Inorganic and Organic Aerogels
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nd Organic Aerogels
température influence the cross-linking chemistry and growth processes taking place prior to gelation. Ultimately, the microstructure of the aerogel is translated from the precursor gel after supercritical extraction. It is the ability to tailor the structure and properties of aerogels at the nanometer level which opens up exciting possibilities for thèse novel materials. Sol-Gel Processing Figure 4.2 outlines a typical sol-gel polymerization. In this reaction, a multifunctional monomer is polymerized in solution and proceeds through three Stages. First, the starting m o n o m e r , undergoes hydrolysis or addition reactions at some of its available reaction sites. Second, the substituted molécules c o n d e n s e into "clusters" that range from 10-200 Â in diameter. Thèse clusters can be polymeric or colloïdal in nature, depending on their degree of crosslinking and the growth processes by which they are formed. Furthermore, they contain surface functional groups (e.g., Si-OH, -CH 2 OH) which can link clusters through covalent bonds. Finally, inter-cluster cross-linking leads to the formation of a transparent gel. Three différent methods can be used to dry cross-linked inorganic or organic sol-gels: (1) solvent evaporation, (2) hightemperature supercritical extraction, and (3) low-temperature supercritical extraction. In the case of solvent evaporation, large capillary forces are exerted as the liquid-vapor interface migrâtes through the gel. This leads to significant shrinkage (greater than 75% by volume), and précautions must be taken to prevent cracking of the gel. Once dried, the solid product may hâve a porosity of 0-50% d e p e n d i n g on the p r e c u r s o r chemistry. Thèse high-density materials are referred to as xerogels, and they are finding applications as sensors and membranes. One way to préserve the low-density structure of an inorganic gel upon drying is to exceed the critical point of the d i l u e n t that occupies the p o r e s (as described in the previous section on Synthesis). Methanol (TC = 240°C, P c = 8.0 MPa (1,150 psi)) is commonly used as the diluent in the sol-gel polymerization of TMOS silica gels. Silica gels can be dried directly in an autoclave with little shrinkage, and the size of the gel that can be processed is limited only by the size of the autoclave. The résultant materials can hâve porosities as great as 99.8% and are referred to as aerogels. Silica aerogels dried under thèse condi-
MRS BULLETIN/DECEMBER1990
R
R-J—R R
R
-h -R
R—h-R
" R
Multifunctional Monomers
Low Yiscosity Sol with Clusters
Cross-linked Gel
Figure 4.2 Generalized reaction scheme for a sol-gel polymerization. "R " is a reactive site.
tions tend to be hydrophobic because methoxy groups (Si-OCH 3 ) groups are formed at the surface of the silica matrix. Alternately, cross-linked inorganic or organic gels may be placed in a pressure vessel where carbon dioxide is fully exchanged for the diluent. The gel is dried by taking the carbon dioxide above its critical point (Tc = 31°C,
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