Designing Multi-Functional Copolymer Nanofiltration Membranes

This dissertation addresses the utility of using rapid functionalization processes for the tailored design of modified nanofiltration membranes. The key need for introducing additional functionalities is to overcome the permeability / selectivity tradeoff seen in size-selective membranes. Many techniques have demonstrated the positive impact of introducing additional interactions between membranes and solutes, such as charged moieties, which can increase the rejection of salt ions.

Beyond singularly functionalized membranes, novel transport properties emerge when multiple functionalities are integrated within a membrane system. This can be due to domains interacting with each other at their interfacial junctions, as well as when isolated domains work independently but in a coordinated manner during operating conditions. This dissertation takes the approach of post-fabrication functionalization as a viable method to develop multi-functional membranes, by investigating the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. First, the desired membrane structure is readily formed by tailoring the polymer chemistry and casting conditions, resulting in membranes with azide moieties lining the pore wall. Second, the addition of a desired functionality is achieved through coupling reactions with reactive solutions of functional alkynes with the azide lined pore walls.

The ability to isolate functional domains using this reaction process was confirmed through Fourier-transform infrared spectroscopy analysis, which developed protocols for synthesizing reactive solutions that achieve high reactive rates compared to the diffusive rate of the solution as it transports through the membrane. Multi-functional membranes with surface anti-fouling chemistry and underlying charged chemistry were made which exhibited equivalent ion rejection performance compared to fully charged membranes, but with a much lower propensity for fouling.

The CuAAC reaction was also used to form surface patterned multi-functional membranes, in which asymmetric coverage of positive and negative charge resulted in unique transport phenomena of asymmetric salts, such as magnesium chloride (MgCl₂) and potassium sulfate (K₂SO₄). The utility of the thiol-yne coupling reaction for surface patterning of multi-functional membranes was also investigated by introducing specific domains for the targeted adsorption of heavy metals.

These results demonstrate the unique abilities and transport properties that multi-functional membranes can achieve. The knowledge for creating multi-functional membranes can improve many separation processes.