Transmission Infrared Spectroscopy as a Probe of Nafion Film Structure: Analysis of Spectral Regions Fundamental to Understanding Hydration Effects

Abstract

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.

Intermolecular Interactions Between Methanol/Water Molecules and Nafion™ Membrane: An Infrared Spectroscopy Study

“…The decrease in the intensity may be due to a decrease in the density of functional groups during the swelling of the membrane. The enlarged views of Figure 1 [25].…”

Vibrational Spectroscopy for the Characterization of PEM Fuel Cell Membrane Materials

“…In particular, bands in ATR spectra suffer distortion due to the wavelengthdependent depth of radiation penetration into samples [15][16][17]. However, transmission methods can be limited, because absorbance bands for the fundamental vibrational modes of ionomer materials become saturated when the membrane thickness is greater than $1 mm [9,19,20]. ATR methods enable ionomer spectra to be recorded from membrane samples that have thicknesses >1 mm, including those that have more practical use in fuel cells where the thickness ranges from tens to hundreds of micrometers [2,5].…”

“…A drawing of a transmission cell configured for use with freestanding membrane is shown in Figure 12.1. Components stack inside an aluminum ring attached to the backplate of a commercial infrared transmission cell [20,29,30]. The windows are held parallel in the cell and separated a fixed distance ($1 cm) by a pair of spacers.…”

“…CaF 2 and ZnSe are often used, because they provide a wide spectral range and are insoluble in water and common organic solvents. In addition to use with freestanding membrane, the cell in Figure 12.1 can also be adapted for measurements on thin (<1-mm) films prepared by casting directly onto one of the windows [19,20].…”

“…The region above 1450 cm À1 is mostly free of polymer bands and enables observation of features from permeating vapors [10,19,20,26,27]. An example is shown in Figure 12.3.…”