Segregative Phase Separation of Strong Polyelectrolytes at High Salt and Polymer Concentrations

Mixtures of oppositely-charged polyelectrolytes typically undergo associative phase separation at low salt concentrations, but recently, it was demonstrated that complexes containing poly(acrylic acid) undergo a hydrophobicity-driven transition to segregative phase separation at high salt and high polymer concentrations. Here, we demonstrate that this phenomenon is not isolated to weak polyelectrolytes. We prepare mixtures of poly(styrene sulfonate) and poly(diallyldimethylammonium) (PSS and PDADMA) at high polymer and salt concentrations, and observe phase separation approximately 1 M above the binodal. Characterization by thermogravimetric analysis, FTIR, and NMR reveal that both phases have significant polymer content, with PSS enriched in the denser, gel-like phase and PDADMA enriched in the less dense, liquid-like phase. We show that this transition is driven by salting out of the PSS and/or partially-neutralized PSS/PDADMA complexes. High salt concentrations dehydrate the polyelectrolytes and increase the polymer-water interaction parameter, leading to hydrophobic aggregation of the PSS chains. Similar behavior is observed in complexes of PDADMA with a non-aromatic sulfonated polymer, suggesting that this behavior is a more universal phenomenon in both strong and weak polyelectrolyte mixtures.