(143c) Kinetic Studies of Alkoxysilane Surface Functionalization On Silica by Thermogravimetric Analysis

The in-situ surface functionalization of silica by alkoxysilane greatly improves the cluster dispersion into nano-particles during mixing in nonpolar polymer matrices. The surface bound alkoxysilane alters the inter-particles and particle-polymer interactions, and aids the silica to stay dispersed in the polymer matrices. The reaction mechanisms between the silica, alkoxysilane and polymer are complex and not fully understood. In order to improve the dispersion process and enhance composite product properties, a fundamental understanding of the bound alkoxysilane formation on polar particles is of paramount interests. We have utilized the thermogravimetric analysis (TGA) to study a bi-functional alkoxysilane binding on silica containing nanopores. This method allows direct quantitative measurements of the bound alkoxysilane for determining the reaction mechanisms and kinetic parameters. We prepare the surface bound alkoxysilane on silica by heating the amorphous silica clusters in excess of the alkoxysilane for different time intervals at 30, 50, 70, 90 and 110°C. The derivative TGA curve of the modified silica clusters shows distinct peaks at two temperature ranges and the peak signals increase with reaction time. We propose that the peaks at the lower and higher temperatures correspond to the bi-functional alkoxysilane bound with one or two surface silanol sites on the silica respectively; subsequently we develop first- and second-order reaction models for the bound alkoxysilane formation. The TGA results fit well with our proposed reaction models which provide the reaction rate constants. We also determine the activation energies of the two models from the Arrhenius plots. The activation energies of the two models are similar, suggesting that both reaction mechanisms occur simultaneously. The results of this work demonstrate that the TGA technique is a powerful tool to investigate the reaction kinetics of processes involving modification of surface functionality of particles with nanopores.