ITHACA, N.Y. -- Using nanoscale chemistry, researchers at Cornell University have developed a new class of hybrid materials that they describe as flexible ceramics. The new materials appear to have wide applications, from microelectronics to separating macromolecules, such as proteins.
What is particularly striking, even to the researchers themselves, is that under the transmission electron microscope (TEM) the molecular structure of the new material -- known as a cubic bicontinuous structure -- conforms to century-old mathematical predictions. "We in polymer research are now finding structures that mathematicians theorized long ago should exist," says Ulrich Wiesner, associate professor of materials science and engineering at Cornell. The structure of the new material appears so convoluted that it has been dubbed "the plumber's nightmare."
Wiesner reports on the new flexible ceramics today (8 a.m.) at the annual March meeting of the American Physical Society at the Indiana Convention Center, Indianapolis. In a talk, "Phase behavior of block copolymer directed nanostructured organic/inorganic hybrids," Wiesner will report that the material "is an exciting, emerging research area offering enormous scientific and technological promise."
Wiesner's research group was attracted to chemistry on the nanoscale (a nanometer is equal to the width of three silicon atoms) by the perfect, symmetrical shapes that are found in nature. An often-cited example is the elegant structure of diatoms, unicellular algae whose shell walls are made of perfectly replicated silica pores. Nature's key to this replication, says Wiesner, "is perfect shape control governed by self-assembly of organic components directing inorganic materials' growth." The Cornell researcher reasoned that the simplest way to mimic nature's pathways was to use organic (or carbon-based) polymers -- more particularly materials known as diblock copolymers -- that have the ability to self-assemble chemically into nanostructures with different symmetries. If the polymer could somehow be melded with an inorganic material -- a ceramic, specifically a silica-type material -- the resulting hybrid would have a combination of properties: flexibility and structure control from the polymer and functionality from the ceramic. This, Wiesner's group has now achieved.
"The resulting material has properties that are not just the simple sum of polymers plus ceramic, but maybe something quite new," says Wiesner. Thus far the Cornell researchers have made only small pieces of the flexible ceramic, weighing a few grams, in petri dishes, but that is enough to test the material's properties. It is transparent and bendable but with considerable strength, and unlike pure ceramic will not shatter. In one form the hybrid material is an ion conductor (an ion is an electrically charged atom), with great promise as highly efficient battery electrolytes. There also is the possibility that the new material could be used in fuel cells, he says.
In some cases, the material's hexagonal symmetry, derived from self-assembly, closely resembles that of the diatom. Indeed, says Wiesner, with the "plumber's nightmare" molecular structure, "you could almost say we have perfected nature."
|Transmission electron micrograph of the cubic structure of the new hybrid material showing pores about 10 nanometers across. Wiesner Research GroupCopyright © Cornell University|
The porous structure of the flexible ceramic forms when the material is heat-treated at high temperatures. In fact, says Wiesner, this is the first material with such a symmetry and narrow pore-size distribution. Because the material has pores only 10 to 20 nanometers across, Wiesner is collaborating with Larry Walker, Cornell professor of biological and environmental engineering, to see if the material can be used to separate live proteins.
Wiesner believes that because of the material's self-assembling ability, it could be produced in large batches. "We have perfect structure control," he says. "We can structure the material down to the nanoscale with unprecedented control. We now know how to make a suite of structures of assorted shapes and pore sizes."
The Cornell researchers can do this by controlling the "phases," or molecular architectures, of the material just by controlling the mix of the polymer and the ceramic. The material goes through several shifts in shape, from cubic to hexagonal to lamellar -- thin and platelike -- to inverse hexagonal and inverse cubic. After the lamella phase and before the inverse hexagonal, the material forms the cubic bicontinuous structure -- the "plumber's nightmare"-- that was not previously known to exist in polymer systems. The "plumber's nightmare" may be only the first of these highly adaptable structures made possible by the specific combination of polymers and ceramics, says Wiesner. "There is a good chance that we will find a whole zoo of other bicontinuous structures that people didn't know existed in polymers. We have opened the avenue to finding further such structures," he says.
Among Wiesner's Cornell collaborators on this research are Sol Gruner, professor of physics and Adam Finnefrock, physics postdoctoral associate. Other collaborators include Ralph Ulrich and Hans Spiess of the Max-Planck-Institute for Polymer Research, Germany. The work is supported by the National Science Foundation, the Max-Planck-Society and the Cornell Center for Materials Research.
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