19F 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, 1H 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. 19F 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 19F–19F 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 β-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.