pubs.acs.org/Macromolecules Published on Web 06/08/2010 r 2010 American Chemical Society 5500 Macromolecules 2010, 43, 5500–5502 DOI: 10.1021/ma100915u Mechanically Coupled Internal Coordinates of Ionomer Vibrational Modes Matthew Webber, † Nicholas Dimakis, ‡ Dunesh Kumari, † Michael Fuccillo, † and Eugene S. Smotkin* ,† † Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Ave, Boston, Massachusetts 02115, and ‡ Department of Physics and Geology, University of Texas- Pan American, 1201 W University Dr., Edinburg, Texas 78539 Received April 26, 2010 Revised Manuscript Received June 1, 2010 Nafion, a sulfonated tetrafluoroethylene copolymer, and its short-side-chain derivative revolutionized low-temperature fuel cell development. Nevertheless, after over 7000 publications on Nafion since 1975, 1 the definitive assignment of key infrared (IR) peaks, including those associated with the SO 3 - exchange group and the ether linkages, has been elusive. Nafion and relevant derivatives are given in Scheme 1. The highlights of reported interpretations of selected IR spectra (Figure 1a-f) provide the context for this Communica- tion. The attenuated total reflectance (ATR) spectra of hydrated Nafion (a) and the short-side-chain ionomer (b) (i.e., Scheme 1) focus on the ∼1060 cm -1 and a multiplet that includes a shoulder at 995 cm -1 and two peaks ∼983 and 970 cm -1 , hereafter referred to as ν h hf and ν h lf , respectively. The ν hf and ν lf have been con- ventionally assigned to ether groups in proximity to the backbone and the sulfonate group, respectively. Cable et al. 2 associated ν h lf to the ether linkage closest to the sulfonate group because of its enhanced sensitivity to ion exchange and the fact that the ν h lf persists in the Dow short-side-chain ionomer spectrum. The short-side-chain ionomer has only one ether group, positioned adjacent to the sulfonate group. The sulfonyl fluoride precursor was also compared to Nafion. In the sulfonyl fluoride spectrum (c), ν h lf diminishes concurrently with the ∼1060 cm -1 peak. These observations were reconciled by invoking solvation effects as responsible for the sensitivity of ν h lf to ion exchange because in hydrated Nafion, the sulfonate group is embedded in an aqueous phase. Therefore, the ether group in closest proximity to the sulfonate group may be subject to solvation as well and thus sensitive to ion exchange. The ν h hf , which is essentially insensitive to ion exchange, has been attributed to the ether link distant from the sulfonate group. Further, Cable 2 concluded that the con- current loss of the 1060 cm -1 peak is due to the loss of the SO 3 - symmetric mode. The association of the 1060 cm -1 peak and ν h lf , solely with SO 3 - and ether link modes, respectively, precludes proper analysis of the spectra. However, if the mechanical coupling of the internal coordinates of the SO 3 - and its near-neighbor COC are considered, the analysis of 1060 cm -1 and ν h lf peaks of Figure 1 can be reconciled without the need for invoking solva- tion of the ether link (vide infra). Warren and McQuillan 3 noted the importance of the considering vibrational contributions from more than one functional group when assigning IR absorp- tions of fluoropolymers. Byun et al. 4 also reported the same loss of the ν h lf upon substitution of the sulfonic acid group for a sulfonyl imide (spectrum f) and assigned ν h lf as did Cable et al. (Figure 1e,f). Transmission infrared spectra of Nafion 112 were obtained on a Bruker Vertex 80V spectrometer (Bruker Optics Inc., Billerica, MA) under dry air or vacuum. All spectra were an average of 100 scans. The Nafion samples were dehydrated on a vacuum line at 10 -2 Torr (under nitrogen) at 135 °C for several hours. Samples were transferred to a drybox for sample holder installation in order to minimize atmospheric exposure. The transmission spec- tra (Figure 2) show a concurrent loss of intensity of 1062 cm -1 and ν h lf due to dehydration of the membrane, simultaneous with evolution of peaks at 1415 and 908 cm -1 . We attribute the transition of the dehydrated (red) to the hydrated (blue) spectrum to a change in the point group symmetry of the sulfonic acid group (vide infra). The following density functional theory (DFT) calculations show that as the proton dissociates from the sulfonic acid group (e.g., with hydration), the local point group symmetry changes from C 1 to C 3v . Unrestricted DFT 5,6 with the hybrid X3LYP 7 functional was used for geometry optimization and calculations of the normal- mode frequencies and corresponding IR spectra of triflic acid, the Nafion side chain (NSC), and the NSC with a PTFE backbone segment (NSCB). The calculations were done at water/sulfonate ratios (λ) 8 from 0 to 10. The X3LYP extension of the B3LYP 9 functional yields more accurate heats of formation. The all- electron 6-311G**þþ Pople triple-ζ basis set is used in all calculations (“**” and “þþ” denote polarization 10 and diffuse 11 basis set functions, respectively). Jaguar 6.5 (Schrodinger Inc., Portland, OR) uses the pseudospectral method 12 for calculation of time-consuming integrals with the same accuracy as the fully analytical DFT codes. Images of the geometry optimized NSC and NSCB anion are shown in Figure 3. Figure 4 shows DFT optimized structures of the triflic acid exchange site as water molecules are sequentially added. The option to include a dielectric in the calculation was not used because the effect of such an option would be a small perturba- tion over the effects due to sequential addition of water molecules to the solvation sphere. The triflic acid calculations reveal a threshold λ (λ d ) where the SO-H bond dissociates (Figure 4, top right), and a λ i-o , where the H 3 O þ loses a direct hydrogen bond to the sulfonic acid anion (Figure 4, bottom right.) Paddison used B3LYP/6-31G** to calculate λ d and λ i-o of 3 and 6, respectively. 13 Although different from our values of 4 and 10, respectively, the near-identical O-H and O-O distances support the converged energies of both Paddison and our calculations (see Table 1). Our higher value of λ d results from the use of diffuse basis set functions. Spitznagel et al. 14 observed significant changes to optimized geometries involving anions and proton affinity when using diffuse functions. To confirm the findings of Spitznagel, 14 the triflic acid structure was optimized for λ = 3 without diffuse functions, resulting in the same λ d as Paddison. This provides confidence in our attribution to diffuse functions for a higher Scheme 1. Structures of Nafion and Derivatives *To whom correspondence should be addressed.