Hybrid organic-inorganic materials synthesized by reaction with alkoxysilanes: Effect of the acid-to-alkoxide ratio on morphology

Abstract

The gelation phenomenon of hybrid organic-inorganic materials is investigated as a function of the temperature and the catalyst-to-alkoxide ratio. These ceramers are prepared in a two-step synthesis: First, an α-hydroxy, ω-methyl poly(ethylene oxide) and an α-hydroxy-terminated hydrogenated polybutadiene (H-PBD) are reacted with isophorone diisocyanate (IPDI); second, these isocyanate-terminated prepolymers are reacted with γ-aminopropyltriethoxysilane (γ-APS) to form alkoxysilane-terminated macromonomers. These macromonomers are cross-linked using the well-known sol-gel process. The gelation time is found to be sensitive to the acid-to-alkoxide ratio. The large difference in the activation energies of the gelation process of the two kinds of macromonomers could be attributed to the amount of the IPDI/γ-APS copolymers. This results in an incomplete reaction of IPDI during the first stages of synthesis: The final morphology of the cured hybrid materials, based on the H-PBD macromonomer, depends on the amount of H⁺ and indicates that the acid-to-alkoxide ratio modifies not only the rate of hydrolysis of the alkoxide, but also the structure of the ceramer. The model of the microstructure of such hybrid materials, described previously, could explain the two relaxation peaks observed by dynamic mechanical spectroscopy. The relaxation close to the glass transition temperature of the initial H-PBD is attributed to the glass transition region of the organic-rich matrix. The second peak, 50°C above, is associated with the glass transition temperature of the interface between the organic-rich region and the inorganic clusters (H⁺/Si, >0.05). The second relaxation is easily observed at a low H⁺/Si ratio (<10⁻³).