Insight into the evolution process from novel polyborazine precursor PPMAB to inorganic BN fiber

Understanding the elemental and structural evolution from green fibers to inorganic fibers is important for fabricating high-performance ceramic fibers via the polymer-derived ceramics (PDCs) route. In a previous study, we found a novel polyborazine precursor, poly[2-propylamino-4,6-bis(methylamino)borazine-co-tri(methylamino)borazine] (PPMAB), which possesses a high ceramic yield and outstanding spinnability. Herein, we provide an insight into the evolution process from PPMAB to inorganic BN fibers. During the curing process, NH3 only reacted with the B–NHCH3 groups in PPMAB to form B–NH2 groups, which further reacted with B–NHCH3 to form B–NH–B bridges. Therefore, crosslinked networks were formed in the cured fibers and the gel content increased from 52.3 to 92.1 wt%. In the nitridation process, NH3 not only reacted with unreacted B–NHCH3 but also reacted with B–NHCH2CH2CH3 groups to form B–NH2 groups, which reacted with N–CH3 bridges in the adjacent PPMAB molecule to form –N(B3N3)3- networks. Simultaneously, CH3NH2 and CH3CH2CH2NH2 escaped from the fibers, and the carbon content of the fibers was reduced from 20.588 wt% to 0.115 wt%. By controlling the curing and nitridation processes, the curing degree can be enhanced, whereas the pore defects resulting from gas molecular escape can be effectively reduced. Thus, dense inorganic BN fibers with excellent dielectric properties (2.33 < ε < 2.5, tan δ < 3.4ⅹ10−3) and high tensile strength (1.6 GPa) were fabricated. The present study not only provides ideas for the design and synthesis of polyborazine precursors, but also for the fabrication of high-performance BN fibers.