Hydration of freestanding Nafion membrane in proton and sodium ion exchanged forms probed by infrared spectroscopy

Transmission infrared spectroscopy was used to follow the uptake of water into Nafion 112 (∼50 μm thick) membrane under conditions that enabled detection of vibrational bands for water in different environments inside membrane pores and channels. The evolution of infrared features for interfacial and weakly hydrogen bonded water were followed upon exposure of initially vacuum dried membranes, exchanged by either Na⁺ or H⁺, to low humidity atmospheres. The rapid uptake of water into H⁺ exchanged Nafion 112 precluded time resolved spectral measurements. However, the considerably slower timeframe for water incorporation into Na⁺ exchanged membrane enabled the evolution of different environments for water to be observed. Under approximately 10% relative humidity, the time dependent increases in absorbance for a mode of interfacial water near 3674 cm^-1 and a mode of more bulk-like, weakly hydrogen bonded water at 3525 cm^-1 in Na⁺ exchanged Nafion 112 could be fit by a pore diffusion model. The results provide a foundation for the application of multivariate analysis techniques to identify different structures that develop in metal cation exchanged Nafion during changes in hydration state.