Molecular insights on Poly(N-isoproylacrylamide) coil-to-globule transition induced by pressure

TL;DR

This study uses molecular dynamics simulations to explore how temperature and pressure influence the coil-to-globule transition of PNIPAM, revealing reentrant behavior and a new globular state at high pressures.

Contribution

It provides novel molecular insights into PNIPAM phase behavior under varying pressure and temperature, including the discovery of a high-pressure globular state and reentrant transition behavior.

Findings

Reentrant lower critical solution temperature with increasing pressure.

Existence of a new structured globular state at high pressures.

Different molecular mechanisms for temperature and pressure effects.

Abstract

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.