Influence of the nature of the R Group on the Hydrolysis and Condensation Process of Trifunctional Silicon Alkoxides, R-
- PDF / 291,212 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 84 Downloads / 143 Views
ABSTRACT
The hydrolysis and condensation reactions of three trifunctional silicon alkoxides, R-Si(OEt) 3 , with various R alkyl chains (R = CH 3 , C2H5, C 8H 17 ), was followed by 2 9Si and 1H Nuclear Magnetic Resonance. The condensation reactions are faster for methyltriethoxysilane and the degree of condensation of the system is higher. The other two systems have similar behaviors during the first hours, with lowest degree of condensation. The condensation reactions slow down for the octyltriethoxysilane, due to the presence of the long alkyl chains which prevent condensation reactions between oligomeric species. The influence of the addition of tetraethoxysilane (TEOS) was studied : for all three systems, the degree of condensation of the network is higher, indicating the role of cross-linking played by TEOS.
INTRODUCTION Trifunctional silicon alkoxides, R-Si(OR') 3 are the most common precursors for the preparation of hybrid siloxane-oxide systems from sol-gel techniques [1]. They permit the introduction of a large number of organic groups, directly bonded via a strong Si-C bond to an oxide-based network. They are commonly used with other precursors, such as metal alkoxides M(OR) 4 (M=Si, Ti, Zr). In order to control of the distribution of the various units in the final materials, it is essential to obtain information on their reaction kinetics under hydrolytic conditions. Some studies have been published on the reactivity of methyltriethoxysilane mainly using 29 Si Nuclear Magnetic Resonance (NMR) [2-5]. For most of the alkoxides used in the preparation of organically modified silicates, the R group is a long chain, usually functionalized at its end. The presence of such groups certainly influence the hydrolysis and moreover condensation reactions. For this reason, the hydrolysis process of various trifunctional alkoxides R-Si(OR') 3 [R=CH 3 , C2H5, C8 H 17 ] has been followed by 1H and 29 Si NMR. The influence of the addition of tetraethoxysilane, TEOS, has also been discussed. 365 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL SECTION The various alkoxides, methyltriethoxysilane (MTS), ethyltriethoxysilane (ETS), octyltriethoxysilane (OTS) and tetraethoxysilane (TEOS) have been used as received. The alkoxide(s) was mixed with ethanol (EtOH/Si = 4) and then hydrolyzed with acidic water (H 2 0/Si = 2; pH [HC1] = 1.2M). 29 Si NMR measurements were performed on a MSL400 Bruker spectrometer (79.5 MHz). The reactants were held in 8-mm tubes which were in turn placed in a 10-mm tube containing C6 D6 in the annulus as lock solvent. We accumulated 120 to 200 scans for each samples with pulse widths of 5 Jts and recycle delays of 5 s. A linebroadening procedure (10Hz) was applied before Fourier Transform. 1H NMR spectra were
recorded on an AC300 Bruker spectrometer. The reactive mixture was removed at given evolution times and diluted in CDC13 before recording the spectra. To avoid the dilution step, a geometry with a 4-mm tube in a 5-mm tube was also used. The results obtained
Data Loading...
