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Influence of Monomer Connectivity, Network Flexibility, and Hydrophobicity on the Hydrothermal Stability of Organosilicas


Dral, Petra; ten Elshof, J;





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It is generally assumed that the hydrothermal stability of organically modified silica networks is promoted by high mono-mer connectivity, network flexibility and the presence of hydrophobic groups in the network. In this study a range of organosilica compositions is synthesized to explore the extent to which these factors play a role in the hydrothermal dis-solution of these materials. Compositions were synthesized from hexafunctional organically bridged silsesquioxanes (OR1)3Si-R-Si(OR1)3 (R = -CH2-, -C2H4-, -C6H12-, -C8H16-, -p-C6H4-; R1 = -CH3, -C2H5), tetrafunctional (OEt)2Si(CH3)-C2H4-Si(CH3)(OEt)2 and Si(OEt)4, trifunctional silsesquioxanes R’-Si(OMe)3 (R’=CH3, n-C3H7, cyclo-C6H11, phenyl), and bifunc-tional Si(i-C3H7)2(OMe)2. The bond strain, connectivity and hydroxyl concentration of all networks were estimated using 29Si cross-polarized magic angle spinning nuclear magnetic resonance and Fourier-transform infrared spectroscopy. The hydrophilicity was characterized by monitoring the water uptake of the materials in moisture treatments with thermo-gravimetric analysis, differential scanning calorimetry and Fourier-transform infrared spectroscopy. The resistance of each network against hydrothermal dissolution in a water/1,5-pentanediol mixture at 80 °C and pH 1, 7 and 13 was analyzed with inductively coupled plasma optical emission spectroscopy and X-ray fluorescence analysis. Bond strain appears to significantly increase the tendency to dissolve under hydrothermal conditions. The stabilizing influences of increased connectivity and hydrophobicity were found to be weak.