Nanophase structure and diffusion in swollen perfluorosulfonate ionomer: An NMR approach

Fluorine-19 NMR was used to study the structure and mobility of the perfluorosulfonate ionomer Nafion (Dupont trademark) in the presence of dimethyl methylphosphonate (DMMP). Fluorine-19 resonances arising from the ionomeric side chain narrowed rapidly with increasing DMMP content while the resonances arising from the perfluoroethylene backbone units remained broad. The plasticization of the side chains indicates that the DMMP is primarily associated with this structural unit which forms its own nanophase in Naflon. To further characterize the morphological structure, fluorine-19 spin diffusion experiments between the side chain and backbone resonances were performed. At low DMMP content, swelling of the pendant group domain is observed while at higher concentrations an unusual change in the spin diffusion indicates the development of a new morphology consisting of larger domains associated with the pendant group plus DMMP. To augment information on this system, proton pulse field gradient experiments were performed as a function of concentration and temperature to obtain DMMP self-diffusion constants. In the same concentration range where morphological change occurred, there was a rapid rise in translational mobility of the DMMP. This reflects the influence of the new larger domains which allow longer range motion of DMMP with fewer obstructions from the backbone domains which are relatively impermeable to DMMP. A fraction of the smaller domains originally present in dry Naflon remain even at high levels of DMMP, indicating an inability of all of the Nafion to undergo the morphological transition to the larger domains.