Fabrication of Sol-gel Materials with Anisotropic Physical Properties by Photo-cross-linking
A method to vary locally the physical properties of a porous material is presented. A wet gel is first prepared following conventional sol-gel techniques. The pore walls are derivatized by adding to the gelling solution a silane carrying a polymerizable moiety such as trimethoxysilylpropyl. The solvent of the wet gel monolith is then exchanged with a solution of a monomer such as styrene and a photoinitiator such as 2,2′-azobis-isobutyronitrile. Illumination with ultraviolet light initiates polymerization which in turn engages the moiety dangling from the pore surfaces. Supercritically dried monoliths were characterized with techniques such as field-emission scanning electron microscopy (SEM), methylmethacrylate atomic force microscopy (AFM), Fourier transform infrared (FT-IR) spectroscopy, and Brunauer-Emmett-Teller surface area measurements. These structural characterization techniques showed that the silica nanoparticles making up the backbone of the monoliths were cross-linked by a polymer conformal coating. Mechanical characterization was carried out with nanoindentation and the three-point flexural method and showed that the properties of uniformly photo-cross-linked monoliths could be varied by varying exposure time. So, for example, the monolith density could be varied between about 0.21 and 0.97 g ·cm-3, the porosity between 6 and 87%, and Young's modulus between 9 and about 1800 MPa. Overall, the characterization techniques show that photo-cross-linked monoliths have physical and mechanical properties comparable and often superior to those of monoliths obtained by thermally initiated cross-linking (see, for example, Leventis, N.; Sotiriou-Leventis, C.; Zhang, G.; Rawashdeh, A.-M. M. Nano Lett. 2002, 2, 957-960). More importantly photo-cross-linking allows fabrication of monoliths with anisotropic physical properties. We demonstrate the modulation capabilities of our method by producing transparent and opaque regions within the same monolith and by producing multifunctional two- and three-dimensional patterns.
C. Wingfield et al., "Fabrication of Sol-gel Materials with Anisotropic Physical Properties by Photo-cross-linking," Chemistry of Materials, American Chemical Society (ACS), Apr 2009.
The definitive version is available at http://dx.doi.org/10.1021/cm803374b
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