Meso-scale modeling of the dynamic behavior of glassy materials
Abstract
The dynamic behavior of glass forming materials is similar for a variety of materials irrespective of the underlying molecular structure. Therefore, it is postulated that the essential physics that controls glassy dynamics is not at the molecular scale, but emerges at the nano-meter length scale. This assumption and its consequences are examined by three approaches: (i) coarse graining of molecular simulations, (ii) a spatially coupled stochastic model, and (iii) meso-scale continuum models to predict super-Arrhenian behavior. A coarse grained analysis of molecular simulations shows that the mobility of atoms can be correlated to the density and potential energy. This correlation is stronger in clusters of high or low mobility atoms as compared to the correlation for individual mobile or immobile atoms, though it is not strong enough to be used as a predictor of mobility. The Mean Field Stochastic Model developed by Medvedev, et al. † was extended to include spatial coupling between domains. The effect of different types of coupling motivated by conservation of mass, momentum and cooperativity was studied. Analysis of these models resulted in the insight that the interactions need to narrow the density distribution in a temperature up-jump without significantly affecting the temperature down-jumps. A model that acknowledges meso-scale heterogeneity was developed, where glassy materials are modeled as a collection of continuum elements which can exist in a number of reference states and interact via conservation of mass and momentum. The evolution of the system is governed by the sum of internal energy and the elastic energy. The individual elements had an Arrhenian dependence, but the model exhibited super-Arrhenian behavior because of the temperature dependent energy landscape that results from the reference states having different specific heats. A simple model based on transitions controlled by local kinetic energy, rather than a thermal bath, was also developed. This model exhibited strong super-Arrhenian behavior attributable to the constraints of conservation of energy and downward flow of kinetic energy imposed on the dynamics. †Medvedev, G., et al., Stochastic Model for Volume Relaxation in Glass Forming Materials (in preparation).
Degree
Ph.D.
Advisors
Caruthers, Purdue University.
Subject Area
Chemical engineering|Condensation
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