Precision Doping of Polyelectrolyte Complexes: Insight on the Role of Ions

The properties of polyelectrolyte complexes and coacervates, both termed PECs, are influenced strongly by their ion and water content. Water plasticizes PECs, reducing their modulus and glass transition temperature, Tg. In this work, a hydrated PEC with a Tg near room temperature, made from poly(diallyldimethylammonium) (PDADMA) and poly(styrene sulfonate) (PSS) was precisely doped with 22Na-labeled sodium salts along a Hofmeister series. A distinctive change in the rate of doping versus added salt concentration was observed for all salts. This transition was interpreted to reflect a change in ion-accessible volume coinciding with a change in the role of added salt from counterions for the polyelectrolytes, paired directly and within one water molecule of the charge on the polymer backbone, to a mix of counterions and co-ions, which do not have a specific location within the PEC. Isothermal calorimetry for PEC made in, and doped by, NaCl showed two clear regions for enthalpy change, ΔH, before and after the doping transition. The higher ΔH region was correlated with the counterion role, an indirect measure of the location of ions within the PEC.