Effect of water content and sulfonation level in a fluorine-free random copolymer on nanoscale morphology and proton transport

Fluorine-free polymers are of interest for use as proton exchange membranes in fuel cells due to lower cost and greater membrane stability. The random copolymer under study consists of a linear polyethylene backbone with either a phenylsulfonic acid (p5PhSH) or phenyl (p5Ph) group pendant to every fifth carbon. We used atomistic molecular dynamics simulations to study the nanoscale morphology as a function of water content (λ = 3, 6, 9, and 12 waters per sulfonic acid) and sulfonation level (p5PhSH:p5Ph ratios of 50, 70, and 90%). The ion exchange capacity (IEC) of the copolymers ranges from 2.69-4.14 mmol/g. At higher λ and sulfonation values, systems exhibited percolated, bicontinuous domains of water and the hydrophobic polymer, as determined by visual analysis and cluster analysis. The water pore size distribution narrows as IEC and λ increase, indicating less tortuous channels. The water diffusion coefficient also increases with increasing IEC and λ. Fractal dimension d_f scales well with the diffusion coefficient because higher d_f indicates more isotropic water domains. When comparing results at 90% sulfonation to p5PhSH (95% sulfonation), the diffusion coefficient normalized by the bulk water diffusion coefficient is nearly identical, indicating similar behavior.