![]() This issue... Sandia Sells Seashells This issue... Sandia Sells Seashells |
Sandia Sells Seashells:New Tougher Transparent CoatingsBy Rosalind Schrempf The quest to create a tough coating that could one day be used to make scratchproof optical lenses or unbreakable windshields has now been realized. Using the resilient seashell as their model, researchers Jeffrey Brinker and Alan Sellinger at Sandia National Laboratories have developed a one-step process to create a layered coating that mimics the durable seashell and is as much as seven times harder than equivalent unlayered coatings. Mollusks produce their tough shells by alternating layers of rigid calcium carbonate (basically chalk) and a natural rubbery polymer. If the calcium layer cracks, the adjacent polymer layer quickly blocks the crack from spreading, and the shell maintains its strength. Reproducing a similar laminated structure in the laboratory previously required layering alternating thin films of calcium and polymer, one at a time. Nature does it in one step, and the researchers set out to do the same. Working with students at the University of New Mexico, they prepared a solution containing inorganic compounds such as silicon dioxide (sand), organic compounds (monomers, crosslinkers, initiators), andthe key ingredientself-organizing detergent molecules that are both hydrophilic (water-loving) and hydrophobic (water-hating). In water, these molecules spontaneously form spherical molecular groups called micelles, which organize themselves so the hydrophilic part of the detergent is in contact with water while the hydrophobic part is shielded in the micelles' interior. When washing dishes with a detergent, for example, this arrangement allows oils to be quickly adsorbed in the micelles' hydrophobic interiors and rinsed away. [graphic, 182 Kb] The researchers dipped a silicon wafer into their solution for several seconds, then removed it. When the excess solution had evaporated and the silicon dioxide and organic polymer had bonded, the wafer retained a transparent, tough coating of alternating hard and soft layerslike a seashell. The solution's organic compounds had been captured in the micelles' hydrophobic interior while the silicon dioxide molecules had attached themselves to the micelles' surface. As the solution evaporated, the micelles formed layered (lamellar) liquid crystals composed of alternating layers of organic and inorganic molecules, forming the layers all at once. Hundreds or thousands of layers could potentially be formed at one time, depending on the solution concentration and coating rate. Such a strong transparent material has many potential applications such as scratch-resistent coatings for optical lenses and automotive finishes. This research was supported by the Office of Basic Energy Sciences/Materials Sciences and the University of New Mexico. Contact: Jeff Brinker, (505) 272-7627, cjbrink@sandia.gov. For more information, see the article in Nature, July 16, 1998. |
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www.pnl.gov/energyscience/ |