No abstract

New second-generation polymeric precursors to BN ceramics have been synthesized in high yield by the reaction of polyborazylene (PB), [B 3 N 3 H ∼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 °C. The DPA-PB polymers were melt-spun using a crude ram extruder to yield continuous polymer fibers 30-40 µm in diameter. After a brief air-cure, pyrolysis of the polymer fibers under ammonia yielded ∼30 µm BN ceramic fibers of good quality, as determined by SEM, DRIFT, XRD, and RBS measurements, as well as oxidation and mechanical studies.

The first borazine/silazane backbone copolymers derived from the parent borazine, B3N3H6, have been obtained by the thermal condensation of borazine with two silazanes, tris-(trimethylsilylamino)silane (TTS), and 1,1,3,3,5,5-hexamethylcyclotrisilazane (HCT). Both series of copolymers are proposed to contain borazine-boron to silazane-nitrogen linkages. Elemental analyses of the TTS copolymers are consistent with an approximate [[(-B3N3H4)-NH]3SiHL structural unit, while the HCT copolymers have compositions ranging from (B3N3H4)i.oo(N)i.8i(SiMe2)i.67(H)i.6 to (B3N3H4)i.oo(N)i.o7(SiMe2)i.28(H)o.9. Molecular weight studies indicate large polydispersities, and when molecular weight data are combined with intrinsic viscosity results, highly branched structures are suggested. Despite their similar compositions, the two series of copolymers yield different types of ceramic materials upon pyrolysis. The TTS copolymers yield BNSi ceramics that are amorphous to 1400 °C. Pyrolyses to 1800 °C result in further loss of silicon to produce ceramics of variable silicon contents with compositions ranging from Bi.ooNo.9oSi

Three new series of processible polymeric precursors (PIN-HPZ, BCP-HPZ, DEB-HPZ) to SiNCB ceramic materials have been synthesized by reaction of hydridopolysilazane (HPZ) with the monofunctional boranes, pinacolborane (PIN-H), 1,3-dimethyl-1,3-diaza-2-boracyclopentane (BCP-H), and 2,4-diethylborazine (DEB-H). Polymers can be prepared with a controllable range of boron contents from ∼1 to 5%. Spectroscopic and chemical studies indicate the boranes are attached to the hydridopolysilazane backbone via B-N linkages that primarily result from dehydrocoupling reactions. The isolation of small amounts of trimethylsilane and Me 3 SiNH-substituted borane side products (i.e., PIN-NHSiMe 3 , BCPNHSiMe 3 , DEB-NHSiMe 3 ) from the polymer reactions, as well as from model reactions of the boranes with hexamethyldisilazane, also suggest borane reactions at the Si-N bonds of the HPZ backbone lead to some polymer chain cleavage. Consistent with these observations, combined molecular weight/infrared spectroscopy studies show that although the polymers are modified throughout the molecular weight distribution, the modified polymers have lower molecular weights than the starting HPZ, with the highest borane concentrations in the lower molecular weight fractions. The glass transition temperatures (T g ) of the PIN-HPZ and BCP-HPZ polymers are in the 100-120°C range, while those of the DEB-HPZ polymers decreased to as low as 25°C with increasing modification. The polymers each showed regions of thermal stability, thus allowing the formation of PIN-HPZ, BCP-HPZ, and DEB-HPZ polymer fibers by melt spinning. Pyrolysis of these fibers to 1200°C then yielded SiNCB ceramic fibers. Studies of the polymer to ceramic conversion reactions showed the modified polymers yield SiNCB ceramics containing ∼1-3% boron at 1400°C, with the highest boron contents in the PIN-HPZ derived samples. At 1800°C, the PIN-HPZ derived ceramic exhibited improved thermal stability with up to 23% nitrogen contents. In comparison, the ceramics obtained from unmodified HPZ, BCP-HPZ, and DEB-HPZ retained less than 4% nitrogen at this temperature. While the BCP-HPZ and DEB-HPZ derived ceramics showed crystallization properties similar to the ceramic obtained from unmodified HPZ, the PIN-HPZ derived ceramic was amorphous to 1600°C and at 1800°C showed only weak diffraction from -SiC.