Thermoresponsivity and cononsolvency of a minimal polymer model in mixed solvents

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.