Separations: Plasma modification of polymer membranes

Membranes are often used in ultrafiltration processes such as purification and pharmaceutical processing. They can also be used to enhance the performance of microfluidic separation devices. However, because these membranes are often made from hydrophobic thermoplastics, they can be difficult to use in aqueous separations. The membranes can be chemically treated with wetting agents to make them more hydrophilic, but these treatments tend to be non-uniform and often lose their hydrophilic character over time.

Plasmas offer an alternative method for hydrophilic modification of these membranes. Our group has investigated this type of modification process by treating membranes in water vapor plasmas as well as with other gas chemistries such as NH3/O2 plasmas and with plasma graft copolymerization. The membranes are held in a barrel-type plasma reactor and are oriented perpendicular to the gas flow to maximize the penetration of active species into the membrane. Membrane materials including polysulfone (PSf), polyethersulfone (PES), and polyethylene (PE) have been studied using contact angle goniometry to measure water contact angles on the membranes as well as surface characterization methods to analyze the morphology and composition of the treated membranes. Our data suggests that the hydrophilic modification observed in these materials is due to the incorporation of OH, CO, and COx functional groups during plasma treatment. The permanency of these treatments varies with material. For example, PSf and PES membranes exhibit permanent retention of their hydrophilic character, whereas structural rearrangement of PE membranes after modification tends to cause some recovery of the original hydrophobicity.