About the Interactions Controlling Nafion's Viscoelastic Properties and Morphology

Interactions controlling the viscoelastic properties of Nafion are identified by investigating morphological changes induced through stretching at a wide range of controlled temperature and relative humidity. H-2-goniometer NMR exploiting the pseudonematic effect in D2O-containing membranes provides information on induced anisotropy on the 70 nm scale while small-angle X-ray scattering (SAXS) is used to reveal local structural correlations on the low nanometer scale. Under highly humidified conditions, stress is suggested to be mainly transmitted through the robust polymeric domain with shearing mainly occurring within the soft aqueous domain. Since the latter deteriorates the uniformity of the flat aqueous domain thickness, also the structural nanocorrelation decays. With decreasing relative humidity, where the weakly hydrated ionic domain is thought to form an increasingly stable Coulomb structure, and with increasing temperature reducing the elastic modulus of the polymeric domain, shearing is suggested to preferentially occur within the mechanically weaker polymer aggregates, leaving the nanoscale structural correlation intact. Structural anisotropy on the 70 nm scale, as evidenced by a residual H-2 quadrupolar splitting, is only observed after stretching. Unstretched Nafion is virtually isotropic on this scale; i.e., any genuine anisotropy of Nafion's morphology must be on a significantly smaller scale.