pNIPAM microgel swelling behavior and suspension structure studied with small-angle neutron scattering

Microgels are of high interest as model systems for soft and deformable colloids, which show a richer and more complex interaction and phase behavior than hard colloids, because their deformability and compressibility provides additional degrees of freedom. This allows microgels to react to changes in their environment and to external stimuli in ways that are not available for hard colloids. Our detailed understanding of this responsiveness is the foundation to model their interaction and phase behavior and further for the development of tailored microgels for applications. We use SANS to measure the form- and structure factor of pNIPAM microgels in dilute and concentrated suspensions and find the microgels to keep a constant size up to a critical concentration and to deswell at higher packing. This happens before the random-close-packing concentration is reached. Our results are compatible with microgel deswelling triggered by the osmotic pressure set by counterions associated to charged groups in the microgel periphery, which sharply increases when the counterion clouds surrounding the microgels percolate. We find that this counterion cloud must be taken into account to model the particle-particle interaction and the phase behavior. Further, the suspension polydispersity must be considered to obtain accurate form- and structure factors.