Thermoresponsivity and cononsolvency of a minimal polymer model in mixed solvents

Abstract

We present computer simulations of a minimal polymer model in water–methanol mixed solvents that captures key features of the cononsolvency behavior of amphiphilic polymers in alcohol–water mixtures. Our results indicate that the effective interactions between polymer segments vary nonmonotonically with the water–methanol composition. Their temperature dependence, however, shows a transition from good-to-poor solvent conditions only in the aqueous, low-methanol regime, up to the solvent composition X* corresponding to the minimum of the lower critical solution temperature. Consistent with this, the polymer radius of gyration decreases sharply with increasing temperature for X < X*. This temperature sensitivity, however, disappears at X = X*, even though the temperature dependence of effective segmental interactions still indicates a good-to-poor solvent transition. This loss of thermoresponsivity, in agreement with previous experimental observations, arises from preferential methanol adsorption, which reduces the role of hydrophobic hydration and leads to a vanishing heat of polymer collapse. Finally, we highlight the role of mixing entropy, arising from methanol–water mixing during polymer collapse, in driving the cononsolvency effect at a fixed temperature.