Molecular scale diffusion of water in cross-linked poly(ethylene glycol) diacrylate

The separation performance of polymer-based water purification membranes is thought to be governed by both molecular and macroscopic water properties near polymer surfaces. However, the role of molecular-level interactions between water and membrane interfaces on macroscopic transport remains unclear. Using aqueous poly(ethylene glycol) (PEG) solution, a common membrane component, we find that the diffusion of water at multiple length scales is mediated by molecular scale water structuring around the polymer chain. A fractional free volume diffusion model accounts implicitly for water-polymer interactions to describe water diffusion in aqueous PEG solution. Furthermore, the impact of polymer interfaces on water dynamics is unknown for cross-linked polymer networks. We measure nanometer scale water diffusivity using Overhauser Dynamic Nuclear Polarization on cross-linked PEG diacrylate to investigate whether the same water structuring drives water diffusion in the cross-linked environment. We determine the effect of water volume fraction in swollen hydrogels on local water diffusion. This study shows how molecular scale interfacial water dynamics impact macroscopic transport relevant to the design of structurally complex polymer membranes.