From Minierals to Materlals: A Facile Synthetic Route to Preceraic Polymers for Aluminum Oxide
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FROM MINI•ERALS TO MATERlALS: A FACILE SYNTHETIC ROUTE TO PRECERA•IC POLY•IERS FOR ALUMINUM OXIDE
ALLEN W.APBLETT, CHRISTOPHER C. LANDRY, MARK R. MASON and ANDREW R. BARRON* Department of Chemistry and Materials Research Laboratory, Harvard University, Cambridge, Massachusetts 02138 ABSTRACT Reaction of boehmite, [AI(O)(OH)]n, with an excess of carboxylic acid (HO2 CR) results in the formation of the carboxy substituted alumoxanes, [AI(O)x(OH)y(O 2 CR)z]n where 2x + y + z = 3 and R = alkyl substituents. The
alumoxanes have been fully characterized by SEM, elemental analysis, IR and multinuclear NMR spectroscopy. The physical properties of the alumoxanes are highly dependent on the identity of R, and range from insoluble crystalline powders, e.g. R = CH3 , to powders which readily form solutions or gels in hydrocarbon solvents, e.g. R = C5H1 1. All of the alumoxanes decompose under mild thermolysis to yield y-alumina.
INTRODUCTION The facile formation of ceramic materials from molecules, either by the pyrolysis of well characterized single-source precursors or via the sol-gel method, has undoubtedly been one of the significant contributions made by chemistry to materials science. 1 However, it is desirable not only to produce the ceramic per se but also to do so in a specific form: for example, a fiber, thin film or three-dimensional body. For this reason, there has been continued research effort aimed at the design of precursors with physical properties suitable for processing prior to pyrolysis. In the case of alumina (A120 3) several groups have employed polymeric alumoxanes as precursors. 2,3 The term alumoxane is used to describe any oligomeric material containing an aluminum-oxygen backbone with pendant organic ligands. Perhaps the most extensive study in this regard is that by Kimura and co-workers 2 in which they demonstrated that carboxy-substituted alumoxanes could be extruded to form preceramic fibers. However, in this and other work, 4 the alumoxanes were prepared via a multistep synthesis involving the hydrolysis of highly pyrophoric aluminum alkyls. Author
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Mat. Res. Soc. Symp. Proc. Vol. 249. 01992 Materials Research Society
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Since this route yields products with variable stoichiometries, it would be desirable to prepare alumoxane preceramic polymers in a one-pot bench-top synthesis. Our recent work has shown that the core structure of alumoxanes is not a linear chain, as often supposed, but similar to that of the mineral boehmite, [AI(O)(OH)]n. 5 From this, the following question may be posed: Can alumoxanes be prepared directly from this mineral? Herein we report the reaction of boehmite with carboxylic acids, and the formation of carboxysubstituted alumoxanes.
EXPERIMENTAL Research grade pseudo-boehmite (100%) was kindly provided by American Cyanamid. All carboxylic acids were obtained commercially and were distilled prior to use. 13C CPMAS and 2 7AI MAS NMR spectra were obtained on a Chemagnetics CMC-200A spectrometer operating at 50.
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