Ultrathin polyamide nanofilm with an asymmetrical structure: A novel strategy to boost the permeance of reverse osmosis membranes

Ultrathin polyamide (PA) nanofilm based separation membranes have attracted drastically increasing attention recently. Typically, PA nanofilms with the thickness of around tens of nanometers are supported by a PSF substrate membrane which provides mechanical support. However, the low surface porosity of the PSF substrate membrane has required the transverse diffusion (parallel to the membrane plane) of water molecules in the nanofilm, which causes much longer mean diffusion paths compared to the thickness of the nanofilm. In this study, we address this problem by introducing a much looser polypiperazinamide (PPA) interlayer in between the PA nanofilm and the PSF support membrane, with the PPA nanofilm serving as a low resistance region for water molecules. A dual interfacial polymerization strategy was applied to create an asymmetrical ultrathin polyamide selective layer comprised of a high permeability loose PPA sublayer and a high selectivity dense PA top layer. Quartz crystal microbalance with dissipation (QCMD) techniques and Doppler broadening energy spectroscopy (DBES) were applied to study the asymmetry structure of the ultrathin polyamide nanofilms. Compared with the home-made traditional ultrathin polyamide (uPA) membrane, the asymmetrical ultrathin polyamide (A-uPA) membrane has 2–2.5 folds higher permeability while maintaining higher salt rejection. Our study demonstrates that the asymmetrical structure can significantly enhance the flux for ultrathin polyamide membranes. Further, the impact of the structure of the top layer and the sublayer on the membrane separation performance was explored by tuning the recipe of the PA top layer and the PPA sublayer.