Designing Polymeric Nanofiltration Membranes that Exhibit Multi-faceted Properties

The tailored design of modified nanofiltration membranes requires rapid functionalization processes to introduce key functionalities that help 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. Utilizing a post-fabrication functionalization method, the desired membrane structure is first formed by tailoring the polymer chemistry and casting conditions and the addition of a desired functionality is achieved through coupling reactions with reactive solutions and functionally lined pore walls.

In this work, the ability to isolate functional domains utilizes a copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction process which was confirmed through Fourier-transform infrared spectroscopy analysis. Here we developed protocols for synthesizing reactive solutions that achieved 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), which resulted in enrichment in ions in the permeate. This type of controlled ion transport is not possible in singularly functionalized materials, thereby demonstrating the unique transport properties that multi-functional membranes can achieve.