Controlling Polydopamine Deposition Morphology on the Surface of Poly(vinylidene fluoride) Microfiltration Membranes for Improved Oil-Water Filtration Performance

Herein, we studied the effect of adding into water-dopamine mixture a small amount of the fourth generation of bis-MPA based BoltornTM hyperbranched polyol polyester (H40) or inorganic nanolayered compound MXene on the development of polydopamine (PDA) coating morphology on the surface of polyvinylidene fluoride (PVDF) microfiltration membranes targeting to further enhance their oil fouling resistance and oil-in-water (O/W) filtration performance. Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray (EDX), Atomic Force Microscopy (AFM), and Water Contact Angle (WCA) techniques were used for characterization of the modified surfaces. The pure water and (O/W) tight emulsion filtration testing was conducted using a custom-made dead-end filtration setup. It was shown that in the case of dopamine only treatment, the formed on the surface PDA coating exhibited largely an orthogonal, planar development which tended to envelope not only the membrane surface irregularities but also the pores thus greatly reducing their size and compromising the permeate flux despite significantly improved surface hydrophilicity. Adding either MXene or H40 into water-dopamine mixture rendered a beneficial effect on the membrane oil fouling resistance and permeation/filtration characteristics. The most spectacular, a hitherto not reported effect was observed after adding into water-dopamine mixture the H40 polymer. In contrast to the planar PDA deposition, the coating morphology in this case consisted of fused together tiny PDA clumps. The ‘clumpy’ PDA coating made less pore blockage and the membranes demonstrated many times greater flux either for pure water or emulsion permeate than after using dopamine only or dopamine with MXene treatments while continued to exhibit oleophobic behavior and high resistance to oil fouling over multiple filtration cycles. The potential nucleating role of this hydroxylated HBP in the development of the PDA clumps is discussed.