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ABSTRACT The time-dependent IR spectra during dehydration of fully hydrated Nafion show the reversible disappearance of the 1061 cm−1 and 969 cm−1 concurrent with the emergence of peaks at ∼928 cm−1 and ∼1408 cm−1. The first pair of group modes is associated with a dissociated exchange group (sulfonate) with a local C3V symmetry. The C3V group modes shift with state-of-hydration: The 969 cm−1 peak completely vanishes and the 1061 cm−1 is reduced to a small shoulder at 1070 cm−1 at end of dehydration. The C3V group modes are replaced by the pair of group modes of an associated exchange group (sulfonic acid) with C1 local symmetry. The density functional theory normal mode analysis confirms that the sulfonic acid/sulfonate site plays a dominant role in the C1 and C3V group modes, respectively. This work clarifies the importance of assigning fluoropolymers peaks as group modes rather than traditional single functional group assignments as is often the case with the ∼1061 cm−1 and ∼969 cm−1 C3V group modes. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1329-1334 [ABSTRACT FROM AUTHOR]

The categorization of Nafion side-chain IR group modes by exchange site local symmetry enables correlation of exchange site structure to state-of-hydration and ion exchange. During dehydration the Nafion-[H] exchange site transitions from the sulfonate to sulfonic acid form, corresponding to C 3V and C 1 symmetry, respectively: C 3V,HF (1061 cm −1 ) and C 3V, LF (969 cm −1 ) bands transition to C 1,HF (1414 cm −1 ) and C 1,LF (910 cm −1 ) bands. Metal ions (M) with ΔH hyd below 550 kJ/mol alter the exchange site at all states-of-hydration. They bind with C 3V symmetry and exhibit high orbital overlap with the sulfonate sulfur and oxygen atoms. The Nafion-[H] C 3V,LF (primarily sulfonate) vanishes at full dehydration with retention of the C 3V,HF (primarily ether link). Theoretical Nafion-[M] C 3V,LF bands (e.g., 940 cm −1 for Li + ) are not experimentally observed. Hydration waters of ions with ΔH hyd > 1800 kJ/mol cannot be displaced by sulfonate oxygens. Thus, the Nafion-[H] C 3V,LF persists at all states of hydration. [ABSTRACT FROM AUTHOR]

Time-dependent FT-IR spectra of sulfonated poly(ether ether ketone) during dehydration show diminishing 1081 cm −1 and 1023 cm −1 band intensities concurrent with the emergence and shifting of bands at 1362 cm −1 and 898 cm −1 . Animations of density functional theory calculated normal modes enable assignment of the 1081 cm −1 and 1023 cm −1 bands as group modes that include a sulfonate exchange site with C 3v local symmetry, while the 1362 cm −1 and 898 cm −1 bands are assigned as group modes that include an associated sulfonic acid with no local symmetry (C 1 ). In contrast to analogously assigned Nafion group mode bands, the SPEEK C 3v and C 1 bands coexist throughout the entire dehydration–hydration cycle, suggesting the presence of associated and dissociated exchange sites in SPEEK at all states-of-hydration. This supports a morphological model for SPEEK featuring branched hydrophilic domains and dead-end aqueous confines. [ABSTRACT FROM AUTHOR]

A high temperature solution processing method was adapted to prepare durable, freestanding, submicrometer thickness films for transmission infrared spectroscopy studies of ionomer membrane. The materials retain structural integrity following cleaning and ion-exchange steps in boiling solutions, similar to a commercial fuel cell membrane. Unlike commercial membrane, which typically has thicknesses of >25 μm, the structural properties of the submicrometer thickness materials can be probed in mid-infrared spectral measurements with the use of transmission sampling. Relative to the infrared attenuated total reflection (ATR) technique, transmission measurements can sample ionomer membrane materials more uniformly and suffer less distortion from optical effects. Spectra are reported for thermally processed Nafion and related perfluoroalkyl ionomer materials containing phosphonate and phosphinate moieties substituted for the sulfonate end group on the side chain. Band assignments for complex or unexpected features are aided by density functional theory (DFT) calculations. [ABSTRACT FROM AUTHOR]