Investigation of Water and Methanol Sorption in Monovalent- and Multivalent-Ion-Exchanged Nafion Membranes Using Electron Spin Resonance

Electron spin resonance (ESR) spectroscopy was used to monitor the local environment of 2,2,6,6-tetramethyl-4-piperidone N-oxide (TEMPONE) spin probe in Li+, Ca2+, and Al3+ ion-exchanged Nafion 117 membranes swollen with mixed methanol/water solvent at varying compositions. The N-14 hyperfine splitting, a(N), which reflects the local polarity of the nitroxide probe, remains nearly steady at higher solvent contents but increases substantially at lower solvent contents, reflecting close contact with the ions. The rotational rate (R) of the probe increased with solvent content, depending strongly on the amount of solvent at low contents but increasing more gradually at higher solvent contents, similar to the behavior of previously measured solvent translation diffusion coefficients. The rotational data from water-containing membranes were fitted using the Fujita free-volume diffusion model, which indicated that multivalent ions tend to increase the free volume fraction of the polymer while decreasing that of the solvent phase. Methanol-containing membranes exhibited greater variation with different exchange ions, but the data could not be fit using the free-volume model, suggesting that the assumption of two phases underlying the free-volume model might not apply to this case. The difference in the trends of swelling between water and methanol is consistent with previous results that have indicated different patterns of penetration for the two solvents. The results are interpreted in terms of changes in membrane morphology with higher-valence ions.