Transmission infrared spectroscopy as a probe of nafion film structure: Analysis of spectral regions fundamental to understanding hydration effects

Transmission infrared spectroscopy was applied to investigate properties of the perfluorosulfonated polymer Nafion. Measurements were made on thin films formed by casting the polymer from solution onto ZnSe windows. Effects of water vapor permeation were studied. A complex band structure between 1350 and 1100 cm ^-1 was analyzed qualitatively by fitting the region to Gaussian functions. Features associated with vibrational modes of -CF ₂ and -SO ₃ ^- groups were identified and observed to be sensitive to film hydration. The intensities of bands for the -SO ₃ ^- modes increased with film hydration, while bands assignable to -CF ₂ modes decreased. The results were applied to interpret infrared difference spectra of Nafion and shed light on the complicated features that appear. Vibrational bands for water were also examined. In partially hydrated films, the stretching mode of the free -OH group for interfacial water present in pores and channels of the polymer and bands for hydrated proton clusters were detected.