Characterizing ionomer morphology from dispersion to film using in situ X-ray scattering

Perfluorinated sulfonic acid ionomers (PFSAs) are polymers comprised of a hydrophobic poly(tetrafluoroethylene) backbone and hydrophilic sulfonic acid-terminated side chains. When hydrated, this material phase separates into a complex nanostructure containing a hydrophilic network that facilitates ion transport, leading to ion conducting properties. Because of this morphology, these materials are widely used in electrochemical energy conversion and storage applications. Understanding how this morphology forms and develops during fabrication is necessary for application purposes because morphology is directly tied to material properties. Here, we perform in situ x-ray scattering measurements to characterize the morphology of ionomer films by blade coating PFSA dispersions in water/nPA mixtures and observe their change as they dry to form solid films. We study three different PFSA chemistries and three dispersion solvent compositions (30:70, 50:50, and 70:30 water to nPA content). We connect feature development at a range of lengths scales to understand the hierarchical nature of this process. From our results, we develop a mechanism that illustrates the morphology formation and development during casting. Through this work, we note relevant processing parameters that drive the final morphology, which are key to in fabricating materials for energy applications.