Polyelectrolyte complexes from poly(<i>N</i>‐ethyl‐4‐vinylpyridinium bromide) and poly(sodium methacrylate). A stopped‐flow/light scattering investigation

Abstract

Abstract Time‐resolved light scattering measurements performed in conjunction with the stopped‐flow method revealed information on the formation of polyelectrolyte complexes (PECs) of the weak polyelectrolyte (PE) couple poly( N ‐ethyl‐4‐vinylpyridinium bromide) and poly(sodium methacrylate). The stability of the PECs depends, apart from other parameters, on the total ionic strength of the solution [ I ], the PE concentration and the concentration ratio (Φ) = [PVPE]/[PMA]. On the basis of Φ‐[ I ] diagrams, obtained for three PE combinations differing in average molar mass, the stability of PECs is characterized by the regimes A, B and C referring to conditions, where PECs are quite stable and water‐insoluble (A), PECs are not very stable and water‐soluble (B) and PECs are instable (not formed). The course of the curves separating regime A from regime B reflects the action of two competing effects regarding the stability of PECs, namely the increase in charge density and the increase in ion condensation with growing total ionic strength: Kinetic measurements revealed that, at low ionic strength right upon mixing, supramolecular complexes and later large aggregates are formed. The aggregates undergo rearrangements leading eventually to smaller complexes. At high ionic strength aggregates are formed rather slowly after mixing. This behavior is related to ion condensation. PECs can form only after the small ions occupying the charged sites at the PE chains are removed. Detachment of small ions from PE chains is a relatively slow process and is the rate‐determining step in PEC formation at moderately high ionic strength of the solution.