Reduction of water-mediated repulsion drives poly(N-vinylcaprolactam) collapse upon heating.

Upon heating, thermo-sensitive aqueous polymers undergo the coil-to-globule transition, where drastic chemical and structural transformations occur that are of great interest for academia and applications. Although it is widely believed that the disruption of the clathrate-like hydration shell drives this polymer collapse, no decisive evidence has yet been provided. Here, we demonstrate, using all-atom molecular dynamics simulations, that poly(N-vinylcaprolactam) in water has a less ordered hydration structure than the bulk liquid and undergoes the coil-to-globule transition without remarkable hydration shell depletion or qualitative transformation. Furthermore, our free energy analyses show that water strongly pushes the "hydrophobic" caprolactam groups apart rather than bringing them together. We find that the reduction of this water-mediated repulsion, arising from the change in free energies for cavity formation, drives the polymer collapse upon heating.