(470f) Sulfonated Oligo-Sulfone Ionomer: Proton Transport-Property Under Thin Film Confinement and Structural Characterization Thereof.

There has been a major research thrust in the last decade to develop low-cost, highly proton conductive ionomers for fuel cells to substitute expensive and not-eco-friendly fluorocarbon-based ones (like Nafion). Understanding thin film behavior is critical to design efficient ionomer-catalyst layers with low ion transport resistance for electrodes of fuel cells. Ionomers in thin films (< 1 micron thick) behave very differently from bulk, free-standing membranes (several tens of micron thick). While the structure and transport characteristics of ionomer bulk membranes have been studied in great detail, the behavior of ionomers, when confined to thin films, is still not well-understood. Systematically exploring the chemical structural factors impacting the thin-film nanostructure, water-polymer mobility, and ion conductivity is thus critical. Here, we made a series of sulfonated oligo- and poly(sulfone) ionomers with low-to-high molecular weight (M_n ranging between 7,000- 80,000 Da). The goal is to explore how the backbone length controls the water-polymer mobility (glass transition temperature, fluorescence spectroscopy), water uptake (quartz crystal microbalance (QCM)), ionic domain characteristics (grazing incidence small-angle X-ray scattering (GISAXS), transmission electron microscopy (TEM)), and proton conductivity (electrochemical impedance spectroscopy (EIS)) in sub-micron thick films on SiO₂ wafer. The study offers useful insights into the potential role of ionomers with low MW in modulating the thin-film ion conductivity.