The synthesis and properties of two polycarbosilanes that have essentially a
“SiH2CH2” composition is described. One of these
polymers is a highly branched hydridopolycarbosilane (HPCS) derived from
Grignard coupling of CI3SiCH2CI followed by
LiAIH4 reduction. This synthesis is amenable to large scale
production and we are exploring applications of HPCS as a source of SiC
coatings and its allyl-derivative, AHPCS, as a matrix source for SiC- and
C-fiber-reinforced composites. These polymers thermoset on heating at
200-400 °C (or at 100 °C with a catalyst) and give near stoichiometric SiC
with low O content in ca. 80% yield on pyrolysis to 1000 °C. The second
method involves ring-opening polymerization of
1,1,3,3-tetrachlorodisilacyclobutane and yields a high molecular weight,
linear polymer that can be reduced to
[SiH2CH2]n (PSE), the monosilicon analog
of polyethylene. In contrast to high density polyethylene which melts at 135
°C, PSE is a liquid at room temperature which crystallizes at ca. 5 °C. On
pyrolysis to 1000 °C, PSE gives stoichiometric, nanocrystalline, SiC in
virtually quantitative yield. The polymer-to-ceramic conversion was examined
for PSE by using TGA, mass spec, solid state NMR, and IR methods yielding
information regarding the cross-linking and structural evolution processes.
The results of these studies of the polymer-to-ceramic conversion process
and our efforts to employ the AHPCS polymer as a source of SiC matrices are
described.
