[Editors] MIT: Nanocomposities yield strong, stretchy fibers

[Elizabeth Thomson]

MIT News Office
Massachusetts Institute of Technology
Room 11-400
77 Massachusetts Avenue
Cambridge, MA  02139-4307
Phone: 617-253-2700
http://web.mit.edu/newsoffice/www

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MIT: Nanocomposities yield strong, stretchy fibers

--Lycra-like materials were inspired by spider silk
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For Immediate Release
FRIDAY, JAN. 19, 2007
Contact: Elizabeth A. Thomson, MIT News Office
Phone: 617-258-5402
Email: thomson at mit.edu

PHOTO, IMAGES AVAILABLE

CAMBRIDGE, Mass.--Creating artificial substances that are both 
stretchy and strong has long been an elusive engineering goal. 
Inspired by spider silk, a naturally occurring strong and stretchy 
substance, MIT researchers have now devised a way to produce a 
material that begins to mimic this combination of desirable 
properties.

Such materials, known as polymeric nanocomposites, could be used to 
strengthen and toughen packaging materials and develop tear-resistant 
fabrics or biomedical devices. Professor Gareth McKinley, graduate 
student Shawna Liff and postdoctoral researcher Nitin Kumar worked at 
MIT's Institute for Soldier Nanotechnologies (ISN) to develop a new
  method for effectively preparing these materials. The research 
appears in the January issue of Nature Materials.

Engineers are already able to create materials that are either very 
strong or very stretchy, but it has been difficult to achieve both 
qualities in the same material. In the last few years scientists have 
determined that the secret behind the combined strength and 
flexibility of spider silk lies in the arrangement of the 
nano-crystalline reinforcement of the silk while it is being produced.

  "If you look closely at the structure of spider silk, it is filled 
with a lot of very small crystals," says McKinley, a professor of 
mechanical engineering. "It's highly reinforced."

The silk's strength and flexibility come from this nanoscale 
crystalline reinforcement and from the way these tiny crystals are 
oriented towards and strongly adhere to the stretchy protein that 
forms their surrounding polymeric matrix.

Liff, a Ph.D. student in mechanical engineering, and Kumar, a former 
MIT postdoctoral associate, teamed up to figure out how to begin to 
emulate this nano-reinforced structure in a synthetic polymer (A 
polymer or plastic consists of long chains composed of small 
repeating molecular units).  Numerous earlier unsuccessful attempts, 
tackling the same issue, relied on heating and mixing molten plastics 
with reinforcing agents, but Liff and Kumar took a different 
approach: They focused on reinforcing solutions of a commercial 
polyurethane elastomer (a rubbery substance) with nanosized clay 
platelets.

They started with tiny clay discs, the smallest they could find 
(about 1 nanometer, or a billionth of a meter thick and 25 nanometers 
in diameter). The discs are naturally arranged in stacks like poker 
chips, but "when you put them in the right solvent, these 'nanosized 
poker chips' all come apart," said McKinley.

The researchers developed a process to embed these clay chips in the 
rubbery polymer-first dissolving them in water, then slowly 
exchanging water for a solvent that also dissolves polyurethane. They 
then dissolved the polymer in the new mixture, and finally removed 
the solvent. The end result is a "nanocomposite" of stiff clay 
particles dispersed throughout a stretchy matrix that is now stronger 
and tougher.

Importantly, the clay platelets are distributed randomly in the 
material, forming a structure much like the jumble that results when 
you try to stuff matches back into a matchbox after they have all 
spilled out.
Instead of a neatly packed arrangement, the process results in a very 
disorderly "jammed" structure, according to McKinley. Consequently 
the nanocomposite material is reinforced in every direction and the 
material exhibits very little distortion even when heated to 
temperatures above 150 degrees Celsius.

In a Nature Materials commentary that accompanied the research paper, 
Evangelos Manias, professor of materials science and engineering at 
the University of Pennsylvania, suggests that "molecular composites" 
such as those developed by the MIT group are especially suitable for 
new lightweight membranes and gas barriers, because the hard clay 
structure provides extra mechanical support and prevents degradation 
of the material even at high temperatures. One possible use for such 
barriers is in fuel cells.

The U.S. military is interested in such materials for use in possible 
applications such as tear-resistant films or other body-armor 
components. The military is also interested in thinner, stronger 
packaging films for soldiers' MREs (meals ready to eat) to replace 
the thick and bulky packaging now used.

Fabric companies have also expressed interest in the new materials, 
which can be used to make fibers similar to stretchy compounds such 
as nylon or Lycra. The new approach to making nanocomposites can also 
be applied to biocompatible polymers and could be used to make stents 
and other biomedical devices, McKinley said.

The research was funded by the U.S. Army through MIT's Institute for 
Soldier Nanotechnologies and by the National Science Foundation. 
McKinley's team was assisted by technical staff at the ISN, including 
research engineer Steven Kooi, who helped prepare special samples for 
transmission electron microscopy.

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Written by Anne Trafton, MIT News Office