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.

Phase separation of polyelectrolyte complexes (PECs) between the polyacid (sodium salt) and polybase (hydrochloride) of poly(acrylic acid) (PAA) and poly-(allylamine) (PAH), respectively, has been investigated in aqueous solution. Chain length of the PAA was varied (25 < P w < 700) holding P w of the PAH constant at 765. The polyacid/ polybase mixing ratio (10−90 wt %) and the ionic strength as salt concentration (0−3,000 mM) were systematically varied. Sample turbidity was utilized as an indicator of PEC formation, complemented by optical microscopy for discrimination between precipitate and coacervate. Salt-free systems always resulted in PEC precipitates; however, coacervates or polyelectrolyte solutions, respectively, were formed upon exceeding critical salt concentrations, the PEC formation also depending on the employed PAA/PAH ratio. The lower the PAA molecular weight, the lower were the critical salt concentrations required for both the precipitate/coacervate and coacervate/solution transitions. The experimental phase behavior established here is explained by molecular models of coacervate complexation, addressing effects of polyelectrolyte molecular weight and salt screening.

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