THERMOLYSIS OF POLYHEDRAL OLIGOMERIC SILSESQUIOXANE (POSS) MACROMERS AND POSS-SILOXANE COPOLYMERS

The pyrolysis of four polyhedral oligomeric silsesquioxane (POSS) macr omers, Cy(8)Si(8)O(11)(OH)(2), Cy(8)Si(8)O(11)(OSiMe(3))(2), Cy(6)Si(6 )O(9), and Cy(8)Si(8)O(12) (where Cy = c-C6H11) and two POSS-siloxane copolymers [Cy(8)Si(8)O(11)-(OSiMe(2))(n)O-] (n = 1, oligomer average 5.4) has been studied in argon, nitrogen, and under vacuum from 30 to 1000 degrees C, Product gases were analyzed by TGA-FTIR and mass spect roscopy. Analysis of the chars was conducted using cross-polarized (CP ) and magic angle spinning (MAS) NMR spectroscopy, X-ray diffraction, density measurements, and gas adsorption analysis. All of the POSS mac romers showed a propensity toward sublimation, while the POSS-siloxane copolymers underwent-a complex depolymerization-decomposition process . For the copolymer [Cy(8)Si(8)O(11)-(OSiMe(2))(5.4)O-] this process i ncluded the evolution of cyclic dimethylsiloxanes at 400 degrees C, cy clohexyl hydrocarbons from 450 to 550 degrees C, and H-2 liberation fr om 700 to 1000 degrees C. Loss of the silsesquioxane ''cage'' structur e occurred upon heating from 450-650 degrees C and after the evolution of most of the pyrolysis gases. Changes in both char porosity and den sity accompanied the structural rearrangements. The activation energie s for pyrolysis under argon or nitrogen was 56 +/- 9 kcal/mol and for oxidation in air was 20 +/- 4 kcal/mol.