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A series of poly [B-(methylamino)borazine] were synthesized by thermolysis of a monomeric B-tri-(methylamino)borazine at various temperatures between 150 and 200 °C and then characterized for suitability as a fiber precursor. Polymerization mechanisms and polymer architectures are discussed. It was shown that poly-[B-(methylamino)borazine] represents a network combining a majority of -N(CH 3 )-bridges with a small proportion of B-N bonds, both connecting borazine rings, and -N(H)CH 3 groups. Both the ratio between flexible -N(CH 3 )-bridges and rigid B-N bonds and the relative amounts of plasticizing -N(H)CH 3 groups cause different responses to thermal properties and spinnability (glass transition, spinning temperatures, melt throughput, and fiber drawing). Based on fiber shape visualization using CCD camera during extrusion, appreciable meltspinnable compounds are prepared between 160 and 185 °C. Such polymers display a chemical formula of [B 3.0 N 4.4(0.1 C 2.0(0.1 H 9.3(0.2 ] n (n ∼ 7.5), a glass transition between 64 and 83 °C, tailored flexibility, and sufficient plasticity to successfully produce fine-diameter green fibers.

Three silicon oxycarbide samples with different carbon contents are analyzed in the present study with respect to their hightemperature creep behavior. The tests were performed in compression at 1100°C, 1200°C, and 1300°C; in this temperature range the mechanism of creep relies on viscoelastic flow within the samples and has been modeled with the Jeffreys viscoelastic model. After the release of the applied mechanical stress, a viscoelastic recovery behavior was observed in all samples. The creep behavior of the investigated samples indicates two rheological contributions in SiOC: (i) a high viscous answer, coming from the silica-rich network, and (ii) an elastic response from the segregated carbon phase within the samples. Furthermore, two distinct effects of the carbon phase on the HT creep behavior of SiOC were identified and are discussed in the present paper: the effect of the carbon presence within the SiOC network (the "carbidic" carbon), which induces a significant increase in the viscosity and a strong decrease in the activation energy for creep, as compared to vitreous silica; and the influence of the segregated carbon phase (the "free" carbon), which has been shown to affect the viscosity and the activation energy of creep and dominates the creep behavior in phase-separated silicon oxycarbides.

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PAPER Laura Gottardo et al. Chemistry, structure and processability of boron-modifi ed polysilazanes as tailored precursors of ceramic fi bers Chemistry, structure and processability of boron-modified polysilazanes as tailored precursors of ceramic fibers †

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A novel non-oxide sol-gel system based on the reaction of bis(trimethylsilyl)carbodiimide with methyltrichlorosilanes has been examined quantitatively with regard to its viscoelastic properties. The chemistry and phenomenology of this process are completely analogous to the well-known oxide solgel process. The changes of the elastic modulus G′ and the viscous modulus G′′ have been measured with low strain amplitude and constant frequency versus time. At the crossover point, i.e., G′ ) G′′, the gel point is reached. The gelation time is independent of the oscillatory frequency ω. The dynamic moduli G′ and G′′ have also been measured versus strain amplitude and shear stress amplitude. A relaxation exponent of n ) 0.65, strength S ) 0.25, and G′ (end of reaction) ) 10 5 Pa‚s have been determined.