Adam-Gibbs model in the density scaling regime and its implications for the configurational entropy scaling

To solve a long-standing problem of condensed matter physics with determining a proper description of the thermodynamic evolution of the time scale of molecular dynamics near the glass transition, we extend the well-known Adam-Gibbs model to describe the temperature-volume dependence of structural relaxation times, ${\tau}_{\alpha} (T,V)$. We employ the thermodynamic scaling idea reflected in the density scaling power law, ${\tau}_{\alpha}=f(T^{-1} V^{-\gamma } )$, recently acknowledged as a valid unifying concept in the glass transition physics, to discriminate between physically relevant and irrelevant attempts at formulating the temperature-volume representations of the Adam-Gibbs model. As a consequence, we determine a straightforward relation between the structural relaxation time ${\tau}_{\alpha}$ and the configurational entropy $S_c$, giving evidence that also $S_c (T,V)=g(T^{-1} V^{-\gamma} )$ with the exponent {\gamma} that enables to scale ${\tau}_{\alpha} (T,V)$. This important finding has meaningful implications for the linkage between thermodynamics and molecular dynamics near the glass transition, because it implies that ${\tau}_{\alpha}$ can be scaled with $S_c$

Why is the change of the Johari-Goldstein β-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 β-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, Eβ. The much smaller change in τβ and practically no change in Eβ 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) β-relaxation is insensitive to densification in USG, since the JG β-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 β-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)/τβ(Ton) to density change at constant τα(Ton), and the other is the same ργ/T-dependence of τβ 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

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 glassformer, N,N0-bis(3-methylphenyl)-N,N0-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 a-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 t0 of the coupling model, or its analogue tb, the Johari–Goldstein b-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 tb and t0 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 tb and t0 further explain approximately the same DS in the different glasses of TPD as found by Fakhraai and coworkers

Two-step aging of highly polar glass

Nonequilibrium processes, including physical aging, belong to the most challenging phenomena of glassy dynamics. One of the fundamental problems that needs clarification is the effect of material polarity on the time scale of the structural recovery of glass. The importance of this issue arises from practical applications and recent findings suggesting a substantial contribution of dipole−dipole interactions to the dielectric permittivity spectra of polar glass-formers. Herein, we use dielectric spectroscopy to investigate structural relaxation and aging dynamics of highly polar glass-former 4-[(4,4,5,5,5- pentafluoropentoxy)methyl]-1,3-dioxolan-2-one (FPC), a derivative of propylene carbonate with εs = 180 and μ = 5.1. We show that ε″(tage) data of FPC at Tage < Tg reveal complex behavior resulting from considerable cross-correlation effects. Namely, two characteristic aging time scales, reflecting the evolution of cross-correlation mode and generic structural relaxation toward equilibrium, are obtained at a given Tage. Furthermore, a single stretched exponential behavior of ε″(tage) has been received for weakly polar carvedilol with negligible dipole−dipole interactions