Collaborative Research: Thermodynamics, Grain Structure, and Ion Transport in Block Copolymer/Salt Mixtures

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NON-TECHNICAL DESCRIPTION:Electrochemical devices related to clean energy such as lithium batteries often rely on polymer membranes to conduct ions between the electrodes. Understanding the factors that limit ion transport will enable the rational design of better membranes that will ultimately lead to more efficient devices. This award specifically examines lithium ion transport through a class of polymers known as block copolymers. These molecules self-assemble to give bicontinuous conducting and non-conducting domains. The domains have specific geometries or morphologies such as hexagonally packed conducting cylinders in a non-conducting matrix or alternating conducting and non-conducting layers. The addition of salt has an effect on the morphologies; a given copolymer may exhibit one morphology at a given salt concentration and another at a higher salt concentration. One objective is to establish the effect of salt on morphology. Sample processing conditions can create defects in the morphology. The morphology and the defects will dictate ionic conductivity, and therefore characterizing the morphology and the defects is an important first step in making better membranes. This work is a step toward building a sustainable clean-energy platform in the US. In addition to the proposed scientific work, one of the investigators will develop courses aimed at helping talented low-income, first-generation-college students develop into scientists and engineers in collaboration with the Berkeley Center for STEM (Science Technology Engineering and Math). The other investigator is involved in broad science-and-technology outreach to high school students in Brooklyn, New York. TECHNICAL DESCRIPTION:This research focuses on mixtures of polystyrene-polyethyleneoxide (PS-PEO) block copolymers, synthesized by anionic polymerization, and bis-trifluoromethylsulfonimide, a salt that is often used in batteries containing polymer electrolytes. Electron microscopy, small-angle X-ray scattering and depolarized light scattering will be used to determine the nature of the order-disorder and order-order phase transitions in these materials. The arrangement of the salt ions in the PEO microphases and the width of windows where ordered and disordered phases coexist are of particular interest. In-situ conductivity measurements will be made on samples using ac impedance spectroscopy while structure is determined by either X-ray or light scattering studies. Measurements will be made over a wide range of block copolymer compositions, salt concentration, temperature, and annealing conditions. The data generated by the project will test recent theories on phase behavior of block copolymer/salt mixtures. The planned work is of considerable intellectual merit as the data generated by the project will provide fundamental insight into the factors that underpin the coupling between structure, thermodynamics, and transport in an emerging class of nanostructured materials for electrochemical devices. The graduate students working on this project will receive scientific training related to sustainable energy use. The multidisciplinary nature of the proposed research, which includes polymer synthesis, characterization, and electrochemical measurements, will provide the students with a rich learning environment. It is coupled with STEM outreach at Berkeley, CA and Brooklyn, NY.