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ther Nature is very adept at maximizing stiffness and strength in low-density materials (e.g., pith, wood), usually through the use of large anisotropic cell structures. 1 Nevertheless, we hâve been unable to identify a single LDMM in nature, and wonder if there is a fundamental limitation on the degree of anisotropy that can be established in LDMMs. The prevailing argument is that capillarity effects tend to favor locally isotropic morphologies. A n o t h e r a p p r o a c h to d e v e l o p i n g LDMMs with improved properties involves the use of selected "building blocks" and a better understanding of sol-gel polymerization mechanisms. For e x a m p l e , S i - O H t e r m i n a t e d polydimethylsiloxane chains can be reacted with TMOS to give silica aerogels with improved toughness. 2 Klemperer et al. 3 reported the synthesis of octamethoxy octasilsesquioxane [Si 8 0 12 ](OCH 3 ) 8 , a multifunctional cubic "building block" that has resulted in xerogels with interesting microstructures. Aerogels based on this "building block" présent the possibility of a long-range ordered structure with a high degree of cross-linking. Using the "condensed silica" approach with tetramethoxysilane, Hrubesh and Tillotson hâve proposed isolating selected ring structures for the production of ultralow-density silica aerogels with improved mechanical and optical properties. New aerogels based on mixed métal oxides (e.g., Zr0 2 /Si0 2 ) and inorganic/organic combinations (e.g., MF/ Si0 2 ) are also of interest. In ail the above cases, further research is needed to convert today's "random" sol-gel polymerizations into tomorrow's "selected pathway" sol-gel polymerizations. Variables such as electrostatic charge, ionic strength, solvent/cluster interactions, and degree of cross linking need to be incorporated into current sol-gel models. Potential applications of LDMMs must emphasize the unique features of thèse materials. For example, the high surface area of aerogels or carbon foams can be used in catalyst and sensor applications. Carbon aerogels loaded with platinum were synthesized for the selected réduction of nitric oxide by ammonia. 4 Teichner 5 investigated the catalytic activity of many doped métal oxide aerogels (e.g., Ni/Al 2 0 3 , Fe3C>4/Si02). Using an innovative combination of biomimetic and solgel chemistry, Droege 6 is attempting to
incorporate active enzymatic sites into inorganic aerogels for the direct conversion of méthane to higher valued products (e.g., olefins and alcohols). We envision derivatized (e.g., -COOH) carbon foams for surface acoustic wave (SAW) devices or other sensor applications. Davis, Weber, and Sylwester 7 infiltrated a carbon LDMM with an epoxy resin to produce an électrode for selected e l e c t r o c h e m i c a l r e a c t i o n s . Other possibilities for carbon LDMMs include the intercalation (diffusion of cations along lattice planes) of partially graphitized structures to give LDMMs with interesting electrical properties. LDMMs may also hâve médical applications. For example, Interpore Int
