Modeling the Response of Soft Microgels to Crowding by Nanoparticles

The internal degrees of freedom of crosslinked polymer networks enable compressible microgels to swell or deswell by absorbing or expelling solvent in solution. The presence of cosolvents or other macromolecules (e.g., nanoparticles) can enrich the swelling response, facilitating applications of “smart” colloidal particles, e.g., as drug delivery vehicles and biosensors. We extend the mean-field Flory-Rehner theory of polymer networks to incorporate, as an implicit species, hard nanoparticles that perturb the polymer network through volume exclusion. Within the free energy landscape of these asymmetric ternary mixtures, we perform Monte Carlo simulations, including novel trial moves [1] that allow microgels to change size and nanoparticles to penetrate microgels. Within our coarse-grained model, we investigate how single-microgel properties (e.g., crosslink density) and solution properties (e.g., microgel and nanoparticle concentrations, solvent quality) influence (1) partitioning of nanoparticles inside and outside of microgels; (2) swelling of microgels; and (3) bulk structure of microgel solutions. Our results can guide experiments and applications by providing insights into how the response of soft (e.g., biological) materials to external stimuli can be tuned by adding nanoparticles.

[1] M. Urich and A. R. Denton, Soft Matter 12, 9086 (2016).