Charge Density-Dependent Phase Behavior and Rheology of Polyelectrolyte Complex Coacervates

We investigate the phase behavior and rheology of polyelectrolyte complex coacervates with varying charge density. The coacervates are prepared from poly(acrylic acid) (PAA) and poly(dimethylamino ethyl methacrylate-stat-di(ethylene glycol)methyl ether methacrylate) (P(DMAEMA-stat-DEGMA)), where the P(DMAEMA-stat-DEGMA) copolymers are prepared with the same number of protonatable DMAEMA units per chain but varying overall charge density. We characterize these coacervates by optical turbidity, NMR spectroscopy, gravimetric analysis, and small-amplitude oscillatory shear rheology. We find that decreasing the polycation charge density decreases both the critical salt concentration and the polymer concentration in the coacervates, and shifts the overall relaxation of the materials to faster timescales. The crossover frequency, for example, increases by an order of magnitude with incorporation of just 25 mol% uncharged DEGMA units in the PDMAEMA chains. Our results indicate that cooperativity between adjacent charged sites plays a significant role in the timescale of the “sticky” interactions between chains, and add to the growing evidence that charge density is an important variable for understanding and controlling the physical properties of coacervate systems.