Infrared Spectroscopy of Bis[(perfluoroalkyl)sulfonyl] Imide Ionomer Membrane Materials

Abstract

Structural properties of the proton-exchanged forms of bis[(perfluoroalkyl)sulfonyl] imide (PFSI) ionomer materials were investigated. The hydration and dehydration of samples prepared as thin films and freestanding membrane were probed by applying transmission infrared spectroscopy. Spectral bands were assigned and effects of water incorporation into membrane pores and channels were understood by drawing upon results from related measurements performed on the structurally similar, perfluorosulfonic acid ionomer, Nafion. Both PFSI and Nafion membrane materials display a prominent infrared absorbance band near 1060 cm(-1) that arises from a vibrational mode of the ionizable group present on the side chains that extend from the poly(tetrafluoroethylene) backbone on the polymers. The mode can be traced to symmetric stretching of the -SO(3)(-) (sulfonate) group in Nafion and to antisymmetric S-N-S stretching within the sulfonyl imide end group (-SO(2)(N(-))SO(2)CF(3)) in the PFSI materials. For Nafion samples, the position and width of the band near 1060 cm(-1) are strongly sensitive to membrane hydration, whereas the band position and shape change only slightly during hydration and dehydration of PFSI materials. The possibility for greater charge delocalization over the sulfonyl imide moiety and shielding of hydrophilic species by the terminal -CF(3) group are suggested to explain the differences. These effects also likely influence the stretching modes of the side chain C-O-C groups. A pair of bands, sensitive to hydration and traceable to different C-O-C groups in a side chain, is present in the 970-990 cm(-1) region of Nafion. However, the two features are not well resolved and are less sensitive to hydration in spectra of PFSI samples. The most intense ionomer spectral bands arise from modes involving C-F stretching motion and appear between 1150 and 1250 cm(-1). Toward the high energy side of the envelope, there is substantial overlap with features of sulfonate group antisymmetric SO stretching modes in Nafion, but SO stretching modes of the sulfonyl imide moiety are higher in energy and better resolved in spectra of the PFSIs. During water uptake from a dry state into PFSI materials, a progression of features characteristic of solvated H(3)O(+) species appears across the water O-H stretching (2800-3800 cm(-1)) and H-O-H bending (1500-2000 cm(-1)) regions, similar to responses observed for water inside proton-exchanged Nafion.