Amine-modified polyborazylenes: Second-generation precursors to boron nitride

New second-generation polymeric precursors to BN ceramics have been synthesized in high yield by the reaction of polyborazylene (PB), [B3N3H similar to 3.5](x), with diethylamine (DEA), dipentylamine (DPA), and hexamethyldisilazane (HMD). Elemental analyses and the spectroscopic data indicate that the resulting DEA-PB, DPA-PB, and HMD-PB polymers contain boron-bonded amino groups attached to the polyborazylene backbone. Analysis of volatile byproducts of the reaction suggests modification of PB with DEA and DPA occurs primarily through dehydrocoupling reactions, while the reaction with HMD involves amine Si-N bond cleavage with elimination of trimethylsilane. Combined molecular weight/infrared spectroscopy studies show the polymers are modified throughout the molecular weight distribution. Modification with HMD results in increased molecular weights due to cross-linking reactions involving the silazane. The DEA-PB and DPA-PB polymers have lower molecular weights than the starting PB, with the highest amine concentrations in the lower molecular weight fractions, suggesting some backbone scission occurs during polymer modification. The modified polymers show increased solubility in organic solvents compared to the parent PB polymer. Also unlike PB, the DPA-PB polymers become fluid, without weight loss, in the range 75-95 degrees C. The DPA-PB polymers were melt-spun using a crude ram extruder to yield continuous polymer fibers 30-40 mu m in diameter. After a brief air-cure, pyrolysis of the polymer fibers under ammonia yielded similar to 30 mu m BN ceramic fibers of good quality, as determined by SEM, DRIFT, XRD, and RBS measurements, as well as oxidation and mechanical studies.