CAREER: Understanding Plasticity In Polymer Glasses at The Molecular Level by Computer Simulation and Solid-State NMR Spectroscopy

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The plasticity of glassy polymers, in spite of its considerable industrial importance, continues to represent a major frontier of materials science and condensed matter physics. Although phenomenological models of the mechanical properties of polymer glasses at large strains do exist, the underlying mechanisms are unkown. The proposed work aims at identifying the elementary processes of plasticity at the nanometer length scale, thus uncovering the relationship between molecular structure and plastic properties. In order to meet this goal, a comprehensive approach is adopted, including molecular simulation as well as experimental work. Modeling efforts will shed light on the role of the topological constraints caused by the entangled polymer chains in determining the spatial extension of the elementary relaxation processes that are activated by plastic deformation. A second target of computer simulation will be the interplay between physical aging and plastic deformation in molecular glasses. Experimental work, in complement to the simulations, will also focus on the role of molecular entanglements. Compatible polymer blend systems offer the opportunity to vary the density of entanglements by simply varying composition. This will be exploited for a systematic solid-state NMR study of how the amount and character of molecular alignment that results from plastic deformation depends on the entanglement density. In addition, the connection between entanglements and the shear activation volume, a key parameter of the plastic response that is linked to the spatial extension of its elementary processes, will be explored. Together, the results from these studies have the potential to take understanding of plasticity in amorphous polymer solids onto a new level. Knowledge of the relation between molecular structure and plastic properties will foster the optimization and development of novel polymer materials and applications, opening many opportunities for further research. By virtue of its interdisciplinary nature between physics, chemistry and materials science, the proposed research program will open a multitude of opportunities for students from several different departments, graduate as well as undergraduate, to become involved. In turn, this will generate a highly stimulating learning environment for fields as diverse as NMR spectroscopy, computer modeling at electronic, molecular and continuum length scales, polymer science, and solid mechanics. %%%Although glassy polymers are ubiquitous in today's technology, many important aspects of their mechanical behavior are not yet well understood. The present project seeks to establish the molecular origins of the ductility of such materials. Advanced magnetic resonance spectroscopy as well as computer simulations will be used for this purpose. The insight gained from this work will be helpful to guide the development of novel materials for structural as well as medical and optoelectronic applications.