By mimicking the brick-and-mortar molecular structure found in seashells, researchers have created a new type of composite plastic that’s as strong as steel, but lighter and transparent. It could be used in microelectromechanical devices, microfluidics and biomedical sensors.
By mimicking the brick-and-mortar molecular structure found in seashells, researchers have created a new type of composite plastic that’s as strong as steel, but lighter and transparent. It’s made of layers of clay nanosheets and a water-soluble polymer that shares chemistry with white glue. Nicholas Kotov, an engineering professor at the University of Michigan (Ann Arbor) who developed the material, says it could be used in microelectromechanical devices, microfluidics and biomedical sensors.
Kotov and his colleagues solved a problem that has confounded engineers for decades: Individual nano-size building blocks, such as nanotubes, nanosheets and nanorods, are ultra strong. But, larger materials made out of bonded nano-size building blocks are comparatively weak.
“When you try to build something you can hold in your arms, scientists have difficulties transferring the strength of individual nanosheets or nanotubes to the entire material,” says Kotov. “We’ve demonstrated that one can achieve almost ideal transfer of stress between nanosheets and a polymer matrix.”
The researchers created this new composite plastic with a machine they developed that builds materials one nanoscale layer after another. The robotic device consists of an arm that hovers over a wheel of vials of different liquids. It holds a piece of glass about the size of a stick of gum on which it builds the new material.
The arm dips the glass into the glue-like polymer solution and then into a liquid that is a dispersion of clay nanosheets. After those layers dry, the process is repeated.
It takes 300 layers of the glue-like polymer and the clay nanosheets to create a piece of material as thick as a piece of plastic wrap. Mother of pearl, the iridescent lining of mussel and oyster shells, is built layer-by-layer like this. It’s one of the toughest natural mineral-based materials.
According to Kotov, the glue-like polymer used in the experiment, polyvinyl alcohol, is as important as the layer-by-layer assembly process. The structure of the nanoglue and the clay nanosheets allow the layers to form cooperative hydrogen bonds, which gives rise to what he calls “the Velcro effect.” Such bonds, if broken, can reform easily in a new place.
The Velcro effect is one reason why the material is so strong. Another is the arrangement of the nanosheets. They’re stacked like bricks, in an alternating pattern.
“When you have a brick-and-mortar structure, any cracks are blunted by each interface,” notes Kotov. “It’s hard to replicate with nanoscale building blocks on a large scale, but that’s what we’ve achieved.”