CAREER: Nanoscale Resolution of Interfacial Materials Physics in Dry, Ionic Polymers

University Of Illinois At Urbana-Champaign, Urbana IL

Investigators

Abstract

NON-TECHNICAL SUMMARY: Polymers are important industrial materials that are flexible, lightweight, and readily processable. When positive or negative electric charges are attached to these polymer molecules they become "ionic polymers" which can enable applications in technologies ranging from flexible batteries to soft robotics to wearable sensors. In all of these areas, a positive and negative electrode will be in contact with the material and the behavior of the polymer/electrode interface will impact the overall performance. In batteries, polymers must adhere to the electrode as it charges/discharges to prevent device failure. For soft robots, the movement of ions and polymer at the interface determines how a polymer bends in response to an electrical signal. This project is pursuing a fundamental understanding of how the structure and dynamics of a polymer are impacted within 10-100 nanometers of a charged surface and how it is different from bulk behavior. Custom made ionic polymers with fluorescent labels will be used to selectively probe the polymer dynamics either adjacent to or far away from the electrode. X-ray experiments will provide further information on how the polymer structure is impacted by electrodes. Insights from this work will be used to design the next generation of improved ionic materials. Such polymers could benefit society by providing cheap and reliable energy storage, advanced real-time wearable health sensors, and soft autonomous robotic systems which could be deployed for defense purposes. The project will contribute to education of students in forefront scientific and technological areas. Outreach activities include a day camp and weekend engineering fair with underrepresented demographics starting at a young age (elementary to middle school). A recurring, off-campus event will host talks open to the public to highlight cutting edge materials research. TECHNICAL SUMMARY: This research seeks to advance the fundamental physical understanding of dry, ionic polymers in the bulk and at electrified interfaces. This work is novel in two key aspects which will enable new fundamental insights. The first is the use of bulky, delocalized ionic groups which weaken the charge interactions and render the polymer processable even at high ion contents and without plasticizers or water (which is incompatible with many battery chemistries). The second is the use of fluorescence and time-correlated X-ray experiments to spatially explore the material physics with sub 10-nm precision. The role of ionic interactions, both intra- and interchain, on influencing ionic polymer mobility and the surface and interfacial tension of polymer films will be investigated. Perturbations to ionic polymers due to electrodes or free surfaces will be directly investigated using nanoscale approaches. First, multilayer films will be constructed where a ca. 10 nm thick layer is placed either at the electrode or in the bulk to selectively probe regions of the stack. In single layer films, coherent X-ray methods will probe the surface fluctuations and provide information on how both surface dynamics and the surface tension depend on ionic correlations and structure (determined in separate scattering experiments) in the material. New insights will provide a better understanding of the physics of polymer electrolytes at electrodes, thin film coatings and adhesives, and self-healing materials. A major contribution of this work is developing a molecular understanding of how ionic associations and electric fields impact the diffusion of dry, charged polymers. Prior work on polyelectrolyte diffusion has been limited to systems with low (< 15 mol%) ionic content or systems that are plasticized with water/solvent, and thus this research is at a fundamentally different regime of ionic polymer physics. 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.