http://www.newscientist.com/article/mg20527445.600-smart-mud-could-be-the-new-plastic.html
Smart mud could be the new plastic
* 20 January 2010 by Colin Barras
Could a mixture of water and clay replace plastics? The desire to wean
the world off oil has sparked all manner of research into novel
transportation fuels, but manufacturing plastics uses large amounts of
oil too. Researchers at the University of Tokyo, Japan, think their
material could be up to the task.
Takuzo Aida and his team mixed a few grams of clay with 100 grams of
water in the presence of tiny quantities of a thickening agent called
sodium polyacrylate and an organic "molecular glue". The thickening
agent teases apart the clay into thin sheets, increasing its surface
area and allowing the glue to get a better hold on it.
This means that, while the mixture is almost 98 per cent water, it forms
a transparent and elastic hydrogel with sufficient mechanical strength
to make a 3.5-centimetre-wide self-standing bridge.
Self-repairing hydrogel
The strength of the material depends on the sum of the forces acting
between the molecules in the clay nanosheets and the glue, says Aida.
These so-called supramolecular forces, such as hydrogen bonds, also help
to trap water molecules between the clay sheets.
Some other hydrogels rely on covalent chemical bonds rather than
supramolecular forces for their strength. One disadvantage of this is
that when the covalent bonds break, the material irreversibly loses its
strength, says Aida. Supramolecular forces, on the other hand, can
easily reform, so if the material fails under stress it can quickly
regain its strength.
The gel takes just 3 minutes to form, and making it requires no
understanding of the chemical process involved, Aida says, – a fact that
impresses Craig Hawker at the University of California in Santa Barbara,
who was not involved with the study. "One of the primary breakthroughs
is the overall simplicity of the procedure coupled with the exceptional
physical properties of the final assemblies," he says.
New class of materials
"Toughness, self-healing and robustness are just some of the initial
physical properties that will be found for this new class of materials,"
Hawker says. "I predict that this approach will lead to the design of
even more impressive materials in the near future."
Polymer scientist Jian Ping Gong at Hokkaido University in Sapporo,
Japan, says the work is "beautiful" but points out that the material's
mechanical strength falls short of what is possible for plastics and
chemically cross-linked gels.
Aida says that strengthening the material is as simple as increasing the
quantities of clay, sodium polyacrylate and glue, provided transparency
is not important.
Journal reference: Nature, DOI: 10.1038/nature08693.
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