Pore morphology control and hydrophilicity of polyacrylonitrile ultrafiltration membranes

The effects of different solvents (dimethyl formamide: DMF and dimethylsulfoxide: DMSO) on the solubility of polyacrylonitrile (PAN) were investigated by the phase diagrams of H₂O/DMF/PAN and H₂O/DMSO/PAN ternary systems through cloud-point titration method at low polymer concentration. The influences of polymer concentrations and temperatures on the morphologies of PAN ultrafiltration membranes were elucidated. The morphologies of fabricated UF membranes were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM), and the basic performance of ultrafiltration including pure water flux and rejection of BSA were explored. At 25°C, the pure water flux of ultrafiltration membranes at the lower PAN content (16 wt % PAN in 84 wt % DMSO) reached 213.8 L/m/bar and the rejection of BSA was 100%. Interestingly, the water flux of UF membranes dramatically decreased to 20.6 L/m/bar (20 wt %) and 2.9 L/m/bar (24 wt %) when increasing PAN concentrations in DMSO. On the other hand, the hydrophilicity of membranes can be enhanced by increasing coagulation temperatures and polymer concentrations which were characterized by static contact angle, fitting well with the variation tendency of roughness. Although there are many works concerning on the effects of phase inversion conditions on the performance of PAN UF membranes, to our best knowledge, there is seldom works focusing on investigating the membrane hydrophilicity trend by adjusting phase inversion conditions. To disclose the reason of the enhanced hydrophilicity, the water and glycol contact angles of various membranes were measured and the surface tensions were presented. The results illustrated that the enhanced hydrophilicity of PAN UF membranes fabricated at higher temperatures or higher polymer concentrations was due to the higher polarity on membrane surface. Since the vast majority of ultrafiltration membranes in labs and in industrial scale have been fabricated by immersion phase inversion method, this work can provide a guidance to obtain hydrophilic PAN UF membranes by adjusting the process of phase inversion.