Synthesis And Characterization Of Polycarbosilane Containing Allyl

SiC fiber has lots of excellent characteristics such as high intensity, high modulus, high temperature resistance, antioxygenation, and so on. It is used as fire-resistant materials and reinforceing fiber in advanced composite and has wide application prospects in the fields of aviation, spaceflight, weapon equipments and fire-resistant components for civil use. Polycarbosilane precursor is key material used to prepare high-performance SiC fiber. Its composition, structure and key physical and chemical properties have the decisive effect on SiC fiber performance importantly. Consequently, the synthesis of precursor plays a key role in the preparation of SiC fiber. It is important for SiC fiber used as high-performance structure material to improve fiber stability in high temperature. Reducing oxygen and free carbon contents in the fiber are the most effective way to improve fire-resistant property of SiC fiber.In order to lower oxygen and free carbon contents in SiC material, polycarbosilanes containing allyl are synthesized by making use of Grignard coupling reaction and LiAlH4 reduction, using chloromethyltrichlorosilane and allyl chloride as starting material, in this paper. Si/C equaling 1 in chloromethyltrichlorosilane accords with the design principle of precursor. Adding allyl chloride to reaction system can easily introduce cross-linking group (allyl) into polycarbosilane, which is beneficial to improving ceramic yield. Polycarbosilane containing allyl, meanwhile, can happen to hot cross-linking reaction in the inert atmosphere, avoiding oxygen being introduced into polycarbosilane during the air cross-linking reaction. Polycarbosilanes containing allyl are analyzed by the use of elementary analysis, IR, 1H-NMR and TGA and the solubility of polymers is also analyzed. The effect of different solvents on the structure of polycarbosilane containing allyl is investigated. Polycarbosilanes containing allyl are synthesized and studied considering the effects of three factors, such as reaction temperature, reaction time and allyl chloride content, on the yield of polycarbosilane containing allyl by means of L9(34) orthogonal design The experimental results show that, at the condition of reaction temperature equaling 45℃, reaction time equaling 12h and allyl chloride content equaling 0.30, the yield of polycarbosilane containing allyl reaches 95.4%. The effects of different content of allyl chloride, reaction temperature and reaction time on Si-H content, allyl content, C/Si, oxygen content and chlorine content are investigated, respectively. The experimental results are as follows. When allyl chloride equals 0.10, polycarbosilane synthesized contains higher Si-H content and C/Si is close to 1. The polycarbosilane is considered as ideal precursor. When reaction temperature equals 65℃polycarbosilane synthesized includes higher Si-H content and allyl content. When reaction time equals 12h, polycarbosilane containing allyl embodies higher Si-H content.