Covalent Polymer Mechanochemistry

Duke University, Durham NC

Investigators

Abstract

With the support from the Macromolecular, Supramolecular and Nanochemistry Program of the NSF Chemistry Division, Professor Stephen Craig at Duke University is learning how to dictate the rates and outcomes of chemical reactions that are accelerated by an applied mechanical force. Organic polymeric materials, or plastics, break down due to the mechanical forces they experience during their use cycles. The mechanical degradation of polymers limits their use in lightweight structural materials, consumer products, and biomedical applications. Reactions that are sensitive to a coupled force therefore impact multiple aspects of materials design, including the macroscopic failure and mechanical limitations of current polymeric materials. In addition, mechanically responsive functional groups might serve as the critical elements in new stress-responsive and self-healing polymeric materials. Professor Craig’s studies will provide insight into how the macroscopic mechanical forces experienced by polymers during use can be effectively channeled into desired chemical responses, providing a foundation for new classes of polymers. Broader impacts of the project include: (1) developing active learning modules and associated laboratory experiences in introductory chemistry and through coupled undergraduate and high school research experiences; (2) broadening the participation of underrepresented groups by engaging and recruiting young scientists early in their scientific careers, before the onset of disproportionate attrition from the sciences; (3) disseminating the results of the research broadly; and, (4) addressing fundamental questions of molecular behavior in a manner that will have an impact on a broad range of fields including polymer chemistry, physical organic chemistry, and self-healing and stress-responsive materials. The overarching technical objective is to lay a quantitative foundation for mechanochemical kinetics by employing state-of-the-art physical measurements and developing new methods for quantitation. The research plan includes the direct, experimental characterization and quantification of the effect of mechanical forces on covalent reactions triggered along overstretched polymer backbones. Because mechanical force, unlike conventional forms of energy input such as heat or light, is directional, the coupling between mechanical force and reactivity is expected to provide insights into the structure of transition states and the shapes of reaction potential energy surfaces. Despite its importance, however, quantitative measures of the effect of force on chemical reactions are rare. The proposed work will further develop a novel approach to quantifying mechanochemical reactivity: pulling on single molecules of muti-mechanophore, non-scissile polymers with an atomic force microscope. Models for the observed mechanochemical activity should permit a quantitative assessment of reactivity, and of the influence of external factors such as the surrounding environment and light-induced changes upon molecular structure. 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.