Polyelectrolyte microgels based on poly-N-isopropylacrylamide: influence of charge density on microgel properties, binding of poly-diallyldimethylammonium chloride, and properties of polyelectrolyte complexes

The influence of the charge density of microgels on the binding of oppositely charged polyelectrolytes was investigated. The charge density in the microgels was varied via the amounts of charged comonomer (as e.g., methacrylic acid) during microgel synthesis and also by changing the reaction conditions in order to influence the distribution of the charged comonomer inside the poly-N-isopropylacrylamide-co-methacrylic acid microgel. The variation in charge density was monitored by taking advantage of the polyelectrolyte effect during acid-base titration. Data of titrations of several microgels were analyzed by a modified Henderson-Hasselbalch equation to monitor the influence of the charge density. The microgels contain either different amounts of cross-linker but same amounts of charged comonomer or the microgels were synthesized with same amounts of cross-linker but different functional monomers with different reactivities yielding different spatial distributions. Charge density and spatial distribution of charges in the microgel strongly influence swelling and interaction with polyelectrolytes. As expected, a highly charged microgel binds more polyelectrolyte than a microgel with low amount of charged groups. The amount, however, does not only scale with the number of charges per microgel but also with the charge density of the microgel. The lower the charge density of the microgel, the more polyelectrolyte per negative charge can bind. In addition, the charge density determines whether and at which composition charge reversal of the microgel-polyelectrolyte complexes occur.