Structure and Dynamics of Ionizable Polymer Melts at the Crossover from Ionomers to Polyelectrolytes

The characteristics of ionizable polymers strongly depend on the fraction of ionizable groups tethered to their backbone, where ionomers consist of well-defined clusters and polyelectrolytes form networks. The structure evolution of the ionic assemblies and their impact on polymer dynamics remain an open fundamental question that affects the integration of these macromolecules into device applications. Here, using fully atomistic molecular dynamics simulations, we probe the structure evolution and its impact on dynamics in this transitional region in sulfonated polystyrene sodium salt SPS-Na in pristine and tetrahydrofuran (THF) swollen melts. We find that across the transition from ionomers to polyelectrolytes, the spherical clusters in the ionomer regime evolve into highly branched assemblies with a broad size distribution in both pristine and swollen melts, with Na+ counterions condensed for all systems. The motion of the polymer is highly constrained and is practically arrested on a time scale of 1000 ns. Though THF is a polar solvent, it drives only small rearrangements of the ionic domains, slightly affecting the packing of the ionizable groups where only some of the larger clusters break up, slightly enhancing the polymer mobility with minimal impact on the motion of fully percolating clusters.