Get thicker when stretched, thinner when
compressed: simulations identify auxetic molecules.
Day-to-day experience teaches us that stretching an
object makes it thinner; pushing it together makes it thicker. However,
there are also materials that behave contrary to our expectations:
they get thicker when stretched and thinner when compressed. Known as
“auxetic” substances, these materials include some foams and special
crystals. Researchers at the Bar-Ilan University and the Israel
Institute of Technology have now used quantum mechanical calculations
to identify the first class of chemical compounds that behave
auxetically on a molecular level.
When a usual material is, for example, hit by a
ball, the material “flows” outward from the impact zone making the
point of impact weaker. However, in auxetic materials, the matter
“flows” inward, thus strengthening this zone. Such materials would be
advantageous for bulletproof vests. Auxetic materials also provide
interesting possibilities for medical technology. The introduction of
implants such as stents to hold open blood vessels would be easier if,
under pressure, the device would get thinner instead of thicker in the
perpendicular direction.
In the auxetic materials known to date, the unusual
behavior is a macroscopic property that stems from a special
arrangement of the particles within the material, such as a particular
weblike structure. Nanoscale auxetic materials are so far unknown.
By using quantum mechanical calculations, a team
led by Shmaryahu Hoz has now predicted that there also exist certain
molecules that behave auxetically: a class of compounds known as
polyprismanes. These are rod-shaped molecules built up of several
three-, four-, five-, or six-membered rings of carbon atoms stacked on
top of each other. The prismanes made of three- and four-membered
carbon rings show roughly equal auxetic effects, regardless of the
number of stacked rings. The ones made of five- and six-membered
carbon rings demonstrate significantly higher auxetic effects. Of all
of the variations for which calculations were carried out, the
prismane made of four six-membered rings showed the strongest effect.
The researchers have not yet been able to unambiguously explain why
prismane molecules behave auxetically.
“Although prismanes were discovered over 30 years
ago, very few representatives of this class of compounds have been
synthesized so far,” says Hoz. “We hope that our insights will act as
an incentive to produce and characterize more prismanes.”
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