Influence of particle softness on the flow properties of colloidal dispersions: A comparison of regularly and ultra-low-crosslinked microgels*

Soft, adaptive microgels are potential building blocks for smart materials with precisely controlled viscoelastic properties that are highly responsive to environmental conditions. We compare the flow properties of different PNIPAM microgels: regularly crosslinked and ultra-low crosslinked (ULC) microgels. We show that the flow properties of ULC microgels reveal a colloid-to-polymer transition with increasing solution concentration. At low concentrations, the viscoelastic properties and phase behaviour of ULC microgels are identical to those of regularly crosslinked microgels. With increasing concentration, the viscosity and plateau modulus display a much weaker increase for ULC relative to regularly cross-linked systems. Oscillatory rheology in the linear viscoelastic region reveals that for regularly crosslinked microgels in concentrated solution, the flow properties are determined by the highly cross-linked core, while for ULC microgels the brush-like interaction is dominant.