Poly(caffeic acid) as interlayer to enhance nanofiltration performance of polyamide composite membrane

It is a challenge to develop nanofiltration (NF) membranes with simultaneously high permeability and excellent selectivity for water desalination. In this work, a composite NF membrane with an interlayer was prepared in order to overcome the upper bound of permeability-selectivity. Poly(caffeic acid) (PCA) was first deposited on the surface of polyacrylonitrile (PAN) ultrafiltration membrane to construct an interlayer, and then polyamide (PA) layer was formed on the PCA interlayer to fabricate the composite membrane (PAN-PCAn-PA). The chemical structure, morphology and surface properties of the composite membranes with and without the interlayer were compared. In addition, the preparation conditions and separation properties of the composite membranes were investigated. The results suggested that a loose "gutter" interlayer and a nanoscale Turing structure on the membrane surface were constructed due to the hydrophilic and electronegative PCA, which optimized the transport channels of water molecules. Hence, the water permeance of PAN-PCA2-PA membrane was 17.7 L·m−2·h−1·bar−1, which was 86% higher than that of PAN-PA membrane. Meanwhile, the PAN-PCA2-PA membrane exhibited high rejection ratio of Na2SO4 (98.9%), excellent selectivity for Cl‐/SO42‐ (75.9) and long-term filtration stability, which showed its potential for practical applications in water desalination.