Exploring the structural transition mechanisms of a pair of poly(N-isopropylacrylamide) chains in aqueous solution through coarse-grained molecular simulations coupled with metadynamics

Abstract

ABSTRACT Thermo-responsive polymers can experience the coil-to-globule transition with the temperature increases through the lower critical solution temperature (LCST). In this work, the coarse-grained molecular dynamics and Metadynamics (MetaD) are employed to investigate the dynamics and thermodynamics of structural change of a pair of poly(N-isopropylacrylamide) (PNIPAM) chains in aqueous solution. The results reveal that the PNIPAM molecules can move freely in aqueous solution below LCST, resulting in many extended conformations moving among several free energy basins on the free energy surface (FES); and these structures are dominated by the interaction force between PNIPAM and the water molecules. However, the PNIPAM molecules aggregate above LCST with collapsed conformations, leading to a deep basin on the FES; and these structures are dominated by the force between neighbouring PNIPAM chains. Each of the dominant mechanisms, namely, PNIPAM–water interaction force and intermolecular interaction force between PNIPAM chains, has an extreme tendency and corresponds to the extended and collapsed structures, respectively. However, the compromise in competition between these two mechanisms inevitably results in different characteristic structures including the extended and collapsed ones. In addition, the evolution of the trajectories at different temperatures are also analysed to examine the pathway of structural change.