Effect of Cation-π Interactions on the Phase Behavior and Viscoelastic Properties of Polyelectrolyte Complexes.

Aromatic rings are a common feature of biological and synthetic polymers that form polyelectrolyte complexes and coacervates. These functional groups can engage in cation-π interactions; however, the impact of such interactions on the physical properties of polyelectrolyte complex materials is not well understood. Here, we investigate the effect of cation-π interactions on the phase behavior and viscoelasticity of polyelectrolyte complexes of poly-(styrenesulfonate) (PSS) and poly-(diallyldimethylammonium), which contain aromatic functional groups on every repeat unit of the PSS polyanion. We prepare samples with matched polymer and/or salt concentrations using salts with different cation-π interaction strengths. Characterization by turbidity, thermogravimetric analysis, and rheology reveals that salts that engage in stronger cation-π interactions destabilize coacervation and speed up the viscoelastic relaxation of the materials. By contrast, removing the aromatic ring by replacing PSS with poly-(2-acrylamido-2-methylpropanesulfonate removes the sensitivity of the phase behavior and viscoelasticity of the complexes to the cation-π interaction strength of the salt. These results reveal that cation-π interactions play a significant role in determining the phase behavior and viscoelasticity of polyelectrolyte complexes and coacervates made from polymers with aromatic functional groups and suggest that cation-π interactions may be a useful molecular handle for tuning coacervate properties.