NMR Spin Diffusion Experiments and Simulations for Model Discrimination and Ionic Domain Size Measurements in Proton Exchange Membranes*

We detail the development of a flexible simulation program (NMR DIFFSIM) that solves the nuclear magnetic resonance (NMR) spin diffusion equation for arbitrary polymer architectures. The simulated ¹H NMR spin diffusion behavior for a range of simple models relevant to proton exchange membranes were compared with the NMR spin diffusion behavior predicted for domain structures obtained from molecular dynamics (MD) simulations. We used DQ ¹H NMR spin diffusion experiments to measure the hydrophilic domain sizes in SDAPP polymers as a function of hydration, and compared them with the domain sizes of Nafion polymers at similar hydration levels. The role of interface regions between polymer domains was also explored. Finally, a direct comparison of experimental spin diffusion results in SDAPP membranes to the spin diffusion behavior predicted for MD-proposed morphologies revealed excellent agreement, providing experimental support for the MD structures at low to moderate hydration levels.