Semipermeable membranes based on polybenzimidazole: Simultaneous improvement in water flux and salt rejection by facile cross-linking

•Semipermeable membranes based on polybenzimidazoles (PBIs).•Cross-linking and N-substitution induced the suppression of the hydrogen bondings.•Facile thermal cross-linking of PBI membranes improved their water flux and salt rejection.

To develop next-generation high-performance semipermeable membranes, novel semipermeable membranes based on the cross-linked polybenzimidazole have been synthesized and characterized. Polybenzimidazole with N-butylsufonate (BSPBI) was prepared by the two steps, that is, deprotonation of PBI using lithium hydride, followed by treatment with 1,4-butanesultone. FTIR measurements showed that the N-substitution and cross-linking suppressed the hydrogen bonding. WAXD analyses also revealed that polymer packing was disrupted by the N-substitution and cross-linking. These results clearly indicate that the suppression of hydrogen bonding induced by the N-substitution and the cross-linking cause disruption of the polymer packing. The cross-linking reaction of BSPBI with divinyl sulfone led to the change of the pore size distribution, where the selective permeation of water molecule was successfully achieved. As a result, simultaneous improvement in water flux and salt rejection was achieved by the facile cross-linking reaction. The cross-linking of BSPBI simultaneously improved both water flux and salt rejection (water flux: 22.1 L m− 2 h− 1 and NaCl rejection: 46%) compared to those of non-cross-linked BSPBI (water flux: 1.88 L m− 2 h− 1 and NaCl rejection: 11%). These findings should contribute to develop high-performance semipermeable membranes for water treatment including reverse osmosis (RO), forward osmosis (FO), and pressure retarded osmosis (PRO).

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