The Preparation And Ultraviolet Irradiation Crosslinking Of The Liquid Hyperbranched Polycarbosilane

Silicon carbide ceramic,which has excellent tensile strength,high modules,low density,high thermal stability and high oxidation resistance,is one of the most important high-temperature structural ceramics,which is widely applied in aerospace,military,nuclear power,machinery manufacturing,engines and other harsh environment.As a key ceramic matrix precursor material in the polymer infiltration and pyrolysis?PIP?process,polycarbosilane?PCS?has gained comprehensive attention for over 40 years.However,this solid polymer precursor still has some shortcomings in the PIP process:?1?a large amount of solvent is needed to solve it;?2?its low ceramic yield;?3?higher crosslinking temperature,longer crosslinking time and?4?more carbon and oxygen elements to cause the deviration of the silicon and carbon ratio.In this thesis,a liquid hyperbranched polycarbosilane?LHBPCS?with the ratio of Si to C equaling SiC stoichiometry but without unsaturated functional groups was prepared by Grignard coupling of chloromethyltrichlorosilane?Cl3SiCH₂Cl?.Different from traditional thermal or catalytic crosslinking for LHBPCS,UV-crosslinking technology was used.Additionally,in order to improve crosslinking degree and ceramic yield,additional divinyldimethylsilane [?CH₃?2Si?CH=CH₂?2] monomer was mixed with the LHBPCS and cured by UV irradiation.The UV-cured samples were high-temperature pyrolyzed to prepare the silicon carbide ceramics.Gel permeation chromatography?GPC?,Fourier transform infrared spectroscopy?FT-IR?,nuclear magnetic resonance?NMR?,thermogravimetric analysis?TGA?and scanning electron microscopy?SEM?et al were performed to characterize the structure,properties and compositions of polymer precursor or SiC ceramic,which investigated the effects of polymerization process,UV irradiation crosslinking and high temperature sintering.The obtained LHBPCS was a hyperbranched polymer,which contained Si-C_x-Si groups as the main structure and Si-Hx or Si-CH₃ groups as the side structure.The precursor structure could be significantly affected by the polymerization temperature and time,and the optimal polymerization conditions of LHBPCS were carried at 60 ? for 12 h.The weight-average molecular weight and number-average molecular weight of as-synthesized LHBPCS were 2210 and 780,respectively.The LHBPCS precursor was a light yellow liquid,which coincided with the characteristics of Newton fluid.The viscosity of LHBPCS was only 0.046 Pa·s,indicating the excellent wetting properties.And,it could be stored for a long time in the 0 ? environment.Different from traditional thermal or catalyst crosslinking,ultraviolet?UV?irradiation crosslinking was taken.It is found that the LHBPCS precursor can be rapid crosslinking under UV irradiation,and the molecular weight,ceramic yield of LHBPCS showed an increase trend with increasing the UV irradiation time.After 30 min of UV irradiation,71.8 wt% ceramic yield was obtained.In the LHBPCS precursor,the active Si-H groups reacted to form Si-Si crosslinking structure by the silicon hydrogen coupling reaction.In addition,extra divinyldimethylsilane was added into LHBPCS to increase the crosslinking content and ceramic yield.Under UV irradiation,both the-Si-H group and vinyl group of divinyldimethylsilane were consumed.A crosslinking reaction might occur between-Si-H and-C=C-to form Si-C-C-Si crosslinking structure.However,the ceramic yield increased first and then decreased with increasing divinyldimethysilane content,and 79.8 wt% ceramic yield was obtained from the mixture of LHBPCS and 5 wt% divinyldimethylsilane after 30 min of UV irradiation.After 1000 ? high-temperature pyrolysis,a stoichiometric SiC ceramic was got with an approximate composition of SiC0.99O0.13.With the increment of thermal-treatment temperature,ceramic crystal structure gradually formed and improved,and the silicon carbide crystals grew to about 10 nm size after 1400 ? high-temperature treatment for 1 h.The obtained SiC ceramic exhibited excellent high-temperature and oxidation resistance at 1400 ? in air atmosphere,and the ceramic surface could keep the good shape.