Interpenetration of polymeric microgels at ultrahigh densities

Soft particles such as polymeric microgels can form ultra-dense phases, where the average center-to-center distance a (s) can be smaller than the initial unperturbed particle diameter σ (0), due to their ability to interpenetrate and compress. However, despite of the effort devoted to microgels at ultrahigh densities, we know surprisingly little about their response to their environment at effective volume fractions ϕ (eff) above close packing (ϕ (cp)), and the existing information is often contradictory. Here we report direct measurements of the size and shape of poly(N-isopropylacrylamide) microgels at concentrations below and above ϕ (cp) using the zero average contrast method in small-angle neutron scattering. We complement these experiments with measurements of the average interparticle distances using small-angle x-ray scattering, and a determination of the glass transition using dynamic light scattering. This allows us to unambiguously decouple interaction effects from density-dependent variations of the particle size and shape at all values of ϕ (eff). We demonstrate that the microgels used in this study significantly interpenetrate and thus change their size and shape only marginally even for ϕ (eff) ≫ ϕ (cp), a finding that may require changes in the interpretation of a number of previously published studies on the structural and dynamic properties of dense soft particle systems.