Cosolvent Control of Lower and Upper Critical Solution Behavior in Polyelectrolyte Complexes

We report that polar cosolvent-water mixtures offer a unique approach to controlling the liquid-liquid phase separation (LLPS) of polyelectrolyte complex solutions formed from degree of polymerization-matched mixtures of strong and weak polyelectrolytes─respectively, quaternary poly(N,N-dimethylaminoethyl methacrylate chloride) (qPDMAEMA) and sodium poly(acrylate) (PA). As observed in prior work, associative LLPS in water exhibits an upper-critical salt concentration with stoichiometric complexes and lower-critical solution temperature (LCST) behavior, where electrostatic correlations are believed to drive phase behavior. However, upon addition of a miscible cosolvent prior to mixing the individual polyelectrolytes at room temperature, we observe a shift in the LCST and the appearance of an upper-critical solution temperature (UCST). This new UCST feature corresponds to a segregative LLPS, whereby the polycation partitions out of the polyanion-rich dense phase and into the supernatant. This behavior arises with cosolvents that decrease (e.g., ethylene glycol) or increase (e.g., N-methyl formamide) the average solvent dielectric constant, suggesting that electrostatic correlations may not primarily control the phase behavior for cosolvated coacervate systems. A conceptual 3D phase surface summarizing these observations for the cosolvated system suggests that two distinct surfaces with critical lines appear on the polymer-salt-temperature phase diagram.