Are Water Swollen Sulfonated Polystyrene Polyelectrolytes Slurries in Equilibrium?Molecular Dynamics Simulations Study*

Ionic aggregates affect the mechanical and transport properties of ionizable polymers and are strongly influenced by changes in their electrostatic environment that occur either in direct exposure to electric fields, or exposure to ions and solvents. With the complex nature of ionizable polymers, any electrostatic perturbation drives the system out of equilibrium. Here, we follow water-sulfonated polystyrene (SPS) complexes using atomistic molecular dynamics simulations.  Water swollen SPS in the polyelectrolyte regime at sulfonation levels f=0.55-1.00 are followed as a function of time and their structure and dynamics and the characteristics of the ionic assemblies are calculated. Addition of water breaks the percolating ionic clusters and concurrently enhances polymer mobility. S(q) consists of an intense broad signature, which shifts to larger dimensions for f ≥ 0.75.  This behavior is attributed to formation of macro aggregates with long-lived domains bordered by water. g(r) shows that the S-S distance increases with time indicative of slow water penetration. With increasing f, for all water concentration, the mobility of the polymers increases, though all components are dynamic and remain far from equilibrium.