Nanofiltration membranes with enhanced microporosity and inner-pore interconnectivity for water treatment: Excellent balance between permeability and selectivity

Highly permeable nanofiltration (NF) membranes with rigorous selectivity are desirable for water treatment, and controlling the porosity of the active layer of thin film composite (TFC) membrane is an effective way to maximize the membrane performance. Herein, a strategy to enhance microporosity and improve microstructure of inner pore by manipulating the molecular structure was employed to prepare NF membranes. Poly(ester-amide) TFC membranes with enhanced microporosity and inner-pore interconnectivity were successfully fabricated by reacting rigidly-contorted bisphenols (two phenols containing spirobisindane and Troger's base structure, respectively) and piperazine (PIP) with trimesoyl chloride (TMC) via interfacial polymerization on a porous substrate. The representative NF membranes fabricated in this work exhibited a 2.7-fold pure water flux (PWF) of the polypiperazine amide (PPA) membrane and the same multivalent salt rejections (PWF, 181.1 Lm(-2)h-1 MPa-1; rejection, 99.2% for Na2SO4). Molecular simulation and N-2 adsorption measurement have demonstrated that the membranes prepared according to the optimal phenol/amine ratio possess more free volumes and mostly interconnected voids, which is resulted from the shape and rigidity of the contorted monomers. Besides, owing to the micropore generation was governed at molecular level, no unselective defects were formed. As a consequence, the membranes show simultaneously high permeability and rejection. Additionally, the obtained membranes also possess great potential for long-time operation, as demonstrated by stability test.