KINETICS OF MORPHOLOGICAL DEVELOPMENT IN ELASTOMER MODIFIED SILICA XEROGELS

Acid-catalyzed, sol-gel chemistry is used to incorporate polysiloxanc clastomcrs in ceramic gels in order to prevent pure collapse on drying. These rubber-modified, ceramic gels can lead to low-density nano-porous materials for uses such as thermal insulation. The morphological mechanisms by which these materials are toughened by elastomers is the focus of this paper. Structural development in these multiphase systems involves a complex interplay between nano-scale hydrolysis/condensation reactions and colloidal to macroscopic phase separation and network formation. The interplay between these morphological features as a function of reaction time is studied using microscopy as well as small-angle x-ray and light scattering. The initial reaction mixture is phase-separated on a micron scale due to the immiscibility of poly-siloxanes and water. At an early stage of the sol-gel reaction, mass-fractal domains in the nano-scale develop, while micron-scale phase-separated domains are diminished. Near the gel-point, a macroscopic network forms whose morphological features are strongly influenced by the presence of polysiloxane elastomer. A model is proposed for the interplay between these structural levels during growth.