Molecular Simulation of Disordered Materials under Stress

Case Western Reserve University, Cleveland OH

Investigators

Abstract

TECHNICAL SUMMARY: This award supports computational materials theory and education. The PI will use molecular simulations to elucidate unresolved theoretical issues associated with the mechanical response of disordered systems. The project is motivated by a set of recent experimental results that are not fully understood; the simulations will address the same systems studied in the experimental investigations, and will be carried out with realistic potential functions to allow meaningful comparison with experimental results. The project will have three thrusts: (1) The interplay of physical aging and mechanical deformation. Experiments on glassy materials, for example polycarbonates, show that physical aging alters the mechanical properties of the material, and conversely, that mechanical deformation appears to alter the physical aging process. (2) The effects of nanoscale structure on the mechanical properties of disordered materials. Recent experimental studies have shown that nanoporous amorphous silica exhibits unusual mechanical properties that can be considered favorable from an applications standpoint for example, failure strains are much higher than in bulk silica, and elastic moduli decrease anomalously slowly with increasing porosity. (3) The effects of disorder on the mechanical properties of crystalline materials. The crystal structures of many oxide materials, for example mullites and perovskites, include some disorder, which may occur naturally or as a consequence of doping. This disorder has an enormous impact on diffusion and conductivity, but it also alters the linear and nonlinear mechanical response. A unifying theme of the investigation will be the use of these specific thrusts to advance the understanding of disordered systems in general. Results will be interpreted in terms of modern theoretical frameworks describing disordered systems, including the energy landscape, configurational entropy, dynamic heterogeneities, and short range structural order. Results for the specific materials will be used to enhance these general theoretical frameworks the unusual and unexplained nature of the motivating experimental observations suggests that there are interesting theoretical underpinnings that are likely to have implications beyond these particular materials. Among the broader impacts of this project is training new scientists. A particular emphasis of this project is a strong effort to mentor undergraduate student researchers. NON-TECHNICAL SUMMARY: This award supports computational materials theory and education. The PI will use computer simulations to understand puzzling aspects of glassy materials like polycarbonates, amorphous materials, like silica, and crystalline materials for which the positions of a significant number of atoms deviate from a regular periodic array. In each case, these materials display unusual mechanical properties that the PI aims to better understand. A unifying theme of the investigation will be to advance the general understanding of materials that are disordered. In particular, the PI's computational results will be interpreted in terms of modern theoretical frameworks describing disordered systems. The project can benefit society by enhancing the understanding of fundamental scientific phenomena, which can lead to improved technology. For example, the proposed project can lead to a better understanding of: (1) the performance degradation of polymer materials due to physical aging, to facilitate the design of materials with superior aging properties; (2) the effects of nanoscale pores on the mechanical integrity of a material, to facilitate development of applications of nanoporous materials; and (3) the effects of disorder on the mechanical properties of ceramics, to facilitate the use of these materials in novel applications. This project will also benefit society by training new scientists. A particular emphasis of this project is a strong effort to mentor undergraduate student researchers.