Collaborative Research: The Role of Sulfonated Polymer Membrane Morphology in Microscale Transport of Organic Molecules

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Advanced chemical sensors enable technologies such as point-of-care medical testing and personal protective equipment against chemical warfare agents. Increasing sophistication of the chemical sensors derives from advances in the materials used as the sensor or in studying existing materials with unusual properties. One such material is the commercial polymer, Nafion, which is used in electrolysis, mineral extraction, specialty chemical synthesis, electrochemical sensors, and fuel cells. The widespread use of Nafion is due to its unique chemical structure, which is comprised of a long hydrophobic (water repellent) backbone of fluorine and carbon atoms and hydrophilic (water loving) branches. These dual functionalities give rise to high chemical stability, high reactivity, and flexibility in the presence of water. The dual functionality also leads to unique transport of molecules through the material, particularly when fabricated into a thin membrane. Transport of small molecules within the Nafion membrane is well described by existing theories. However, transport of larger organic compounds, such as those found in chemical warfare agents, cannot be explained within existing theoretical frameworks, particularly when the water-induced flexibility of Nafion is considered. This project will use advanced experimental techniques to deduce transport of large organic molecules that are representative of chemical warfare agents through the dynamic Nafion membrane. This project will develop a direct mechanistic understanding of water-enabled transport of organic molecules through Nafion membranes. Preliminary data suggests large organic molecules are effectively immobilized in a tertiary interphase region between hydrophobic and hydrophilic domains in dry Nafion. The source of this interphase is hypothesized to be the fluoroether linkage between the backbone and the perfluorosulfonic acid side chains. However, phenols and other weak organic acids remain immobilized under both dry and wet conditions. This project will probe the interphase, and the role of bulk sulfonated polystyrene copolymers on transport of large organic molecules in mixed solvent systems. An interdisciplinary team will employ small angle neutron scattering, X-ray scattering, and nuclear magnetic resonance at high magnetic fields. If successful, the proposed research will resolve the relationship between the structural and dynamic properties of the distinctly different domains in perfluorosulfonic acid and sulfonated polystyrene membranes. Specifically, the investigation will determine impact of domain morphology on the transport, immobilization, and reactivity of organic molecules. The project will support graduate education, create new teaching modules, and community outreach activities that demonstrate opportunities at the intersection of transport, catalysis and structure optimization of polymeric membranes.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.