John Hoffman

Chemical and Biomolecular Engineering

Faculty Advisor: William Phillip

Spatially-Controlled Functionalization of Nanofiltration Membranes

Membrane separations provide the most energy efficient means for producing clean water. As of 2017, an estimated 90 million m3 of water was produced via desalination daily. Of this capacity, 70% is produced by thermal processes through distillation, which requires large amounts of energy.  However, because it is approaching the thermodynamic limit on separation efficiency,desalination by reverse osmosis (RO) membranes is rapidly displacing thermal methods. Despite the efficiency of RO, it remains energy intensive, which drives continued interest in wastewater reuse. In this regard, nanofiltration membranes (NF), which possess pores with diameters on the order of 1-8 nm have been developed. However, this porous nature results in a permeability/selectivity tradeoff, whereby balancing the performance of NF membranes to maximize selectivity with the highest possible permeability is needed. The functionality lining the pore walls of NF membranes offers a means through which to increase selectivity without sacrificing permeability.

A rapid and controllable reaction process that operates on time-scales relevant to modern roll-to-roll manufacturing is needed to functionalize the pore walls of NF membranes. “Click” reaction mechanisms are well-suited to address this need for precise, high throughput membrane functionalization techniques. This class of reactions is characterized by rapid kinetics as well as high conversions and selective yields. As such, click reaction mechanisms enable a vast number of functionalities to be incorporated with high fidelity on time scales consistent with membrane manufacturing rates. Copper (I)-catalyzed azide-alkyne cycloaddition (CuAAC) reactions are the premier example of a “click” reaction. Previous work within our group has shown the utility of this reaction for controlling the functionalization of nanofiltration membranes. Controlling the distribution of reactive sites across the membrane surface through inkjet printing applications result in domains that are larger than desired due to the physical limitations of the deposition process (i.e. droplet size, printer spacing, etc.). In order to better pattern the membrane surface, thiol-ene reactions, which are UV initiated, can be controlled and patterned through the use of photomasks. This offers an appealing opportunity to reduce the length scale of these patterns to better match the domain space in which interactions between the membrane and solutes exist. This work seeks to further develop the potential of thiol-ene based post-fabrication functionalization processes by developing a framework for understanding the factors that affect the ability to execute this reaction in a controlled manner and quantifying the resulting effects on the performance of patterned membranes.