Synthetic Control of Molecular Structure in Organic Aerogels
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SYNTHETIC CONTROL OF MOLECULAR STRUCTURE IN ORGANIC AEROGELS
RICHARD W. PEKALA Lawrence Livermore National Laboratory, Livermore, CA 94550
ABSTRACT Organic aerogels have been formed from the aqueous, sol-gel polymerization of resorcinol with formaldehyde. These materials are transparent and have continuous porosity with cell/pore sizes of less than 1000 A. Their microstructure is composed of interconnected colloidal-like particles with diameters of 30-200 A. The particle size, cell size, surface area, and density of the aerogels are predominantly controlled by the catalyst concentration used in gel preparation.
INTRODUCTION The sol-gel processing of metal alkoxides (e.g. tetramethoxy silane, aluminum sec-butylate) is a convenient method for tailoring the properties of inorganic materials at the molecular level. Sol-gel research has principally focused on the manipulation of silicate precursors to form polymeric or colloidal structures in solution. The hypercritical drying of crosslinked silica gels leads to the formation of a special class of open-celled foams referred to as aerogels. Aerogels have an ultrafine cell/pore size (< 1000 A) and a solid matrix composed of interconnected colloidal-like particles or lightly crosslinked polymer chains. These particles or chains have characteristic diameters of less than 100 A. The above microstructure is responsible for the unusual optical, thermal, and acoustic properties of these materials [1,21. Our research has focused on organic syntheses which proceed through a sol-gel transition and can be controlled to give aerogels with specific properties. Organic aerogels have been synthesized from the base catalyzed, aqueous reaction of resorcinol with formaldehyde. In this reaction, resorcinol (1,3 dihydroxy benzene) is a trifunctional monomer capable of adding formaldehyde in the 2,4, and/or 6 ring positions. These intermediate products condense into polymeric "clusters" with diameters ranging from 30-200 A. The resorcinol-formaldehyde (RF) "clusters" contain surface functional groups (-CH 2 OH) which lead to further crosslinking and eventual gel formation. RF gels are dark red in color and transparent, indicative of their ultrafine pore size. If the solvent in the pores of the gel is exchanged with a monomer such as methyl methacrylate, it is possible to form transparent molecular composites by polymerizing the monomer with a free-radical initiator or UV light. The final molecular composite consists of two continuous phases with poly(methyl Mat. Res. Soc. Symp. Proc. Vol. 171. ©1990 Materials Research Society
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methacrylate) being the dominant phase (80-95% by volume). In order to obtain organic aerogels, the RF gels are hypercritically dried from carbon dioxide. The resultant aerogels are dark red in color and transmit light. Because RF aerogels consist of a highly crosslinked aromatic polymer, they can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. In a sense, both RF and carbon aerogels are molecular composites with air being the dominant ph
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