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
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
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
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.