Molecular Dynamics Modeling of Proton Transport in Nafion and Hyflon Nanostructures

Classical molecular dynamics modeling studies at 363 K are reported of the local atomic-level and macroscopic nanostructures of two well-known perfluorosulfonic acid proton exchange polymer membrane materials: Nafion and Hyflon. The influence of the different side-chain lengths in the two polymers on local structure is relatively small: Hyflon exhibits slightly greater sulfonate-group clustering, while Nafion has more isolated side chains with a higher degree of hydration around the SO(3)(-) side-chain ends. This results in shorter mean residence times for water molecules around the end groups in Nafion. Hyflon also displays a lower degree of phase separation than Nafion. The velocities of the water molecules and hydronium ions are seen to increase steadily from the polymer backbone/water interface toward the center of the water channels. Because of its shorter side chains, the number of hydronium ions is approximately 50% higher at the center of the water channels in Hyflon, and their velocities are approximately 10% higher. The water and H(3)O(+) diffusion coefficients are therefore higher in the shorter side-chain Hyflon system: 6.5 x 10(-6) cm(2)/s and 25.2 x 10(-6) cm(2)/s, respectively; the corresponding values for Nafion are 6.1 x 10(-6) cm(2)/s and 21.3 x 10(-6) cm(2)/s, respectively. These calculated values compare well with experiment: 4 x 10(-6) cm(2)/s for vehicular H(3)O(+) diffusion.