Molecular dynamics simulation of a single polymer in hydrophilic nano-slits

The behavior of a single polyethylene polymer in aqueous solution confined between two hydrophilic walls is studied with molecular dynamics (MD) simulations. The thickness of the nano-slit ranges from 1.26 to 3.15 nm, which is comparative to the polymer dimension. A monotonic transition from 3D-to 2D-like configurations is observed as the distance between the two walls narrows. Monomers are compressed into several layers and the preferred bond orientations alternate between parallel and normal to the walls accordingly. The diffusivity in the direction parallel to the wall is always larger than the one perpendicular to it. Calculation of the entropy and enthalpy changes during the folding of the polymer chain alone cannot explain the spontaneous process. The corresponding increase in water entropy due to volume expansion may be large enough to result in the overall free energy decrease.