Composite membranes consist of a selective skin on a highly permeable support. The minimal thickness of the skin layer affords relatively high flux along with selectivity, whereas the support provides mechanical strength. This dissertation presents methods for improving the function of composite membranes prepared by either polymerization from a porous substrate or alternating polyelectrolyte deposition on ultrafiltration membranes. Specifically, this work aims to create gas-separation membranes that selectively remove CO2 from H2 streams and nanofiltration membranes that resist biofouling. Steam reforming of hydrocarbons followed by the water gas shift reaction yields H2 streams that contain 20 % CO2 as a byproduct. Ideally, in membrane-based purification of these streams, the CO2 should preferentially pass through the membrane so the H2 remains at high pressure. Because H2 is smaller than CO2, membranes that selectively pass CO2 must have a high CO2/H2 solubility selectivity and minimal diffusive resistance. Amorphous poly(ethylene glycol) (PEG) possesses these properties, but crystallization of PEG chains dramatically decreases flux and eliminates selectivity. One goal of this research is to create ultrathin, PEG-containing membrane skins in which PEG chains do not crystallize. Surface-initiated atom transfer radical polymerization of PEG-methyl ether methacrylate (PEGMEMA) monomers produces thin (<200 nm) membrane skins on porous substrates, but crystallization of long PEG side chains (22-23 ethylene oxide units) restricts both flux and selectivity. However, copolymerization of monomers containing 8-9 and 22-23 ethylene glycol units yields membranes that maintain a CO2/H2 selectivity of 13 or more, which is equal to the highest reported room-temperature CO2/H2 selectivities. The short PEG chains in these films frustrate crystallization, and the high PEG content in the larger monomers leads to high selectivity. Formation of nanoparticles in membrane skins also inhibits room-temperature PEG crystallization. The incorporation of tetraisopropoxytitanate into a poly(PEGMEMA) film and subsequent condensation to TiO2 nanoparticles yields a nanocomposite membrane that maintains a CO2/H2 selectivity of 6 over time. A small amount of crystallization in these films likely decreases selectivity from ∼ 13 to 6. Unfortunately, the use of higher nanoparticle concentrations to better frustrate crystallization leads to defect formation and elimination of selectivity. Membranes employed in aqueous environments commonly exhibit a decrease in permeability over time due to fouling. Biofouling, the formation of a microbial foulant layer, is particularly intrusive because even a few cells can multiply to create a film that reduces permeability. One goal of this research is to create skin layers that both kill bacteria and provide useful ion-transport selectivities. Alternating layer-by-layer deposition of poly(styrene sulfonate) and polycations containing antibacterial guanidine groups generates a surface that resists biofouling. Reflectance FTIR spectra confirm the layer-by-layer growth of these polyelectrolyte films on Al-coated Si wafers, and filtration of bacterial solutions show that multilayer films can be 100% bactericidal at short times. However, preliminary dead-end filtration studies reveal MgCl2 and NaCl rejections <50%. Future work will examine the permeability of these membranes in cross-flow filtration and investigate methods to improve ion-transport selectivity.
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