Functionalization is an excellent way of adding new properties and capabilities to well-established and understood membranes. For example, sulfonation of a conventional polyethersulfone membrane will improve its hydrophilicity and inhibit protein binding. If the idea of surface functionalization is extended to the grafting of functional polymer chains, anti-fouling characteristics, surface charge, and permeability control can be enhanced by orders of magnitude. When this idea is applied to membrane adsorbers, very high sorption capacity can be achieved while minimizing the internal mass transfer resistance encountered in resin materials.
Professor Dibakar Bhattacharyya is a pioneer in the field of functionalized membranes. The work began with a challenge: Is it possible to create a sorbent material for heavy metals with a capacity of 1 g metal per g material? Building from his experience with immobilized enzymes, Prof. Bhattacharyya looked to biopolymers for a suitable candidate for the selective adsorption of lead and other heavy metals over calcium. Two candidates emerged – polyglutamic and polyaspartic acid – polyamino acids with carboxylate functionality and a natural selectivity for transition metals based ion acidity.
Since this early beginning, we have moved to fully synthetic polymer grafts based on polystyrene. Styrene is a highly versatile monomer, both for methods of polymerization and in diversity of side functional groups. We have focused on cationic polymerization of styrene and functional styrenes to add charged groups for catalysis and protein separation. We have also designed custom grafts based on Protein A chemistry for selective adsorption of antibodies. These membranes have also been scaled up for large scale production. Field trials have shown that functionalized membranes can give improved flux with no change in permeate quality for greater than 1 year of operation. These applications, as well as the future of functionalized membranes will be discussed.