Anti-fouling membranes for organic solvent nanofiltration (OSN) and organic solvent ultrafiltration (OSU): graft modified polybenzimidazole (PBI)

Fouling is one of the major obstacles to successful membrane-based separations. In aqueous systems, fouling is well characterised and fouling reduction techniques are established. However, in organic solvents, membrane fouling is not well understood. In this work we manufactured anti-fouling organic solvent stable polybenzimidazole (PBI) membranes by graft modifying the surface with linear poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG). Graft modification provided an effective method of reducing fouling. However, the anti-fouling properties were dependent on the interactions between the modifying polymer and the solvent. In water, the hydrophilic PEG graft modifiers had favourable modifier-water interactions and provided an anti-fouling steric layer on the surface of the membrane. However, hydrophobic PPG graft modifiers had unfavourable modifier-water interactions and provided no anti-fouling properties. Conversely, in polar organic solvents, graft modification with PPG showed superior modifier-solvent interactions, and superior anti-fouling properties, relative to graft modification with the same molecular weight PEG. In non-polar solvents, both the PEG and PPG modifiers had unfavourable modifier-solvent interactions and provided minimal anti-fouling properties. The anti-fouling properties after graft modification were closely correlated to the hydrodynamic diameter of the grafting polymer in solution, and the selection of an optimal grafting polymer for manufacturing anti-fouling OSN and OSU membranes is significantly solvent dependant.