Polyelectrolyte Molecular Weight and Salt Effects on the Phase Behavior and Coacervation of Aqueous Solutions of Poly(acrylic acid) Sodium Salt and Poly(allylamine) Hydrochloride

Abstract

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.