4. Membrane Modification

Hydrophilization by treating the membrane surface with water soluble solvent (acids, alcohols, and mixtures of acids, alcohols, and water) is one of the surface modification techniques. Heating and coating are also used to modify the membrane surface.

In chemical treatment, the polymeric membrane surface is modified through covalent bonding. Among many such approaches, grafting to and grafting from are two distinct surface modification approaches used to modify membranes with responsive polymers. The grafting-to technique is based on the attachment of end-functionalized responsive materials to the membrane surface, whereas grafting-from involves a polymer chain grown from initiator sites on the membrane surface through a surface-initiated polymerization process. The membrane surface is activated by high energy radiation, followed by the grafting of hydrophilic modifiers. In this way, the membrane bulk is not significantly affected, but the membrane surface properties can be improved. LBL is also the most common method to modify the membrane surface. It can be conducted by intercalation of positive- and negative-charged polyelectrolytes on the external surface of a porous support.

Membrane surface modification using responsive nanomaterials is a promising technique to overcome the limitations of existing membranes. Depending upon the mode of application, a wide variety of foreign materials has been incorporated into the dope solution for improving the functional properties of membranes. These materials include metal or metal oxide nanoparticles, zeolites, clay nanoparticles, carbon nanotubes and metal organic frameworks (MOFs). The surface of nanofibers fabricated via electrospinning can also be modified by the methods described above.