Thermolysis of Polyhedral Oligomeric Silsesquioxane (POSS) Macromers and POSS-Siloxane Copolymers.
Thermolysis of Polyhedral Oligomeric Silsesquioxane
(POSS) Macromers and POSS-Siloxane Copolymers.
Mantz, R. A.; Jones, P. F.; Chaffee, K. P.; Lichtenhan,
J. D.; Gilman, J. W.; Ismail, I. M. K.; Burmeister, M.
Chemistry of Materials, Vol. 8, No. 6, 1250-1259, 1996.
pyrolysis; siloxanes; monomers; vapor phases
The pyrolysis of four polyhedral oligomeric
silsesquioxane (POSS) macromers, Cy8-Si8O11(OH)2,
Cy8Si8O11(OSiMe3)2, Cy6Si6O9, and Cy8Si8O12 (where Cy =
c-C6H11) and two POSS-siloxane copolymers
[Cy8Si8O11-(OSiMe2)nO-] (n = 1, oligomer average 5.4)
has been studied in argon, nitrogen, and under vacuum
from 30 to 1000 deg C. Product gases were analyzed by
TGA-FTIR and mass spectroscopy. Analysis of the chars
was conducted using cross-polyarized (CP) and magic
angle spinning (MAS) NMR spectroscopy, X-ray
diffraction, density measurements, and gas adsorption
analysis. All of the POSS macromers showed a propensity
toward sublimation, while the POSS-silioxane copolymers
underwent a complex depolymerization-decomposition
process. For the copolymer [Cy8Si8O11-(OSiMe2)5.4O-]
this process included the evolution of cyclic
dimethylsiloxanes at 400 deg C, cyclohexyl hydrocarbons
from 450 to 550 deg C, and H2 liberation from 700 to
1000 deg C. Loss of the silsesquioxane "cage" structure
occurred upon heating from 450-650 deg C and after the
evolution of most of the pyrolysis gases. Changes in
both char porosity and density accompanied the
structural rearrangements. The activation energies for
pyrolysis under argon or nitrogen was 56 + 9 kcal/mol
and for oxidation in air was 20 + 4 kcal/mol.