Synthesis And Pyrolysis Property Of Polyborazine Precursor For Boron Nitride Fibers

Boron nitride (BN) ceramic fibers are of technological importance because of their potential uses in the fabrication of composite materials with enhanced oxidation resistance, thermal stability, infrared and microwave transparency properties, while polymer derived ceramics (PDCs) route is an efficient method to produce high performance BN fibers. In this dissertation, B-trichloroborazine (TCB) with boron-nitrogen ring structure was synthesized in high yield using boron trichloride (BCl3) and ammonium chloride (NH4Cl) as raw materials. Then polyborazine with good spinnability was also obtained by substitution reaction of TCB with amine and thermal polymerization. Further the h-BN fibers were prepared derived from the polyborazine through melt-drawing, curing process and pyrolysis in NH3.First of all, TCB was synthesized using BCl3 and NH4Cl as raw materials and toluene as solvent by means of improvement of reaction equipments and synthesis conditions, and the yield was 98%. Compared with the conventional methods for TCB, using toluene in place of chlorobenzene as the solvent, this method could reduce the reaction temperature, facilitate the experimental process and improve the synthesis yield evidently.Four kinds of substitution monomers were obtained through the substitution reaction of TCB with propylamine, isopropylamine, methylamine and dimethylamine respectively. Using FTIR, 1H-NMR, 13C-NMR and 11B-NMR, the process of thermal polymerization was studied. The results indicated that the polymerization property was varied with different monomers, and the polymerism activity was weaker according to TMAB, TPiAB, TPAB and TDMAB in sequence. However, the precursor polymer prepared for BN fibers was not acquired from these monomers.Based on the studies above, the co-substitution reaction was presented with propylamine/isopropylamine and methylamine/dimethylamine system. Different monomers were obtained by substitution reaction of TCB with amine in different molar ratio, then two kinds of corresponding polymers with chemistry stability were synthesized by thermal polymerization. These polymers were dissolvable and fusible, and could produce polymer fiber with average diameter 10~15μm by melt-drawing. As a result, the amorphous h-BN was acquired by sintered at 1200℃in N2, which suggested the as-achieved polymers could be used as the precursor for BN fibers.On account of mixture structure with different monomer molecular, the co-substitution reaction influenced the spinnability of polymer badly. Hence choosing propylamine/methylamine system, the chemical component and structure of monomer were well controlled by two-step substitution reactions. When the molar ratio of propylamine/methylamine was 1:2, this synthetical monomer (PMAB1-2) was boron-nitrogen ring with one propylamino and two methylamino. And then, the polyborazine (Poly(PMAB1-2)) was prepared through thermal polymerization at 150℃. The results proved the B-N(CH3)-B bridge bond was formed to build the polymer structure through deamine condensation reaction of methylamino during the polymerization process. This polymer product showed good spinnability, and the continuous polyborazine fibers were obtained with 25~30μm through melt-spinning, which indicated this polymer was suitable precursor for BN fibers.Using FTIR, FTIR, XPS, TG-MS, XRD and element analysis, the curing and pyrolysis process of polyborazine (Poly(PMAB1-2)) were studied. It was found that the polyborazine could not fulfill curing in N2, while it could realize curing heated at 70~80℃for 2hrs in NH3. During the curing process in NH3 atmosphere, the both transamination reaction and condensation reaction occurred, resulting in cross-link between monomer molecular.The pyrolysis process of cured polyborazine was influenced by atmosphere significantly. In N2 atmosphere, depending on the active group in polymer, the polyborazine took place cross-link and decomposition reactions generating methylamine, CH4 and H2, and pyrolysate was mainly h-BN structure remaining a little nitrogen and carbon element at the same time. Compared with N2 atmosphere, NH3 atmosphere promoted the pyrolysis process of cured polyborazine, and the polymer-to-ceramic conversion temperature was lowered by 100~200℃. Moreover the decarbonization effect was evident, and mineralization and crystallization were excellent, which indicated pyrolysis in NH3 atmosphere was beneficial for preparation of high purity h-BN ceramic. In addition, the research showed the pyrolysis process could be divided into four stages, and the trait of each stage was analyzed in detail.Finally, the polyborazine (Poly(PMAB1-2)) with good spinnability was synthesized through two-step substitution reactions of TCB with propylamine/methylamine. And then, BN fibers were prepared derived from the polyborazine through melt-drawing, curing process and pyrolysis at 1200℃in NH3. The as-achieved BN fibers were white and flexible, smooth on surface, and the tensile strength was 0.63GPa.