  By means of atomistic molecular dynamics simulations we investigate the
behaviour of poly(N-isopropylacrylamide), PNIPAM, in water at temperatures
below and above the lower critical solution temperature (LCST), including the
undercooled regime. The transition between water soluble and insoluble states
at the LCST is described as a cooperative process involving an intramolecular
coil-to-globule transition preceding the aggregation of chains and the polymer
precipitation. In this work we investigate the molecular origin of such
cooperativity and the evolution of the hydration pattern in the undercooled
polymer solution. The solution behaviour of an atactic 30-mer at high dilution
is studied in the temperature interval from 243 to 323 K with a favourable
comparison to available experimental data. In the PNIPAM water soluble states
we detect a correlation between polymer segmental dynamics and diffusion motion
of bound water, occurring with the same activation energy. Simulation results
show that below the coil-to-globule transition temperature PNIPAM is surrounded
by a network of hydrogen bonded water molecules and that the cooperativity
arises from the structuring of water clusters in proximity to hydrophobic
groups. Differently, the perturbation of the hydrogen bond pattern involving
water and amide groups occurs above the transition temperature. Altogether
these findings reveal that even above the LCST PNIPAM remains largely hydrated
and that the coil-to-globule transition is related with a significant
rearrangement of the solvent in proximity of the surface of the polymer. The
comparison between the hydrogen bonding of water in the surrounding of PNIPAM
isopropyl groups and in bulk displays a decreased structuring of solvent at the
hydrophobic polymer-water interface across the transition temperature, as
expected because of the topological extension along the chain of such
interface.
