Categorization of sulfonated ionomer vibrational modes by exchange site symmetry
Permanent URL:
http://hdl.handle.net/2047/D20248678
Zhang, Ke (Committee member)
Davies, Geoffrey (Committee member)
Dimakis, Nicholas (Committee member)
The time-dependent IR spectra during dehydration of fully hydrated Nafion show the reversible disappearance of the 969 cm-1 and 1061 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 and are referred to as the C3V,LF (969 cm-1) and C3V,HF (1061 cm-1) bands, respectively. The C3V,LFand C3V,HF 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 theC1 and C3V group modes, respectively. The time-dependent IR spectra during dehydration of fully hydrated sulfonated poly(ether ether ketone) show similar group modes to that of Nafion. The SPEEK C3V,LF and C3V,HF bands are located at 1023 cm-1 and 1061 cm-1, respectively. With dehydration, the C3V bands diminish, while C1 bands (898 cm-1 and 1362 cm-1) emerge.
Metal ions bind to the ionomer sulfonate exchange sites according to their enthalpy of hydration. In Nafion, metal ions with a Hhyd below 550 kJ/mol alter the sulfonate exchange site at all states of hydration, binding to the site with C3V symmetry and exhibiting 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 C3V,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 Hhyd >1800 kJ/mol cannot be displaced by sulfonate oxygens. Thus, the Nafion-[H] C 3V,LF persists at all states of hydration.
In SPEEK, the metal ions also bind to the exchange site with a C3V symmetry: the C3V,HF persists at all states-of hydration, while the C3V,HF vanishes for ions with Hhyd >1450 kJ/mol. The persistence of C3V bands, even at a low state-of-hydration, can be reconciled by charge balancing the exchange site-metal ion pair. A divalent metal ion can be formally charge balanced by (1) another crosslinking exchange site or (2) a hydroxide ion. The degree of cross-linking would be expected to be inversely related to the intensity of C1,HF band. Ions with lower enthalpies of hydration (Ba2+ and Sr2+) have smaller C1,HF intensity and cause a larger degree of crosslinking. Higher enthalpy ions such as Cd2+ and Cu2+ bind more weakly to the exchange site because the exchange site cannot compete with waters of hydration.
A complex-molecule vibrational normal mode consists of small molecule pure modes whose internal coordinates are mechanically coupled. A functional-group pure mode is a descriptor of oscillatory motions (stretching, wagging, etc.). A subset of generalized coordinate eigenvectors, corresponding to atom participants of selected pure modes, are used to quantify normal mode displacement contributions of pure modes to vibrational group modes. An algorithm applied to subsets of normal mode displacement contributions enables quantitative decomposition of vibrational group modes (normal modes) into contributing pure modes. The color coding of pure mode intensities provides provide for three dimensional spectra (i.e., intensity vs. wavenumber vs. pure mode) that clearly elucidate the make-up of broad IR bands in terms of pure mode contributions. The method is applied to Nafion and its side chain fragments.
ionomer
nafion
operando spectroscopy
vibrational modes
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