Unexpected Behavior of Ultra-Low-Crosslinked Microgels in Crowded Conditions

Ultra-low-crosslinked (ULC) microgels are among the softest colloidal particles nowadays routinely synthesized experimentally. Despite a growing literature of experimental results, their microscopic behavior under crowded conditions is yet to be revealed. To this aim, we resort to realistic monomer-resolved computer simulations to investigate their structural, mechanical, and dynamical properties across a wide range of packing fractions. Using particle-resolved analyses, we unveil the role of outer chains in the ULCs, which manifest in peculiar behaviors, utterly different from those of regularly crosslinked microgels. In particular, we report the absence of faceting and the dominance of interpenetration between microgels at high densities. Furthermore, we observe a strong suppression of the structural reentrance characteristic of Hertzian-like particles, that is accompanied by the lack of a dynamical arrest transition, even well above random close packing. We further explore the change of behavior of the suspensions by lowering the crosslinker concentration and the single-particle density, providing strong evidence of the uniqueness of ULCs in the current landscape of microgels. Altogether, our results establish ULCs as a distinct class of soft colloids in which polymeric degrees of freedom are highly predominant over colloidal ones, providing for the first time a robust, microscopic framework to interpret their unusual behavior.