A novel single-scan printing approach for polyamide membranes by electrospray technique on polydopamine pre-coated substrate

In the quest for the next generation desalination/water reuse membranes, there is an increasing interest in the potential of 3-dimensionally (3D) printed membranes; one promising candidate method is the electrospray (ES) technique. This work proposes a new approach based on the ES technique to print polyamide thin-film composite membranes in a single scan. Herein, we first dip-coated a polydopamine (PDA) layer to tailor the surface properties of the supporting substrate so that a layer of m-phenylenediamine (MPD) aqueous solution can be loaded on top of the coated substrate. We then utilized different electrohydrodynamics (EHDs) of the ES technique to deliver the trimesoyl chloride (TMC) organic solution. Two key factors, the PDA coating duration and the EHD conditions of the TMC solution, were systematically studied. The substrate with a 4-h PDA coating was found to be optimal to enable consistent ES printing given its enhanced surface wetting property. Using this substrate, 4 selected EHD conditions of the TMC solution at 4 spray distances resulted in different membrane morphologies, surface chemistries, and separation performance. While a focused jet of TMC solution at 1 cm spray distance resulted in an interesting polyamide stripe pattern, a cone-jet spray at 2.5 cm spray distance showed the highest NaCl rejection at 98.1%. The membrane formation mechanism is also elucidated; a proposed ‘capping’ effect can explain the performance trend based on the morphology and chemical characterization. Overall, the printed polyamide membranes in our approach showed better combinations of water permeance and solute rejection than a polyamide membrane prepared by conventional interfacial polymerization. Thus, this study offers a promising and innovative method to print patterned polyamide membranes potentially with enhanced separation performance.