Effect of crosslink-induced heterogeneities on the transport and deformation behavior of hydrophilic ionic polymer membranes

Hydration and crosslinking in hydrophilic ionic polymers give rise to microstructural features that affect the diffusion of water and proton conductivity. In this work, we show that the local heterogeneities arising from domains that are more cross-linked and regions that are less cross-linked result in differential swelling behavior during diffusion of water and differential stresses during oscillatory deformation. Distinct signatures in the water uptake kinetics and the dynamic mechanical behavior are shown to be due to these heterogeneities, which are prominent at high and intermediate water contents and are unnoticeable or absent at low water contents. Using polyvinyl alcohol (PVA) cross-linked with sulfosuccinic acid (SSA) as a sample system, we show that differential swelling can lead to anomalous diffusion. In addition, the deformation of these polymers, carried out through large amplitude oscillatory shear experiments, results in strain hardening behavior due to the increased viscous dissipation arising from the differential movements of the polymer network due to the local heterogeneities. The proton conductivity of these materials is affected not only by these heterogeneities but also by the water distribution among the two types of hydrophilic groups (–SO3H and –OH) present in the system. Overall, we demonstrate the important role of crosslink heterogeneities and competitive hydration on the transport and deformation behavior of cross-linked hydrophilic ionic polymer systems. Hydration and crosslinking in hydrophilic ionic polymers give rise to microstructural features which affect diffusion of water and proton conductivity in them. Crosslinking in these systems gives rise to the presence of crosslink heterogeneities resulting in more-crosslinked domains and less-crosslinked regions. Because of the difference in elasticity between the two regions the polymer matrix swells differentially resulting in anomalous water sorption kinetics. The diffused water is distributed among hydroxyl and sulfonic groups and the crosslinking alters this distribution resulting in an increase in conductivity.