The "ultrapermeable, reverse-selective, nanocomposite membranes" are described in a paper published in the April 19 issue of the journal Science. According to the authors, the membranes should be useful in environmental remediation, seawater desalinization, biological purification and other molecular separations - including gas and petroleum production.
At NC State, the research team included Drs. Timothy Merkel and Benny Freeman, who with Spontak conducted this work in the Department of Chemical Engineering. Merkel is now at the Research Triangle Institute and Freeman has since joined the University of Texas at Austin.
The membranes are called reverse-selective because, contrary to expectations, they permit the preferential passage of larger molecules - such as butane - relative to small molecules, such as methane.
"In conventional membranes, increasing permeability invariably leads to decreased selectivity," said Spontak. "To use an analogy, if you make the holes in a net big enough, ping-pong balls, tennis balls, and basketballs will all make it through. With our membranes, we can achieve both high permeability and reverse selectivity - in other words, our net preferentially lets basketballs get through."
The research team achieved their results by embedding very fine silica particles into high-free-volume, glassy polymer membranes. As the researchers discovered, the resulting membranes behave unlike similar membranes embedded with metal oxides, carbon black or other nanoscale particles. Instead of the reduced permeability typical of "filled" membranes, the chemical engineers found both dramatically increased permeability and enhanced selectivity.
"The fact that both permeability and vapor selectivity increase when we add fumed silica to the polymer," said Merkel, "indicates that these particles modify transport properties without introducing gross defects or selectivity-destroying gaps into the membrane."
The research should benefit industries like natural-gas suppliers and petroleum processors now struggling with energy-intensive and expensive methods of separating gases.
For examples of reverse-selective membrane applications, the authors cite "the removal of higher hydrocarbons from methane in the purification of natural gas, organic monomer separation from nitrogen in the production of polyolefins, and hydrocarbon removal from hydrogen in refineries."
The NC State research team worked with Zhenjie He and Dr. Ingo Pinnau, both of Membrane Technology and Research in Menlo Park, Calif., and Drs. Pavla Meakin and Anita Hill at the Commonwealth Scientific and Industrial Research Organization in Clayton, Australia.
Editor's note: Included below is the abstract of the paper published in
Science. To see this and related articles, visit www.sciencemag.org. "Ultrapermeable, Reverse-Selective Nanocomposite Membranes"
Authors: R.J. Spontak, North Carolina State University; T.C. Merkel, Research Triangle Institute; B.D. Freeman, University of Texas at Austin; Z. He, I. Pinnau, Membrane Technology and Research; P. Meakin, A.J. Hill, Commonwealth Scientific and Industrial Research Organization.
Published: April 19, 2002, in Science
Abstract: Polymer nanocomposites continue to receive tremendous attention for application in areas such as microelectronics, organic batteries, optics, and catalysis. We have discovered that physical dispersion of nonporous, nanoscale, fumed silica particles in glassy amorphous poly (4-methyl-2-pentyne) simultaneously and surprisingly enhances both membrane permeability and selectivity for large organic molecules over small permanent gases. These highly unusual property enhancements, in contrast to results obtained in conventional filled polymer systems, reflect fumed silica-induced disruption of polymer chain packing and an accompanying subtle increase in the size of free volume elements through which molecular transport occurs, as discerned by positron annihilation lifetime spectroscopy. Such nanoscale hybridization represents an innovative means to tune the separation properties of glassy polymeric media through systematic manipulation of molecular packing.
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Science