not been answered. By studying the structural α-relaxation over a wide range of temperatures in several aqueous mixtures without crystallization and with glass transition temperatures Tg close to 136 K, we deduce the properties of the α-relaxation and the temperature dependence of τα,bulk(T) of bulk water. The frequency dispersion of the α-relaxation is narrow, indicating that it is weakly cooperative. A single Vogel-Fulcher-Tammann (VFT) temperature dependence can describe the data of τα,bulk(T) at low temperatures as well as at high temperatures from neutron scattering and GHz-THz dielectric relaxation, and hence, there is no fragile to strong transition. The Tg-scaled VFT temperature dependence of τα,bulk(T) has a small fragility index m less than 44, indicating that water is a "strong" glass-former. The existence of the JG ß-relaxation in bulk water is supported by its equivalent relaxation observed in water confined in spaces with lengths of nanometer scale and having Arrhenius T-dependence of its relaxation times τconf(T). The equivalence is justified by the drastic reduction of cooperativity of the α-relaxation in nanoconfinement and rendering it to become the JG ß-relaxation. Thus, the τconf(T) from experiments can be taken as τß,bulk(T), the JG ß-relaxation time of bulk water. The ratio τα,bulk(Tg)/τß,bulk(Tg) is smaller than most glass-formers, and it corresponds to the Kohlrausch α-correlation function, exp[-(t/τα,bulk)1-n], having (1-n) = 0.90. The dielectric data of many aqueous mixtures and hydrated biomolecules with Tg higher than that of water show the presence of a secondary ν-relaxation from the water component. The ν-relaxation is strongly connected to the α-relaxation in properties, and hence, it belongs to the special class of secondary relaxations in glass-forming systems. Typically, its relaxation time τν(T) is longer than τß,bulk(T), but τν(T) becomes about the same as τß,bulk(T) at sufficiently high water content. However, τν(T) does not become shorter than τß,bulk(T). Thus, τß,bulk(T) is the lower bound of τν(T) for all aqueous mixtures and hydrated biomolecules. Moreover, it is τß,bulk(T) but not τα(T) that is responsible for the dynamic transition of hydrated globular proteins.

Why is the change of the Johari-Goldstein ß-relaxation time by densification in ultrastable glass minor?

Ultrastable glasses (USG) formed by vapor deposition are considerably denser. The onset temperature of devitrification, Ton, is significantly higher than Ton or Tg of ordinary glass (OG) formed by cooling, which implies an increase of the structural α-relaxation time by many orders of magnitude in USG compared to that in OG at the same temperature. However, for a special type of secondary relaxation having properties strongly connected to those of the α-relaxation, called the Johari-Goldstein ß-relaxation, its relaxation time in USG is about an order of magnitude slower than that in OG and it has nearly the same activation energy, Eß. The much smaller change in τß and practically no change in Eß by densification in USG are in stark contrast to the behavior of the α-relaxation. This cannot be explained by asserting that the Johari-Goldstein (JG) ß-relaxation is insensitive to densification in USG, since the JG ß-relaxation strength is significantly reduced in USG to such a level that it would require several thousands of years of aging for an OG to reach the same state, and therefore the JG ß-relaxation does respond to densification in USG like the α-relaxation. Here, we provide an explanation based on two general properties established from the studies of glasses and liquids at elevated pressures and applied to USG. The increase in density of the glasses formed under high pressure can be even larger than that in USG. One property is the approximate invariance of the ratio τα(Ton)/τß(Ton) to density change at constant τα(Ton), and the other is the same ργ/T-dependence of τß in USG and OG where ρ is the density and γ is a material constant. These two properties are derived using the Coupling Model, giving a theoretical explanation of the phenomena. The explanation is also relevant for a full understanding of the experimental result that approximately the same surface diffusion coefficient is found in USG and OG with and without physical aging, and ultrathin films of a molecular glass-former.

Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses.

Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via the Coupling Model, secondary relaxations having such connection have been identified by properties similar to the primitive relaxation of the Coupling Model and are called the Johari-Goldstein (JG) ß-relaxations. They involve the motion of the entire molecule and act as the precursor of the structural α-relaxation. The change in dynamics of the secondary relaxation by aging an ordinary glass is one way to understand the connection between the two relaxations, but the results are often equivocal. Ultrastable glasses, formed by physical vapour deposition, exhibit density and enthalpy levels comparable to ordinary glasses aged for thousands of years, as well as some particular molecular arrangement. Thus, ultrastable glasses enable the monitoring of the evolution of secondary processes in case aging does not provide any definitive information. Here, we study the secondary relaxation of several ultrastable glasses to identify different types of secondary relaxations from their different relationship with the structural relaxation. We show the existence of two clearly differentiated groups of relaxations: those becoming slower in the ultrastable state and those becoming faster, with respect to the ordinary unaged glass. We propose ultrastability as a way to distinguish between secondary processes arising from the particular microstructure of the system and those connected in properties to and acting as the precursor of the structural relaxation in the sense of the Coupling Model.

Secondary relaxation in ultrastable etoricoxib: evidence of correlation with structural relaxation.

Secondary relaxations are fundamental for their impact in the properties of glasses and for their inseparable connection to the structural relaxation. Understanding their density dependence and aging behavior is key to fully address the nature of glasses. Ultrastable glasses establish a new benchmark to study the characteristics of secondary relaxations, since their enthalpy and density levels are unattainable by other routes. Here, we use dielectric spectroscopy at ambient and elevated pressures to study the characteristics of the secondary relaxation in ultrastable etoricoxib, reporting a 71% decrease in dielectric strength and one decade increase in relaxation time compared to the ordinary glass. Interestingly, we find an unprecedented connection between secondary and structural relaxations in ultrastable etoricoxib in exactly the same manner as in the ordinary glass, manifested through different properties, such as aging and devitrification. These results further support and extend the general validity of the connection between the secondary and structural relaxation.

Contrasting two different interpretations of the dynamics in binary glass forming mixtures.

In a series of papers on binary glass-forming mixtures of tripropyl phosphate (TPP) with polystyrene (PS), Kahlau et al. [J. Chem. Phys. 140, 044509 (2014)] and Bock et al. [J. Chem. Phys. 139, 064508 (2013); J. Chem. Phys. 140, 094505 (2014); and J. Non-Cryst. Solids 407, 88-97 (2015)] presented the data on the dynamics of the two components studied over the entire composition range by several experimental methods. From these sets of data, obtained by multiple experimental techniques on mixtures with a large difference ΔTg ≈ 200 K between the glass transition temperatures of two starting glass formers, they obtained two α-relaxations, α1 and α2. The temperature dependence of the slower α1 is Vogel-Fulcher like, but the faster α2 is Arrhenius. We have re-examined their data and show that their α2-relaxation is the Johari-Goldstein (JG) ß-relaxation with Arrhenius T-dependence admixed with a true α2-relaxation having a stronger temperature dependence. In support of our interpretation of their data, we made dielectric measurements at elevated pressures P to show that the ratio of the α1 and α2 relaxation times, τα1(T,P)/τα2(T,P), is invariant to variations of T and P, while τα1(T,P) is kept constant. This property proves unequivocally that the α2-relaxation is the JG ß-relaxation, the precursor of the α1-relaxation. Subsequently, the true but unresolved α2-relaxation is recovered, and its relaxation times with much stronger temperature dependence are deduced, as expected for the α-relaxation of the TPP component. The results are fully compatible with those found in another binary mixture of methyltetrahydrofuran with tristyrene and PS with ΔTg ≈ 283 K, even larger than ΔTg ≈ 200 K of the mixture of TPP with PS, and in several polymer blends. The contrast between the two very different interpretations brought out in this paper is deemed beneficial for further progress in this research area.

Deviations of dynamic parameters characterizing enthalpic and dielectric relaxations in glass forming alkyl phosphates.

In our recent study [T. Wu et al., J. Chem. Phys. 147, 134501 (2017)], an alkyl phosphate glass former was studied and it suggested that the enthalpy relaxation involving the motions of all parts of the molecule is global, while the dielectric relaxation detects the local rotation of the polar core. In this work, we study a series of trialkyl phosphates using calorimetric and dielectric measurements over a wide temperature range. The results indicate a departure of the dielectric fragility indexes from the enthalpic ones as the length of the branch chain increases in the trialkyl phosphates. The Kirkwood correlation factor (g k ) is found to coincide at ∼0.6 at glass transition temperature (T g ) from triethyl phosphate to tributyl phosphate, indicating a similar structural alignment. The enthalpic relaxation serving as the more fundamental relaxation relevant to the structural relaxation is confirmed. Strikingly, we observed the relation of T g to the chain length in alkyl phosphates, revealing a minimum T g behavior, and its explanation assists in the understanding of the glass transition in relation to the structure of the glass-formers.

Atorvastatin as a Promising Crystallization Inhibitor of Amorphous Probucol: Dielectric Studies at Ambient and Elevated Pressure.

The aim of this article was to check the physical stability of the amorphous form of probucol at both standard storage and manufacturing conditions. Our studies clearly show that disordered form of the examined, cholesterol lowering, agent stored at ambient pressure does not reveal any tendency toward recrystallization. The physical stability of neat probucol stored at ambient pressure has been investigated (i) at room temperature by means of X-ray diffraction technique (XRD) as well as (ii) at T = 333 K by means of broadband dielectric spectroscopy (BDS). Due to the fact that compression is an important stage of drugs manufacturing we additionally performed physical stability tests of amorphous probucol at elevated pressure. The recrystallization tendency of the examined pharmaceutical has been tracked online from the initial and further up to a few hours after compression by means of the high pressure BDS technique. These experiments indicate that even very small pressure applied during the sample compression immediately induce its recrystallization. Since, the sensitivity on pressure eliminates probucol from the group of physically stable amorphous APIs, its stabilization is required. Taking into account that there are many scientific reports describing the positive effect of coadministration of probucol with the drug atorvastatin, we used the latter as probucol's crystallization inhibitor.

Dynamics of hydrated proteins and bio-protectants: Caged dynamics, ß-relaxation, and α-relaxation.

BACKGROUND: The properties of the three dynamic processes, α-relaxation, ν-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences. METHODS: Collection of various experimental data enables us to characterize the structural α-relaxation of the protein coupled to hydration water (HW), the secondary or ν-relaxation of HW, and the caged HW process. RESULTS: From the T-dependence of the ν-relaxation time of hydrated myoglobin, lysozyme, and bovine serum albumin, we obtain Ton at which it enters the experimental time windows of Mössbauer and neutron scattering spectroscopies, coinciding with protein dynamical transition (PDT) temperature Td. However, for all systems considered, the α-relaxation time at Ton or Td is many orders of magnitude longer. The other step change of the mean-square-displacement (MSD) at Tg_alpha originates from the coupling of the nearly constant loss (NCL) of caged HW to density. The coupling of the NCL to density is further demonstrated by another step change at the secondary glass temperature Tg_beta in two bio-protectants, trehalose and sucrose. CONCLUSIONS: The structural α-relaxation plays no role in PDT. Since PDT is simply due to the ν-relaxation of HW, the term PDT is a misnomer. NCL of caged dynamics is coupled to density and show transitions at lower temperature, Tg_beta and Tg_alpha. GENERAL SIGNIFICANCE: The so-called protein dynamical transition (PDT) of hydrated proteins is not caused by the structural α-relaxation of the protein but by the secondary ν-relaxation of hydration water. "This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

Recently Fakhraai and coworkers measured surface diffusion in ultrastable glass produced by vapor deposition, ordinary glass with and without physical aging, and ultrathin films of the same molecular glass-former, N,N'-bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD). Diffusion on the surfaces of all these glasses is greatly enhanced compared with the bulk diffusion similar to that previously found by others, but remarkably the surface diffusion coefficients DS measured are practically the same. The observed independence of DS from changes of structural α-relaxation due to densification or finite-size effect has an impact on the current understanding of the physical origin of enhanced surface diffusion. We have demonstrated before and also here that the primitive relaxation time τ0 of the coupling model, or its analogue τß, the Johari-Goldstein ß-relaxation, can explain quantitatively the enhancement found in ordinary glasses. In this paper, we assemble together considerable experimental evidence to show that the changes in τß and τ0 of ultrastable glasses, aged ordinary glasses, and ultrathin-films are all insignificant when compared with ordinary glasses. Thus, in the context of the explanation of the enhanced surface diffusion given by the coupling model, these collective experimental facts on τß and τ0 further explain approximately the same DS in the different glasses of TPD as found by Fakhraai and coworkers.

Presence of global and local α-relaxations in an alkyl phosphate glass former.

The dynamics of a molecular glass former, tributyl phosphate (TBP), with an alkyl phosphate structure (three alkyl branches emanating from a polar core of PO4) is studied in the supercooled regime by dielectric and thermal (or enthalpic) relaxations. The dielectric fragility index md and the stretching exponent ßd of the Kohlrausch-Williams-Watts correlation function are determined. Analyses of the enthalpic relaxation data by the Tool-Narayanaswamy-Moynihan-Hodge formalism yield the enthalpic fragility index mH and stretching exponent ßH. The large difference between the dielectric md and the enthalpic mH, as well as between ßd and ßH, is a remarkable finding. The differences are interpreted by the formation of molecular self-assemblies. The interpretation is supported by the quite comparable fragility determined by viscosity and the enthalpic relaxation. The Kirkwood factor calculated at low temperatures is also consistent with the interpretation. The results suggest that the enthalpic relaxation involving the motions of all parts of TBP is global, while the dielectric relaxation detects the local rotation, which might originate from the rotation of the dipole moment of the core. The presence of two structural α-relaxations, one global and one local, with a large difference in dynamics is revealed for the first time in a molecular glass former.

Universal Behavior of Dielectric Responses of Glass Formers: Role of Dipole-Dipole Interactions.

From an exhaustive examination of the molecular dynamics in practically all van der Waals molecular glass formers ever probed by dielectric spectroscopy, we found that the width of the α-loss peak at or near the glass transition temperature T_{g} is strongly anticorrelated with the polarity of the molecule. The larger the dielectric relaxation strength Δε(T_{g}) of the system, the narrower is the α-loss peak. This remarkable property is explained by the contribution from the dipole-dipole interaction potential V_{dd}(r)=-Dr^{-6} to the attractive part of the intermolecular potential, making the resultant potential more harmonic, and the effect increases rapidly with the dipole moment µ and Δε(T_{g}) in view of the relation, D∝(µ^{4}/kT_{g})∝kT_{g}[Δε(T_{g})]^{2}. Since the novel correlation discovered encompasses practically all van der Waals molecular glass formers studied by dielectric spectroscopy, it impacts the large dielectric research community as well as those engaged in solving the glass transition problem.

Modifications of Structure and Intermolecular Potential of a Canonical Glassformer: Dynamics Changing with Dipole-Dipole Interaction.

By systematic modifications of the canonical propylene carbonate, a family of van der Waals glass-formers with similar chemical structures is generated for dielectric studies of the dynamics of the structural α-relaxation with the purpose of critically testing the correlation of dynamic properties with the dipole-dipole interaction contribution to the intermolecular potential. With the dielectric strengths at Tg varying over a vast range from 4.2 to 182, the modified propylene carbonates provide strong support of the correlation by themselves and in conjunction with 88 van der Waals glassformers previously considered ( Phys. Rev. Lett. 2016 , 116 , 025702 ).

A fast dynamic mode in rare earth based glasses.

Metallic glasses (MGs) usually exhibit only slow ß-relaxation peak, and the signature of the fast dynamic is challenging to be observed experimentally in MGs. We report a general and unusual fast dynamic mode in a series of rare earth based MGs manifested as a distinct fast ß'-relaxation peak in addition to slow ß-relaxation and α-relaxation peaks. We show that the activation energy of the fast ß'-relaxation is about 12RTg and is equivalent to the activation of localized flow event. The coupling of these dynamic processes as well as their relationship with glass transition and structural heterogeneity is discussed.