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

Abstract

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.

Thermoreversible Gelation of a Vinylidene Fluoride‐Based Copolymer in Methyl Ethyl Ketone: Dynamics and Structure

“…Moreover, the 19 F DQ build‐up curve shows a maximum for µs. The static value of the apparent 19 F‐ 19 F dipolar couplings for the vicinal fluorines of the backbone CF 2 groups, which are the major types of fluorine present in our P(PDV‐co‐HFP) copolymer, are was recently calculated for Nafion and a value 16.06 kHz was obtained . Interestingly, the value µs is very close to the at the maximum of the 19 F DQ build‐up curve.…”

X-ray Absorption and Solid-State NMR Spectroscopy of Fluorinated Proton Conducting Polymers

“…The alternative PFSA materials, Aquivion98 (Figure b) and 3M-725 (Figure c), have not been studied in detail previously using 19 F SSNMR. Importantly, our meticulous deconvolution routine ,, is an effective tool for distinguishing the molecular structure of the PFSAs. In the spectra of all three PFSAs, the signal at −122 ppm is dominant, and is assigned to the backbone CF 2 moieties, which are the major building blocks of PFSAs.…”

Understanding Surface and Interfacial Chemistry in Functional Nanomaterials via Solid‐State NMR

“…It was discovered that the motional amplitude of the branches is greater in hydrated Nafion than that in dry samples. Later work based on the 19 F– 19 F double‐quantum effect (caused by 19 F– 19 F dipolar couplings) also confirmed that hydration has a greater effect on the branching sites …”