Spray coating polymer substrates from a green solvent to enhance desalination performances of thin film composites

Toxic solvents like n,n-dimethylformamide (DMF), n,n-dimethylethanamide (DMAc), and 1-methyl-2-pyrrolidone (NMP) are commonly used to fabricate polymer support membranes. Replacing these toxic solvents with green solvents such as Cyrene (TM) can imbue sustainability into membrane fabrication, but at the expense of poor membrane separation performances. Here we overcome this limitation by spray coating Cyrene (TM)-based polymer dope solutions to form highly porous asymmetric membranes. The pure water permeance of spray-coated polyethersulfone (PES) membranes reached 68.9 L m(-2) h(-1) bar(-1), 7-fold higher than knife cast membranes. This significant increase in permeance was ascribed to a porous, thin skin layer and macrovoids interconnected with finger-like pores in spray-coated PES films. However, this did not impact on the ability to yield thin film composites (TFCs) with high separation performances. Through interfacial polymerisation, we deposited a polyamide selective layer on to the surface of spray-coated PES films to yield TFCs for desalination of a 2000 ppm NaCl solution. The salt rejection rate and permeance of such TFCs reached 93% and 1.76 L m(-2) h(-1) bar(-1), respectively. This desalination performance was similar to knife cast membranes produced from DMF-, NMP- and DMAc-based polymer dope solutions, but fabricated here in a more sustainable manner. This indicated that spray coating can overcome the trade-off between poor membrane separation performance and sustainability.