(452b) Hydrated Fluorine-Free Terpolymers with Percolated Nanoscale Water Channels

Designing polymers with controlled nanoscale morphologies and scalable synthesis is of great interest in the development of fluorine-free materials for proton-exchange membranes in fuel cells. We recently reported a precision polyethylene with phenylsulfonic acid branches at every fifth carbon, p5PhSA, with a high ion-exchange capacity (Chemistry of Materials, 2021). When hydrated this polymer self-assembles into hydrophilic and hydrophobic co-continuous nanoscale domains, wherein the hydrophilic domain, composed of polar sulfonic acid moieties and water, serves as a pathway for efficient mesoscopic proton conductivity. At 40 C and 95% relative humidity, the proton conductivity of p5PhSA is quite promising at 0.28 S/cm. In this study, we simulated 14 terpolymers across a wide range of ion-exchange capacities (IEC = 1 - 4.2) using atomistic molecular dynamics simulations. While the terpolymers with IEC below 2.1 exhibit discrete water domains suggesting macrophase separation of the water (water:sulfonate ratio of 9), terpolymers with higher IEC values form aperiodic percolated nanostructures. The diffusion coefficient of water normalized by the bulk water diffusion coefficient increases from about 0.17 to 0.33 with increasing with IEC across the 8 terpolymers with percolated water channels. The fractal dimension of the water channels and the solvent accessible surface area also correlate strongly with the normalized diffusion coefficient. These simulations are guiding the synthetic effort to adapt the ring-opening metathesis polymerization to produce a family of terpolymers designed to balance proton conductivity, processability and durability for hydrogen fuel cells.