Unraveling how nanoconfinement and phase-separation affect the transport properties of ionomer membranes*

Transport of water and ions through ionic polymer membranes depends on multi-scale membrane structure. This talk will focus on the nano-scale structure-transport relations of ionomer membranes. We will present studies on the temperature dependence of diffusion as a function of water uptake in a perfluorosulfonic acid (PFSA) membrane. Using NMR diffusometry and MD simulations, we are able to probe the activation energy of water diffusion (E_a), which arises from molecular scale interactions and reflects the nano-scale environment of a water molecule. By comparing E_a in the PFSA membrane to E_a in a free liquid environment, we are able to identify two structural primary features of ionomer membranes, nanoconfinement and nano-scale phase separation, that affect membrane transport. Nanoconfinement alters local energetics of water molecules and can prompt formation of more ordered water structures. Nano-scale phase separation creates a local environment for water molecules that is closer to that of pure water, and thus gives rise to E_a values that approach that of pure water. These findings shed light on the fundamental aspects of structure-transport interplay in ionomer membranes.