Polymer membranes are critically important in addressing urgent global needs in the 21st century for energy efficient gas separations as well as reliable, sustainable, efficient access to clean energy and clean water. In the gas separation field, polymer membranes are now well established for air separation, hydrogen purification, and, increasingly, natural gas processing. We are working on new membranes based on so-called thermally-rearranged polymers, which have among the highest combinations of gas permeability and selectivity. We are also extending the range of applications where polymer membranes are used for gas separations to applications such as olefin/paraffin separation and bioethanol purification. We have an ongoing interest in exploring fundamentals of gas transport through polymers, including studies of multicomponent transport and exploring the physics underlying long-term changes in polymer transport properties, a process called physical aging.
Polymer membranes have also emerged as a leading technology to desalinate water (e.g., reverse osmosis) and are being explored for energy generation in applications such as reverse electrodialysis and pressure retarded osmosis. Furthermore, efforts are under way to develop additional applications of membranes for water purification, such as forward osmosis and membrane-assisted capacitive deionization. In each of these applications, control of small molecule transport (gases, water and ions) across polymer membranes is critically important for optimizing performance of such membranes. One aspect of our work focuses on the fundamentals of small molecule transport in polymers obeying the solution/diffusion model. Structure/property correlations have been developed for a variety of polymers, including uncharged and charged materials. The role of free volume in governing diffusion of solutes through polymers is explored. Consistent with the so-called upper bound relations in gas separation membrane materials, the existence of a water-salt permeability/selectivity tradeoff relation is observed for polymers being considered for water purification and energy generation applications. Areas where the physics of water and ion transport are both similar to and different from those of gas transport in polymers are highlighted. Additional areas of study include development of desalination and gas separation membranes via melt processing, rather than conventional processing from organic solvents.
Across many platforms of membranes, fouling mitigation is a major challenge to be addressed to achieve the most energy-efficient, cost-effective membrane filtration processes. Previously, many surface modifications and functionalized polymers were reported to prevent fouling. However, most of these techniques and materials are practically difficult to implement in water purification membranes. We have discovered surface treatment methodologies that can be used to prepare high permeability polymeric membranes from all common water purification membrane classes. These surface-modified membranes have persistent tolerance to fouling by emulsified oil, a ubiquitous contaminant in a variety of wastewaters. These membranes were prepared by depositing bio-inspired, self-polymerized, hydrophilic polydopamine.
We are also working on new barrier materials based on oxygen scavenging technology. These materials promise to extend the range of use of polymers for many high barrier packaging applications in areas as diverse as food packaging or flexible electronic displays.