Computational Calorimetry of PNIPAM Cononsolvency in Water/Methanol Mixtures.

We revisit the mechanism for cononsolvency of PNIPAM in water/methanol mixtures. Using extensive molecular dynamics simulations, we calculate the calorimetric enthalpy of the PNIPAM collapse transition and observe a unique fingerprint of PNIPAM cononsolvency which is analyzed in terms of microscopic interactions. We find that polymer hydration is the determining factor for PNIPAM collapse in the cononsolvency regime. In particular, it is shown that methanol frustrates the ability of water to form hydrogen bonds with the amide proton and therefore causes polymer collapse.