Synthesis and characterization of silicon-based polymers for use as organic/inorganic hybrids and silicon carbide precursors.

摘要

Organic/inorganic hybrids from silsesquioxanes. This Dissertation describes the synthesis and characterization of methacrylate, epoxy and liquid crystalline (LC)-containing organic/inorganic hybrid materials based on silsesquioxanes. While the methacrylate and epoxy groups provide polymerizable moieties to the hybrids, the LC component is anticipated to provide toughness, and oxidative stability as well as minimize shrinkage during curing. The inorganic silsesquioxane portion, ((RSiO{dollar}sb{lcub}1.5{rcub})sb8{dollar}, cubes), which closely resembles specific crystalline forms of silica and zeolites, may be covalently linked to a variety of organic functional groups. As a result, single-phase organic/inorganic hybrids are formed that when polymerized mimic silica-reinforced composites. The resultant hybrids are liquids at room temperature, and hence allow for single-phase composite processing, ideal for abrasion-resistant coatings and filling molds, as in dental restorative applications. The reactions are based on inexpensive starting materials, have high yields ({dollar}>{dollar}80%), and form soluble products containing up to 65% masked silica. The hybrids were characterized using NMR spectroscopy ({dollar}rmsp1H, sp{13}C, sp{lcub}29{rcub}{dollar}Si), FTIR, size exclusion chromatography (SEC), and thermal analysis (TGA, DSC).; A modified polymethylsilane as a precursor of silicon carbide. It is generally known that polymer precursor routes to silicon carbide (SiC) are very important in the processing of SiC fibers and high performance SiC parts with specific shapes. It is further known that commercial SiC precursor polymers are often not resistant to oxidation, and are based on monomers rich in carbon. As a result of this, their pyrolysis yields SiC rich in oxygen and carbon, a feature which drastically reduces the final materials' ultimate properties (high temperature resistance, tensile strength, modulus). To remedy this, we describe in this work the synthesis and characterization of a modified polymethylsilane (mPMS) which: (1) is derived from low cost materials, (2) provides adequate rheology for fiber spinning and monolith infiltration (M{dollar}rmsb{lcub}W{rcub}{dollar} and PDI of {dollar}approx{dollar}16000 g/mol and {dollar}approx{dollar}8.0, respectively), (3) is resistant to oxidation and (4) provides phase-pure SiC with high ceramic yields ({dollar}>{dollar}80%) upon pyrolysis. The mPMS was synthesized in three steps: (1) the sodium-coupling reaction (Wurtz reaction) of methyldichlorosilane ({dollar}rm CHsb3SiHClsb2{dollar}) yielding PMS, (2) the synthesis of oligomers based on the hydroboration of tetravinylsilane with borane{dollar}cdot{dollar}THF (TVSB) and (3) the Pt-catalyzed coupling of PMS with TVSB (10 wt. %) to give mPMS. The PMS, TVSB and mPMS were characterized using NMR ({dollar}rmsp1H, sp{13}C, sp{lcub}29{rcub}Si{dollar},), TGA, FTIR, and SEC.