Improved Synthesis Of Polycarbosilane

Polycarbosilane (PCS) is used as a precursor to prepare SiC fibers and ceramic matrix composites. The structure and the properties of PCS directly affect the subsequent preparation as well as the quality of the final products. National University of Defense and Technology successfully synthesized PCS precursor by pyrolysis under normal pressure at high temperature, from which continuous SiC fibers were prepared. The oxygen and free carbon in SiC fiber deteriorate the properties of the fiber. Those components come from the PCS directly or indirectly, such as low amount of Si-H active group and high content of free carbon in PCS. In addition, low yield leads to high cost of PCS and restricts its applications. Therefore, to modify the composition and structure of PCS as well as improving synthesis yield had become more and more important. LPS, which is used as feed stock to synthesize PCS under normal pressure at high temperature, is composed of the backbone structure of polysilane (Si-Si) and PCS (Si-C). Active group Si-H in the chain can easily react with double bonds by hydrosilication. If adding some divinylbenzene (DVB) into LPS, hydrosilication will happen between the side group of CH₂=CH- in DVB and Si-H bond in LPS. Therefore, it accelerates the splitting and rearrangement of Si-Si bond and improves the content of Si-H group. At the same time, yield and molecular weight of PCS can also be increased.In this work, a series of DVB modified PCS (PPCS) were synthesized and studied considering the effects of five factors, such as the reaction temperature (T_R), heating speed (V_H), weight percent of DVB (W_PD), pyrolysis temperature (T_P) and heat retaining time (t_H), on the yield of PPCS by means of ¹⁶L₄⁵orthogonal design. The experimental results showed that, at the comditions of TR = 420℃,V_H = 6℃/hr,W_PD = 1.5%, t_H = 5hrs, T_P = 500℃, the yield of PPCS reached to 50%. In addition, the order of influence priority of the 5 factors is V_H > T_R > W_PD > > T_P > t_H. In addition, under this optimized conditions, the effect of DVB content was studied systematically by changing W_PD from 1.5wt.% to 2.5 and 4.5wt.%.Using FT-IR, UV, ¹H-NMR, ¹³C-NMR, ²⁹Si-NMR, XPS and TG-DTA, the structure and the composition of the modified PPCS were characterized. The reactants and reflux liquid at different temperature stage were also studied on-line. The mechanism of LPS/DVB reaction was discussed according to the analysis results. The PPCS synthesized by LPS/DVB under normal pressure and at high temperature was a polymer consisted by Si-C main chain. Two kinds of silicon groups, SiC₄ and SiC₃H, were found in PPCS which are made up of Si-CH₃, Si-H, Si-CH₂-Si. The C/Si value of the PPCS with melting point of 200℃was 1.70. The ratios of Si-H/C-H and SiC₃H/SiC₄ of PCS...