Infrared Spectroscopy of Bis[(perfluoroalkyl)sulfonyl] Imide Ionomer Membrane Materials Chang Kyu Byun, † Iqbal Sharif, ‡ Darryl D. DesMarteau, ‡ Stephen E. Creager, ‡ and Carol Korzeniewski* ,† Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409-1061, and Department of Chemistry, Clemson UniVersity, Clemson, South Carolina 29634-0973 ReceiVed: January 7, 2009; ReVised Manuscript ReceiVed: March 14, 2009 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 SdO stretching modes in Nafion, but SdO 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. Introduction Bis[(perfluoroalkyl)sulfonyl] imide (PFSI) ionomers are a class of perfluorinated materials that have attracted interest for use as anode-cathode separation media in polymer electrolyte membrane (PEM) fuel cells. 1-3 PFSI ionomers are structurally similar to the perfluorosulfonic acid ionomer Nafion 4 (Scheme I). Relative to Nafion, however, PFSIs have potential for greater thermal stability and stronger acidity and provide more op- portunities for synthetic variation. 1-3,5 In the present study, structural properties of PFSI ionomers cast as thin films and freestanding membrane were investigated through application of infrared spectroscopy. For the analysis of polymer membrane materials, infrared spectroscopy provides a nondestructive probe of the environment surrounding ionomer functional groups and water molecules within pores and channels (c.f., refs 4, 6-8, and references therein) and can be adapted for in situ measurements on operating fuel cells. 9-11 The reported work examines infrared spectra of PFSI ionomers in comparison to Nafion. Band assignments for PFSI ionomers are proposed and effects of hydration state changes on features of ionomer and membrane-bound water are revealed for the first time. The experiments lay groundwork for the use of vibrational spec- troscopy in the development of new PFSI ionomers and other PEM materials. Experimental Section PFSI ionomer materials were prepared as described previ- ously. 3 Briefly, the copolymers of perfluorovinylether sulfon- imide with tetrafluoroethylene were prepared by aqueous emulsion polymerization using a redox initiator. The semibatch polymerization employed a continuous addition of the monomers * To whom correspondence should be addressed. Phone: (806) 742-4181. Fax: (806) 742-1289. E-mail: carol.korzeniewski@ttu.edu. † Texas Tech University. ‡ Clemson University. SCHEME I: Structures for the Protonated Forms of Nafion and Bis[(perfluoroalkyl)sulfonyl] Imide (PFSI) Ionomers J. Phys. Chem. B 2009, 113, 6299–6304 6299 10.1021/jp900164x CCC: $40.75  2009 American Chemical Society Published on Web 04/14/2009