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Functional polymers or copolymers have been added to separations membranes by incorporating them in the membrane dope prior to casting, byin situpolymerization, and by postsynthesis surface modification of existing membranes. Here, a postsynthesis membrane functionalization that targeted decreasing the molecular weight cutoff (MWCO) and increasing the hydrophilicity without significantly decreasing the operating flux was studied. Hybrid bisamide molecules with added amine and carboxylic acid functionalities as end groups were synthesized to form a selective layer on membrane surface via covalent attachment to the membrane. Fourier transform infrared spectroscopy analysis showed the functional groups corresponding to bisamide molecules were present on modified membranes. Furthermore, modified membranes displayed MWCO of 400 Da as compared to 1000 Da MWCO of unmodified membranes, along with an increase in the hydrophilicity of modified membranes. Modified membranes showed an improvement in divalent salt rejection and percent flux recovered after reverse‐flow filtration as compared to unmodified membranes. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2020,137, 48327.