|
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 |