Nanotech 2010

Nanohybrid silica/polymer subcritical aerogels

A. Fidalgo, L.M. Ilharco, J.P.S. Farinha, J.M.G. Martinho
Centro de Química-Física Molecular, Instituto Superior Técnico, PT

Keywords: aerogels, hybrid nanocomposites, insulators


Novel nanohybrid silica/polymer aerogels were produced by the soft sol-gel process, under subcritical conditions. They are hydrophobic, monolithic, and with improved mechanical properties. These materials may well be the answer to low cost-high performance insulators. Previous work proved the possibility of obtaining pure silica aerogels by subcritical drying of aged alcogels.[1] The inclusion of core-shell polymer nanoparticles (PNP) at the first stages of the sol-gel process builds up a structure stiff enough to withstand capillary pressure, yielding machineable monoliths with better mechanical properties (Figure 1).[2-4] The purpose of the present work was to further improve the mechanical and thermal behavior of those hybrid aerogels, and to decrease their production costs in view of a broader application range. These goals were achieved by using different silica precursors and different polymer nanoparticle sizes. The hybrid alcogels were prepared by a two step acid/basic catalyzed co-hydrolysis/co-condensation of a silica precursor and cross-linked core-shell particles. These consist of a trimethoxysilyl-functionalized poly(butyl methacrylate) shell and a poly(butyl methacrylate-co-butyl acrylate) core (TMS-PNP). Aqueous sodium silicate was used in alternative to tetraethoxysilane as silica precursor. After convenient aging, the gels were washed with isopropanol and dried in a quasi-saturated solvent atmosphere (Figure 2). The chemical and physical properties of the dry gels have been analyzed by volume shrinkage upon drying, envelope density determinations, scanning electron microscopy (SEM), nitrogen sorption isotherms and diffuse reflectance infrared spectroscopy (DRIFT). Envelope densities as low as 0.20 g cm-3 were achieved, corresponding to a porosity of 89%, distributed in a bimodal pore structure of interconnected macropores and mesopores. The mechanical properties were measured by unidirectional compression tests, and the thermal conductivities were determined using a modified Lee’s disk method. Aqueous sodium silicate proved to be an excellent silica precursor with cost benefits, and the size of the polymer nanoparticles was a key parameter of the aerogel’s mechanical resistance. The high performance of these nanostructured composites meets the most severe standards of modern insulators. References: 1. Fidalgo, A.; Rosa, M.E.; Ilharco, L.M., Chem. Mater. 2003, 15, 2186-2192. 2. Fidalgo, A.; Farinha, J.P.S.; Martinho, J.M.G.; Rosa, M.E.; Ilharco, L.M., Chem. Mater. 2007, 19, 2603-2609. 3. Ilharco, L.M.; Fidalgo, A.; Farinha, J.P.S.; Martinho, J.M.G.; Rosa, M.E., J. Mater. Chem. 2007, 17, 2195-2198. 4. Martinho, J.M.G.; Ilharco, L.M.; Farinha, J.P.S.; Fidalgo, A.M.; Martinho, P.O., PCT-2006-000010, 04/04/2006 and U.S. Patent Application No. 11/910,716.
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