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

Abstract

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 no signs of local ordering in the radial distribution functions, nor the structural reentrance characteristic of Hertzian-like particles. This is accompanied by the lack of a dynamical arrest transition, even well above random close packing. 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.

Figures (8)

• Figure 1: Deswelling behavior Snapshots of a.I) an ultra-low crosslinked microgel (ULC), and a.II) a microgel with a crosslinker concentration of $c=5\%$ at the lowest generalized packing fraction $\zeta$, respectively; (b) hydrodynamic radius $R_H$ normalized by its dilute limit $R_{H0}$ for ULC microgels (circles) and microgels with $c=5\%$ (triangles). The vertical dashed line denotes the random closed packing (RCP) value $\zeta_{\mathrm{RCP}}\sim 0.64$. The inset shows that ULCs begin shrinking already well below $\zeta_{\mathrm{RCP}}$, in a so-called chain regime (CR) dominated by their long-free chains. Here, the size reduction follows an apparent power law $R_H/R_{H0}\sim \zeta^{-1/6}$ (dash-dotted line). At high enough $\zeta$, deswelling becomes isotropic also for ULCs as well as for $c=5\%$ microgels, with $R/R_{H0}\approx \zeta^{-1/3}$, indicated by a dotted line.
• Figure 2: Shape evolution a) Density profiles $\rho(r)$ for different values of $\zeta$. Lines correspond to fuzzy-sphere fittings. b) Shape anisotropy $S_p$ as a function of $\zeta$ for I) a system of ULC and II) microgels with $c=5\%$ accounting for different structural components, respectively. The ULC chain-dominated regime is shown in dark-gray, while the isotropic regime with light-gray. Dashed green line represent the random-closed packing of hard-spheres, $\zeta_{\mathrm{RCP}}=0.64$. c) Snapshot of a ULC at the lowest $\zeta$, showing I) all monomers and II) monomers with $|r_i-r_{\mathrm{cm}}| < l_h$. d) I-II corresponding surface meshes.
• Figure 3: Absence of faceting in ULCs Representative snapshots of a system of a) microgels at $c=5\%$ and b) ULCs at a $\zeta\approx0.65$ (top) and $\zeta\approx2.5$ (bottom).
• Figure 4: Many-body overlaps a) Many-body overlap volume fraction $\phi_{nb}$ as a function of generalized packing fraction $\zeta$ for ULCs, for $n_b=2$, $n_b=3$, and $n_b=4$. Open symbols correspond to the $n_b=2$ of microgels with crosslinker concentration $c=5\%$. The vertical dashed line indicates the random closed-packing fraction of hard spheres $\zeta_{\mathrm{RCP}}$. The inset reveals that, even at low values of $\zeta<\zeta_{\mathrm{RCP}}$, the ULCs already exhibit multiple overlaps. b) Representative snapshots of two and three overlaps of ULCs at $\zeta=1.3$ and $\zeta=2.0$.
• Figure 5: Mechanical response a) Bulk modulus $K$ as a function of generalized packing fraction $\zeta$ for a system of ULCs and $c=5\%$ microgels. b) Single-particle bulk modulus $K_p$ obtained from spherical compression of isolated particles. c) Ratio $K/K_p$ highlighting the connection between collective and single-particle responses. Dashed lines in all panels denote $\zeta_{\mathrm{RCP}}$.