Self-Assembling in Nafion Perfluorinated Ionomers Based on ESR Spectra of Novel Fluorinated Nitroxide Spin Probes

Abstract

ESR spectra of three fluorinated nitroxide radicals with different lengths of the fluorinated side chain were measured in neat solvents and in aqueous Nafion solutions and membranes swollen by water. The probes were prepared by condensation of the 3-carboxy-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxyl acid chloride (acid probe, AP) with 1H,1H-perfluoroalkanols, CF3(CF2)nCH2OH, where n = 6, 10, and 16. The corresponding notation for the probes is FP8, FP12, and FP18. The 14N hyperfine splittings (Azz and aN) are sensitive to the local site: Azz in the range 33.3−36.5 G and aN in the range 13.95−16.48 G were measured for solvents ranging from perfluorinated n-hexane to 10 M LiCl/water solution. The line shapes in the probe solutions at and near 300 K are sensitive to the presence of oxygen; exceptionally narrow signals (peak-to-peak width 0.1 G) were detected in carefully deaerated probe solutions, thus allowing the measurement of small hyperfine splittings (typically 0.24 G) from the methyl protons. ESR spectra of the fluorinated probes in Nafion solutions and membranes suggested the presence of multiple sites where the probes exhibited a range of dynamics. A possible reason for this effect is the location of probes in a range of amorphous phases where the dynamics is restricted by the proximity to crystalline polymer domains. The Azz values for the “slow” component of the probes in Nafion solutions and in membranes swollen by water indicated the location of the nitroxides in polar sites, where the local polarity is similar to that in the 10 M LiCl/water system. Probes with longer fluorinated segments penetrate deeper into the assembled polymer chains, farther away from the interface between the polymer aggregate and the solvent. Structural information that can be deduced from protiated and fluorinated probes intercalated in Nafion systems was compared, based on present results and previous studies.