RALEIGH, N.C. North Carolina State University professor Jan Genzer has hit on a method for creating the ultimate nonstick surface.
Potential applications include covering adjacent disk-drive components to prevent scratching, improving the biocompatibility of medical implants by eliminating interactions with surrounding cells and coating airplanes with a water repellent that would automatically de-ice wings.
The basic technique is simple: a substrate is stretched slightly and then coated with a polymer. When the tension is relieved, the substrate pulls the polymer molecules so close together that no other material is able to bond to the polymer molecules. Devices coated with the friction-free polymers can bang against each other without scratching and cannot become coated with anything even liquids.
"These researchers have discovered a clever way of packing molecules more tightly than nature does on its own," said Andrew Lovinger, who is program manager at the National Science Foundation-funded project.
Genzer's technique coats a shrinkable polymer with a nonstick material, such as Teflon. When the polymer shrinks, it squeezes the already smooth molecules into a monolayer so dense that even water molecules can't penetrate it.
"By increasing a material's surface area before you chemically attach the layer of molecules that forms its final coating, you can tailor the material's physical and chemical surface properties, such as water resistance and durability," Genzer said. Genzer and his postdoctorate research associate at North Carolina State University, Kirill Efimenko, stretched a substrate material before applying the nonstick coating, thereby depositing more of the desired molecules than on a nonstretched substrate.
When the tension was released, the chemically grafted molecules were squeezed together into a "locked" configuration that excluded all previous irregularities on its surface. With just the right amount of stretching too much spoiled the effect all the extra room between molecules that previously made even nonstick surfaces irregular at the molecular level was squeezed out.
The resulting surface not only had much greater density and "smoothness," but also proved to be more chemically inert than natural substances. Without any of the ordinary irregularities in its surface, even down to the atomic level, nothing could attach itself to the material, even water molecules or solvents for the coating material.
"We were stunned by the results, which we discovered accidentally. Ordinarily the only way to improve surface density is to rely on environmentally dirty oxygen plasma treatments, which are also very hard on the substrate. Our method is much cleaner and cheaper, plus it's a much more controllable process with very uniform densities," said Genzer.
The researchers chose to perform their experiment on polydimethyl siloxane (PDMS) an elastic polymer because PDMS is made of cross-linked molecules that are typical of elastic industrial materials. They consider their success with PDMS as strong evidence that the shrinking process will work with other widely used industrial polymers.
The PDMS networks were fabricated using an array of rigid, semifluorinated units aligned perpendicular to the substrate. The perpendicularly aligned molecules were able to slightly adjust their orientation as the elastic shrank, resulting in maximally dense surfaces that proved completely impermeable.
The team spent six months testing the new material to see if environmental exposure would undo the miraculously smooth and impermeable surfaces. With various damp, wet, dusty, hot and cold environments, including six months of submersion, the PDMS networks remained inert and unaffected by their environment.
The orientation of molecules, molecular density and physical integrity remained virtually unchanged by the passage of time, changes in temperature or humidity, barometric pressure or exposure to normal wear and tear. "The molecules on the surface of most self-assembled monolayers become disorganized after prolonged exposure to humidity, but we observed very little deterioration in our mechanically assembled monolayers, even after submersion for six months," said Genzer.
Genzer's next step will be to test the coating's long-term stability and resistance to industrial acids and extreme environmental conditions. He predicts that it will be five years or more before commercial formulas for the process can be put into mass production.