Charged hollow microgel capsules.

Responsive hollow microgels are a fascinating class of soft model systems at the crossover between polymer capsules and microgels. The presence of the cavity makes them promising materials for encapsulation and controlled release applications but also confers them an additional softness that is reflected by their peculiar behaviour in bulk and at interfaces. Their responsivity to external stimuli, such as temperature, pH, and ionic strength, can be designed from their synthesis conditions and the choice of functional moieties. So far most studies have focused on "small" hollow microgels that were mostly studied with scattering or atomic force microscopy techniques. In our previous study, we have shown that large fluorescent hollow poly(-isopropylacrylamide) (PNIPAM) microgels could be synthesized using micrometer-sized silica particles as sacrificial templates allowing their investigation confocal microscopy. In this work, we extend this approach to charged large hollow microgels based on poly(-isopropylacrylamide--itaconic acid) (P(NIPAM--IA)). Hereby, we compare the structure and responsivity of "neutral" (PNIPAM) and "charged" (P(NIPAM--IA)) hollow microgel systems synthesized under similar conditions with the same sacrificial template using confocal and atomic force microscopy and light scattering techniques. In particular, we could demonstrate the extremely soft character of the swollen charged hollow microgels and their responsivity to pH, ionic strength, and temperature. To conclude this study, the buckling behavior of the different capsules was investigated illustrating the potential of such systems to change its conformation by varying the osmotic pressure and pH conditions.