In addition to the studies of water dynamics in PEM materials, inelastic scattering techniques have been used to examine the dynamics of ions in neutralized mem­branes as well as the polymer-chain dynamics. The aim of these studies is to gain a deeper fundamental insight into the mechanisms of charge transport and to examine the correlation between ion motion and polymer-chain dynamics. In the earliest studies of this kind, Rollet and co-workers [74, 75] used QENS to probe the dynamics of N(CH3)+ ions in hydrated (D2O) Nafion® membranes. Nafion® and D2O both have very low incoherent scattering cross-sections, and hence the mea­sured scattering in these studies is dominated by the hydrogenated counter-ion, allowing for directionality of ion motions, as a function of ion concentration, to be determined. In order to extend the time-range over which the ion dynamics could be probed, nuclear magnetic resonance and radiotracer experiments were also per­formed. For the given set of experimental conditions outlined, they found that at short time-scales the self-diffusion of the ions within the water domains of Nafion® was similar to that found in non-confined solutions. With increasing electrolyte concentration the self-diffusion coefficient of N(CH3)+ was found to decrease, a phenomenon thought to arise from viscosity effects. For long-range diffusion, the transport of ions was found to be limited by the tortuosity of the diffusion path which is in large part determined by the channels connecting the water domains.

Page and co-workers [76—79] have also used inelastic neutron methods to study the molecular dynamics of Nafion® as part of an effort to further the fundamental understanding of the role of electrostatic interactions in relaxation phenomena observed in these materials. QENS was used to measure the dynamics of the counter-ions in perfluorosulfonate ionomers (PFSIs) neutralized with various alkyl ammonium ions. While the work by Rollet and co-workers focused on measuring the counter-ion dynamics at room temperature in hydrated systems, Page and co-workers measured the counter-ion dynamics in dry systems over a range of temperatures in the vicinity of the corresponding alpha-relaxation temperature, which is highly dependent of the choice of counter-ion. Transitions in the counter­ion dynamics were correlated with bulk mechanical-relaxations in an effort to directly observe the ion-hopping process thought to be the mechanism for long — range diffusive motions of polymer chains and ions in these systems. This work explicitly showed that the counter-ion dependent alpha-relaxation, observed in thermo-mechanical analysis, is linked to the onset of mobility of the counter-ions on the length-scale of 2-3 nm. These data, taken together with other studies, dem­onstrate that the motions of the ions and the polymer chains are highly correlated in these systems (Fig. 10.11).

While work on dry, neutralized forms of Nafion® has helped to further the fundamental understanding of the complex molecular-relaxation behaviour, i. e., the relationship between electrostatic interactions, counter-ion dynamics, and polymer chain relaxations, these model studies are less useful in understanding the inter­dependence of water dynamics/transport and polymer-chain motions in the more device-relevant, acid-form hydrated Nafion® membranes. More recently, Page and co-workers have begun using NSE spectroscopy to probe the relationship between water content and polymer-chain dynamics in Nafion®. NSE enables the study of slow relaxation processes in polymeric systems [80] and for Nafion® samples hydrated with D2O, the polymer-chain dynamics can be measured by monitoring the intermediate-scattering function, I(Q, t), at a Q corresponding to the length-scale associated with the distance between the individual polymer chains. An example of the results from an ongoing study can be found in Fig. 10.12. The I(Q, t) decay was fit with a Kohlrausch-Williams-Watts (KWW) function (Fig. 10.12) and it was determined that with increasing water content and temperature, the relaxation time for chain motions decreased (i. e. chain motions became faster). Interestingly, the timescale associate with the polymer-chain motions plateaus at value near a k of 6, which is also where other studies have shown a transition in the dynamics and transport of water [51]. This application of NSE indicates that there is a degree of coupling between the local water dynamics/transport and the polymer-chain dynamics. This influence of water on the polymer-chain dynamics provides a molecular-level understanding of the observed decrease in Young’s modulus with increasing humidity and temperature. The increased mobility in molecular relax­ations induced by the presence of water points to the molecular origins of the temperature — and humidity-dependent softening mechanisms in Nafion® and other poly (perfluorosulfonic acid) membrane materials.