posted on 2020-09-17, 15:33authored byAdlai Katzenberg, Debdyuti Mukherjee, Peter J. Dudenas, Yoshiyuki Okamoto, Ahmet Kusoglu, Miguel A. Modestino
The role of fluoropolymer physicochemical
properties in the dynamic evolution of nanostructure and ionic conductivity
in perfluorinated sulfonic acid ionomer thin films was investigated
by in situ water sorption experiments. The properties
and mass fraction of the ionomer matrix were systematically varied
between Nafion and a perfluorodioxolane ionomer with the same sulfonic
acid side chain and mass fractions ranging from 0.26 to 0.57. Swelling
rate constants attributed to Fickian mass transport (∼10–2 s–1) decreased with increasing
ionic strength and humidity (i.e., with increased swelling) while
rate constants associated with morphological rearrangement (∼10–3 s–1) increased. The rate of deformation,
in nm s–1, was primarily dictated by the matrix
segmental mobility. Transient hydration-driven conductivity exhibited
a single rate constant (∼10–3 s–1) corresponding to the morphological process. In situ grazing incidence X-ray scattering experiments reveal a rapid formation
of ionomer domains during Fickian water sorption, followed by a slower
ordering of these domains during hydration. This relationship between
the rates of swelling and morphological changes confirm/pinpoint transient
changes controlling ion conduction mechanisms in ionomer thin films.