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 C_3V 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 C_3V 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 C_3V modes during dehydration. Membrane IR spectra feature a total absence of C_3V modes when totally dehydrated, overlapping C₁ and C_3V 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 (λ_Avg) values of 1, 3, 10, 15, and 20 waters/exchange site were sectioned into subcubes to enable the manual counting of the distribution of λ_local values that integrate to λ_Avg values. This work suggests that at any state of hydration, IR spectra are a consequence of a distribution of λ_local values. Bond distances and the threshold value of λ_local, for exchange site dissociation, were determined by DFT modeling and used to correlate spectra to manually counted λ_local distributions.