Impact of dynamic stretching exponent on the correlation between liquid fragility and nonexponentiality at the glass transition.

The presence of large scatter in linear response data has cast doubt on the existence of an inverse correlation between liquid fragility and nonexponentiality, as originally proposed by Böhmeret al(1993J. Chem. Phys.994201). We present a model for the temperature dependence of the stretching exponent based on the Mauro-Yue-Ellison-Gupta-Allan model for supercooled liquid viscosity and discuss the factors impacting the relationship between fragility and the stretching exponent at the glass transition. The proposed model exhibits distinct advantages over previous models in terms of interpretability and limit behaviors for the temperature dependence.

Beyond the Average: Spatial and Temporal Fluctuations in Oxide Glass-Forming Systems.

Atomic structure dictates the performance of all materials systems; the characteristic of disordered materials is the significance of spatial and temporal fluctuations on composition-structure-property-performance relationships. Glass has a disordered atomic arrangement, which induces localized distributions in physical properties that are conventionally defined by average values. Quantifying these statistical distributions (including variances, fluctuations, and heterogeneities) is necessary to describe the complexity of glass-forming systems. Only recently have rigorous theories been developed to predict heterogeneities to manipulate and optimize glass properties. This article provides a comprehensive review of experimental, computational, and theoretical approaches to characterize and demonstrate the effects of short-, medium-, and long-range statistical fluctuations on physical properties (e.g., thermodynamic, kinetic, mechanical, and optical) and processes (e.g., relaxation, crystallization, and phase separation), focusing primarily on commercially relevant oxide glasses. Rigorous investigations of fluctuations enable researchers to improve the fundamental understanding of the chemistry and physics governing glass-forming systems and optimize structure-property-performance relationships for next-generation technological applications of glass, including damage-resistant electronic displays, safer pharmaceutical vials to store and transport vaccines, and lower-attenuation fiber optics. We invite the reader to join us in exploring what can be discovered by going beyond the average.

Theory of structural relaxation in glass from the thermodynamics of irreversible processes.

This work proposes a fundamental thermodynamic description of structural relaxation in glasses by establishing a link between the Prony series solution to volume relaxation derived from the principles of irreversible thermodynamics and asymmetric Lévy stable distribution of relaxation rates. Additionally, it is shown that the bulk viscosity of glass, and not the shear viscosity, is the transport coefficient governing structural relaxation. We also report the distribution of relaxation times and energy barrier heights underpinning stretched exponential relaxation. It is proposed that this framework may be used for qualitative and quantitative descriptions of the relaxation kinetics in glass.

Predicting Ionic Diffusion in Glass from Its Relaxation Behavior.

In low-viscosity liquids, diffusion is inversely related to viscosity via the Stokes-Einstein relation. However, the Stokes-Einstein relation breaks down near the glass transition as the supercooled liquid transitions into the non-ergodic glassy state. The nonequilibrium viscosity of glass is governed by the liquid-state viscous properties, namely, the glass transition temperature and the fragility. Here, a model is derived to predict the ionic diffusivity of a glass from its nonequilibrium viscosity, accounting for the compositional dependence of the glass. The free energy activation barrier for diffusion is related to the activation enthalpy for viscous flow using the Mauro-Allan-Potuzak model of nonequilibrium viscosity [Mauro, J. C.; Allan, D. C.; Potuzak, M. Nonequilibrium Viscosity of Glass. Phys. Rev. B 2009, 80, 094204]. These insights allow for accurate prediction of activation barriers for diffusion of alkali ions. The model is supported by experimental results and nudged-elastic band calculations applied to sodium silicate and borate glasses.

Topological Control of Water Reactivity on Glass Surfaces: Evidence of a Chemically Stable Intermediate Phase.

Glass surfaces are of considerable interest due to their disproportionately large influence on the performance of glass articles in many applications. However, the behavior of glass surfaces has proven difficult to model and predict due to their complex structure and interactions with the environment. Here, the effects of glass network topology on the surface reactivity of glasses have been investigated using reactive and nonreactive force field-based molecular dynamics simulations as well as density functional theory. A topological constraint-based description for surface reactivity is developed, allowing for improved understanding of the physical and chemical origins of surface reactivity. Results show evidence for the existence of a chemically stable intermediate phase on the surface of the glass where the glass network is mechanically isostatic.

Prediction of the Glass Transition Temperatures of Zeolitic Imidazolate Glasses through Topological Constraint Theory.

A topological constraint model is developed to predict the compositional scaling of glass transition temperature ( Tg) in a metal-organic framework glass, agZIF-62 [Zn(Im2- xbIm x)]. A hierarchy of bond constraints is established using a combination of experimental results and molecular dynamic simulations with ReaxFF. The model can explain the topological origin of Tg as a function of the benzimidazolate concentration with an error of 3.5 K. The model is further extended to account for the effect of 5-methylbenzimidazolate, enabling calculation of a ternary diagram of Tg with a mixture of three organic ligands in an as-yet unsynthesized, hypothetical framework. We show that topological constraint theory is an effective tool for understanding the properties of metal-organic framework glasses.