Nanoscale Structure and Morphology of Sulfonated Polyphenylenes via Atomistic Simulations

Abstract

We performed atomistic simulations on a series of sulfonated polyphenylenes systematically varying the degree of sulfonation and water content to determine their effect on the nanoscale structure, particularly for the hydrophilic domains formed by the ionic groups and water molecules. We found that the local structure around the ionic groups depended on the sulfonation and hydration levels, with the sulfonate groups and hydronium ions less strongly coupled at higher water contents. In addition, we characterized the morphology of the ionic domains employing two complementary clustering algorithms. At low sulfonation and hydration levels, clusters were more elongated in shape and poorly connected throughout the system. As the degree of sulfonation and water content were increased, the clusters became more spherical, and a fully percolated ionic domain was formed. These structural details have important implications for ion transport.

Hydrophilic domain structure in polymer exchange membranes: Simulations of NMR spin diffusion experiments to address ability for model discrimination

“…The NMR_DIFFSIM program also allows the NMR spin diffusion recovery curves to be simulated for more complex structural models, including those obtained directly from computational studies. As an example, we have included the predicted spin diffusion recovery curves for structures extracted from atomistic MD simulations of the SDAPP polymer . Visual inspection of the domain morphology in these MD simulations reveals irregularly shaped, nonpercolating hydrophilic domains at low hydration levels, that become larger and percolated with increasing sulfonation and/or hydration levels .…”

“…As an example, we have included the predicted spin diffusion recovery curves for structures extracted from atomistic MD simulations of the SDAPP polymer . Visual inspection of the domain morphology in these MD simulations reveals irregularly shaped, nonpercolating hydrophilic domains at low hydration levels, that become larger and percolated with increasing sulfonation and/or hydration levels . The irregular shaped hydrophilic domains preclude the mapping of the simple geometric models to correctly represent the MD proposed structures.…”

“…The NMR spin diffusion recovery curves were obtained for the MD‐proposed structures using a single time increment snap shot from the SDAPP polymer MD simulations . The size of the MD unit cell varied from 60 3 to 90 3 Å 3 depending on the sulfonic acid content and hydration level, with the MD simulation employing 70 polymer chains composed of 3 monomer repeat units.…”

“…For the MD simulations, the sulfonic groups were unprotonated with the number of hydronium molecules varied to maintain neutrality in the unit cell. Additional MD simulation details have previously been described . Table lists some of the relevant properties and results of the MD‐generated structures used for the current investigation.…”

“…There have also been pulsed field gradient NMR studies measuring diffusion rates of the hydrating waters, along with quasi‐elastic neutron scattering investigations of water dynamics in SDAPP membranes . Recent atomistic MD simulations for a series of SDAPP polymer compositions revealed an evolution of the hydrophilic domain nanostructures as a function of sulfonation and hydration conditions . The MD‐generated SDAPP structures were not readily related to any of the simple structural models previously considered for PEM systems, and await experimental verification.…”