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

N Loupe and K Abu-Hakmeh and ST Gao and L Gonzalez and M Ingargiola and K Mathiowetz and R Cruse and J Doan and A Schide and I Salas and N Dimakis and SS Jang and WA Goddard and ES Smotkin, CHEMISTRY OF MATERIALS, 32, 1828-1843 (2020).

DOI: 10.1021/acs.chemmater.9b04037

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 form of a the exchange site is mechanically coupled to an ether link with no local symmetry. This gives rise to C-1 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 C-1 and C-3V modes when partially hydrated and a total absence of C-1 modes when fully hydrated. DFT calculated normal-mode analyses complemented with molecular dynamics simulations of Nafion with overall lambda (lambda(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 lambda(local) values that integrate to A(Avg) values. This work suggests that at any state of hydration, IR spectra are a consequence of a distribution of lambda(local) values. Bond distances and the threshold value of lambda(local), for exchange site dissociation, were determined by DFT modeling and used to correlate spectra to manually counted lambda(local) distributions.

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