Quaternized microgels as soft templates for polyelectrolyte layer-by-layer assemblies

A cationic microgel with quaternized amine groups was prepared and used as a soft template for layer-by-layer (LbL) assemblies. The presence of quaternized amine groups inside the microgels was necessary to prevent rearrangements and subsequent bridging between the coated microgels, which were observed for the precursor microgels containing protonated primary amino groups. Sequentially, negatively charged polyelectrolyte poly(sodium styrene sulfonate) (PSS) and positively charged polyelectrolyte poly(diallyldimethylammonium chloride) (PDADMAC) were added to a suspension of quaternized microgels. This leads to an odd-even effect with respect to particle size and surface charge of the formed microgel–polyelectrolyte complexes (MPECs). MPECs with an even number of layers exhibit positive surface charge due to PDADMAC as the outermost layer and are larger compared to complexes with an odd number of layers, which are negatively charged having PSS in the outermost layer. Taking into account previous results (Macromolecules, 2009, 42, 1229), these observations show that electrostatic interactions are the major force for the odd-even effect in polyelectrolyte multilayers on microgels: the cationic groups of PDADMAC compete with the cationic moieties of the microgel for binding with the sulfonate groups of PSS. Concomitantly, a fluctuating size of the MPECs is induced by an osmotic pressure modulation within the microgel. In contrast, surface tension effects invoked by a possible varying hydrophilicity of the different terminal layers are negligible.