BERKELEY, CA -- Nature has used self-assembling materials forstructures measured in nanometers (billionths of a meter) for hundredsof millions of years -- as components of living cells -- but humanattempts at nanoscale manufacture have been confined mostly to buildingstructural materials a few atoms or molecules at a time. That state ofaffairs may be on the verge of change.
Douglas Gin of the Materials Sciences Division at the Ernest OrlandoLawrence Berkeley National Laboratory, Assistant Professor of Chemistryat UC Berkeley, has devised a general technique for engineeringnanocomposites that begins with the self-assembly of synthetic startingmaterials.
Early in the twentieth century chemists began coaxing simplematerials to assemble themselves into microscopic structures such aslayered 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 differentmaterials into a variety of useful composites at the cellular level.Bone, tough but not brittle, consists of layers of collagen proteinincorporating crystals of inorganic calcium phosphate; the samematerials in a different ratio, with only a few percent protein, yieldthe hardest material produced by living things, tooth enamel.
Not proteins but polymerizable liquid crystals form the skeleton ofDouglas Gin's unique new composites, matrices containing stacks ofhexagonally packed tubes whose diameter and spacing is measured innanometers. These ordered tubes contain a chemical precursor insolution, which can be converted to solid filler material after thearchitecture of the liquid-crystal matrix has been locked into place bypolymerization.
Unlike the sort of liquid crystals found in digital displays, whichchange in response to temperature or an electromagnetic field, Gin useslyotropic liquid crystals; in addition to changes in temperature, theserespond to additives and changes in the chemical solution in which theyare immersed.
"The design of unique lyotropic liquid crystals is the key toeverything that follows," says Gin. Basically, he works with chemicalsknown as polymerizable surfactants. "Like laundry soap, they're made ofamphiphilic monomers" -- molecules, each of which has a hydrophilic(water-loving) end and a hydrophobic (water-fearing) end. When theamphiphilic molecules of laundry soap form a droplet in water, all theirwater-loving heads point outward and their water-fearing tails pointinward -- where they may surround a glob of grease or dirt. Thetechnical name for a soap droplet is "micelle;" by adding more and moremonomers, spherical micelles can self-organize and lengthen intocylinders.
Instead of submerging his monomers in water, Gin reduces the amountof water in his system and designs monomers to form "inverse"cylindrical micelles with their water-loving heads inward. Meanwhile thewater-fearing tails on the outside of the tubes seek each other'scompany, and the tubes pack themselves into hexagons, the tightestpossible geometric packing arrangement. After the hexagonal architectureis locked in place, says Gin, "We can do ordinary synthetic-organicchemistry inside the channels."
Using two different kinds of monomers and two different fillerprecursors, Gin and his colleagues have already demonstrated two novelself-organizing nanocomposites with unique properties. In one techniquethe liquid-crystal matrix has been formed in a solution containing aprecursor to poly(para-phenylenevinylene) -- a light-emitting,electrically conducting polymer, more often called PPV -- which fillsthe tubes. When Gin turns up the heat, the precursor converts to PPVinside the tubes to form what is effectively a bundle of long, discrete,exceedingly fine wires. His group has made uniformly oriented films ofthis material up to eight centimeters wide, yet only 30 to 100 micronsthick. Nanoscale materials often show markedly different properties fromthe same materials in bulk, and PPV is no exception: GinÕs hexagonalmatrix of PPV has over twice the fluorescence, per unit volume, of PPVin bulk.
In related work, Gin is studying an entirely different liquid-crystalsystem, which uses a different monomer to build the hexagonal-tubeframework and a different filler precursor, tetraethyl orthosilicate, ina solution of water and ethanol. The solution also includes a smallamount of a chemical that generates an acid when illuminated. In thepresence of the acid the precursor converts to silicate glass -- even atroom temperature.
Because of the hexagonal array of confining channels, the glassycomposite has a fine, nanoscale structure quite unlike that of normalamorphous glass or plastic. Gin and his colleagues describe it as "atough, pale-yellow, slightly opaque, glassy material . . . completelyinsoluble in common organic solvents and water." It promises unusualproperties, including hardness, now under investigation.
The two composites so far created using custom-made lyotropic liquidcrystals are promising steps on the path to true nanometer-scalematerials engineering.
"Three years ago I started with this crazy idea that self-assemblingliquid crystals could be used to make nanomaterials in bulk," saysDouglas Gin. "Now my new graduate students make a hundred grams a weekof some of these liquid crystals, just as a training exercise. I thinkwe have a viable system."
Berkeley Lab is a U.S. Department of Energy national laboratorylocated in Berkeley, California. It conducts unclassified scientificresearch and is managed by the University of California.
The above story is based on materials provided by Lawrence Berkeley National Laboratory. Note: Materials may be edited for content and length.
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