# Physical origin of the expansion of polymer coils in a binary solvent in the vicinity of its demixing critical point

## TL;DR

This paper investigates how critical Casimir forces cause polymer coils to expand near a solvent's critical point, revealing a reswelling phenomenon due to finite-size effects not predicted by earlier theories.

## Contribution

It introduces a new explanation for polymer coil reswelling near critical points based on finite-size effects and critical Casimir forces, extending previous collapse theories.

## Key findings

- Polymer coils initially collapse near the critical point.
- Large critical Casimir forces induce coil expansion.
- Coils eventually reshrink due to micro-phase separation.

## Abstract

Critical fluctuations are known to induce a collapse of polymer chains in a mixed solvent upon approaching its liquid-liquid critical point, as originally predicted by Brochard and de Gennes. Recently, we have found that closer to the critical point this collapse is followed by a reswelling of the polymer coils well beyond the original dimensions, a phenomenon not predicted by the theory of Brochard and de Gennes. We submit that upon approaching the critical temperature more closely, the correlation length of the critical fluctuations inside the polymer coils can no longer further increase due to the finite size of the coils, resulting in the appearance of large critical Casimir forces that cause a significant expansion of the polymer coils. Eventually, micro-phase separation inside the coils will appear and the coils will reshrink. This entire process takes place while the bulk solution is still in the one-phase region.