New insights into tailoring polyamide structure for fabricating highly permeable reverse osmosis membranes

Abstract Herein, plasticizer-assisted interfacial polymerization was developed to fabricate thin-film composite (TFC) reverse osmosis (RO) membranes with improved water permeability and comparable salt selectivity. Three representative phthalate acid esters, including dimethyl phthalate (DMP), dibutyl phthalate (DBP), and dioctyl phthalate (DOP), served as the plasticizer. FTIR results revealed that the introduction of plasticizers disrupted original amide-amide bonding between polyamide chains, converting polyamide structure from stiff to flexible, thus significantly promoting water diffusion efficiency through the polyamide matrix. Besides, the presence of plasticizers led to the creation of more voids in the polyamide layer, where water transport resistance was relatively low. Specifically, with the addition of 0.6 wt% DMP, water flux enhanced from 19.1 L m−2 h−1 of the virgin membrane to 72.5 L m−2 h−1, with NaCl rejection declining from 99.3% to 98.3%. And 0.6 wt% DBP-modified membrane showed water flux of 44.0 L m−2 h−1 and NaCl rejection of 98.9%. It was interesting to discover that DOP possessed the poorest plasticizing capacity, which contradicted the traditional cognition that DOP is one of the most resultful plasticizers for polymers. We believed it was the sizable alkyl moieties in DOP that suppressed the interaction between DOP molecules and polyamide chains.