Polymer membranes for gas pair separations exhibit an inverse relationship between gas selectivity and gas flux. Maximizing both would be a significant advance. The idea of combining the selective separation properties of porous inorganic materials with the processability of polymers has resulted in several advances. Nevertheless, it appears that the performance of such membranes has reached a plateau, and a new approach will be needed to achieve a revolutionary breakthrough in membrane-based gas separations. The PIs have discovered that membranes constructed from otherwise immiscible polymers can be engineered at the nanoscale through the addition of small amounts of porous nanoparticles.
The major limitation with current mixed matrix membranes (MMMs) for gas separations is their low gas flux, primarily due to the membrane thickness (>several tens of micrometers) that is required to accommodate the porous additives. Colloidal ZIFs with particle diameters of <60 nm will enable the selective polymer layer to be submicrometer in thickness, and the matrix-droplet geometry will increase the interfacial surface area by 50 to 100X compared to a layer-by-layer morphology, greatly increasing flux while maintaining superior permselectivity. In order to fully utilize this novel membrane architecture, a thorough understanding of the thermodynamic and kinetic factors that control the structure will also be researched.
Combining the selective separation properties of inorganic molecular sieves with the processability of polymers to form mixed-matrix membranes (MMMs) has resulted in several advances including the incorporation of porous additives such as metal organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs). The organic-inorganic hybrid nature of these additives has afforded improved interfacial contact with the polymer matrix, enabling very high loadings in the MMMs.
The PIs propose a unique membrane architecture comprising blends of otherwise immiscible polymers that can be engineered at the nanoscale through the addition of small amounts of ZIFs. By choosing component materials with appropriate interfacial/surface tensions, the ZIF nanoparticles localize at the interface between the polymers. This has the advantage of compatibilizing high performance immiscible polymers thus greatly expanding the number of polymer combinations that can be utilized.
The proposed research will develop membranes comprising thin, continuous ribbons of a highly selective polymer embedded in a discontinuous matrix of a second, highly permeable polymer, somewhat akin to the marbling in USDA Prime Beef. Such architectures will significantly improve the performance of membranes, especially by increasing flux and selectivity at lower additive loadings, thus reducing cost. This project on energy and the environment includes numerous tasks that will lead to the integration of research and multilevel education in the area of membrane science and novel nanomaterials. The replacement of energy intensive separations with membranes could result in economic savings. This level of structural control could also be potentially useful for fuel cell applications and other separations. Additionally, the strong educational component coinciding with the research activities will engage students at both the graduate and undergraduate levels, as well as students from underrepresented groups and women. The skills acquired by students during this project will enhance their preparation for careers in membrane engineering, nanotechnology, energy, and materials science. We are also committed to high school student research experiences and we anticipate that this project will also impact the community at large by educating our high school teachers and students.
SIGNATURE
Name: Rosemarie D. Wesson Title: Program Director Program: Chemical and Biological Separations
DATE: April 2014
