Polysiloxane-POSS systems as precursors to SiCO ceramics

In the work, two linear polysiloxanes (D2V and V3 polymers) containing vinyl (Vi) groups regularly distributed along their chains were cross-linked by hydrosilylation with SiH groups containing polyhedral oligomeric silsesquioxane (POSS) molecules, namely: dimethylsiloxy-substituted octahedral oligosilsesquioxane (Q8M8H). The reactions were carried out at equimolar SiVi: SiH groups ratios in methylene chloride, tetrahydrofuran and toluene in order find out the optimum solvent for achieving the highest possible polymer cross-linking levels. In toluene, cross-linking was also performed at lower reactive groups molar ratios. Investigation of the cross-linked systems allowed establishing that hydrosilylation in toluene was the most efficient. Independently of the SiVi: SiH groups molar ratio in the starting reaction medium, D2V polymer reacted with Q8M8H to a higher extent than the V3 one. In some systems, Q8M8H acted not only as a cross-linker, but also as filler. Pyrolysis of the cross-linked polymers conducted in Ar atmosphere led to formation of SiCO ceramics with high yields (63.4-85.1 wt.%). Particularly advantageous effect of incorporation of POSS molecules on ceramic yield was observed for the D2V polymer. SiCO materials obtained are pore- and crack-free and can be applied as chemically and thermally stable protective layers on various substrates.