Viscoelasticity and Sol–Gel Transition via Multiscale Self-Assembled Nanostructures in Short-Side-Chain PFSA Dispersions

The morphology and property of the high-temperature proton exchange membranes (PEMs) based on short-side-chain perfluorosulfonic acid (SSC-PFSA) are determined by the polymer structure in dispersion during solution casting. In this work, by using rheological analysis and structural characterization techniques, including Cryo-transmission electron microscope (Cryo-TEM) and small-angle X-ray scattering (SAXS), the rheology and microstructure of SSC-PFSA dispersions were collectively studied to spotlight the concentration dependent viscoelasticity across a large scale from dilute solution to gelation. Initially, SSC-PFSA forms rod-like primary aggregates exhibiting a scaling exponent (0.63) that deviates from the theoretical values of 0.5 (for semidilute solutions). As the concentration increases, these primary aggregates assemble into secondary aggregates, where the viscosity-concentration relationship deviates from the predicted scaling behavior. Further increasing the concentration, the secondary aggregates interact to form a percolating network, leading to gelation. This new multiscale self-assembly mechanism elucidates the fundamental connections underlying the gelation process toward membrane formation. It provides the first comprehensive understanding of the nonequilibrium morphology evolution across multiple magnitudes of concentration and length scales and finds the origin of the physical properties for SSC-PFSA electrolyte membranes.