AMINE-MODIFIED POLYBORAZYLENES - 2ND-GENERATION PRECURSORS TO BORON-NITRIDE

New second-generation polymeric precursors to BN ceramics have been sy nthesized 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 c ontain boron-bonded amino groups attached to the polyborazylene backbo ne. Analysis of volatile byproducts of the reaction suggests modificat ion of PB with DEA and DPA occurs primarily through dehydrocoupling re actions, while the reaction with HMD involves amine Si-N bond cleavage with elimination of trimethylsilane. Combined molecular weight/infrar ed spectroscopy studies show the polymers are modified throughout the molecular weight distribution. Modification with HMD results in increa sed molecular weights due to cross-linking reactions involving the sil azane. The DEA-PB and DPA-PB polymers have lower molecular weights tha n the starting PB, with the highest amine concentrations in the lower molecular weight fractions, suggesting some backbone scission occurs d uring polymer modification. The modified polymers show increased solub ility in organic solvents compared to the parent PB polymer. Also unli ke PB, the DPA-PB polymers become fluid, without weight loss, in the r ange 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 diamete r. After a brief air-cure, pyrolysis of the polymer fibers under ammon ia yielded similar to 30 mu m BN ceramic fibers of good quality, as de termined by SEM, DRIFT, XRD, and RBS measurements, as well as oxidatio n and mechanical studies.