Neutron reflectivity, water dynamics and performance of polyamide nanofilms for water desalination

We investigate the structure and hydration of polyamide (PA) membranes, with a combination of neutron and X-ray reflectivity, and benchmark their performance in reverse osmosis water desalination. Neutron spectroscopy provides further insight into the dynamics of water and the PA membrane under various hydration states. PA membranes were synthesised by the interfacial polymerization of m-phenylenediamine (MPD) and trimesoyl chloride (TMC), varying systematically reaction time, concentration and stoichiometry, to yield large-area, planar films of ≈10 nm thickness. Reflectivity precisely determined membrane thickness and roughness, as well as the (TMC/MPD) concentration profile, and response to hydration in the vapour phase. The resulting film thickness is found to be predominantly set by the MPD concentration, while TMC regulates water uptake. A favourable correlation is found between higher swelling and water uptake with permeance. The confined water dynamics were resolved and found to be coupled with the polymer relaxation dynamics. Our data provide quantitative insight into the film formation mechanisms and correlate reaction conditions, cross-sectional nanostructure and performance of the PA active layer in RO membranes for desalination.