Aging phenomena in silica-filled polydimethylsiloxane

The aging behavior of silica-filled polydimethylsiloxanes with molecular weights ranging from 9000 to 140,000 g/mol was examined for several filler surface treatments. A fumed silica with 130 g/m(2) surface area was used as received and after treatment with hexamethyldisiloxane and hexamethyldisilazane. The rheological properties, bound rubber, and state of dispersion of these systems were monitored as a function of time. The samples exhibited softening instead of crepe hardening which is typically reported for silica filled systems. These experiments indicate that polymer adsorption onto the surface plays an important role in determining the overall stability of these systems. Significant changes in the dispersion of filler were observed and these changes were dependent on polymer molecular weight and filler surface treatment. The addition of a surface treating agent, either physically adsorbed or covalently bound to the silica surface, that inhibits the adsorption of polymer was found to significantly increase the stability of the filler dispersion and therefore the rheological properties. An increase in the number of bridging chains during aging is responsible for the force which allows the filler migration to occur. (C) 1999 Academic Press.