Scientist Designs Self-Assembling Composite Materials at Nanometer Scale |
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| By Paul Preuss, paul_preuss@lbl.gov August 25, 1997 |
BERKELEY, CA -- Nature has used self-assembling materials for structures measured in nanometers (billionths of a meter) for hundreds of millions of years -- as components of living cells -- but human attempts at nanoscale manufacture have been confined mostly to building structural materials a few atoms or molecules at a time. That state of affairs may be on the verge of change.Douglas Gin of the Materials Sciences Division at the Ernest Orlando Lawrence Berkeley National Laboratory, Assistant Professor of Chemistry at UC Berkeley, has devised a general technique for engineering nanocomposites that begins with the self-assembly of synthetic starting materials.
Early in the twentieth century chemists began coaxing simple materials to assemble themselves into microscopic structures such as layered films and liquid crystal phases, but remarkable as they were, these structures lacked the sophistication of natural composites. Teeth, bones, and shells demonstrate how cleverly nature assembles different materials into a variety of useful composites at the cellular level. Bone, tough but not brittle, consists of layers of collagen protein incorporating crystals of inorganic calcium phosphate; the same materials in a different ratio, with only a few percent protein, yield the hardest material produced by living things, tooth enamel.
Not proteins but polymerizable liquid crystals form the skeleton of Douglas Gin's unique new composites, matrices containing stacks of hexagonally packed tubes whose diameter and spacing is measured in nanometers. These ordered tubes contain a chemical precursor in solution, which can be converted to solid filler material after the architecture of the liquid-crystal matrix has been locked into place by polymerization.
Unlike the sort of liquid crystals found in digital displays, which change in response to temperature or an electromagnetic field, Gin uses lyotropic liquid crystals; in addition to changes in temperature, these respond to additives and changes in the chemical solution in which they are immersed.
"The design of unique lyotropic liquid crystals is the key to everything that follows," says Gin. Basically, he works with chemicals known as polymerizable surfactants. "Like laundry soap, they're made of amphiphilic monomers" -- molecules, each of which has a hydrophilic (water-loving) end and a hydrophobic (water-fearing) end. When the amphiphilic molecules of laundry soap form a droplet in water, all their water-loving heads point outward and their water-fearing tails point inward -- where they may surround a glob of grease or dirt. The technical name for a soap droplet is "micelle;" by adding more and more monomers, spherical micelles can self-organize and lengthen into cylinders.
Instead of submerging his monomers in water, Gin reduces the amount of water in his system and designs monomers to form "inverse" cylindrical micelles with their water-loving heads inward. Meanwhile the water-fearing tails on the outside of the tubes seek each other's company, and the tubes pack themselves into hexagons, the tightest possible geometric packing arrangement. After the hexagonal architecture is locked in place, says Gin, "We can do ordinary synthetic-organic chemistry inside the channels."
Using two different kinds of monomers and two different filler precursors, Gin and his colleagues have already demonstrated two novel self-organizing nanocomposites with unique properties. In one technique the liquid-crystal matrix has been formed in a solution containing a precursor to poly(para-phenylenevinylene) -- a light-emitting, electrically conducting polymer, more often called PPV -- which fills the tubes. When Gin turns up the heat, the precursor converts to PPV inside the tubes to form what is effectively a bundle of long, discrete, exceedingly fine wires. His group has made uniformly oriented films of this material up to eight centimeters wide, yet only 30 to 100 microns thick. Nanoscale materials often show markedly different properties from the same materials in bulk, and PPV is no exception: Gin's hexagonal matrix of PPV has over twice the fluorescence, per unit volume, of PPV in bulk.
In related work, Gin is studying an entirely different liquid-crystal system, which uses a different monomer to build the hexagonal-tube framework and a different filler precursor, tetraethyl orthosilicate, in a solution of water and ethanol. The solution also includes a small amount of a chemical that generates an acid when illuminated. In the presence of the acid the precursor converts to silicate glass -- even at room temperature.
Because of the hexagonal array of confining channels, the glassy composite has a fine, nanoscale structure quite unlike that of normal amorphous glass or plastic. Gin and his colleagues describe it as "a tough, pale-yellow, slightly opaque, glassy material . . . completely insoluble in common organic solvents and water." It promises unusual properties, including hardness, now under investigation.
The two composites so far created using custom-made lyotropic liquid crystals are promising steps on the path to true nanometer-scale materials engineering.
"Three years ago I started with this crazy idea that self-assembling liquid crystals could be used to make nanomaterials in bulk," says Douglas Gin. "Now my new graduate students make a hundred grams a week of some of these liquid crystals, just as a training exercise. I think we have a viable system."
Berkeley Lab is a U.S. Department of Energy national laboratory located in Berkeley, California. It conducts unclassified scientific research and is managed by the University of California.
