SYNTHESIS, CHARACTERIZATION, AND CERAMIC CONVERSION REACTIONS OF BORAZINE SILAZANE COPOLYMERS - NEW POLYMERIC PRECURSORS TO SINCB CERAMICS/

The first borazine/silazane backbone copolymers derived from the paren t borazine, B3N3H6, have been obtained by the thermal condensation of borazine with two silazanes, tris-(trimethylsilylamino)silane (TTS), a nd 1,1,3,3,5,5-hexamethylcyclotrisilazane (HCT). Both series of copoly mers are proposed to contain borazine-boron to silazane-nitrogen linka ges. Elemental analyses of the TTS copolymers are consistent with an a pproximate [[(-B3N3H4)-NH]3SiH](x) structural unit, while the HCT copo lymers have compositions ranging from (B3N3H4)(1.00)(N)(1.81)(SiMe(2)) (1.57)(H)(1.6) to (B3N3H4)(1.00)(N)(1.07)(SiMe(2))(1.28)(H)(0.9). Mole cular weight studies indicate large polydispersities, and when molecul ar weight data are combined with intrinsic viscosity results, highly b ranched structures are suggested. Despite their similar compositions, the two series of copolymers yield different types of ceramic material s upon pyrolysis. The TTS copolymers yield BNSi ceramics that are amor phous to 1400 degrees C. Pyrolyses to 1800 degrees C result in further loss of silicon to produce ceramics of variable silicon contents with compositions ranging from B1.00N0.90Si<0.01C<0.01 to B1.00N0.91Si0.14 C0.07. While their DRIFT spectra are consistent with the presence of b oron nitride, the XRD spectra of these materials showed no crystalline boron-containing species, but instead, peaks due to small amounts of crystalline beta-Si3N4, beta-SiC and elemental Si. The ceramics derive d from the HCT copolymers at 1400 degrees C were also amorphous, but s howed greater retention of silicon and carbon with typical composition s of B1.0N1.5Si0.4C0.2. Pyrolyses of the HCT copolymers to 1800 degree s C yielded materials with compositions ranging from B1.00N2.58Si0.94C o0.30 to B1.00N0.86Si0.16C0.19. The DRIFT spectra of the ceramics obta ined at both temperatures again indicate the presence of boron nitride . XRD spectra, however, show crystalline phases of beta-Si3N4, beta-Si C, and Si, but no diffraction from any boron-containing species.