  Poly(N-isopropylacrylamide), PNIPAM, is a widely studied polymer, which
serves as a key constituent of nanostructured soft materials with responsive
properties. Upon increasing temperature the PNIPAM polymer chain undergoes a
reversible coil-to-globule transition at ~305K, which is reflected by a volume
phase transition in cross-linked architectures, such as microgels, valuable for
many practical applications. The addition of a cosolvent is a simple method to
tune the transition temperature according to the specific purpose. In this
study, we use atomistic molecular dynamics simulations to explore the solution
behavior of a PNIPAM chain in a mixture of water and ethanol, acting as
cosolvent, at low alcohol concentration. Our simulations reproduce the
occurrence of the coil-to-globule transition of the polymer chain at 289 K, a
temperature lower than that measured in water, in full agreement with
experimental findings. By monitoring the temperature evolution of structural
and dynamical properties of the PNIPAM-water-ethanol ternary system, we detect
a localization of ethanol molecules at the polymer interface, mainly due to
interactions between isopropyl and ethyl groups. We observe that the transition
occurs without a release of adsorbed ethanol molecules, but with a loss of
water molecules from the surrounding of PNIPAM hydrophobic moieties that
favours the aggregation of ethanol molecules close to the polymer. Our results
support the idea that both the decreased chemical potential of water in the
bulk of the mixture and the competition between water and ethanol molecules in
the interactions with the polymer play a driving role in the transition.

Soft Condensed Matter