By JUDY SIEGEL-ITZKOVICH
Everybody knows that when you stretch something, it gets thinner. But there are materials that behave contrary to expectations: they thicken when stretched and get thinner when compressed. Known as "auxetic' substances, these include some foams and special crystals. Now researchers at Bar-Ilan University and the Technion-Israel Institute of Technology have used quantum mechanical calculations to identify chemical compounds that behave auxetically on a molecular level.
The degree to which an object thins upon stretching is characterized by something called Poisson's Ratio. "Normal" materials have a positive ratio; an exception is the cork in a wine bottle, which has a Poisson's ratio near zero, so its transverse contraction remains essentially unchanged when stretched or compressed - which is why corks are so easily slipped into bottles.
When a non-auxetic material is hit by a ball, the material flows outward from the impact zone, making the point of impact weaker. But in auxetic materials, the matter flows inward, thus strengthening this zone, as in a bulletproof vest.
Auxetic materials have a negative Poisson's ratio. Such materials, which constrict rather than expand under pressure, would be advantageous for a number of applications, such as seals and cable anchors. They also provide interesting possibilities for medical technology. The introduction of stents to hold blood vessels open would be easier if the material contracted under pressure.
In the auxetic materials known to date, the negative Poisson's ratio is a macroscopic property that stems from a special arrangement of particles within the material, such as a particular web-like structure. Nanoscale auxetic materials have been unknown so far.
By using quantum mechanical calculations, a team led by BIU chemistry Prof. Shmaryahu Hoz predicts that there are also certain molecules - a class of compounds known as polyprismanes - that behave auxetically. These are rod-shaped molecules comprised of several three-, four-, five- or six-membered rings of carbon atoms on top of each other. The prismanes made of three- and four-carbon rings show roughly equal auxetic effects, regardless of the number of stacked rings. The ones made of five- and six-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 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 more."
FEATHERY FABRIC
In the future, we might be wearing clothes made of chicken feathers or rice straw. Scientists at the University of Nebraska plan to develop these agricultural wastes into conventional-looking fabrics as a way to reduce the use of petroleum. The feather-based fabric will resemble wool, while the rice straw fabric will look and feel more like linen or cotton, according to the researchers.
The studies describing these fabrics were presented recently at the 232nd national meeting of the American Chemical Society. Both fabrics are still in early development, and may not reach the consumer for several years.
"We hope that the research reported here will stimulate interest in using agricultural byproducts as textile fibers, which would add value to agricultural crops and make the fiber industry more sustainable," says textile science Prof. Yiqi Yang.
Millions of tons of chicken feathers and rice straw available represent an abundant, cheap and renewable alternative to petroleum-based synthetics, Yang says. And unlike petroleum-based fibers, agro-fibers are biodegradable. The development could be a boon to the nation's rice and chicken farmers, he says.
Rice straw, which consists of the stems left over after rice grains are harvested, is the most developed of the fabric concepts so far. Like cotton and linen, rice straw is composed mostly of cellulose. Using a special combination of chemicals and enzymes - a process now under patent review - Yang and colleagues developed fibers from the straw. The properties of the fibers indicate that they are capable of being spun into fabrics that look like cotton or linen using common textile machinery.
Chicken feathers are composed mostly of keratin - the type of protein found in wool. The researchers are particularly interested in the barbs and barbules - the thin, filamentous network that forms the fluffy parts of a feather. They have a honeycomb architecture with tiny air pockets, making the filaments extremely lightweight and resilient. Those properties offer the potential for fabrics with lighter weight, better shock absorption and superior insulation - properties that may mean an improvement over wool.
This is not the first time Yang's team has attracted attention. Two years ago, it developed a way of turning corn husks into fabric with properties similar to linen or cotton. Fabric based on feathers or rice straw might offer even better performance, including the ability to withstand washing.
They could also become "green" fabrics in carpets, automobiles, building materials and a host of other everyday applications, all at potentially less cost and with novel and sometimes superior properties than their synthetic counterparts.
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