Nanostructure/Swelling Relationships of Bulk and Thin-Film PFSA Ionomers

Perfluorinated sulfonic acid (PFSA) ionomers are the most widely used solid electrolyte in electrochemical technologies due to their remarkable ionic conductivity with simultanous mechanical stability, imparted by their phase-separated morphology. In this work, the morphology and swelling of PFSA ionomers (Nafion and 3M) as bulk membranes (>10 μm) and dispersion-cast thin films (<100 nm) are investigated to identify the roles of equivalent weight (EW) and side-chain length across lengthscales. Humidity-dependent structural changes as well as different PFSA chemistries are explored in the thin-film regime, allowing for the development of thickness-EW phase diagrams. The ratio of macroscopic (thickness) to nanoscopic (domain spacing) swelling during hydration is found to be affine (1:1) in thin films, but increases as the thickness approaches bulk values, revealing the existence of a mesoscale organization governing the multiscale swelling in PFSAs. Ionomer chemistry, in particular EW, is found to play a key role in altering the confinement-driven structural changes, including thin-film anisotropy, with phase separation becoming weaker as the film thickness is reduced below 25 nm or as EW is increased. For the lower-EW 3M PFSA ionomers, confinement appears to induce even stronger phase separation accompanied by domain alignment parallel to the substrate.