A Livermore scientist needs a support system with virtually no mass for a project he is working on. He is certain to end up using an aerogel. No mass at all is an impossibility, but aerogels come pretty close. Researchers at Livermore have already synthesized a silica aerogel only twice as dense as air.
Sometimes called frozen smoke, aerogels are open-cell polymers with pores less than 50 nanometers in diameter. In a process known as sol-gel polymerization, simple molecules called monomers suspended in solution react with one another to form a sol, or collection, of colloidal clusters. The macromolecules become bonded and cross-linked, forming a nearly solid, transparent sol-gel. An aerogel is produced by carefully drying the sol-gel so that the fragile network does not collapse.
The complicated, cross-linked internal structure gives aerogels the highest internal surface area per gram of material of any known material. Aerogels also exhibit the best electrical, thermal, and sound insulation properties of any known solid.
For about the last 15 years, Livermore has been developing and improving aerogels for national security applications. Livermore scientists have also synthesized electrically conductive inorganic aerogels for use as supercapacitors and as a water purifier for extracting harmful contaminants from industrial waste or for desalinizing seawater. For a time, Livermore was involved with a NASA project in which an aerogel was to be installed in a satellite to collect particles of meteorites as they flew by.
Given aerogels' many sterling qualities, one would expect to find them in use everywhere. Indeed, there has been major industrial interest in aerogels. However, using them in everyday applications presents practical problems, specifically the cost of fabrication and processing. Several years ago, a Livermore team won an R&D 100 Award for developing a new fabrication method that was faster and cheaper. (See S&TR, December 1995, pp. 22-25.)

But another problem still stood in the way. Sol-gel polymerization is a bulk process with no way to control the size of the sols or the way they come together. The structure and density of the final aerogel are dictated to some extent by the conditions during polymerization such as temperature, pH, type of catalyst, and so on. But with current fabrication methods, the aerogel's structure cannot be controlled at the molecular level.
Chemist Glenn Fox is leading a project at Livermore that aims to bring more control to the design and synthesis of organic aerogels. "Laboratory programs would find many more uses for aerogels if only we could fabricate them to precise specifications," Fox says. "They could be used as sensors for biological agents, in environmental remediation, as catalysts for chemical reactions, or in experiments on the National Ignition Facility. Aerogels have also been of interest for insulating appliances and homes and for a plethora of other uses. Nanostructured materials are attracting increased scientific and practical interest. But control of the material's structure all the way down to the molecular level is needed first." Fox and a small team obtained funding from the Laboratory Directed Research and Development program to apply a relatively new polymerization method to this problem.

Starting with a Tree
Dendrimers are highly branched, treelike macromolecules that can be synthesized "generationally" to produce perfectly regular structures (dendron is the Greek word for tree). Conventional polymers are chains of differing lengths with a range of molecular weights and sizes, while dendrimers have a precise molecular size and weight. Large, multigenerational dendrimers tend to form tidy spherical shapes with a well-defined structure that makes them particularly strong.
Fox's team has begun applying dendritic methodology to the creation of sol-gels and aerogels in the hope of achieving structural control. The Livermore team is one of the first to use dendritic technology in the organic sol-gel process.
Says Fox, "We are trying to understand and control the sol-gel polymerization process on a molecular level. Using dendrimers allows us to separate the clustering and gelling processes when an aerogel is being formed, something that has not been possible before. If we succeed, the payoff for Laboratory programs will be extremely important. We may be able to script the physical properties of the aerogel or build specific tags on molecules in a uniform way."
Organic aerogels are currently formed by combining either resorcinol (1,3-dihydroxybenzene) or melamine (2,4,6-triaminotriazine) with formaldehyde. Fox's team is synthesizing and experimenting with a whole collection of new starting materials that are being assembled into dendrimers. Some are based on resorcinol to take advantage of its well-documented reactive attributes. Another set of new dendrimer systems with rigid cores could give the resulting aerogel greater structural efficiency, improving the ease of processing and lowering the cost of aerogel production. Other experiments involve the synthesis of new organometallic materials and ways to evenly disperse metal ions in an organic aerogel.
These tailored dendritic monomers are being combined with preformed, dendritic, sol-gel clusters whose outer surface has been coated to react with the monomer. Two kinds of dendrimer precursors have been studied, amino-based and aromatic-based, each having different advantages. Amino-based dendrimers are available commercially and have been studied extensively. Reactants can be added relatively easily to their outer surfaces to "functionalize" them, prompting them to cross-link as desired. Benzyl ether dendrimers, on the other hand, are structurally similar to the colloidal sols of the resorcinol-formaldehyde mix. They are not commercially available but can be prepared readily in the laboratory.
Controlling the size and composition of the clusters formed during gelation as well as the type of cross-linking involved should give Fox's team a new-found architectural control over aerogels. Analysis of the structures with infrared spectroscopy, nuclear magnetic resonance spectroscopy, and mass spectroscopy will provide a better understanding of how chemistry can affect the composition and structural efficiency of these nanostructured materials.
—Katie Walter

Key Words: aerogels, dendrimers, polymers.

For more information contact Glenn Fox (925) 422-0455 (fox7@llnl.gov).

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