High water permeable nanofiltration membranes via post pore expansion of polyamide thin layer introducing methoxycarbonyl groups

Improvement in the water permeability of nanofiltration (NF) membranes is desirable for saving energy in separation processes. Polyamide (PA) membranes formed via interfacial polymerization are commonly used as NF membranes because of their high water permeability and controllable selectivity. Typically, water permeability and rejection of target molecules are trade-offs, making simultaneous improvement challenging. In this paper, we demonstrate a new methodology called "post pore expansion" of PA NF membranes to enhance water permeability using a cleavable amine molecule, methyl 3,5-diaminobenzoate (MDAB). PA membranes were formed using piperazine, MDAB, and m-phenylenediamine as amine molecules. They were then treated with aqueous NaOH to convert the hydrophobic methoxycarbonyl groups to hydrophilic carboxyl groups via hydrolysis. Water permeability drastically increased after the NaOH treatment, reaching values 10 times higher than the initial value, while maintaining Na2SO4 rejection. The fabricated membranes exhibited a maximum water permeability of 72.3 LMH/bar and Na2SO4 rejection of 91.1 %. Various surface analyses confirmed the presence of carboxyl groups, and positron annihilation lifetime spectroscopy results indicated removing methoxycarbonyl groups and increasing pore size of the PA layer, suggesting that the pores were expanded. The presented "post pore expansion" methodology has the potential to drastically improve water permeability maintaining salt rejection in PA NF membranes.