Organic solvent filtration by brush membranes: Permeability, selectivity and fouling correlate with degree of SET-LRP grafting

A novel surface modification technique for polyimide membranes, their characterization and organic solvent filtration performance is described. The process involves treating crosslinked polyimide support membranes with surface initiators followed by Single Electron Transfer - Living Radical Polymerization (SET-LRP) with three alkyl methacrylates. Proof of grafting is established using ATR-FTIR, XPS and FESEM. The degree of grafting (DG) and brush height were a function of monomer type and polymerization time and provide a unifying basis for correlating selectivity and permeability. The permeability of these composite membranes, evaluated with 6 organic solvents (alcohols), correlated with solvent properties and can be tailored with DG. Filtration performance was assessed by measuring the selectivity and fouling of the organic dye Rose Bengal (RB) in ethanol. The polarized wall concentration increased with DG, whereas the RB sieving coefficient decreased with DG. Flux decline during RB filtration was best described by pore blocking and by heterogeneous and Langmuir sorption models. These studies provide a basis for designing polymeric membranes comprising branched brush selective layers with defined microstructure not possible with the widely popular phase inversion process.