Insight into polydopamine coating on microfiltration membrane with controlled surface pore size for enhanced membrane rejection

Membrane modification with polydopamine (PDA) is a common practice to enhance membrane selectivity and improve membrane compatibility with nanoparticles or thin films. Depending on the membrane surface properties, dopamine can deposit or penetrate into microporous membranes, leading to alteration in the membrane's physicochemical properties and consequently its performance. In this study, microfiltration (MF) polyethersulfone membranes with varying surface pore sizes were fabricated by manipulating the precasting time (solvent evaporation time of 3s, 15s, 30s and 60s) prior to non-solvent induced phase separation (NIPS) process. Notably, the coating of PDA slightly reduces the membrane's surface pore size and membrane's porosity. M60 membrane which possess the smallest mean surface pores (∼300 nm), exhibits PDA clusters predominantly formed on top of the membrane surface. With an increase in surface pore size, dopamine can penetrate the porous structure of the MF membrane, leading to the polymerization of dopamine into PDA aggregates on both the membrane surface and inner pores. The M30 membrane has the largest surface pore size (∼500 nm), showing the formation of large PDA globules in the finger-like pores. The membrane (M30P) experienced significant narrowing of the inner pores, resulting in reduced water permeability. Overall, all PDA-coated membranes exhibited improved sucrose rejection (85–95%) compared to the uncoated ones (64–83%). These finding provides insight into the polymerization mechanism of PDA on porous membranes prior to the incorporation of nanomaterials or thin film coating. [Display omitted] •Previous studies less focused on penetration of dopamine in membrane interior.•Large surface pore size facilitates diffusion of dopamine prior to polymerization.•Polydopamine forms on both membrane surface and finger-like pores' wall.•Polydopamine coating enhances hydrophilicity on both surface and inner pores.•Polydopamine-coated membranes showed enhanced sucrose rejection up to 90%.