Layer-by-Layer Assembly of Polyelectrolyte Multilayers on Thermoresponsive P(NiPAM-co-MAA) Microgel: Effect of Ionic Strength and Molecular Weight

Abstract

The layer-by-layer assembly of polyelectrolyte multilayers of poly(diallyldimethylammonium chloride) (PDADMAC) and poly(sodium styrenesulfonate) (PSS) on soft and porous thermoresponsive poly(N-isopropylacrylamide-co-methacrylic acid) (P(NiPAM-co-MAA)) microgel was studied by dynamic light scattering and electrophoretic measurements. Polyelectrolyte multilayers were prepared from solutions of polyelectrolytes in different salt concentrations of sodium chloride and from polycations of different molecular weights in order to analyze the influence of the ionic strength and the polyelectrolyte chain length, respectively, on the properties of the polyelectrolyte-coated microgels. We show that the latter retain their thermoresponsive property and that the swelling and deswelling processes are nearly reversible. There is an “odd−even” effect of the hydrodynamic radius of the coated microgel depending on the type of polyelectrolytes in the outermost layer: PSS-terminated microgels are more swollen and more temperature-sensitive than PDADMAC-terminated microgels. Upon assembly of a strong polyelectrolyte shell around the pH-responsive microgel core, the ensemble is no longer pH-responsive. The core−shell microgel shows a clear increase of the size with increasing salt concentration from which the layer (shell) is being deposited, although no strong dependence of the size on the polycation molecular weight has been found. The electrophoretic mobility results reveal charge reversal after each layer deposition and the magnitude of which decreases with increasing number of layers as well as with increasing salt concentration in the depositing polyelectrolyte solutions.