Understanding the lower critical solution temperature of amphiphilic synthetic polymers in water: The role of conformation and hydration

The thermoresponsiveness of polymer-based soft materials opens perspectives in many applicative fields. This property in hydrated systems has the simple origin of a reversible de-mixing across a lower critical solution temperature, and has been explored for several chemically different amphiphilic polymers, each of them with specific critical conditions and phase behaviours. This work investigates for the first time by extensive atomistic molecular dynamics simulations the temperature dependence of the infinitely diluted aqueous solution of two such thermoresponsive macromolecules, poly(N-isopropyl-methacrylamide), PNIPMAM, and poly(2-isopropyl-2-oxazoline), PIPOX, in comparison to poly(N-isopropylacrylamide), PNIPAM, the much-more studied prototype system for polymers exhibiting a lower consolute boundary in water. The evolution of conformation and hydration water of the macromolecules is detected in a temperature range from well-below to well-above the transition temperature, with a favourable comparison to available experimental data. The resulting molecular description provides an explanation for deviations in solution properties of these macromolecules, including the effect of chaotropic anions and of solvent isotopic composition on the value of the lower critical solution temperature and the transition enthalpy. Overall, simulation findings show how the trade-off between gain in translational entropy of water molecules, associated with the reduction in solvent-excluded volume during the transition, and polymer-specific conformational features, determines the temperature response of these macromolecules in aqueous solution. In light of this molecular characterisation, a correlation between the single chain behaviour across the transition and the type I or type II phase diagram of the polymer aqueous solution can be postulated.