Diffusion of water in aqueous poly(ethylene oxide) at molecular to macroscopic length scales

Poly(ethylene oxide) (PEO) is a ubiquitously used water-soluble polymer with applications from drug delivery to water treatment membranes. Despite widespread use of PEO, models for water diffusion near the PEO surface and in the bulk lack molecular detail. We measure bulk diffusion coefficients of water in PEO solutions from 0 to 90 wt% PEO using Pulsed-field gradient NMR and local water diffusion within 1 nm of spin-labeled PEO by Overhauser Dynamic Nuclear Polarization (ODNP). We find that the bulk water diffusion coefficient scales with PEO concentration as described by a free volume model that accounts for interstitial space. Molecular dynamics simulations confirm that slower translational water dynamics correlates with enhanced tetrahedral water structure. ODNP experiments reveal that water diffusivity is slower near PEO surfaces than in bulk, but converges at the overlap concentration. PEO is not simply an obstruction to water transport but integrates into the water network to modify the structure and dynamics of the hydration shell.