Cationic polyvinylamine binding to anionic microgels yields kinetically controlled structures

Abstract

Polyvinylamine (PVAm) binding (absorption and adsorption) to carboxylated microgels gave colloidally stable, cationic microgels that can be centrifuged, washed, freeze dried, and redispersed in water with no loss in colloidal stability. Because both PVAm and the carboxylated microgels are pH sensitive, changes in microgel swelling and electrophoretic mobility in response to pH change can be positive or negative depending upon pH and the PVAm content of the microgels. For a given PVAm molecular weight, the steady-state saturated mass fraction of bound PVAm in the microgels varied by a factor of four in our experiments. We proposed that the PVAm content at saturation was controlled by the relative rates of the initial attachment of PVAm chains versus the rate of attached chain spreading on and into the microgel structure. This explanation was further supported by a series of quartz crystal microbalance measurements.Finally, PVAm binding to two types of PNIPAM microgels shows general features recently reported for other polyelectrolyte types. Specifically: (1) for surface localized anionic charges on the microgels, the mass fraction of bound PVAm increased with PVAm molecular weight and vice versa; (2) in virtually all conditions, the quantity of adsorbed cationic ammonium groups was much greater than the carboxylate content of the microgel; and (3) sodium chloride additions lowered the mass fraction of bound PVAm.