| TECHNOLOGY |
Picture this: you've just been involved in a fender bender. You grab your heat gun, apply it to the dent, and the dent disappears. The fender has "remembered" its original shape. This futuristic fender isn't metal, of course. It is made from a shape-memory polymer like those recently constructed by researchers. The research team's goal was to develop a polymer system that was easier to shape and offered more applications than the shape-memory materials already in use. The latter include the nickel-titanium alloy Nitinol, used, for example, in making orthodontic wires that self-adjust, flexible eyeglass frames, and pliant guidewires and tools for "bloodless" surgery. Nitinol and other shape-memory alloys are able to undergo a so-called martensitic phase transformation that enables them to switch from a "temporary" shape to a "parent" shape at temperatures above a transition temperature. Below that temperature, the alloy can be bent into various shapes. Holding a sample in position in a particular parent shape while heating it to a high temperature programs the alloy to remember the parent shape. Upon cooling, the alloy adopts its temporary shape, but when heated again above the transition temperature the alloy automatically reverts to its parent shape. There's plenty of room for improvements in such materials, notes lead author. "Programming a metal alloy is not only a time-consuming procedure, it also involves heat treatment at temperatures of several hundred degrees Celsius," he says. "Another drawback for shape-memory alloys is that the maximum deformation they can undergo is only about 8%. Furthermore, they are much more expensive than polymers." Authors began prospecting polymers for more versatile materials They identified two monomeric components that, when combined, generate a family of polymers that display "excellent" shape-memory properties. Moreover, the polymers are programmed into shape in seconds at about 70 ºC. And they can withstand deformations of "several hundred percent." One component, oligo(e-caprolactone) dimethacrylate, furnishes the crystallizable "switching" segment that determines both the temporary and permanent shape of the polymer. By varying the amount of the comonomer, n-butyl acrylate, in the polymer network, the cross-link density can be adjusted. In this way, the mechanical strength and transition temperature of the polymers can be tailored over a wide range. "Sometimes it doesn't make sense to have shape transition at 32 ºC. You might want to have it at 42 ºC instead." "With a polymer, you can easily adjust the transition temperature. With a metal, that's difficult. Now, with one set of monomers, you can have a whole set of shape-memory materials." |
| UPDATE | 02.01 |
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