Theory Of Glass Formation Research Articles

Letter to the Editor: Observations related to the free volume theory of glass formation

Letter to the Editor: Observations related to the free volume theory of glass formation

Energy-Renormalization for Achieving Temperature Transferable Coarse-Graining of Polymer Dynamics.

The bottom-up prediction of the properties of polymeric materials based on molecular dynamics simulation is a major challenge in soft matter physics. Coarse-grained (CG) models are often employed to access greater spatiotemporal scales required for many applications, but these models normally experience significantly altered thermodynamics and highly accelerated dynamics due to the reduced number of degrees of freedom upon coarse-graining. While CG models can be calibrated to meet certain properties at particular state points, there is unfortunately no temperature transferable and chemically specific coarse-graining method that allows for modeling of polymer dynamics over a wide temperature range. Here, we pragmatically address this problem by "correcting" for deviations in activation free energies that occur upon coarse-graining the dynamics of a model polymeric material (polystyrene). In particular, we propose a new strategy based on concepts drawn from the Adam-Gibbs (AG) theory of glass formation. Namely we renormalize the cohesive interaction strength and effective interaction length-scale parameters to modify the activation free energy. We show that this energy-renormalization method for CG modeling allows accurate prediction of atomistic dynamics over the Arrhenius regime, the non-Arrhenius regime of glass formation, and even the non-equilibrium glassy regime, thus allowing for the predictive modeling of dynamic properties of polymer over the entire range of glass formation. Our work provides a practical scheme for establishing temperature transferable coarse-grained models for predicting and designing the properties of polymeric materials.

Deconstructing the glass transition through critical experiments on colloids

The glass transition is the most enduring grand-challenge problem in contemporary condensed matter physics. Here, we review the contribution of colloid experiments to our understanding of this problem. First, we briefly outline the success of colloidal systems in yielding microscopic insights into a wide range of condensed matter phenomena. In the context of the glass transition, we demonstrate their utility in revealing the nature of spatial and temporal dynamical heterogeneity. We then discuss the evidence from colloid experiments in favor of various theories of glass formation that has accumulated over the last two decades. In the next section, we expound on the recent paradigm shift in colloid experiments from an exploratory approach to a critical one aimed at distinguishing between predictions of competing frameworks. We demonstrate how this critical approach is aided by the discovery of novel dynamical crossovers within the range accessible to colloid experiments. We also highlight the impact of alternate routes to glass formation such as random pinning, trajectory space phase transitions and replica coupling on current and future research on the glass transition. We conclude our review by listing some key open challenges in glass physics such as the comparison of growing static lengthscales and the preparation of ultrastable glasses, that can be addressed using colloid experiments.

Atomistic Theory of Bulk Metallic Glass Formation

ABSTRACTBulk metallic glass can be formed only when the critical cooling rate for glass formation is reduced to 100–2 K/sec. However, a cooling rate achievable with molecular dynamics simulation is higher by many orders of magnitude, so the gap has to be abridged by analytical theories. We propose a theory of bulk metallic glass formation based upon our early theories of glass formation composition. The critical concepts include the idea of local glass transition, distributed local glass transition temperatures and coincident local fluctuation for atomic transport. Strong repulsion between small atoms was recognized for the first time as the necessary condition for bulk glass formation.

Ion beam-induced amorphization in MgO–Al 2O 3–SiO 2. II. Empirical model

Ion beam-induced crystalline-to-amorphous transition was studied for crystalline MgO (periclase), α-Al 2O 3 (corundum), SiO 2 (quartz), MgSiO 3 (enstatite), Al 2SiO 5 (sillimanite, andalusite, kyanite), 3Al 2O 3 · 2SiO 2 (mullite), Mg 3Al 2Si 3O 12 (pyrope), and Mg 2Al 4Si 5O 18 (cordierite). A model for ion-beam induced amorphization is proposed based on the theory of glass formation. In this model, a quantitative parameter of glass-forming ability, S, is developed. The calculation of S is based on geometric constraints, bonding, and phase transition temperature. S reflects the resistance to crystallization and correctly predicts the relative susceptibility to radiation-induced amorphization.

Relevance of a glassy nanocrystalline state of Mo4VO14 for its action as selective oxidation catalyst

The oxide Mo4VO14, in its active state, is a partly reduced material with an ``amorphous'' structure. Temperature-programmed methods reveal that the active lattice oxygen gives rise to a desorption signal at 500 K, which is at the lower end of the temperature window for activity in the selective oxidation of n-propanol. The phase is metastable with respect to oxidative crystallisation into isotropic crystals of the title structure occurring at 781 K. Photoemission revealed in the zero bandgap semiconductor oxide of the amorphous state the MO6 octahedra to be distorted and the oxygen pumping activity to be related to oxidation-reduction cycles of metal d-states situated close to the Fermi energy. Structural studies by XRD and HRTEM showed that the oxygen-pumping state is a nanocrystalline modification of the title structure. The relation of the structural motif to the catalytic function is discussed for this compound and for the whole family of systems in the framework of the theory of glass formation.

Glass formation in ionic systems

In the past five years, glass formation has been observed in many chemical systems involving zirconium tetrafluoride1–5. These ionic glasses were unexpected as no theory of glass formation had predicted their occurrence. Most common glasses can be explained by the Zachariazen theory6 which gives criteria for vitreous structures with highly directed three-dimensional bond arrays, according to energy and kinetic considerations. However, the theory does not encompass some new amorphous systems such as amorphous germanium or the new ionic glasses. I review here the structural and energetic conditions for glass formation in ionic systems with particular reference to fluoride glasses.

CONTRIBUTIONS TO THE THEORY OF GLASS FORMATION AND THE GLASSY STATE*

ABSTRACTAn attempt has been made to assimilate the most important results of recent glass research into a theory of glass formation and the glassy state and to discuss it in relation to present knowledge of physics, chemistry, and colloidal chemistry.The formation of glass from the liquid state, in contrast to crystallization, is a continuous path from the liquid, through the viscous (supercooled) to the brittle state. The transformation point (Tjp) represents the boundary temperature between viscous and brittle glass, at which boundary the property‐temperature coefficients more or less suddenly change. Property values are influenced by thermal history which can not be explained by mechanical strains. Therefore, while a melt is cooled, an inner change of state, dynamic aggregation, is assumed which takes more and more time as the transformation point is approached and is completely checked at lower temperatures. The question of the thermodynamic stability of the glassy state thus becomes a problem. The difference between “unchecked” (lasting) and “checked” displacements of state is explained with respect to a series of properties.From measurements of the viscosity and of the electric conductivity at temperatures from 1300 down to 300°C, it is shown that the liquid or viscous state follows a simple hyperbolic law, according to which these properties can only continue down to a lower limiting temperature (7V) above the absolute zcro point. The brittle state begins at the transformation point and follows a new law, knowledge of which is made more difficult by sooner or later “checking” the displacements of state. From the previous two different laws, it follows that brittle glass is something other than a supercooled liquid.The transformation point can not be thermodynamically explained. Molecular kinetic considerations make it appear to be that temperature at which densest packing with a cessation of free molecular kinetic motion of the particles is reached. Changes in the brittle state are explained from an atom‐kinetic point of view (shrinking of particle volumes) provided no changes in the state of aggregation occur. Dynamic aggregation takes place by the formation of primary and secondary particles. The structure of secondary particles is compared to a roll of coins and this picture is used to explain negative expansion under pressure, peculiarities in expansion curves, and the beginning of brittleness.According to colloidal chemical considerations, the glassy or viscous states represent a solution of secondary particles as the disperse phase in a dispersing medium of primary particles and individual molecules, and the transformation point represents the boundary temperature between viscous and brittle glass paralleled by the gelatinizing temperature characteristic of the change from a sol to a gel. Support for the colloidal‐chemical conception is obtained from observations on rapidly cooled glass melts and from reaction kinetic. Changes of property in the brittle and viscous states are proportional to the logarithm of the time, z, a law which is also found in elastic after‐working, in isothermal dissociation of carbonates, and in some other instances. This law can he deduced from v. Smoluchowski's theory of coagulation. The disappearance of, double refraction of chilled glasses due to annealing (cooling) can also be linked to the log Z law.