<p>Limitations in fuel cell electrode
performance have motivated the development of ion-conducting binders (ionomers)
with high gas permeability. Such ionomers have been achieved by
copolymerization of perfluorinated sulfonic acid (PFSA) monomers with bulky and
asymmetric monomers, leading to a glassy ionomer matrix with chemical and
mechanical properties that differ substantially from common PFSA ionomers
(e.g., Nafion™). In this study, we use perfluorodioxolane-based ionomers to provide
fundamental insights into the role of the matrix chemical structure on the
dynamics of structural and transport processes in ion-conducting polymers. Through
<i>in-situ</i> water uptake measurements, we
demonstrate that ionomer water sorption kinetics depend strongly on the properties
and mass fraction of the matrix. As the PFSA mass fraction was increased from
0.26 to 0.57, the Fickian swelling rate constant decreased from 0.8 s<sup>-1</sup>
to 0.2 s<sup>-1</sup>, while the relaxation rate constant increased from 3.1×10<sup>-3</sup>
s<sup>-1</sup> to 4.0×10<sup>-3</sup>. The true swelling rate, in nm s<sup>-1</sup>,
was determined by the chemical nature of the matrix; all dioxolane-containing
materials exhibited swelling rates ~1.5 - 2 nm s<sup>-1</sup> compared to ~3 nm
s<sup>-1</sup> for Nafion. Likewise, Nafion underwent relaxation at twice the
rate of the fastest-relaxing dioxolane ionomer. Reduced swelling and relaxation
kinetics are due to limited matrix segmental mobility of the dioxolane-containing
ionomers. We demonstrate that changes in conductivity are strongly tied to the
polymer relaxation, revealing the decoupled roles of initial swelling and
relaxation on hydration, nanostructure, and ion transport in perfluorinated
ionomers. </p>
