The general objective is to develop smart materials, specifically damage tolerant polymers and composites. This requires a fundamental understanding of the failure mechanisms and the structural changes that precede failure. A multiscale analysis of the structure by representing the structure at several length scales, and describing their response at different time scales, has been found to be most effective in such investigations. Deformation of polymers at high strain rates (>100 strains/s) will be investigated using split Hopkinson bar apparatus. Dynamic mechanical information will be obtained using ultrasound spectroscopy. Micromechanical measurements will be performed on atomic force microscope (AFM). AFM will also be used for structural studies at sub-mm length scales. Small- and wide-angle X-ray scattering will provide structural details at sub-mm to nm length scales, i.e., aggregates of crystals and assemblies of molecules, and Raman measurements will reveal structural features at sub-nm or molecular length scales. Changes in soft domains will be followed by studying the redistribution of deuterated probes after deformation using small-angle neutron scattering. The goal is to control the bulk characteristics of polymers and composites by manipulating their structure at nm and sub-mm length scales. One approach is to fabricate materials that tolerate damage by embedding damage-limiting devices analogous to crazing that is generated plastics. A second approach is in-situ repair by using lasers, and development of materials that heal fractures or self-repair. A final means for imparting intelligence to materials is by detection of local damage before it goes global by monitoring structural changes of the weakest links, tagged when possible, during deformation using either Raman or ultrasound methods. These investigations will be carried out using commercially available high-strength fibers, and nano- and micro-composites prepared using engineering plastics.
NON-TECHNICAL SUMMARY: Strong materials are designed not to fail, but smart materials have to be designed to respond to a stimulus. The strategy that has evolved in biological structures, and the one that is being emulated in synthetic materials, is to monitor in real time the health of a structure, alert the user of impending failure, or enable a material to adapt to the environment, and eventually to self-repair. The research will identify the key structural features that contribute to strength and influence failure, and use this information to develop smart materials. The focus is on failure of the type that occurs when a bullet hits a fabric or when bone fractures under impact. Materials behave very differently at such high deformation rates. One objective is to explain this difference in terms of the structural changes that occur at several length- and time- scales in polymers and in nanocomposites. The methods and tools developed here will be integrated into collaborative projects on biomechanics, organ replacement and prosthetics, and into a green-chemistry program for using biomass. This research will enhance the ongoing effort at the University of Vermont (UVM) to expand materials research by developing the infrastructure for polymer research at UVM, its curriculum, and substantially enrich the educational environment for high school, undergraduate and graduate students. The goal, being the only research university in Vermont, is to be the focal point for polymer research and development for many of the small- and medium-sized polymer industries, thus supporting local businesses while contributing to the advancement of science.
