AI 中文总结
研究聚合物折返转变所需最少成分，通过分子动力学模拟发现拥挤剂体积分数$\phi_c$可驱动此转变，低$\phi_c$时桥连致塌缩，高$\phi_c$时饱和致膨胀，还研究了排斥性拥挤剂及带电聚合物情况，揭示可饱和几何桥连是相关最小机制。
AI 中文摘要
在各种软物质系统中都观察到了折返的线圈-球状体-线圈转变，即随着单个参数的变化聚合物先塌缩然后再膨胀，但产生这种转变所需的最少成分仍不清楚。通过对与单一吸引性拥挤剂相互作用的粗粒化聚合物进行分子动力学模拟，发现仅拥挤剂体积分数$\phi_c$就足以驱动完整的折返转变。低$\phi_c$时，拥挤剂桥连远处单体并驱动协同塌缩；高$\phi_c$时，单体结合位点饱和抑制桥连连通性并产生折返膨胀。纯排斥性拥挤剂不存在这种密度驱动转变，只会产生单调压实。桥连打破了自回避行走（SAW）通用性。对于有明确抗衡离子的带电聚合物，静电作用增强而非抑制折返。可饱和几何桥连是连接拥挤环境中中性和带电聚合物折返现象的最小机制。
英文摘要
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 $ϕ_c$ alone is sufficient to drive a complete reentrant transition. At low $ϕ_c$, crowders bridge distant monomers and drive cooperative collapse; at high $ϕ_c$, 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 $ϕ_c$ 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.
Comments12 pages, 7 main figures, 6 supp figures
