Unraveling Mechanics of High Strength and Low Stiffness in Polymer Nanocomposites through Integrated Molecular Modeling and Nanomechanical Experiments

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The storage and release of elastic strain energy in materials, along with mechanical strength, play important roles in both natural and engineered mechanical actuation systems, such as biological tissues responsible for the fast and high-powered locomotions in animals. The modulus of resilience provides the measure of a material?s ability to absorb and release elastic strain energy, determined by the ratio between strength and stiffness of these materials. In general, engineering the modulus of resilience is extremely difficult because it requires asymmetrically increasing strength and stiffness against their mutual scaling behavior. As an alternative route, this award supports fundamental research to elucidate how nanoparticles can be used to tune mechanical behaviors of their polymer composites for simultaneously achieving high strength and low stiffness. This knowledge will accelerate the design of high strength, yet compliant polymeric materials for a broad range of applications, such as light-weight elastic energy storage devices, protective coatings, flexible and foldable optoelectronics, and artificial muscles. Thus, this research will not only promote the progress of science, but also advance the national health, prosperity and welfare. It can also help to secure national defense through technological innovations, e.g. light-weight energy absorption and protection systems for aircrafts. By integrating multiple disciplines, this project will train a diverse group of students in the areas of mechanics of materials, polymer science, mechanical engineering, and material science and engineering for next-generation workforce development. The educational objectives of the project will be realized through curriculum development, undergraduate research opportunities, summer research program for high school students, research experience for K-12 teachers program, and K-12 outreach program. Special efforts will be made to involve underrepresented students in this project. The objective of this project is to test the hypothesis that the combination of high strength and low stiffness is attributed to the unique microstructure of polymer nanocomposites that contains spherical nanoparticles weakly interacting with soft polymer matrix. To achieve this objective, the research plan consists of two major aims. The goal of Aim 1 is to establish a research framework integrating multiscale molecular modeling and complementary nanomechanical experiments for studying polymer nanocomposites with high strength and low stiffness. It will be used to elucidate how the size of nanoparticles, in comparison with free volume elements of the polymer matrix, can influence mechanical behaviors of polymer nanocomposites. In Aim 2, the underpinning mechanics principle of attaining the combination of high strength and low stiffness will be generalized to guide exploration of a novel class of polymer nanocomposites. These nanocomposites with ultra-high elastic energy storage capability can be used for superior mechanical protection, artificial muscles, soft robotics and flexible electronics. The interdisciplinary effort will open promising avenues for quantitatively understanding the anomalous high strength and low stiffness behaviors, and offer mechanistic insights into the design principles of a novel class of nanocomposites.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.