Polymer Conformations with Attractive Bridging Crowder Interactions: Role of Crowder Size

We use coarse-grained molecular dynamics to map how attractive crowders control the conformation of an electrostatically neutral polymer at low crowder volume fraction. We vary three parameters: (i) the monomer–monomer interaction (good-vs poor-solvent-like), (ii) the monomer–crowder attraction strength ϵmc, and (iii) the crowder size σc. Two distinct mechanisms emerge that are selected by σc. For small crowders (σc ≲ σm), adsorption onto multiple monomers produces bridging that collapses the chain at comparatively low ϵmc; in this regime, the collapsed conformations are similar whether the bare polymer is weakly self-repulsive or strongly self-attractive. For large crowders (comparable to or larger than the polymer), the polymer instead occupies interstitial spaces and effectively bridges multiple crowders, leading to a confinement-assisted collapse with a different dependence on ϵmc. Near the threshold for weakly self-repulsive chains, increasing σc can generate a nonmonotonic sequence (collapse → transient extension → recollapse) due to competition between monomer cohesion and crowder adsorption. In contrast, for strongly self-attractive chains, only small crowders substantially modify the collapsed state, while large crowders tend to suppress further changes. Purely repulsive crowders do not induce collapse at the studied densities. These results clarify when bridging versus confinement governs crowding-induced collapse and provide a simple basis for tuning polymer states via crowder size and attraction.