Depleted Depletion Drives Polymer Swelling in Poor Solvent Mixtures

TL;DR

This paper uncovers the microscopic origin of paradoxical polymer swelling in poor solvent mixtures, revealing how depletion interactions cause collapse and swelling cycles at constant pressure.

Contribution

It introduces a novel microscopic explanation for polymer swelling in solvent mixtures, combining simulations and theory to explain the collapse-swelling-collapse behavior.

Findings

Polymer swelling occurs at intermediate solvent ratios.

Depletion interactions drive the collapse and swelling cycles.

The phenomenon is generic and occurs in purely repulsive mixtures.

Abstract

Macromolecular solubility in solvent mixtures often exhibit striking and paradoxical nature. For example, when two well miscible poor solvents for a given polymer are mixed together, the same polymer may swell within intermediate mixing ratios. We combine computer simulations and theoretical arguments to unveil the first microscopic, generic origin of this collapse-swelling-collapse scenario. We show that this phenomenon naturally emerges at constant pressure in mixtures of purely repulsive components, especially when a delicate balance of the entropically driven depletion interactions is achieved.