Characterization of the Products of the Pyrolysis of Inorganic Polymers Used as Dispersants
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CHARACTERIZATION OF THE PRODUCTS OF THE PYROLYSIS OF INORGANIC POLYMERS USED AS DISPERSANTS WILLIAM D. SAMUELS%, GREGORY J. EXARHOS. SARAH D. BURTON, BARBARA J. TARASEVICH, AND DANIEL J. STASKO* *Pacific Northwest Laboratory, Richland, WA 99352 "**Student Trainee from Kent State University,
A series of alumina particles coated with inorganic polymers, that have utility as preceramic dispersants, have been pyrolyzed and the products of the pyrolysis studied to help determine the possible decomposition routes. The inorganic polymers, bis(amino)polyphosphazenes and siloxanes, appear to undergo loss of the organic sidechains via a nucleophilic process which would incorporate inorganic backbone into the alumina matrix. The bis(alkoxy)polyphosphazenes decompose by nucleophilic attachment but also exhibit polymer loss by formation of cyclic oligomers. WTODUCION Preparation of densified ceramic materials by the usage of polymer dispersant in the colloidal processing stage has influenced the efficiency of particle packing[1-3]. The formation of the stabilized colloidal dispersion is thought to be controlled by the localized interactions among the colloid particle, polymer and the solvent molecule. Preliminary work on the microstructural interactions of inorganic polymers, solvents and oxide ceramics has been an area of study in our research group[4,5]. Non-destructive testing of colloidal-polymer interactions has been accomplished by the spectroscopic methods of nuclear magnetic resonance(NMR) and laser Raman scattering. The utility of the phosphazenes for spectroscopic analysis is due the high magnetic susceptibility and natural abundance of the 31p isotope. The other possible candidates for inorganic polymers, siloxanes, silazanes, borazenes, etc., are more difficult to probe using NMR methods. The inorganic polymer of choice for our studies are based upon the polyphosphazene. Such materials are composed of a phosphorous nitrogen backbone with two substituents attached at the phosphorous atom. The basic synthetic method for the formation of phosphazenes has been developed by H.R. Allcock.[6] Polyphosphazenes are isoelectronic with siloxanes. One of the drawbacks to the usage of organic dispersant in colloidal processing has been the failure of the dispersant, upon sintering of the green body, to be completely removed. Sun[7] has shown that poly(methyl methacrylate)(PMMA) thermally decomposes by "unzipping" to give methyl methacrylate; a second possible pathway leads to the formation-of methanol and aluminum carboxylates, as shown in Figure 1. This second pathway leads to the formation of residual carbon during decomposition of the polymer. Polyphosphazenes also were noted to be thermally unstable and undergo depolymerization to form rings of 3 and 4 PN repeat units. Two modes of decomposition of polyphosphazenes are envisioned as in Figure 2. Certain substituents are known to be susceptible towards nucleophilic attack.[8] The polyphosphazene might have an added advantage of the polymer backbone being incorporated into th
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