The Effect of Cation-π Interactions on the Relaxation Dynamics of Polyelectrolyte Complex Coacervates

We investigate the effect of cation-π interactions in determining the relaxation dynamics of polyelectrolyte complex (PEC) coacervates by studying their rheology in the presence of salts with different cation-π interaction strengths. We prepare a bulk, salt-free PEC composed of poly(styrene 4-sufonate) (PSS) and poly(diallyldimethylammonium) (PDADMA) and use a salt addition method to prepare coacervate samples with identical salt and polymer concentrations but differing inorganic salts. We find that the viscosity and relaxation times of the coacervates increase as the cation is changed from K⁺, to Na⁺ to Li⁺. This trend is consistent with the known cation-π interaction strengths of the three cations, with K⁺ engaging in the strongest binding with aromatic rings on the PSS and thus the most effectively out-competing stabilizing cation-π interactions between the chains. These findings show that cation-π interactions can play an important role in determining the dynamics of PECs and should be accounted for when interpreting rheological data on these materials.