Phase Behavior and Coacervation of Aqueous Poly(acrylic acid)−Poly(allylamine) Solutions

Phase separation and coacervate complex formation of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were investigated as model pair of oppositely charged, weak polyelectrolytes in aqueous solution. Both fully or partially neutralized PAA (sodium polyacrylate) and PAH were employed. Important factors affecting the complexation were systematically varied including the polyacid/polybase mixing ratio (10−90 wt %), ionic strength as salt concentration (0−4700 mM), polymer concentration (0.02−2.0 wt %), pH (5 and 7), and temperature (30−75 °C). Sample turbidity was utilized as an indicator of polyelectrolyte complex formation. Phase separation in the solution was also observed by optical microscopy in the distinguishable forms of either precipitate or coacervate. In the absence of salt, polyelectrolyte complexation always resulted in the formation of a precipitate. In the presence of sodium chloride, complex formation does not take place (neither precipitate nor coacervate) when either polyelectrolyte is present in large excess. Increasing salt concentration causes a change from solid precipitate to fluid coacervate phase, and finally a one-phase polyelectrolyte solution is obtained. Temperature affected the precipitate-to-solution transition only in the case of samples with low concentrations of either PAA or PAH. The data generated led to the construction of phase diagrams that illustrate how the various parameters control the demixing and the precipitate−coacervate−solution phase transitions. We find such phase diagrams for simple, flexible synthetic macromolecular systems to be rare in the polymer science literature. Ternary phase diagrams were prepared, which showed the influence of relative polymer and salt concentration on the phase behavior of the aqueous PAA/PAH system. We believe data such as these will both improve both the reliable applications of polymer coacervates and the development of new macromolecular assemblies based on charge complexation.