LIQUID-STATE SI-29 NMR-STUDY ON THE SOL-GEL REACTION-MECHANISMS OF ORMOSILS

Sol-gel reaction mechanisms of organically modified silicates (ormosils) were conducted by liquid state Si-29 NMR (nuclear magnetic resonance) spectroscopy. Samples were prepared by mixing tetraethoxysilane (TEOS), polydimethylsiloxane (PDMS) and water with isopropanol, tetrahydrofuran (THF) and hydrochloric acid (HCl). Several series of solutions with different reaction times were examined by liquid state Si-29 NMR spectroscopy. A new proposed site, D(Q), was assigned from the spectrum of the solution of the dimethyldiethoxysilane (DMDES)/TEOS system. By the reaction between hydrolyzed TEOS and -O-Si(CH3)2-O- in the middle of PDMS chains as well as the silanol end groups, HO-Si(CH3)2-O-, of PDMS, bonds between PDMS and TEOS were formed and the PDMS chains were found to exist also as chains (D) and/or cyclic D4 tetramers (D4c) during the reaction. Results of this study also display the expected effects of temperature, acid and water on the reaction rates. The higher the reaction temperature (room temperature to 70-degrees-C), the more copolymerization between PDMS and TEOS occurs. The water content (H2O/TEOS = 2 to 4) had little effect on the rate of copolymerization but significantly increased the rate of condensation of TEOS. Increasing acid content (HCl/TEOS = 0.1 to 0.3) increased the rates of both copolymerization and condensation of TEOS. Structural models of the sol-gel reaction mechanisms of ORMOSILs are proposed.