A plastic as solid as steel
A new composite plastic built layer by layer has been created by engineers at the University of Michigan. This plastic is as strong as steel. It has been built the same way as mother-of-pearl, and shows similar strength. Interestingly, this 300-layer plastic has been built with 'strong' nanosheets of clay and a 'fragile' polymer called polyvinyl alcohol (PVA), commonly used in paints and glue, which acts as 'Velcro' to envelop the nanoparticles. This new plastic could soon be used to design light but strong armors for soldiers or police officers. The researchers also think this material could be used in biomedical sensors and unmanned aircraft.
You can see above an illustration showing the various atoms composing this ultrastrong plastic. "Energy-optimized geometry of bonding between PVA [Polyvinyl alcohol] and MTM [Na+-Montmorillonite] via hydrogen bonding obtained by computer calculations using AM1 semi-empirical algorithm. Atoms: Al - purple, O – red, H- light grey, Si - dark grey, C – green. The distance between the OH groups on PVA matches very well that between oxygen atoms in the coordination sphere of silicon atoms on the surface of clay platelets. No defects or substitution atoms were included in the consideration of the hydrogen bonding." (Credit: University of Michigan)
This interdisciplinary project has been led by engineering professor Nicholas Kotov and the members of his lab. He also was helped by various collaborators such as mechanical engineering professor Ellen Arruda or aerospace engineering professor Anthony Waas.
Apparently, the researchers have solved an old problem. While nanostructures are very strong, it's very difficult to carry this strength at the human scale. "When you tried to build something you can hold in your arms, scientists had difficulties transferring the strength of individual nanosheets or nanotubes to the entire material," Kotov said. "We've demonstrated that one can achieve almost ideal transfer of stress between nanosheets and a polymer matrix."
So how did the team build this composite plastic? They used "a machine they developed that builds materials one nanoscale layer after another. The robotic machine consists of an arm that hovers over a wheel of vials of different liquids. In this case, the arm held a piece of glass about the size of a stick of gum on which it built the new material. The arm dipped the glass into the glue-like polymer solution and then into a liquid that was a dispersion of clay nanosheets. After those layers dried, the process repeated. It took 300 layers of each the glue-like polymer and the clay nanosheets to create a piece of this material as thick as a piece of plastic wrap."
And why the combination of PVA and MTM was so successful? "The glue-like polymer used in this experiment, which is polyvinyl alcohol, was as important as the layer-by-layer assembly process. The structure of the 'nanoglue' and the clay nanosheets allowed the layers to form cooperative hydrogen bonds, which gives rise to what Kotov called 'the Velcro effect.' Such bonds, if broken, can reform easily in a new place. The Velcro effect is one reason the material is so strong. Another is the arrangement of the nanosheets. They're stacked like bricks, in an alternating pattern.
If you are interested by this 'plastic steel,' this research work has been published in Science under the name "Ultrastrong and Stiff Layered Polymer Nanocomposites" (Volume 318, Issue 5847, Pages 80-83, October 5, 2007). Here are two links to the abstract and to some supporting online material (PDF format, 12 pages, 1.17 MB), from which the above picture has been extracted.
Sources: University of Michigan news release, October 4, 2007; and various websites
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