Glasses can be formed by many routes. In some cases, distinct polyamorphic forms are found. The normal mode of glass formation is cooling of a viscous liquid. Liquid behavior during cooling is classified between "strong" and "fragile," and the three canonical characteristics of relaxing liquids are correlated through the fragility. Strong liquids become fragile liquids on compression. In some cases, such conversions occur during cooling by a weak first-order transition. This behavior can be related to the polymorphism in a glass state through a recent simple modification of the van der Waals model for tetrahedrally bonded liquids. The sudden loss of some liquid degrees of freedom through such first-order transitions is suggestive of the polyamorphic transition between native and denatured hydrated proteins, which can be interpreted as single-chain glass-forming polymers plasticized by water and cross-linked by hydrogen bonds. The onset of a sharp change in d dT( is the Debye-Waller factor and T is temperature) in proteins, which is controversially indentified with the glass transition in liquids, is shown to be general for glass formers and observable in computer simulations of strong and fragile ionic liquids, where it proves to be close to the experimental glass transition temperature. The latter may originate in strong anharmonicity in modes ("bosons"), which permits the system to access multiple minima of its configuration space. These modes, the Kauzmann temperature T(K), and the fragility of the liquid, may thus be connected.

https://science.sciencemag.org/content/sci/267/5206/1924.full.pdf

Formation of glasses from liquids and biopolymers.

Glasses are disordered materials that lack the periodicity of crystals but behave mechanically like solids. The most common way of making a glass is by cooling a viscous liquid fast enough to avoid crystallization. Although this route to the vitreous state-supercooling-has been known for millennia, the molecular processes by which liquids acquire amorphous rigidity upon cooling are not fully understood. Here we discuss current theoretical knowledge of the manner in which intermolecular forces give rise to complex behaviour in supercooled liquids and glasses. An intriguing aspect of this behaviour is the apparent connection between dynamics and thermodynamics. The multidimensional potential energy surface as a function of particle coordinates (the energy landscape) offers a convenient viewpoint for the analysis and interpretation of supercooling and glass-formation phenomena. That much of this analysis is at present largely qualitative reflects the fact that precise computations of how viscous liquids sample their landscape have become possible only recently.

Supercooled liquids and the glass transition

Although it has long been recognized that dynamics in supercooled liquids might be spatially heterogeneous, only in the past few years has clear evidence emerged to support this view. As a liquid is cooled far below its melting point, dynamics in some regions of the sample can be orders of magnitude faster than dynamics in other regions only a few nanometers away. In this review, the experimental work that characterizes this heterogeneity is described. In particular, the following questions are addressed: How large are the heterogeneities? How long do they last? How much do dynamics vary between the fastest and slowest regions? Why do these heterogeneities arise? The answers to these questions influence practical applications of glass-forming materials, including polymers, metallic glasses, and pharmaceuticals.

Spatially Heterogeneous Dynamics in Supercooled Liquids

The field of viscous liquid and glassy solid dynamics is reviewed by a process of posing the key questions that need to be answered, and then providing the best answers available to the authors and their advisors at this time. The subject is divided into four parts, three of them dealing with behavior in different domains of temperature with respect to the glass transition temperature, Tg , and a fourth dealing with ‘‘short time processes.’’ The first part tackles the high temperature regime T.Tg ,i n which the system is ergodic and the evolution of the viscous liquid toward the condition at Tg is in focus. The second part deals with the regime T;Tg , where the system is nonergodic except for very long annealing times, hence has time-dependent properties ~aging and annealing!. The third part discusses behavior when the system is completely frozen with respect to the primary relaxation process but in which secondary processes, particularly those responsible for ‘‘superionic’’ conductivity, and dopart mobility in amorphous silicon, remain active. In the fourth part we focus on the behavior of the system at the crossover between the low frequency vibrational components of the molecular motion and its high frequency relaxational components, paying particular attention to very recent developments in the short time dielectric response and the high Q mechanical response. © 2000 American Institute of Physics.@S0021-8979~00!02213-1#

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1065&context=mse_pubs

Relaxation in glassforming liquids and amorphous solids

A theoretical perspective is provided on the glass transition in molecular liquids at thermal equilibrium, on the spatially heterogeneous and aging dynamics of disordered materials, and on the rheology of soft glassy materials. We start with a broad introduction to the field and emphasize its connections with other subjects and its relevance. The important role played by computer simulations in studying and understanding the dynamics of systems close to the glass transition at the molecular level is given. The recent progress on the subject of the spatially heterogeneous dynamics that characterizes structural relaxation in materials with slow dynamics is reviewed. The main theoretical approaches are presented describing the glass transition in supercooled liquids, focusing on theories that have a microscopic, statistical mechanics basis. We describe both successes and failures and critically assess the current status of each of these approaches. The physics of aging dynamics in disordered materials and the rheology of soft glassy materials are then discussed, and recent theoretical progress is described. For each section, an extensive overview is given of the most recent advances, but we also describe in some detail the important open problems that will occupy a central place in this field in the coming years.

Theoretical perspective on the glass transition and amorphous materials

Deviations from thermally activated and from exponential response are typical features of the vitrification phenomenon and previously have been studied using viscoelastic, dielectric, calorimetric, optical, and other techniques. Linear response data from literature on about 70 covalent glass formers, ionic melts, supercooled liquids, amorphous polymers, and glassy crystals are surveyed. Except for orientational glasses and monohydric aliphatic alcohols a distinct but broad correlation of non‐Debye behavior with non‐Arrhenius relaxations is found. Within the broad trend several groups of materials, distinguished by their respective molecular complexity, can be identified and are shown to exhibit narrow correlations. At a given degree of deviation from Arrhenius behavior externally imposed stresses are relaxed with a departure from exponential behavior which is stronger the more the molecular or atomic subunits of the glassforming material are interconnected with each other.

Nonexponential relaxations in strong and fragile glass formers

The viscoelastic properties of amorphous polymers are reviewed with emphasis on the glass to rubber dispersion (often referred to as the transition zone). Deviations from thermorheologieal simplicity (where molecular retardation and relaxation mechanisms have the same temperature dependence) are identified. Most theories and models of polymer chain dynamics do not address or acknowledge thermorheological complexities and correlations, such as that between the temperature dependence and the breadth of viscoelastic and dielectric dispersions of the local segmental motion. Without successful theories of these phenomena the understanding of polymer chain dynamics must be considered incomplete. In this review, old and new experimental data are used to identify the different modes of molecular motions and the domains of their contributions to the time and frequency dependence of the mechanical response of amorphous polymers. The different modes are then shown generally to have their own dependence on t...

Identification of Different Modes of Molecular Motion in Polymers That Cause Thermorheological Complexity

Kincs and Martin [Phys. Rev. Lett. 76, 70 (1996)] have recently discovered a ubiquitous non-Arrhenius temperature dependence of the ionic conductivity in glassy fast ionic conductors when the conductivity exceeds the high level of ${10}^{\ensuremath{-}3}$ to ${10}^{\ensuremath{-}2}{\ensuremath{\Omega}}^{\ensuremath{-}1}{\mathrm{cm}}^{\ensuremath{-}1}$. This important experimental discovery is readily explained and reproduced quantitatively by the coupling scheme without the use of any adjustable parameter.

Parameterless explanation of the non-Arrhenius conductivity in glassy fast ionic conductors.

The restrictions on the configurations available to the cross-link junctions of a network, arising from its presence in a dense phase, have a well-established effect on the mechanical properties of elastomers. These constraints give rive to intermolecular cooperativity of the junction motions and hence can be well described by models of constraint dynamics in relaxation phenomena. This connection between junction dynamics and elasticity of networks is illustrated by comparing the predictions of the coupling model of relaxation with the constrained junction model of Flory. The comparison is borne out by recent 31 P NMR results on network

/pdf/junction-dynamics-and-the-elasticity-of-networks-320ectq3u9.pdf

K. L. Ngai

Papers

Nonexponential relaxations in strong and fragile glass formers

Relaxation in glassforming liquids and amorphous solids

Identification of Different Modes of Molecular Motion in Polymers That Cause Thermorheological Complexity

Parameterless explanation of the non-Arrhenius conductivity in glassy fast ionic conductors.

Junction dynamics and the elasticity of networks