Structural investigation of the first stages of pyrolysis of Si-C-O preceramic polymers containing Si-H bonds

The polymer-to-glass transformation in the Si-C-O system usually involves cross-linking reactions at the C sites, which transform for example =Si-(C) under bar H-3 sites into (C) under bar(Si)(4) sites, leading to the formation of a silicon oxycarbide network. Identification of the various C sites that form during pyrolysis under inert atmosphere is rather difficult from C-13 MAS-NMR spectra, due to strong overlap of resonance lines, which prevents an accurate determination of the chemical shift values. This paper reports on the use of a spectral editing technique called Inversion Recovery Cross Polarization (IRCP), which, when combined with Cross Polarization (CP) technique, allows us to identify and quantify the formation of Si-(C) under bar H-2-Si bridges during the early stages of the pyrolytic transformation of a polysiloxane containing Si-CH3 and Si-H groups. Several Si-C-O systems characterized by the same initial number of Si-CH3 bonds but with increasing number of Si-H bonds, have been investigated by MAS-NMR as well as thermogravimetric analysis coupled with mass spectrometry (TG/MS). The Si-H bonds play clearly an active role in the insertion of C atoms within the silica network.