Unusual transformation of polymer coils in a mixed solvent close to the critical point

TL;DR

This study observes a novel collapse-reswelling-expansion-collapse transition of polymer coils near a solvent's critical point, revealing complex behavior influenced by micro-phase separation in a binary solvent system.

Contribution

It reports the first observation of a new expansion-collapse transition of polymer coils near the critical point, extending understanding of polymer-solvent interactions.

Findings

Polymer coils undergo a unique expansion-collapse transition near the critical temperature.

The observed phenomena are attributed to micro-phase separation within the polymer coil.

The behavior aligns with existing theories initially but introduces a new transition not previously documented.

Abstract

We have discovered unusual behavior of polymer coils in a binary solvent (nitroethane+isooctane) near the critical temperature of demixing. The exceptionally close refractive indices of the solvent components make the critical opalescence relatively weak, thus enabling us to simultaneously observe the Brownian motion of the polymer coils and the diverging correlation length of the critical fluctuations. The polymer coils exhibit a collapse-reswelling-expansion-collapse transition upon approaching the critical temperature. While the first stage (collapse-reswelling) can be explained by the theory of Brochard and de Gennes, the subsequent expansion-collapse transition is a new unexpected phenomenon that has not been observed so far. We believe that this effect is generic and attribute it to micro-phase separation of the solvent inside the polymer coil.