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, NH 3 only reacted with the B–NHCH 3 groups in PPMAB to form B–NH 2 groups, which further reacted with B–NHCH 3 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, NH 3 not only reacted with unreacted B–NHCH 3 but also reacted with B–NHCH 2 CH 2 CH 3 groups to form B–NH 2 groups, which reacted with N–CH 3 bridges in the adjacent PPMAB molecule to form –N(B 3 N 3) 3 - networks. Simultaneously, CH 3 NH 2 and CH 3 CH 2 CH 2 NH 2 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. [ABSTRACT FROM AUTHOR]