Depleted Depletion Drives Polymer Swelling in Poor Solvent Mixtures

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.