Complexation, Structure, and Rheology of Designer Polyelectrolytes

Polyelectrolyte complexes that form solid-like precipitates in water strongly resemble kinetically trapped solid mixtures and present formidable challenges in reproducing reliable properties and functionality. However, salt can be employed to break intermolecular ion pairs and process these nonequilibrium assemblies from a rubbery to a viscous liquid-like state. Here, we explore this transition with a pairing of RAFT-derived poly(styrene sulfonate sodium) (PSS) and poly(vinyl benzyl trimethylammonium chloride) (PVBTMA), using a combination of rheology, small angle X-ray scattering (SAXS), optical imaging, and thermal characterization. PSS and PVBTMA homopolyelectrolytes were synthesized at different molar masses with low dispersity; sodium bromide was chosen to probe individual chain and complex features. With no added salt, solid-like aggregates precipitated from solution. At 2.5 M salt, viscoelastic behavior was observed – shifting of the storage/loss moduli exhibited excellent time-salt superposition. Systematic SAXS studies were in good agreement with morphologies at various combinations of molar mass and salt. Finally, thermogravimetric analysis of the separate complex and supernatant phases enabled us to quantify the partitioning of salt and water.