Aperiodic Percolated Nanostructures in Hydrated Fluorine-free Terpolymers for Proton Exchange Membranes

Fluorine-free polymers with mechanical and transport properties rivaling or exceeding that of perfluorosulfonic acid polymers are sought for hydrogen fuel cells. This work expands upon a recent study that investigated the proton transport properties of a precise polyethylene with phenylsulfonic acid branches on every fifth carbon. Novel terpolymers with a range of ion-exchange capacties (IEC) could be synthesized by copolymerization and varying the degree of sulfonation on pendant phenyl groups. We applied atomistic molecular dynamics simulations to explore this vast compositional space of terpolymers consisting of segments of five-carbon polyethylene, five-carbon polyethylene with a phenylsulfonic branch, and five-carbon polyethylene with a phenyl branch. Under hydration, many of the terpolymers exhibit strong nanophase separation into polymer and water domains due to the flexible carbon backbone. The diffusion coefficient of the hydronium ion decreases with IEC as the percolated water channels become more tortuous. Fractal dimensional analysis of the nanostructured material correlates well with the normalized water diffusion coefficient. Finally, bounds for hydrogen-conducting compositions for this terpolymer are established via the investigation of mean-squared displacements.