Principal Investigator Michael Rubner
Membrane fouling, which can simplistically be described as the clogging of a membrane due to the adsorption of feed components, is one of the major obstacles faced by the membrane industry. It drives up energy consumption as well as cleaning and membrane replacement, and is especially severe in processes where the feed has high concentrations of biomolecules, such as in wastewater treatment, and in food and pharmaceutical industries. The most common way to prevent fouling is to graft a hydrophilic polymer from the membrane surface, but this is often an expensive and poorly controlled process. My project aims to develop improved membranes that resist fouling making use of the self-organization of copolymers, specifically comb copolymers with a hydrophobic backbone and hydrophilic side-chains.
Polyacrylonitrile-graft-poly(ethylene oxide) (PAN-g-PEO) is such a polymer: It has a backbone of PAN, a glassy polymer used in membrane industry, and side-chains of PEO, a hydrophilic polymer well known for its resistance to adsorption. When this copolymer is added to the casting solution during the manufacture of porous ultrafiltration (UF) membranes, the side-chains are driven to the polymer/water interface due to their hydrophilic nature. The polymer is pinned down by the PAN backbone, creating a brush of PEO chains on the membrane surface as well as lining all the pores. Membranes prepared using this method were found to resist irreversible fouling completely to a range of foulants, including protein, humic acid and alginate solutions, and oil-well produced water: The membrane can be cleaned simply by water, potentially decreasing cleaning costs and increasing membrane life. These membranes are very promising for waste-water treatment and membrane bioreactors.
Another application, this time used to prepare membranes with selectivity in the small molecule scale, relies on the microphase separation of PAN-g-PEO. In this process, a porous support membrane is coated with a thin (0.2-2 micron) film of PAN-g-PEO. The copolymer microphase separates into a bicontinuous network of each phase. The PEO phase, which is hydrophilic, allows the passage of water and molecules smaller than its diameter, acting as PEO-lined "nanochannels". The membranes produced have size-based selectivity in the nanofiltration scale, and can be used to fractionate small-molecule dyes by size. They also have a high pure water permeability, and complete resistance to irreversible fouling. Furthermore, the size cut-off of these membranes is responsive to a range of factors that affect the conformation of PEO chains, such as temperature, pressure, and ionic strength.
Amphiphilic comb copolymers impart fouling resistance to polymer membranes, an increasingly important part of the world's water supply sustainability. Research is underway to expand the application of the morphology and properties of these combs to create membranes with regenerative fouling-resistant surfaces, nanosieving membranes, and improved desalination membranes.