Interfacial layers of stimuli-responsive poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) microgels characterized by interfacial rheology and compression isotherms

Stimuli-sensitive emulsions stabilized by microgel particles consisting of poly-(N-isopropylacrylamide-co-methacrylicacid) (PNIPAM-co-MAA) and being responsive to both pH and temperature have been investigated with respect to the visco-elastic properties of the interfacial layer. Properties of the interfacial layer were probed by means of shear and dilatational rheology as well as by compression isotherms and are related to the microgel packing at the interface as visualized by cryogenic scanning electron microscopy. The corresponding pH dependent emulsion stability is strongly correlated with the visco-elastic properties of the microgel covered oil-water interface. At high pH when the microgels are charged, a structure of partially interconnected microgels is found that provides elastic, soft gel-like interfaces. At low pH, however, the uncharged microgels are densely packed and the interface is rather brittle. Obviously, these pH dependent visco-elastic properties of the microgel layer at the oil-water interface play a determining role in the stability of emulsion droplets and allow us to prepare very stable emulsions when the microgels are charged and to break the emulsion by changing the pH.