  Poly-N-isopropylacrylamide (PNIPAM) phase diagram is explored in a wide range
of temperature and pressure using extensive all-atom molecular dynamics
simulations. By exploiting a simple model of an atactic linear polymer chain,
we provide novel insights into PNIPAM coil-to-globule transition addressing the
roles played by both temperature and pressure. We find that a coil-to-globule
transition exists up to large pressures, undergoing an intriguing reentrant
behavior of the lower critical solution temperature with increasing pressure in
agreement with experimental observations. Furthermore, we report the existence
of a new kind of globular state at high pressures, again confirming
experimental results: this is characterized by a more structured hydration
shell, that is closer to PNIPAM hydrophobic domains, with respect to the
atmospheric pressure counterpart. Our results clearly show that temperature and
pressure induce PNIPAM coil-to-globule transition through different molecular
mechanisms, opening the way for a systematic use of both thermodynamic
parameters to tune the location of the transition and the properties of the
associated swollen/collapsed states.
