Investigating the Effects of Charge Arrangement in Stimuli-Responsive Polyelectrolytes

The transition from self-coacervation to complex coacervation of a series of charge-balanced polyampholyte (PA) and polyelectrolyte (PE) pairs based on the copolymers of methacrylic acid and N-(3-aminopropyl) methacrylamide hydrochloride is described, with focus on the effects of charge distribution on coacervate properties. Turbidimetric pH titration was used to compare the response of different charge-balanced coacervates to changes in pH. At high molecular weight, resistance to stimuli such as ionic strength and pH increased from the liquid self-coacervate formed from the stoichiometric PA, through complex coacervates formed from complementary nonstoichiometric PAs, to less hydrated coacervates or precipitates formed from two PE homopolymers. In addition, optical microscopy was used to map the compositional regions as a function of ionic strength corresponding to liquid–liquid phase separation by both self- and complex coacervation and liquid–solid phase separation or precipitation. Coacervates were thermally responsive and showed lower critical solution temperatures that were sensitive to composition and ionic strength. Isothermal titration calorimetry (ITC) was used to explore the energetics of complexation for select complex coacervates, confirming that complexation is dominated by an entropic contribution. Coacervate droplets were shown to reversibly bind ionic dyes and thus have potential use as drug release systems. The coacervate droplets also reacted with genipin to form cross-linked coacervate microgels. The synthetic coacervates used here are easily accessible by free-radical copolymerization and can serve as model systems for phase separation of natural polyampholytes.