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

Abstract

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™ can imbue sustainability into membrane fabrication, but at the expense of poor membrane separation performances. Here we overcome this limitation by spray coating Cyrene™-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.