Molecular origins of adhesive failure:: Siloxane elastomers pulled from a silica surface

To gain insight into the molecular basis of failure processes in adhesives, we have performed a first-principles molecular dynamics study on siloxane oligomers adsorbed on a silica surface. We model covalent bond rupture in the high force regime of single molecule atomic force microscopy experiments where an elastomer is pulled from a substrate at constant velocity. Our system consists of covalent silicon-oxygen bonds which differ in the extent to which they are constrained by their location depending on whether they are in the elastomer, at the attachment or in the substrate. We find that the magnitude of the rupture force is dependent on whether the ruptured bond is in the polymer or at the attachment to the substrate. We determine a rupture force of 2.0 nN for an Si-O bond at the attachment when a polydimethylsiloxane hexamer is pulled from a beta-cristobalite (100) slab.