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By means of atomistic molecular dynamics simulations we investigate thebehaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperaturesbelow and above the lower critical solution temperature (LCST), including theundercooled regime. The transition between water soluble and insoluble statesat the LCST is described as a cooperative process involving an intramolecularcoil-to-globule transition preceding the aggregation of chains and the polymerprecipitation. In this work we investigate the molecular origin of suchcooperativity and the evolution of the hydration pattern in the undercooledpolymer solution. The solution behaviour of an atactic 30-mer at high dilutionis studied in the temperature interval from 243 to 323 K with a favourablecomparison to available experimental data. In the PNIPAM water soluble stateswe detect a correlation between polymer segmental dynamics and diffusion motionof bound water, occurring with the same activation energy. Simulation resultsshow that below the coil-to-globule transition temperature PNIPAM is surroundedby a network of hydrogen bonded water molecules and that the cooperativityarises from the structuring of water clusters in proximity to hydrophobicgroups. Differently, the perturbation of the hydrogen bond pattern involvingwater and amide groups occurs above the transition temperature. Altogetherthese findings reveal that even above the LCST PNIPAM remains largely hydratedand that the coil-to-globule transition is related with a significantrearrangement of the solvent in proximity of the surface of the polymer. Thecomparison between the hydrogen bonding of water in the surrounding of PNIPAMisopropyl groups and in bulk displays a decreased structuring of solvent at thehydrophobic polymer-water interface across the transition temperature, asexpected because of the topological extension along the chain of suchinterface.

On the Molecular Origin of the Cooperative Coil-to-globule Transition of Poly(N-isopropylacrylamide) in Water