Density-driven reentrant polymer transitions via saturable bridging crowders
arXiv:2607.14838
The paper uses coarse‑grained molecular dynamics simulations to show that the volume fraction of attractive crowders alone can drive a polymer to collapse and then re‑expand (reentrant coil‑globule‑coil transition) via saturable bridging, with enhanced effects for charged polymers.
Abstract
Reentrant coil-globule-coil transitions, in which a polymer collapses and then reexpands as a single parameter is varied, have been observed across diverse soft matter systems, yet the minimal ingredients required to produce them remain unclear. Using molecular dynamics simulations of coarse-grained polymers interacting with a single species of attractive crowder, we show that crowder volume fraction alone is sufficient to drive a complete reentrant transition. At low , crowders bridge distant monomers and drive cooperative collapse; at high , saturation of monomer binding sites suppresses bridging connectivity and produces reentrant expansion. This density-driven transition is absent with purely repulsive crowders, which produce only monotonic compaction while preserving self-avoiding walk (SAW) chain statistics. In contrast, bridging breaks SAW universality: the rescaled size distributions no longer collapse onto a universal curve, and the conformational distributions trace the full coil-globule-coil trajectory as is varied. For charged polymers with explicit counterions, electrostatics amplifies rather than suppresses reentrance: bridging crowders displace counterions from the chain, and upon saturation the unscreened backbone charges drive expansion well beyond the original chain size. Saturable geometric bridging thus emerges as a minimal mechanism linking reentrant phenomena across neutral and charged polymers in crowded environments.
12 pages, 7 main figures, 6 supp figures