Investigation of perfluorosulfonic acid ionomer solutions by ¹⁹ f NMR and DLS: Establishment of an accurate quantification protocol

Abstract

Long side chain perfluorosulfonic acid (PFSA) aqueous solutions with various degrees of substitution and trifluoroacetic acid solutions were investigated by liquid 19 F nuclear magnetic resonance (NMR) to propose a new and efficient quantification protocol. These two examples were selected to respond to a specific need for quantification in fuel cell applications. Classical quantification revealed a systematic underestimation of the PFSA concentrations at room temperature, in contrast to small fluorinated molecules. Dynamic light scattering data suggested the presence of "NMR silent aggregates" up to 50 8C. These aggregates appeared as large particles of about 30 mm in diameter, resulting from the agglomeration of primary aggregates through hydrogen bonds. An increase of the measurement temperature to 80 8C was sufficient to take apart the secondary aggregates, and get the correct quantification. In parallel, a rapid 19 F NMR quantification method was developed using a careful analysis of the Signal-to-Noise ratio. This new method provided, in at least a six decades range, an estimation of the PFSA concentration without the need for an external reference. This approach may be successfully applied to determine the fluorine atom content of any small molecule or polymer.

Time-resolved monitoring of composite Nafion™ XL membrane degradation induced by Fenton's reaction

“…For accurate quantitative NMR experiments, the recycle delay should be higher than five times the relaxation time T 1 of the target molecule. The relaxation time of Nafion ™ ionomer solutions J o u r n a l P r e -p r o o f was determined by Yuan et al [51] to be 0.92 ± 0.05 s and that of TFT was determined by El Kaddouri et al [50] to be 1.5 ± 0.1 s. Consequently, the recycle delay was fixed to 8 s for all quantitative NMR experiments.…”