Probing the buried structure at the silica/rubber interface*

Silica (SiO₂) fillers have been recently paid great attention due to their potential for improving the processability of rubbers and are being used as partial or even complete replacements for carbon black (CB) fillers in automobile tires. Since the chemical interaction between SiO₂ and PB is much weaker than that between CB and PB, silane coupling agents are typically compounded together into an elastomer matrix, resulting in the formation of a monolayer on the filler surface. However, the detailed structure of the filler-rubber interface including the formation of bound rubber (BR) remains unsolved. To provide insight into it, we mimic the SiO₂/rubber interface using a planar silicon (Si) substrate with a native oxide layer. Monodisperse polybutadiene (PB) and a bis[3-(triethoxysilyl)propyl]tetrasulfide (Si69) were used as a rational model. PB thin films (~200 nm in thickness) onto the Si substrates were prepared by spin coating of a PB/Si69 (with 16 parts per hundred rubber) and toluene mixture solution and were subsequently annealed at 140°C/1hr to facilitate silanization. The films were then leached by toluene to extract the BR layer on the substrate. An X-ray reflectivity technique allows detailed characterization of the BR and silane coupling layer. The details will be discussed in the presentation.