An NMR study of the state of ions and diffusion in perfluorosulfonate ionomer

Abstract

Proton and sodium-23 spectra are reported on acid and sodium neutralized forms of the perfluorosulfonate ionomer (Nafion), respectively. This local structural view is compared with pulse field gradient diffusion measurements of water, methanol and dimethyl methylphosphonate (DMMP) in several Nafion films. Both the proton and sodium-23 spectra indicate that the counter ions are primarily in clusters or aggregates with no evidence for isolated ions. The proton resonance from the sulfonic acid group reflects hydrogen bonding but these protons rapidly exchange with sorbed water. There is also evidence for small amounts of water not in contact with the ionic clusters. Pulse field gradient diffusion measurements of water show little evidence of tortuous diffusion though a tortuous pathway might be expected for diffusion through domains of ionic clusters. A cast film shows greater evidence of a tortuous pathway than commercially prepared Nafion sheets though the level of tortuosity is still low. Methanol diffusion shows more tortuosity in both commercial films and cast films with the most tortuosity in a cast film containing fumed silica nanoparticles. Even greater tortuosity is observed for DMMP. The greatest tortuoisty is observed for DMMP in cast films with nanoparticles and this is an improvement in terms of potential applications of Nafion in chemical protection. Though the high-resolution proton spectra show only modest differences between commercial film, cast film and nanocomposite film, more significant structural changes on a larger length scale are detected in the pulse field gradient NMR experiments. Cast films may display more tortuous diffusion because of better-organized domains of ionic groups, backbone units and side chains even though this is not clearly indicated by focusing on the structure of the ionic cluster itself.