Structure and Dynamics of THF Swollen Polystyrene Ionomer Melts

The structure of ionomers is often trapped in far-from-equilibrium states as a result of clustering of the ionizable groups. Solvent tweaking of the ionic clusters to allow rearrangements to form thermodynamic stable systems at ambient temperature is a promising path for constraint release, offering a path to form equilibrium structures. However, it requires a common solvent for the highly segregated ionic clusters and the polymer matrix. The current study probes the structure and dynamics of sulfonated polystyrene ionomer melts swollen with tetrahydrofuran (THF), a hydrophobic yet polar solvent, by using fully atomistic molecular dynamics simulations. The structure and dynamics of these swollen melts were calculated and correlated with the characteristics of the ionic clusters. We find that THF partitions to the polystyrene-rich domains as well as to the interface of the ionic domains, resulting in enhanced dynamics of the chains on all length scales measured, as expected for a swollen melt. Surprisingly though, the cluster size increases with time in the presence of THF, indicating that the solvent releases some of the constraints, enabling the growth of ionic assemblies.