Interpenetration of Rubber and Silane Coupling Agent on an Inorganic Substrate Revealed by Spin-Contrast-Variation Neutron Reflectivity

We elucidated the interpenetration of silane coupling agent (SCA) molecules bound to Si substrates and overcoated rubber using spin-contrast-variation (SCV) neutron reflectivity (NR), which determines the nanostructure of multilayer films from polarized NR curves that change with polarization of hydrogen nuclei in the film samples. In an integral blend film of 15 wt % SCA of trimethoxy(octadecyl)silane and deuterated polybutadiene (dPB) annealed at 120 degrees C, the (1.7 +/- 0.2) nm thick SCA layer was generated between the Si substrate and dPB layer. The SCA layer was composed of 70 vol % SCA molecules extended perpendicularly from the silicon substrate and 30 vol % of dPB molecules. The diffuseness of the interface between the SCA and dPB layers of (1.0 +/- 0.2) nm was comparable to the thickness of the SCA layer. These results indicate that SCA and dPB molecules interpenetrate into each other. By contrast, in an SCA-precoated polybutadiene film sample, the layer thickness, diffuseness of interface, and composition of the SCA layer strongly suggest that the densely coated SCA molecules formed a crystal-like structure, and thus, the postcoated polybutadiene molecules could not penetrate into the precoated SCA layer. This poor penetration resulted in peeling off of the polybutadiene layer upon cooling to the cryogenic temperature.