Backbone dynamics of the Nafion ionomer studied by 19F-13C solid-state NMR

The chain dynamics of a perfluorinated ionomer, Nafion (R), have been studied by F-19 and F-19-C-13 solid-state NMR at 295 K. The backbone of Nafion is essentially poly(tetrafluoroethylene) (PTFE), which was investigated for reference. Fast uniaxial rotations of the helical backbone were confirmed in PTFE and detected similarly in Nafion, though with a distribution of amplitudes. The rotations produce motionally averaged F-19(-13) C dipolar couplings and chemical shift anisotropies (CSAs) that are line-arly correlated. Additional narrowing of the CSAs indicated that the backbone axis in hydrated Nafion moves with an amplitude >15 degrees. Motional amplitudes of various backbone and side-branch sites were inferred from motionally averaged F-19 CSA parameters measured with CSA recoupling. They increase with the distance from the branch point, e.g., to >25 degrees inthecenteroitheside branch. Implications for the chain and supramolecular structure of Nafion are discussed.