Characterization of Organically Modified Silicates by 17 O Solid State MAS and MQ-MAS NMR
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ABSTRACT Organically modified silicates have been prepared, using methyltriethoxysilane and tetraethoxysilane in various molar ratios. The final network is thus composed of trifunctionnal T units and tetrafunctionnal Q units. Three types of oxo bridges can be formed through the competitive self-condensation or co-condensation reactions : (T)-O-(T), (Q)-O-(Q) and (T)-O(Q). 17U-enriched water was used as reactant in order to selectively enrich the various oxo bridges, and to perform '70 solid state MAS-NMR. The combination of MAS experiments recorded at two different magnetic fields, and of the recently discovered MQ-MAS experiments allowed us to identify clearly the three types of oxo bridges. Quantitative analysis was extracted from the MAS-NMR spectra, and compared with calculations based on randomly distributed Si units. INTRODUCTION Hydrolysis and condensation of various organosilanes (RSi(OR') 4 _,,R'=Me, Et, x=0,1.2) lead to the formation of hybrid networks in which organic R groups are directly grafted on the silicate network. The spatial distribution of the various Si units in the final gel network depends on the chemical reactivity of the starting monomeric precursors. Two types of condensation reactions can occur: - self-condensation: --SiA-OX + HO-SiA- -=SiA-O-SiA- + XOH (X H, R) (1) - co-condensation : -SiA-OX + HO-Si- -- =-SiA- 0 -SiB-= + XOH (X H, R) (2) The competition between these reactions will lead to an homogeneous distribution of units within the network (1) or to phase separation (2). These differences in architectures can influence the properties of the final materials (mechanical strength, hydrophobicity...). It is thus important to find characterization techniques to probe the spatial arrangement of the various sites.
NMR has proved to be a valuable tool to characterize in solution, the formation of cocondensed species in systems such as (CH 3)2Si(OEt) 2/Si(OEt) 4 and CH 3Si(OEt) 3/Si(OEt)4 [1]. Hydrolysis and condensation reactions have also been followed in-situ for the same two systems [2] and this study has revealed the stability of the -SiA-O-SiB- bonds in solution versus time. The results obtained in solution prompted us to undertake a 170 solid state MAS-NMR study on the final gels. 170 MAS-NMR spectra have already been obtained in natural abundance on different crystalline oxides (TiO 2, ZrO 2..) [3]. But due to the low natural abundance of this isotope (0.037%), most of the experiments are usually performed on enriched samples (20 to 50 at.%). The enrichment is achieved in sol-gel derived samples simply by using 170 enriched water for hydrolysis. which will lead to specific enrichment of the silanol groups, and oxo bridges. 170 is 170
also a quadrupolar nucleus (S = 5/2) : the second order quadrupolar interaction which affects the central transition is only partially averaged by Magic Angle Spinning (MAS).
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21 Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
The resulting signal has a characteristic shape, which depends on the isotropic chemical shif
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