Conversion of Polymers of Methyl- and Vinylsilane to Sic Ceramics
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ABSTRACT
Poly(methylsilane) and poly(vinylsilane) were synthesized using a titanocene catalyst, and their pyrolytic conversion to ceramics was followed using a combination of thermal analysis and infrared spectroscopy. The two polymers have distinctly different backbone structures, as determined by 29 Si NMR; methylsilane polymerizes to a polysilane, while vinylsilane polymers have a predominately polycarbosilane, backbone, with some polysilane structure as well. The pyrolysis path and char yield were dependent primarily on backbone structure, with little influence of polymer molecular weight. The majority of the weight loss on conversion occurs below 650 'C, although bond rearrangement continues to 1400 'C. Poly(vinylsilane) produced a C-rich Si-C ceramic in which the carbon was dispersed on a sufficiently fine level to show resistance to oxidation on heating in air to 1400 'C.
INTRODUCTION Use of organometallic polymers as precursors to ceramics enables fabrication techniques not attainable with more traditional powder sintering approaches, particularly in the areas of fiber and film formation and in the development of fiber reinforced ceramic matrix composites having complex shapes. The composition and structure of the precursor polymer, as well as the pyrolysis cycle and atmosphere, determine the composition and morphology of the final ceramic, as well as its physical and mechanical properties. Control of polymer structure, molecular weight and curing mechanism permits tailoring the polymer rheology for fiber spinning or composite matrix fabrication, and can determine ceramic yield and ultimate properties. We previously 13 have reported on the synthesis and characterization of a number of polymers of alkyl and alkenylsilanes. The present work is concerned with characterizing the pyrolytic conversion of these polymers to Si-C ceramics, with the objective of understanding the relationship between polymer structure and the conversion process, as well as on ceramic stoichiometry. The intent of the work is the development of a series of polymers having tailorable rheological behavior that are suitable precursors to ceramic composite matrices and ceramic coatings.
623 Mat. Roes. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL Poly(methylsilane) and poly(vinylsilane) were synthesized using a titanocene catalyst 1 ' 3-6. Two types of synthesis conditions were employed: a room temperature synthesis, for periods of nominally 30 days (condition A), and a protocol in which polymerization was started at room temperature for a period of seven days, and then continued at a temperature of 50 'C for an additional seven days (condition B). One of two solvents, tetrahydrofuran (THF) or cyclohexene, was used for polymerization. The polymers were characterized by 1 H, 13C and "S5i nuclear magnetic resonance to determine backbone structure, using a Bruker AC-300 NMR spectrometer and chloroform-d as the solvent. FTIR spectra of as-synthesized polymer samples were obtained using a Perkin-Elmer Model 1750
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