Significance: Diverse viewpoints have been developed to understand the scientifically fascinating and universal dynamics of glass-forming fluids. Currently, there are several prevailing models in the scientific literature based on seemingly different physical conceptions of glass formation, a fact that limits both theoretical and technological development in many scientific fields. We address this fundamental problem by simulating polymer glass-forming materials having a wide variation in the temperature dependence of structural relaxation (“fragility”), and we show by direct comparison that existing models equally describe our data, revealing deep relations between them. In this way, we achieve a greater theoretical unity of understanding glass-forming materials that should aid many applications in materials development and biology, the preservation and aesthetic properties of food, and medical science.

The stabilizing effect of sugar-glass matrix materials for freeze-drying proteins or nucleic acids has been variously ascribed to the thermodynamic effect of ‘water replacement’ by sugar molecules or to the kinetic effect of slowed α relaxation associated with sugar matrix vitrification. While evidence for each of these hypotheses exists, we show that neither can adequately account for the observed stabilization of proteins embedded in sugar-glasses. Instead, we find firm evidence that protein stability in these glasses is directly linked to high frequency β relaxation processes of the sugar matrix. Specifically, we observe that when the β relaxation time, τ_β, of sugar-glasses is increased with antiplasticizing additives, protein stability increases in linear proportion to the increase in τ_β, even though these same additives simultaneously decrease the glass transition temperature, T_g, and the α relaxation time, τ_α, of the sugar matrix materials. Moreover, we find that while sugars ‘replace’ water by stabilizing protein native-like conformation in the dry state, the resulting enhanced protein conformational stability does not have a significant impact on the degradation rate of the proteins in sugar-glasses. We discuss implications of these findings for the fundamental physics of glass formation and for effective engineering of protein stabilizing glasses through the modification of τ_β.

Polycrystalline materials are composites of crystalline particles or “grains” separated by thin “amorphous” grain boundaries (GBs). Although GBs have been exhaustively investigated at low temperatures, at which these regions are relatively ordered, much less is known about them at higher temperatures, where they exhibit significant mobility and structural disorder and characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GBs at elevated temperatures, and we exploit MD to explore basic aspects of GB dynamics as a function of temperature. It has long been hypothesized that GBs have features in common with glass-forming liquids based on the processing characteristics of polycrystalline materials. We find remarkable support for this suggestion, as evidenced by string-like collective atomic motion and transient caging of atomic motion, and a non-Arrhenius GB mobility describing the average rate of large-scale GB displacement.

The term gel is central to many scientific fields, including polymer science, biophysics, and supramolecular chemistry. In polymer science, a gel is said to be formed when polymer chains are linked into a permanent three-dimensional (3-D) network by cross-links that are either chemical bonds or strong physical associations. Linear chains in the absence of such cross-links are expected to form a network that is only defined by topological (entanglement) interactions between the chains; accordingly, such a network is expected to show viscoelastic rather than gel-like, rheology. On the other hand, many systems consisting of extended nanoscale fibers/chains (e.g., supramolecular organo- and hydro-gels, wormlike micelles, and protein filaments like F-actin) do exhibit the rheology of permanent gel networks, even in the absence of putative cross-links. We argue here that linear fibers can indeed form gels through their topological interactions alone, i.e., without cross-links, provided the fibers are sufficiently long (contour length much larger than cross-sectional size), sufficiently stiff (worm-like or rod-like) and temporally persistent, i.e., “unbreakable”. This hypothesis is supported by recent experimental and theoretical studies. In particular, we review recent experiments on wormlike micelles that show a smooth rheological transition from viscoelastic (relaxing) to gel (non-relaxing) behavior upon varying temperature.

Polymer–nanoparticle composites play a vital role in ongoing materials development. The behavior of the glass transition of these materials is important for their processing and applications, and also represents a problem of fundamental physical interest. Changes of the polymer glass transition temperature T_g due to nanoparticles have been fairly well catalogued, but the breadth of the transition and how rapidly transport properties vary with temperature T – termed the fragility m of glass-formation – is comparatively poorly understood. In the present work, we calculate both T_g and m of a model polymer nanocomposite by molecular dynamics simulations. We systematically consider how T_g and m vary both for the material as a whole, as well as locally, for a range of nanoparticle (NP) concentrations and for representative attractive and repulsive polymer–NP interactions. We find large positive and negative changes in T_g and m that can be interpreted in terms of the Adam–Gibbs model of glass-formation, where the scale of the cooperative motion is identified with the scale of string-like cooperative motion. These results provide a molecular perspective of fragility changes due to the addition of NPs and for the physical origin of fragility more generally. We also contrast the behavior along isobaric and isochoric approaches to T_g, since these differing paths can be important to compare with experiments (isobaric) and simulations (very often isochoric). Our findings have practical implications for understanding the properties of nanocomposites and have fundamental significance for understanding the properties glass-forming materials more broadly.