Effect of addition of MTES on the structure of siloxane-PPG nanocomposites

Hybrid transparent and flexible siloxane-polypropyleneglycol (PPG) material s with covalent bonds between the inorganic (siloxane) and organic (polymer ic) phases were prepared by sol-gel process. In order to improve the qualit y of the mechanical properties of these materials, different amounts of met hyltriethoxysilane (MTES) were added to the initial sol. The effect of MTES addition on the structure of the composites was studied by Small-Angle X-R ay Scattering (SAXS) and Si-29 Nuclear Magnetic Resonance (Si-29 NMR). In a bsence of MTES, SAXS spectra exhibit a peak that is assigned to spatial cor relation due to short range order between the siloxane clusters embedded in the polymeric phase. The experimental results indicate that, for low MTES concentrations ([MTES]/[O] less than or equal to 0.8, O: ether-type oxygen of PPG), the silicon species resulting from hydrolysis and condensation of MTES fill the open spaces between polymeric chains, interacting with the et her-type oxygens. For larger MTES content ([MTES]/[O] greater than or equal to 0.8), the number of "free" ether-type oxygen sites avalaible for reacti on with such silicon species is not large enough. Consequently, a fraction of silicon species resulting from MTES addition graft to siloxane clusters formed by hydrolysis and condensation of the hybrid precursor. For all MTES concentrations the condensation degree of the siloxane phase, determined f rom Si-29 NMR spectroscopy, is high (> 69%), as expected under neutral pH s ynthesis conditions.