Molecular Structure and Vibrational Spectra of Water Molecules Sorbed in Poly(2-methoxyethylacrylate) Revealed by Molecular Dynamics Simulation.

Molecular dynamics (MD) simulations of water sorption in poly(2-methoxyethylacrylate) (PMEA) are carried out to elucidate the hydrogen bonding (H-bonding) structures of the water molecules and the side chains of PMEA. A PMEA model incorporating lone-pair virtual sites on the carbonyl and methoxy oxygens of the side chain of PMEA, which are the key interaction sites in a biocompatible polymer, is newly developed. The PMEA model well reproduces the experimentally observed features in the infrared spectra of the hydrated polymer, as well as the radial distribution function of the water molecules in contact with the polymer, as calculated by ab initio MD simulations. The MD simulation results reveal that water molecules tend to form H-bonds with the carbonyl oxygen and the methoxy oxygen of the side chain of PMEA simultaneously, which enhance the "head-to-tail" stacking structure of the side chains at a low concentration range of water. Further penetration of water into the PMEA structure gradually increases the water-water H-bonding state and promotes the formation of water clusters even below the equilibrium water content.