Zwitterion Polymer Brushes on Porous Membranes: Characterization, Tribology, Performance, and the Effect of Electrolyte Anions

Herein, we present a two-step activation process to achieve surface-initiated atom transfer radical polymerization (ATRP) and covalently grafted poly­(sulfobetaine methacrylate) polyzwitterion brushes with controlled density and thickness on polyacrylonitrile (PAN) ultrafiltration membrane surfaces. The activation is based on the known amidoxime synthesis on PAN membrane surfaces, with subsequent mixed oxadiazole formation to incorporate the ATRP initiator. Successful grafting and the differences in brush density and thickness were verified by X-ray photoelectron spectroscopy, attenuated total reflection Fourier transform infrared spectroscopy, and atomic force microscopy (AFM). Specifically, we qualitatively show the differences in the brush interface layers by using lateral force microscopy (LFM) with a colloidal AFM probe and through nanowear experiments using a sharp AFM tip. All brush interfaces showed reduced friction in LFM and unique stick-slip behavior and ripple formation as a nanowear mode. We also show that the molecular weight cutoff and permeability vary with the brush properties and that both high- and low-density brushes offer superior antifouling properties against alginate compared to a pristine PAN membrane. Finally, we analyze how Hofmeister series anions affect filtration with 0.01, 0.1, and 0.5 mol L^–1 KCl, KBr, KSCN, K₂SO₄, and K₂HPO₄ solutions. The results reveal a positive flux effect for KCl, KBr, and KSCN and a negative one for K₂SO₄ and K₂HPO₄ solutions, suggesting that the brushes are grafted mostly to the surface, crowding the pore mouth in the media with strongly hydrated counterions. Overall, this work provides a method for preparing well-controlled polymer brushes on porous membranes to produce membranes with excellent antifouling properties.