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Using a simple process, North Carolina State University chemical engineers have discovered a way to make flexible coating materials more durable and water-resistant, without the use of environmentally harmful solvents.
Coatings enhanced through the process would be a boon for use in wet environments where a nonpermeable barrier must function without fail for prolonged periods, such as on ship hulls and buried or submerged cables. Because the coatings are biocompatible, they also could be used to extend the useful life of surgical implants, or in cosmetics.
Dr. Jan Genzer, assistant professor of chemical engineering, and Dr. Kirill Efimenko, a research associate in Genzer's lab, published their finding in the Dec. 15 issue of Science. Genzer also presented their finding at the American Chemical Society's PacifiChem 2000 science conference in Honolulu on Dec. 15. The research – the first to show that long-lasting superhydrophobic polymer surfaces can be created through mechanically assembled monolayers (MAMs) – is funded by the National Science Foundation and NC State's College of Engineering.
"We discovered, quite by accident, that you can tailor control a flexible material's physical and chemical surface properties, such as water resistance and durability, by increasing its surface area before you chemically attach the layer of molecules that form its final coating," Genzer said.
Stretching the substrate material allows you to fit more of the desired coating molecules – up to a critical point – onto its surface, he explains. Then, when you release the tension and the material resumes its original size, the chemically grafted molecules are squeezed and locked together in a much greater density and with much greater chemical stability than would occur naturally.
"The relative simplicity of it is stunning," Genzer said. "For years, scientists have relied on oxygen plasma treatments, which are quite harsh on the substrate, to attach the molecules without being able to control the density of the anchored molecules. Here's a mechanical way that's cheaper, easier, less harsh and more controllable."
Genzer's method is also better for the environment because it doesn't require the use of solvents that produce harmful fumes or by-products.
He and Efimenko have used polydimethyl siloxane (PDMS) networks – an elastic polymer film widely used in industry and research – as a substrate material in their experiments. Because PDMS is a model material for other polymer films made of crosslinked molecules, Genzer believes the process will work on other elastic materials as well.
In their experiments, the NC State researchers fabricated their material's surface using an array of rigid, semifluorinated units composed mainly of CF2, and one CF3 group. "Aligning molecules perpendicular, or close to perpendicular, to the substrate rather than keeping them laying in the surface plane gave us the tool to control the density of such molecules on the surfaces, and thus achieve such superior surface impermeability," Genzer explained.
To test the durability and impermeability of the experimental coating, Genzer and Ekimenko submerged strips of the MAM material in water for controlled periods of time and subsequently stored them in normal ambient laboratory conditions, and then studied their stability. "To our surprise, the surfaces of MAMs stayed virtually unchanged, even after six months in a dusty and humid atmosphere. The MAMs chemical properties, such as orientation and molecular density, remained the same, and there was very little physical deterioration," Genzer said. In contrast, strips of coating materials made the conventional way usually begin to decay and deteriorate after sitting in water a relatively short time. "Their molecules become disorganized. In some cases, we have observed that surface properties are degraded after barely more than a day," he said.
Despite the initial results, Genzer stresses that more research is needed before the mechanically assembled monolayer process can be put into commercial use. Next, the researchers have to explore the coating's stability and resistance to acids and other extreme environments.
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Editors' note: Genzer and Efimenko's Science abstract follows on page three. For a copy of the full paper after the embargo date, contact Genzer at (919) 515-2069 or jan_genzer@ncsu.edu, or Tim Lucas at (919) 515-3470 or tim_lucas@ncsu.edu. For a copy before Dec. 14, call Science at (202) 326-6440.
Genzer will be at the ACS meeting in Hawaii from Dec. 13 through Dec. 18. For assistance reaching him there, contact Lucas at NC State News Services.
"Creating Long-Lived Superhydrophobic Polymer Surfaces Through Mechnically Assembled Monolayers"
By J. Genzer and K. Efimenko, NC State University
Published Dec. 15, 2000, in Science
Abstract: We show that elastomeric surfaces can be tailored using "mechanically assembled monolayers" (MAMs), structures that are fabricated by combining self-assembly of surface grafting molecules with mechanical manipulation of the grafting points in the underlying elastic surface. The versatility of this surface modification method is demonstrated by fabricating MAMs with semi-fluorinated (SF) molecules. These SF-MAMs have superior nonwetting and barrier properties in that they are "superhydrophobic" and nonpermeable. We also establish that these material characteristics do not deteriorate even after prolonged exposure to water, which usually causes surface reconstruction in conventionally prepared SF self-assembled monolayers.
Journal
Science