UCLA

This body of work addresses the ways in which water interacts with materials on a molecular scale and the scalable fabrication of membranes for water purification applications.Threats to water security including global population growth, climate change, environmental pollution, and decades of unsustainable water usage making the generation of new freshwater sources imperative. Seawater desalination along with beneficial purification and reuse of municipal, agricultural, and industrial wastewaters is crucial to achieve water-stress alleviation. These aqueous separations are now almost universally achieved using reverse osmosis (RO) membrane technology, which offers lower energy and cost than thermal distillation technologies; and hence, RO is now widely deployed throughout the world in these applications. Still, there is a lot of interest in improving the performance of RO membrane processes through engineering, and membrane capabilities through materials science. Better materials could improve liquid and solid recovery from complex aqueous and non-aqueous solutions and overcome some of the ongoing problems facing membrane operations. In particular, improved membrane materials can help reduce the cost, extend the lifespan, and broaden the applicability of membrane processes for water recovery. Not only can new materials be better suited for removal of a particular solute and have particular selectivity, but the robustness (chemical and physical) of membranes can be improved to reduce the amount of pre-treatment and membrane passes required to generate potable water. Chapter 1 of this work, addresses one approach to these challenges taken by developing a new membrane fabrication technique that enables the employment of almost any functional material as a membrane active layer. An in-depth discussion of the development of support-layer chemistry to resist compaction and optimization of support layer chemistry for the lift-off of thin films is provided. Beginning with the development of this technique, advances were made in the use of epoxy-based membranes as both a supporting material for thin-film composite membranes and for standalone membranes. A demonstration of how porogens can be used while curing and their effect on the resulting microstructure of membranes is provided along with other factors relevant to the membrane industry such as flux, compaction, and solute removal. Chapter 2 builds on this chemistry through the development of “all-epoxy” membranes. A series of membranes are developed and using similar technology to that discussed in Chapter 1, are optimized to achieve high-performance nanofiltration membranes. A proposal for new, more sustainable membranes is made that utilizes our newly developed techniques and including monomers with reversible bonds.

Conjugated polymers have access to multiple charged states and are interesting in the development of salt-rejecting membranes. Unlike the nanofiltration membranes discussed in Chapter 2 where membrane selectivity is largely governed by the pore-flow model, reverse-osmosis membrane separation mechanisms are based on the solution-diffusion model, where a molecule’s solubility in a dense membrane material in addition to its diffusivity determines the efficiency of its removal. Chelation effects, hydrogen bonding, and charge-charge interactions are all important components to the rejection mechanism of a membrane. There is still a lot of room for building an improved understanding of membrane separation mechanisms. By systematically tuning structure and performance of novel materials, mechanistic relationships between these and a physical understanding of transport through them can be elucidated.Chapter 3 contains studies of active-layer materials, in particular, approaches to their fabrication and the surface properties that affect membrane performance. Polybenzimidazoles and Polyimides are difficult to process polymers, but of great interest due to their chemical tolerance. They produce very dense films and often highly crystalline networks. A number of membranes are made using these materials and some are tested for desalination of waters with different degrees of acidity. Correlations between interfacial surface tension components and rejection of NaCl at 0.20% NaCl and 3.5% NaCl, as well as the potential to predict the performance of particular materials using these correlations, are discussed. Water treatment and energy go hand-in-hand. Membrane treatment requires large amounts of power and energy and the optimization of materials to reduce energy requirements have thermodynamic limits. Chapter 4 discusses the development of inexpensive electrochemical capacitors, utilizing many of the characterization techniques discussed earlier in this work.

In Chapter 5 of this work, a technique to create functionally graded materials is discussed. The method involves the casting of graphene-oxide films onto foils of metals that have different reduction potentials. The films then undergo graded conversion to reduced graphene oxide and the foil may be recovered for continued use. The electrochemical and actuation characteristics of these materials are discussed, but these novel methods of making functional materials are very important to creating scalable, roll-to-roll methods for fabricating materials for novel membranes.