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, but 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(styrene sulfonate) (PSS) and poly(diallyldimethylammonium) (PDADMA), 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 cationπ interactions destabilize coacervation and speed up the viscoelastic relaxation of the materials. By contrast, polyelectrolyte complexes composed of polymers that do not contain aromatic rings (poly(2-acrylamido-2-methyl propane sulfonate) (PAMPS) and PDADMA) are found to be insensitive 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.