Concentration dependence of water dynamics in poly(ethylene oxide)/water solutions from molecular dynamics simulations

We have performed molecular dynamics simulations of aqueous solutions of poly(ethylene oxide) (PEO) in order to investigate the influence of the polymer on water dynamics. Simulations were performed on 12 repeat unit CH3-capped PEO chains (530 Da) at 318 K covering a composition range (polymer weight fraction) from 0.17 to 1.0. The simulations employed an ab initio quantum-chemistry-based PEO/water and PEO/PEO force field together with the TIP4P (four-point transferable intermolecular potential) water model. Water translational and rotational diffusion were found to slow monotonically, whereas water-water and water-ether hydrogen bond lifetimes increased with increasing polymer concentration. The slowing of water dynamics in PEO/water solutions was associated with PEO-water interactions that are moderated in concentrated solutions by the formation of water clusters. Water translational motion could be ascribed to a combination of free water (water not involved with PEO hydration) that exhibited bulklike water dynamics and bound (hydrating water) whose motion was strongly correlated with that of the PEO molecule. Water rotational motion was found to be strongly correlated with translation motion and exhibited increasing anisotropy with increasing PEO concentration indicative of preferred rotation of the water molecules around their dipole moment vector. At high PEO concentration, water and ether oxygen atoms exhibited well pronounced subdiffusive behavior occurring on a picosecond time scale that disappears upon dilution. The characteristic length scale for the water subdiffusive behavior associated with water caging correlates well with nearest-neighbor ether oxygen-ether oxygen distance.