Preparation of silicon carbide fibers from polysilanes: (I) Role of polyvinylsilazane as a spinning aid for polycarbosilane. (II) Synthesis and characterization of polymethylsilane

The effect of the addition of polyvinylsilazane (PSZ) on the characteristics (i.e., spinnability, rheological behavior, wetting behavior, evaporation behavior, etc.) of polycarbosilane (PCS) solutions was investigated. Spinnability of PCS solution was characterized by number of breaks occurring during spinning and amount of fibers formed after spinning. PCS and PCS+PSZ solutions were characterized by measuring surface tension, contact angles and rate of solvent evaporation. Effect of PSZ on mechanical properties of SiC fibers prepared from PCS and PCS+PSZ solutions was also investigated. Chemical changes taking place in PCS and PCS+PSZ fibers during heat treatment from 40-600{dollar}spcirc{dollar}C in nitrogen were studied by Fourier transform infrared spectroscopy (FTIR).; Addition of PSZ to PCS greatly improved spinnability of PCS solutions. Significant differences in wetting characteristics were observed for PCS solutions and PCS+PSZ solutions deposited on stainless steel and teflon substrates, as well as on PCS-coated and PCS+PSZ-coated stainless steel substrates. The rate of evaporation of solvent was higher for PCS solution than for PCS+PSZ solution at identical polymer concentrations. As-spun PCS and PCS+PSZ fibers developed similar tensile strengths and rupture strains. After heat treatment at 400{dollar}spcirc{dollar}C in nitrogen, PCS+PSZ fibers showed higher tensile strength and rupture strain compared to PCS fibers. Based on FTIR spectra of PCS and PCS+PSZ fibers during heat treatment from 40-600{dollar}spcirc{dollar}C, it is suggested that PSZ acts as a cross-linking aid for PCS. PCS+PSZ fibers developed higher tensile strengths than PCS fibers at all heat treatment temperatures between 200-1150{dollar}spcirc{dollar}C.; SiC fibers were fabricated from polymethylsilane (PMS) and PMS/PCS polymer blends. PMS polymers were synthesized by a Wurtz-coupling polymerization of methyldichlorosilane (MDCS) and methyltrichlorosilane (MTCS) in 70:30 wt% proportion with sodium in refluxing toluene. The addition of polar solvents (i.e., THF and 1,4-dioxane) to toluene improved yields and increased the molecular weight of PMS polymers. As-prepared PMS polymers with additives were heat-treated to increase molecular weight to permit fiber spinning. As-prepared PMS polymers were also fractionally-precipitated to isolate higher molecular weight fractions suitable for fiber spinning. The heat treatment approach was ineffective in obtaining molecular weight increases reproducibly. Fractionally- precipitated PMS polymers were useful in preparation of high strength pyrolyzed SiC fibers. These fibers, however, exhibited poor thermal stability at high temperatures. More investigations will be needed to address this problem.