Neutron spectroscopy of water dynamics and polymer relaxation in polyamide RO desalination membranes

We investigate model polyamide (PA) membranes with a combination of quasi-elastic and inelastic neutron scattering to elucidate the translational dynamics of confined water and the relaxation of the polymeric matrix under conditions relevant to reverse osmosis (RO). Specifically, PA membranes were synthesised by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), and hydrated with both H₂O and D₂O to decouple the water dynamics from the polymer network relaxation. We hydrate the membranes with water vapor at precise RH conditions, in order to elucidate the translational dynamics of confined water. For comparison, we also measure PA membranes prepared by direct immersion in water and careful drying, in order to examine the relative contributions of confined and ‘bulk’ water. We isolate contributions from localized diffusion, translational jump-diffusion, and long-range diffusion mechanisms. Overall, our results quantify this multimodal diffusive nature of water in PA membranes that can be related to bulk transport via coarse-grained engineering models.