The researchers created this new composite plastic with 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.
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.
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.
"When you have a brick-and-mortar structure, any cracks are blunted by each interface," Kotov explained. "It's hard to replicate with nanoscale building blocks on a large scale, but that's what we've achieved."
Collaborators include: mechanical engineering professor Ellen Arruda; aerospace engineering professor Anthony Waas; chemical, materials science and biomedical engineering professor Joerg Lahann; and chemistry professor Ayyalusamy Ramamoorthy. Kotov is a professor of chemical engineering, materials science and engineering, and biomedical engineering.
The nanomechanical behavior of these materials is being modeled by professor Arruda's group; Waas and his group are working on nanomechanical behavior and applications in aviation.
The University of Michigan
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