Hydrolyzed polyacrylonitrile nanofibers as interlayers for ultrathin nanofiltration membranes of high permeance and salt rejection

Abstract

Interfacial polymerization (IP) is one of the most common approaches to prepare polyamide (PA) nanofiltration (NF) membranes. However, these membranes usually lack permeability because of their dense PA layers created by superfast IP. A strategy to use interlayers has gained widespread popularity because it can regulate IP processes and consequently make loose and thin PA layers. Polymer interlayers are usually thick and dense, lowering the permeance of the NF membranes. Herein, we demonstrate a novel and versatile approach to prepare hydrolyzed polyacrylonitrile (HPAN) nanofiber interlayers that are thin and porous, resulting in highly permeable ultrathin PA NF membranes. HPAN nanofibers were prepared by hydrolysis of PAN and the innovative approach of fast freeze-extraction. These interlayers help to retain more piperazine (PIP) monomers by electrostatic attraction, slow down the IP rate by impeding diffusion of the PIP monomers towards the oil−water interface, and consequently create ultrathin and wrinkled PA layers. For example, ultrathin PA NF membrane (∼34 nm thickness, 13.85 cm2 membrane area) using 20 mL nanofiber dispersion as interlayer has a favorable water permeance (33.7 L m−2 h−1 bar−1) and remarkable salt rejections (98.9 % for Na2SO4 and 99.7 % for MgSO4), suggesting great potential for rapid desalination.