Surface modification of dense membranes using radical graft polymerization enhanced by monomer filtration

Surface graft polymerization is a promising way to modify membranes for improved performance. Redox-initiated graft polymerization of vinyl monomers is a facile and inexpensive method carried out at room temperature in aqueous media; however, its use is often limited by slow kinetics, low surface specificity, and excessive consumption of chemicals on undesired homopolymerization. It is shown that in the case of RO or NF membranes these drawbacks may be eliminated by utilizing the selectivity of the membranes toward monomers and carrying out the polymerization while applying pressure, i.e., under filtration conditions. Concentration polarization that ensues raises the concentration of reagents near the membrane surface and thereby drastically increases the rate of reaction and preferentially directs it towards surface grafting. Grafting experiments using 2-hydroxyethyl methacrylate and other monomers and characterization of modified membranes using permeability measurements, ATR-FTIR, AFM, XPS, and contact angle demonstrate that the required monomer concentrations can be drastically reduced, particularly when a small fraction of a cross-linker is added. As an additional benefit, this approach enables broadening the spectrum of utilizable monomers to sparingly soluble hydrophobic, charged, and macro-monomers, as was demonstrated using sparingly soluble ethyl methacrylate and 2-ethoxyethyl methacrylate and other monomers. Even though the kinetics of the process is substantially complicated by evolution and concentration polarization of oligomeric and polymeric species, especially in the presence of a cross-linker, it is well offset by the benefits of higher rate, specificity, and reduced monomer consumption.