Temperature and concentration dependence of the microgel Counter-ion Cloud configuration with increasing particle stiffness studied with Small-Angle Neutron Scattering (SANS)

Microgels are stimuli-sensitive colloids formed by cross-linked polymer networks and are a good model system for soft colloids. Unlike hard colloids, their interaction and phase behavior is not well understood, especially at high concentrations. Although pNIPAM is an uncharged polymer, pNIPAM microgels are peripherally charged due to charges remaining from particle synthesis. The corresponding counterion cloud has been found to play a crucial role in the observed spontaneous deswelling behavior at high concentrations; particles deswell before reaching random closing packing density Φrcp. Our recent SANS measurements have confirmed that the counterion cloud indeed locates at the particle surface, which corroborates our model explaining the observed spontaneous deswelling at high concentrations. Here, we have studied microgels with various softness, set by the crosslink density of 0.05%, 2.5% and 5%, and suspensions with concentrations from ζ=0.02 to ζ=1 were measured. We replace the counterions with either Na⁺ or NH₄⁺ via dialysis, which changes the microgel form factor and allows obtaining detailed information on the configuration of the counter-ions and charged groups. All measurements were done at 18°C, 30°C, and 45°C, which are below, around and above the lowest critical solution temperature 32°C. We find that the deswelling ratio at high concentrations varies for these samples and the counterion cloud persists at the particle surface at all conditions. Our results allow exploring the temperature-, concentration-, and particle-stiffness dependence of the deswelling behavior, which leads to a grasp of the interplay among the osmotic pressures due to polymer-solvent mixing, chain elasticity and ionic contribution, which control the microgel volume phase transition. We find that the osmotic pressure due to the counterions has to be included for a comprehensive understanding of microgels at high concentrations and for formulating new models for their phase behavior that take spontaneous deswelling at high concentrations into account.