The incredible shrinking process

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When it comes to new ways of processing, nanotechnology continues to "lower the bar"

anotechnology processes

are often defined as those involving components no larger than 100 nm in size. In this context, these processes are nothing new to chemical engineers - catalysis and crystallization spring immediately to mind. What is new is the way engineers are designing processes and materials to increasingly exploit the rules of nature on the nanoscale. And, it's also a growing focus - a $422 million 2001 budget for the National Nanotechnology Initiative, a proliferation of new consortia and centers in U.S. universities and research institutions, and major commitments by governments worldwide - from Canada to Korea.

Most media attention, however, has highlighted futuristic views. "These dreams about building nanorobots are conceptually beautiful, but it might take a million years to make them," says researcher Lajos Balogh at the Center for Biologic Nanotechnology at the Univ. of Michigan (UM; Ann Arbor). "I'd like to harvest knowledge for the sake of others and myself, now." Researchers are patiently working to better understand the way things work at nanoscale, using that knowledge to improve materials and their applications.

A perfect example is nanoclays, platelets of clay materials about 1 nm thick and 100 run in diameter, which can be dispersed in a matrix (e.g., a polymer) to form a composite. Like straw used to reinforce bricks, the platelets dissipate mechanical stress throughout the material. But, because the reinforcement occurs on such a tiny scale, nanocomposites are more effective. "You could use a low amount, say 5%, of nanoclays for applications requiring 20% of traditional fillers, like glass fibers," says Al Rossi, a consultant with Principia Partners (Exton, PA). Nanocomposites can also take loadings above 10% without sacrificing ductility, which isn't the case with traditional fillers, says consultant Phillip Wilson (Commerce Township, MI). Under experimental conditions, nanocomposites have been produced with five to seven times the flexural modulus, with no ductility loss, says Wilson.

Although it's been 10 years since Toyota Central R&D researchers first successfully dispersed Montell's montmorillonitrile into nylon-6, commercial applications have taken almost that long to appear. Myriad challenges range from separating and dispersing the platelets uniformly, to dealing with impurities in naturally occurring clays and compatibilizing them with the matrix material. "Natural clays, like smectire, are hydrophilic," Rossi explains. "To work with polymers, they have to be made organophilic."

But successful applications could confer a long chain of benefits. Even a three-fold increase in modulus, for example, would also mean greater tensile and flexural strengths, and scuff and mar resistance, plus a much lower thermal expansion coefficient, says Wilson. Injection-molded parts could be thinner, leading to reductions in weight, material and cycle times; and, because the platelets are so small, the nanocomposites would be completely recyclable. With these improvements, Wilson says, commodity resins could someday replace high-performance engineering ones.

Platelets can also act as nanoscale gas barriers, increasing the distance that gas molecules must travel. At Honeywell Engineering Applications and Solutions (Morristown, NJ), researchers have combined this capability with an oxygen-reactive component to produce highly impermeable packaging. According to plastics R&D team leader Kris Akkapeddi, the Nanomero synthetic clay platelets (supplied by Nanocor, Arlington Hts., IL) are incorporated into caprolactam, a nylon6 monomer, via in-situ polymerization. The platelets, says Akkapeddi, are exfoliated and dispersed as the clay swells within the molten monomer. A proprietary oxygen-scavenger is then added. "By blending in a twin-screw extruder, and playing with the rheology on reaction, we reduce the size of the scavenger to about 10-run-size particles." The nanoclays increase the gas barrier fourfold; the addition of an oxygen scavenger decreases the net permeability about 100-1,000 times over that of the original matrix material.

Honeywell uses the composite, Aegis, as a barrier layer between PET layers for beverage bottles. Aegis and PET streams are extruded separately, flowed into a preform, then stretchblow-molded to about 10-12 mil thickness. The barrier layer, comprising about 5-8 wt% of the bottle, limits oxygen permeability below 1 ppm for 4-6 months. This, says Akkapeddi, meets regulatory requirements for beer and juice containers. Because there is no chemical bonding between layers, the bottle's components can be shredded and air-separated for recycling. The bottles are currently undergoing qualification trials.

Nanoclays are also serving up tennis balls with more bounce. A nanoplatelet-based butyl coating from InMat (Hillborough, NJ) decreases airflow from the core of DoubleCore balls by 200%, says Wilson Racquet Sports (Chicago), doubling the balls' playing life. The coating starts with vermiculite, exfoliated in water, and mixed with butyl rubber to form a composite with loadings higher than 20 wt% - resulting in a 30-35-fold decrease in permeability, with no loss of flexibility, says president Harris Goldberg.

Big things in nano packages

Nanoparticles' high surface/volume ratios offer many advantages - enhanced reaction rates, for one. "At a certain particle size, say 100 nm for some materials, the surface effects begin to dominate what's going on," explains Dan Coy, group leader of advanced engineering at Nanophase Technologies (Argonne, IL). At Argonide Technologies (Sanford, FL), for example, 100-nm aluminum nanoparticles are used to enhance combustion in rocket fuels, and copper nanoparticles are incorporated in an automotive lubricant additive to reduce engine wear, according to president Fred Tepper. The company also supplies nanoparticle-containing pastes for capacitors and microelectronics, which allow for higher-density circuits and lower sintering temperatures. Argonide's process for making the nanometal powders is similar to what happens when the filament in an incandescent light bulb burns out, Tepper explains. Metal wire is "overloaded" with electrical energy until an explosion occurs. "The metal clusters get blown at supersonic speed into an inert gas, and you get superquenching," he says.

Nanophase uses a physical vapor-synthesis process (Figure 1). A plasma is used to heat a metal or other conductive precursor, which vaporizes. A gas then cools the vapor, which condenses into substantially spherical nanoparticles typically smaller than 100 run. The cooling gas can be inert or reactive.

Because of their size, nanoparticles do not behave as many customers would expect. "You'd think they would be subject to Brownian motion, like smoke," says Coy. "But at the concentrations we make them in, they condense by electrostatic forces into a very fine, light, fluffy powder with a specific gravity of 0.1 or so." The powder needs to be tailored for customers' solvents and resins, says Gina Kritchevsky, vice president of technology and engineering, by adding hydrophilic or hydrophobic coatings to allow for dispersion, for instance.

Nanophase recently introduced abrasion-resistant, alumina-based coatings for wood and plastics, as well as a material for very fine polishing in chemical-mechanical planarization (CMP) applications for electronic components, a market in which DuPont and Air Products recently announced a joint venture, under the name of DA Nanomaterials LLC.

Perspective matters

Size reduction, which nanotechnologists call a "top-down" approach, is a good way to make certain nanomaterials. "We've gone through an evolutionary change of these processes, to the point that we can do this semicontinuously, approaching six sigma," points out John Mendel, materials commercialization manager at Kodak (Rochester, NY). "Our inline sensors allow us to look at particle size in real time, to monitor throughput and many process parameters," he says, "all of which are necessary for successful scaleup." To make filter dyes and inkjet materials, Kodak uses two size-reduction techniques: one using a modified mill device, and another using crosslinked polymer beads about 50 pin in size for low-impact grinding.

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Figure 1.

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Figure 2.

Then, there's "bottom-up" nanoprocessing, or self assembly. "You start with nothing, and build your materials up to a certain size," explains Mendel. The basic approach involves two components in solution, which meet in a nucleation zone to create a precipitate. When the nuclei reach a certain size in the nano range, growth is terminated. This is the process Kodak is investigating to make scratch-resistant coatings for possible use in x-ray films.

To create polarizer coatings for LCDs, Optiva Inc. (South San Francisco, CA) uses a bottom-up technique, where dye molecules dissolved in water self-assemble to form crystal structures like stacks of coins, says vice president of manufacturing Jerry Jones. A high-shear force is applied to the materials to "comb" the crystals into the proper alignment. After the water dries, the coating is about 300-400 nm thick.

The coating is targeted to replace traditional polarizers, which are typically 100-200 pm thick. An LCD uses two sepcrate, perpendicularly oriented polarizer layers on the outside of its glass or plastic display. By contrast, the Optiva coating can be applied directly to the inside of the glass. The coating also provides a wider viewing angle and better color control than current sheet polarizers, says Jones.

"Sheet polarizers are still made by the same process developed by E. H. Land in 1928," Jones says, adding that the process requires an iodine-based compound to be dipped in polyvinyl alcohol and stretched, and then laminated to a film. "Because of the complex nature of this process, an LCD polarizer can cost $15/m2, or as much as $60-70/m2 for a high-performance version." Since only about Sg of the Optiva coating is needed to cover I m2, says Jones, "We think we can offer a value proposition and a substantially lower price."

Some self-assembly methods are the stuff of science fiction. At The Scripps Research Institute's Skaggs Institute for Chemical Biology (La Jolla, CA), researchers have designed cyclic peptides that self-assemble inside a bacterium, forming nanotubes that puncture the cell membrane and kill the organism (Figure 2). The peptides target certain bacteria, such as E. coli, while leaving host cells unscathed. "You can make lots of things that break the membrane, but this has to be selective so it doesn't kill the host," points out principal investigator M. Reza Ghadiri. The peptides are made of alternating left- and right-handed amino acids to form short chains, the ends of which are joined to form a ring. Inside the target organism, these rings stack to form a nanotube. Because only the lefthanded form of amino acids are found in naturally occurring proteins, the bacteria have never seen these types of peptides before, which means that they may have a hard time developing resistance.

Carbon copies

Video screens could also benefit from a quite different technology. At Carbon Nanotechnologies (Houston), singlewall carbon nanotubes offer the electrical conductivity of copper or silicon, says CEO Bob Gower. Field-emission flat-panel displays incorporating the nanotubes could replace traditional cathode ray tubes, he says, by using nanotubes as as individual electron guns for each pixel, since he claims buckytubes are the best-known field emitters. The conductive properties of the buckytubes could serve in shielding applications for military equipment and cell phones, says Gower, or in energy storage.

Carbon Nanotechnologies uses a hightemperature, high-pressure catalytic process to form the buckytubes, which are long, tubular fullerenes. "Me theory is that the carbon attaches to the iron catalyst, building up in rings, and then the ends close up with the heat," explains Gower. One challenge, he says, is prying them apart - they tend to adhere to one another in the form of ropes by van der Waals forces. Undeniably a premium product, the high-purity buckytubes are now made in small quantities. A new unit will go onstream late this year, production is targeted at 0.5 lb/d.

Multiwall carbon nanotubes, on the other hand, are made in multiton quantities via a catalytic process at Hyperion Catalysis International (Cambridge, MA), says marketing director Patrick Collins. The firm provides its Fibril nanotube product (10-15 nm in diameter) dispersed in polymer masterbatches to eliminate customers' dispersion problems. The present markets are in automotive and electronic applications, says Collins, explaining that conductive nylon composites with about 1-5% nanotubes are used in auto fuel lines and in electrostatically painted exterior body parts.

Customers' uses for multiwall carbon fibers are quite imaginative, says Max Lake, president of Applied Sciences (Cedarville, OH). The high surface area of his firm's fibers, which range from 100 nm to 100 pm in diameter, makes them good uranium absorbers, for example, which made one customer suggest using them to treat contaminated groundwater at nuclear facilities.

A finer shade of scale

Nanomaterials even offer new prospects for enhancing reaction chemistry, as reactions are more likely to be homogeneous and therefore faster and more precise. They can even be tailored to have specific properties.

Polyhedral oligomeric silsesquioxanes and silicates (POSS) are a family of hybrid molecules having an inorganic core and eight organic side groups, each of which can be unreactive or reactive, depending on the desired functionality. Typically 1.5 nm in radius, the molecules are often dispersed in polymer matrices to form composites, or used to surface-modify nanoclay particles to promote adhesion with matrix materials, says Joe Lichtenhan, CEO of Hybrid Plastics (Fountain Valley, CA).

According to Miriam Rafailovich, principal investigator at the Materials Research Science & Engineering Center of the State Univ. of New York (Stony Brook), POSS is very useful in polymer grafting. Copolymers, by themselves, don't allow for strong grafts, but POSS's side groups can be tailored to help blend the polymers together. POSS could also prove useful for adhering biological materials to polymers, says Rafailovich, eventually leading to the development of prosthetics, implants and artificial tissues. Since cell adhesion is preceded by protein adsorption, she says, POSS could act as an intermediary to help the protein adhere to a polymer.

Like POSS, polymer nanocomposites are organic/inorganic hybrids. However, dendrimer nanocomposites are branched polymeric structures, holding immobilized atoms or small clusters of inorganic materials, and combining properties of all constituents. For medical imaging applications, UM's Balogh has developed such nanocomposites consisting of a poly(amidoamine) (PAMAM) matrix containing gold atoms, which may also act as markers. These can be injected into a site and imaged by transmission electron microscopy. Balogh points out that these nanocomposites could also be useful as delivery vehicles for medicines (eg., radioisotopes, such as beta-emitter Au- 198). Since tumor microvasculature is more porous than that of healthy tissues, he explains, dendrimer composites can penetrate tumor tissues to deliver an exact dose of medicine.

In a similar application, Balogh is developing silver-carrying dendrimers to combat bacterial infections. Silver, which kills microbes, is also toxic to humans. If it's encapsulated in nontoxic dendrimers, however, that's another story. Because they are smaller than the bacteria, the dendrimers can travel to the bacteria, where they are ingested, releasing the silver.

Balogh is also exploring the use of dendrimers to remove toxic metal ions from industrial wastewater, in a collaboration with Mamadou Diallo, director of Molecular Environmental Technology at the Materials and Process Simulation Center of the Beckman Institute of Caltech (Pasadena, CA). Dendrimers with 4th- and 5th-generation PAMAM structures possess a high density of functional nitrogen and amide groups. This makes them particularly attractive as high-capacity chelating agents for toxic metal ions, such as copper. Diallo and Balogh have developed a process that exploits this feature. Dendrimers are mixed with contaminated water, binding the target metal ions, and sent to an ultrafiltration membrane. Because of their large size, 98% of the metal-laden dendrimers are retained by the membrane, says Diallo. The dendrimers are sent to another stage, where pH swing releases the metal from the dendrimers, which are then recycled. "Basically, the dendrimer is used as a nanoscale separation and reaction medium."

As nanotechnology increasingly becomes a household word, researchers are working to overcome technical and marketplace hurdles, not to mention managing expectations. "There's an awful lot of hype - most of it is Buck Rogers stuff - and the industry is in the infant stages," says Tepper. In reality, Lichtenhan points out, nanotechnologies are just another tool for engineering materials and processes. - Irene Kim