Solid-State NMR Investigations of a Perfluorinated Ionomer (Nafion)

The chain dynamics and supramolecular structure of Nafion$^{\mbox{{\textregistered}}}$, a perfluorinated ionomer which is widely used as a hydrophilic permselective membrane in fuel cells and chloralkali electrolysis, have been studied by solid-state NMR. With 1D and 2D NMR under 30-kHz magic-angle spinning (MAS), the $^{19}$F and $^{13}$C NMR peak widths and positions are determined, which corrects several previous assignments. The peak widths reveal static disorder around the branch point, increasing mobility towards the side group end, and a conformationally ordered backbone, which is essentially polytetrafluoroethylene (PTFE). Fast rotations of the helical backbone segments around their axis are confirmed in PTFE and observed similar in Nafion. The equal $^{19}$F chemical shifts within parallel packed rotating chains in PTFE crystals result in slow $^{19}$F spin diffusion between differently oriented chains. This spin diffusion is observed very fast for a majority of backbone segments in Nafion and the orientational correlations of the remainder backbones are weak. The typical diameter of backbone ``clusters'' in Nafion was 1 - 3 nm estimated by $^{19}$F spin diffusion. Relatively fast $^{19}$F spin exchange from any site in the side group to the backbone is observed. Absorbed water increases the side-group dynamics and conformational averaging, but not the segments near the branch point.