Unraveling the Complex Hydration Behavior of Ionomers under Thin Film Confinement

The mechanical and transport properties of submicron thick ionomer films are crucial in the study and understanding of the ionomer-catalyst interfaces in the porous electrodes of energy conversion and storage devices, such as fuel cells and batteries. Interestingly, ionomers in thin films behave differently from bulk ionomer membranes, and the behavior of these thin films is not well understood. The complex hydration behavior of thin ionomer films in the confined state is described in this work. Here, thin films (∼25-250 nm) of sulfonated Radel (S-Radel) were investigated to understand thickness and hydration effects on the density and mechanical properties of these sulfonated thin polymer films. The density values obtained from quartz crystal microbalance and spectroscopic ellipsometry showed a thickness trend similar to density values obtained from neutron reflectometry with thin films of ∼25 nm thickness having lower densities than thicker, ∼250 nm films. Thicker films were always more dense and less water-rich at the interface compared to thinner samples. The mechanical properties of nanoscale thick polymer films are challenging to probe with traditional techniques. A fluorescent rotor probe was thus incorporated into the polymer samples to infer the distribution of material stiffness from fluorescence lifetime measurements. The lower density of the thin films as measured by ellipsometry and neutron reflectometry rationalized greater mobility from fluorescence measurements in the thinner films when dry. When the thin sample was hydrated, the film density and interfacial water volume fraction significantly increased. Also, the samples with lower thicknesses antiplasticized, indicating poor water-polymer mobility inside the film. The plasticization properties and water-polymer mobility appeared to be controlled by both film density and competing interfacial phenomena, between the air-polymer and polymer-substrate interfaces, depending on the level of hydration.