F-19 Double Quantum NMR Spectroscopy: A Tool for Probing Dynamics in Proton-Conducting Fluorinated Polymer Materials

Solid-state NMR spectroscopy is an important technique for probing the structure and local dynamics of materials at the molecular level. For example, H-1 double quantum (DQ) NMR is a well-established probe of local dynamics. Here, this concept has been extended to characterize fluorinated ionomer materials for the first time. F-19 DQ recoupling NMR experiments are applied to investigate the site-specific local dynamics of the polymer electrolyte material, Nafion 117, under various conditions with respect to temperature and hydration level. The initial rise of the normalized double quantum (nDQ) build-up curves generated from NMR dipolar recoupling experiments is compared as a measure of the motionally averaged F-19-F-19 dipolar couplings for spectroscopically resolved domains of the polymers. Since the side-chain and backbone fluorines can be distinguished by their chemical shifts, it was possible to demonstrate a difference between the side-chain and backbone local dynamics profiles. The side chain is shown to be more sensitive toward the temperature and relative humidity (\%RH) changes, and generally the side chain exhibits greater local dynamics as compared to the hydrophobic backbone, which is consistent with subsegmental motion known as beta-relaxation. Elevated temperature and increased relative humidity give rise to increased local dynamics, which is reflected by the slower initial increase of the nDQ build-up curves. This NMR technique has been validated as a comparative analysis tool, suitable for a range of perfluorinated ionomers.