Solid-state NMR investigations of a perfluorinated ionomer: Nafion
One and two-dimensional (1D and 2D) 19F, 1H, and 13C solid-state NMR techniques are developed and applied to investigate the structure, morphology, and dynamic information of Nafion, a perfluorinated ionomer. The 19F magnetic resonances of Nafion are assigned to their corresponding chemical groups using various 1D and 2D NMR experiments. The inhomogeneous broadening of the 13C and 19F signals is the result of the static disorder near the branch point, the high conformational order of the backbone, and the mobility of the sidechains. Fast rotations of the backbone segments around the local helix axis are confirmed in polytetrafluoroethylene (PTFE) and observed similarly in Nafion, in terms of motionally reduced C-F dipolar couplings and uniaxial 13C and 19F chemical shift anisotropies (CSAs) that are parallel to each other. The mobility difference of various sites in Nafion is revealed by 2D iso-anisotropic chemical shift separation NMR and quantified by the site-specific CSA dephasing technique. Fast frequency changes measured by centerband-only detection of exchange (CODEX) NMR confirm short lateral order in Nafion compared with the parallel packed chains in PTFE crystallites. The location of organic counterions in Nafion depends on the length of their alkyl chains: smaller counterions are closer to the end of the sidechains, while larger counterions with flexible alkyl chains can reach the backbone regions. The different influences of the organic counterions on the molecular motions of sidechains and backbones of Nafion are revealed by 2D exchange and CSA dephasing experiments. 1H wideline and 2D 1H-13C wideline separation (WISE) NMR illustrate the different dynamics of organic counterions in Nafion: smaller counterions undergo overall fast motions, larger counterions undergo more heterogeneous segmental motions. The size of the ionic clusters is estimated to be at least 10 nm3 containing around 100 counterions. The packing order of Nafion backbone segments was probed by 2D CODEX with the detailed simulation on the spin diffusion along the backbones. The diameter of backbone segments in Nafion was estimated to be 1--3 nm. A model with alternating curvatured backbones for Nafion is proposed based on the structural and dynamics data.