Polymer crosslinked aerogels have generated tremendous interest as multifunctional materials because of their superior mechanical properties in comparison to traditional silica aerogels (refs. 1 to 3). The high degree of porosity makes them attractive candidates as insulation for extreme environments, whereas their ability to bear load makes the materials amenable to applications requiring insulation as part of a load-bearing structure. One significant drawback to these materials is their method of production, a lengthy process requiring multiple steps and about 15 days to fully complete. Despite their promising properties, application of the polymer crosslinked aerogels in future space missions depends on simplification of this process.
Traditional method to prepare crosslinked aerogels. Each wash step takes 24 hr, and heating to crosslink can take as long as 72 hr. It takes approximately 1 liter of solvent to prepare one ~20-ml cylindrical monolith.
Long description of figure 1.
The preceding figure demonstrates the traditional manner in which crosslinked aerogels are prepared. The gel is produced after the hydrolysis and condensation of common silica precursors tetramethyl orthosilicate and 3-amino-propyltriethoxysilane (TMOS and APTES) and is subject to several washing steps. Polymer crosslinking is introduced as a secondary step, requiring a nonstoichiometrically controlled soaking process where polymer precursors diffuse through the aerogels’ porous network and await reaction with heat. This is largely dependent on the cross-sectional area of the monolith, making larger pieces somewhat more difficult to crosslink in this manner. Finally, the crosslinked aerogel is washed four more times in preparation for supercritical fluid extraction of the solvent. The overall process is very effective for making reproducible polymer crosslinked aerogel monoliths; however, it is not very efficient.
An alternative reaction scheme is under development at the NASA Glenn Research Center in which the precursors for crosslinking are included in the sol at the start (see the next figure). This simplifies the process to only a few steps, eliminating most of the time-consuming wash steps and totally eliminating diffusion-driven crosslinking. When the polymer precursors are added in this manner, the gelation continues as normal. After a brief aging period, the gel is ready to be crosslinked. Depending on the type of crosslinking mechanism, the monoliths might be heated to start a chain growth reaction or be exposed to ultraviolet light to begin a radical polymerization reaction. In either case, not only does the one-pot method produce crosslinked aerogels in many fewer steps, but more control of stoichiometry and uniformity of the crosslinked gels is possible since diffusion-driven crosslinking is eliminated.
One-pot synthesis of crosslinked aerogels. Gray constituents indicate prepolymers in the sol that are inert to gelation. Once the gel is formed, it can be reacted to initiate crosslinking. After one wash, which removes any unreacted components, the monolith, whose scanning electron microscopy (SEM) image is shown here, was supercritically dried and demonstrated nanoscale morphology similar to that of the well-characterized multistep-prepared crosslinked aerogels.
For this one-pot synthesis to proceed effectively, the polymer precursors must remain inert to gelation by either being introduced as a co-reacting agent with the oxide gel like an acrylate-modified siliane, or as a soluble monomer or oligomer that does not interfere with gelation, such as a bismaleimide; both of which we have demonstrated. As seen in the following photograph, the one-pot-created isocyanate crosslinked aerogels look very similar to the traditionally prepared monoliths.
Visual comparison of aerogels, showing similar diameters and moderate transparency. Left: One-pot-processed isocyanate crosslinked aerogel. Right: Traditionally processed version.
Ongoing work involves full development of the factors contributing to the one-pot process, such as aging time, reaction times, optimal stoichiometric conditions, and optimum combinations of reaction initiator in cases where appropriate. A patent application for this new technology has been filed.
Last updated: December 14, 2007
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