(722b) 3D Printing Polyamide Films:  an Additive Approach to Making Thin Film Composite Membranes with Tunable Thickness and Roughness

The thin film composite (TFC) membrane is a commonly used platform for making membranes with the capability to desalinate. TFC membranes employ a thin, highly selective aromatic polyamide layer that is formed in-situ onto a porous supporting membrane through a poly-condensation reaction between two monomers: a diamine (typically m-phenylene diamine) and an acid chloride (typically trimesoyl chloride). The reaction occurs at the interface between an organic phase containing the acid chloride and the aqueous phase containing the diamine. Remarkably, in 30 years of using this approach, the RO industry has yet to create a polyamide film onto a supporting membrane with controllable thickness, roughness and independence from the substrate. While the film formation using this approach essentially is “self-controlled” as the film formation further monomer reaction, the reaction itself is simply allowed to proceed. Monomers can be changed or additives included to affect selectivity, but generally features of the film, like thickness and roughness, are uncontrolled. Thickness and roughness impact membrane permeance and fouling propensity, respectively, and controlling these parameters would be a great benefit to the membrane community

This work entails demonstration of a new method for making aromatic polyamide TFC membranes using electro-hydrodynamic spray, or, more simply, electrospray. This additive approach involves depositing diamine and trifunctional acid chloride monomers sequentially onto a porous supporting surface where they form an ultra-thin, highly selective, and tunable polyamide layer. We demonstrate the use of electrospray to form polyamide selective layers with adjustable thickness and roughness while decoupling the film formation from the supporting substrate structural and chemical properties. We demonstrate the ability to control thickness with a resolution of 20 nm and the control of roughness in between 3.5 nm to 35 nm which is comparable to the other novel technique such as m-LBL technique. Additional benefits are garnered from making the polyamide selective layer formation “support independent” by ensuring that support layer properties do not affect film formation. Using this approach, we have fabricated TFC membranes on PAN50 UF membranes (Nanostone Water) and have demonstrated a 66-97 % and 80-98.7 % rejection for sodium chloride, and magnesium sulfate, respectively, with pure water permeance ranging from 0.1 to 0.75 LMH/bar and we believe that such an approach can yield even higher performance. We noted that polyamide films made through our method were more cross-linked compared to those made through the conventional method which may be a reason for their low permeance. Lastly, an additional benefit to this approach is that it is far more environmentally friendly than conventional interfacial polymerization since it requires very little monomer and solution.