A New Synthetic Route to Organic Aerogels
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A NEW SYNTHETIC ROUTE TO ORGANIC AEROGELS R.W. PEKALA AND C.T. ALVISO, Department of Chemistry & Materials Science, Lawrence Livermore National Laboratory, Livermore, CA 94550 ABSTRACT The aqueous, sol-gel polymerization of melamine with formaldehyde, followed by supercritical extraction, leads to the formation of a new type of organic aerogel. Synthetic conditions (e.g. reaction time, pH) affect the density, transparency, and microstructure of the resultant aerogels. Unlike previous organic aerogels based upon resorcinol-formaldehyde, the melamine-formaldehyde aerogels are both colorless and transparent. Low densities (0.1-0.8 g/cc), high surface areas (-1000 m 2 /g), and optical clarity are only a few of the promising characteristics of this new material. INTRODUCTION Aerogels are a special class of low density, open-cell foams. These materials have continuous porosity, ultrafine cell/pore sizes (< 500 A), and a solid matrix composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 A. This microstructure is responsible for the unusual acoustic, mechanical, optical, and thermal properties of these materials [1,2]. The hydrolysis and condensation of metal alkoxides (e.g. tetramethoxy silane) is the most common synthetic route for the formation of inorganic aerogels [3-5]. Our research has focused on sol-gel polymerizations that lead to organic aerogels. The reaction pathway, microstructure, and properties of these new materials are analagous to their inorganic counterparts. Furthermore, our organic aerogels are distinctly different from the 'aerogels' that Kistler prepared from nitrocellulose, cellulose, agar, and egg albumin [6]. Previous work has shown that organic aerogels can be formed from the base catalyzed, aqueous polymerization of resorcinol (1,3 dihydroxy benzene) with formaldehyde [7-10]. Although resorcinol-formaldehyde (RF) aerogels exhibit minimal light scattering, these materials are dark red in color and have a large absorption coefficient within the visible spectrum. The color centers in RF aerogels result from oxidation products (e.g. quinones) formed during the sol-gel polymerization. Their presence has limited the use of these materials in certain optical applications where an aerogel is required to be both colorless and transparent. In order to circumvent this problem, we are forming colorless gels from the aqueous polycondensation of melamine (2,4,6 triamino s-triazine) with formaldehyde. Synthetic conditions (e.g. pH, reaction time) affect both the transparency and microstructure of the resultant melamine-formaldehyde (MF) aerogels. This paper describes in detail the synthesis and characterization of these new organic aerogels.
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
792
EXPERIMENTAL The polycondensation of 3-6 moles of formaldehyde (37.6 %; methanol stabilized; J.T. Baker) with 1 mole of melamine (99+ %; Aldrich Chemical) was carried out in deionized and distilled water using 10-100 millimoles of sodium hydr
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