Group Vibrational Mode Assignments as a Broadly Applicable Tool for Characterizing Ionomer Membrane Structure as a Function of Degree of Hydration

Infrared
spectra of Nafion, Aquivion, and the 3 M membrane were
acquired during total dehydration of fully hydrated samples. Fully
hydrated exchange sites are in a sulfonate form with a C3V local symmetry. The mechanical coupling
of the exchange site to a side chain ether link gives rise to vibrational
group modes that are classified as C3V modes. These mode intensities diminish concertedly
with dehydration. When totally dehydrated, the sulfonic acid form
of the exchange site is mechanically coupled to an ether link with
no local symmetry. This gives rise to C1 group modes that emerge at the expense of C3V modes during dehydration. Membrane IR spectra
feature a total absence of C3V modes when totally dehydrated, overlapping C1 and C3V modes when partially hydrated and a total absence of C1 modes when fully hydrated. DFT calculated normal-mode
analyses complemented with molecular dynamics simulations of Nafion
with overall λ (λAvg) values of 1, 3, 10, 15,
and 20 waters/exchange site were sectioned into subcubes to enable
the manual counting of the distribution of λlocal values that integrate to λAvg values. This work
suggests that at any state of hydration, IR spectra are a consequence
of a distribution of λlocal values. Bond distances
and the threshold value of λlocal, for exchange site
dissociation, were determined by DFT modeling and used to correlate
spectra to manually counted λlocal distributions.