Computational Calorimetry of PNIPAM Cononsolvency in Water/Methanol Mixtures

Poly(N-isopropylacrylamide) (PNIPAM) is a widely used thermoresponsive polymer which dissolves in aqueous solutions and in pure methanol, however precipitates at low methanol concentrations of water/methanol mixtures. This interesting phenomenon where mixtures of two good solvents act as a poor solvent is called cononsolvency and the molecular mechanism for PNIPAM cononsolvency in water/methanol mixtures is still debated. Using extensive computational simulations we find a fingerprint for the cononsolvency of PNIPAM in water/methanol solutions in the calorimetric enthalpy of the collapse transition.[1] The microscopic interactions reveal that polymer hydration is the determining factor for chain collapse. In particular, the hydrogen bonds between water molecules and the amide proton are frustrated by the addition of methanol leading to a collapse. [1]C.Dalgicdir, F.Rodríguez-Ropero, N.F.A. van der Vegt J. Phys. Chem. B, 2017, 121 (32)