Molecular dynamics study of the thermosensitive properties of poly (N-vinyl caprolactam) in water

Abstract

In this work, we present molecular dynamics (MD) simulations of single poly (N-vinyl caprolactam) (PNVCL) in the water at different temperatures, from 275 to 335 K with a gradient of 10 K. The force fields of OPLS-AA and SPC model were used for PNVCL and water molecules, respectively. Simulations with durations of 200 ns were performed at each temperature for a 20-mer PNVCL chain in an isothermal-isobaric (NPT) and canonical (NVT) ensembles. The results showed that the phase transition of PNVCL chain during heating is driven by H-bonding transformation of amide groups, followed by the hydrophobic dehydration of CH group in the caprolactam ring. PNVCL exhibited a coil-to-globule transition at 305 K, corresponding to its low critical solution temperature (LCST). Below LCST, H-bonding predominated while hydrophobic interaction was dominant above LCST. PNVCL collapsed at elevated temperatures due to the weakening of the attractive forces between its hydrophobic/hydrophilic group and water molecules. The analysis of the H-bonds formation showed that PNVCL is hydrophilic and only carboxyl oxygen (C = O) in the amide group formed H-bonds with water molecules. In the collapsed state, PNVCL formed no intra H-bonds between its residues and the globule conformation was mainly due to the hydrophobic interactions between PNVCL residues. The H-bond formation played an important role in the coil-to-globule transition of PNVCL and is believed to be the key factor of its solubility.