Effect of the dimethylsilyloxy co-monomer “D” on the chemistry of polysiloxane pyrolysis to SiOC

0461612 - ÚSMH 2017 RIV NL eng J - Journal Article
Havelcová, Martina - Strachota, Adam - Černý, Martin - Sucharda, Zbyněk - Šlouf, Miroslav
Effect of the dimethylsilyloxy co-monomer “D” on the chemistry of polysiloxane pyrolysis to SiOC.
Journal of Analytical and Applied Pyrolysis. Roč. 117, JAN (2016), s. 30-45. ISSN 0165-2370. E-ISSN 1873-250X
Grant - others:OPPK(XE) CZ.2.16/3.1.00/21538
Program: OPPK
Institutional support: RVO:67985891 ; RVO:61389013
Keywords : silicon oxycarbide * siloxane * pyrolysis * composites * fibers
Subject RIV: CD - Macromolecular Chemistry
Impact factor: 3.471, year: 2016

The effect of the dimethylsilyloxy co-monomer "D" on the chemistry of polysiloxane pyrolysis to silicon oxycarbide (SiOC) glass was studied with the aim of its optimization for the preparation of refractory composites with ceramic fibers. Reasonably small weight losses (shrinkage), but also some temporary plasticity of the material during its pyrolysis was sought. The pyrolysis chemistry was varied by preparing precursors from methyltriethoxysilane (T; main monomer) and dimethyldiethoxysilane (D; co-monomer) in different ratios, but also by using an alternative composition based on tetraethoxysilane (Q) and D. Pyrolysis temperatures between 300 and 1000 degrees C were studied, and the escaping gases-polar, as well as non-polar-were analysed by means of chromatography/mass spectrometry. It was demonstrated, that the co-monomer D, which undergoes thermal elimination and subsequent reactions with the siloxane skeleton, seems to be responsible for the generally useful "micro-creep" ability of the pyrolyzing material. At higher D contents, where the pyrolysis weight losses strongly increase, the pyrolysis gases were shown to contain also polycyclic oligomers, which consist not only of D, but of the branching T or Q "main" monomers as well. The Si-O/Si-C exchange reactions of escaping D with the skeleton were found to be highly efficient not only in T/D, but also in Q/D polysiloxanes. It was further found, that the SiOC products after completed pyrolysis at 1000 degrees C still can release gases detectable by GC/MS upon repeated heating, although no significant weight losses are observed: this indicates the presence of small amounts of pyrolysable sediments in micro-and nano-pores of the SiOC glass.
Permanent Link: http://hdl.handle.net/11104/0261219