Colloidal Reinforcement: The Influence of Bound Polymer

The reinforcement of rubbers by colloidal filler particles has often been associated with the presence of ‘bound polymer’, that polymer which cannot be removed by solvent extraction from the filled material. This paper demonstrates to the contrary that, for filled polyethylene, bound polymer is observed but reinforcement is not. Indeed, the bound polymer appears to embrittle the filled material. A new model of the filler-polymer interface is proposed to account for these results. It is suggested that there are two interfaces in a filled polymer; one between filler and bound polymer, and a second interface between bound polymer and soluble polymer. This second interface appears to be weak in the case of filled polyethylene and causes failure some 2.8 nm from the true filler surface. The model explains why cross-linking the polyethylene, or increasing its molecular weight, overcomes the embrittlement problem.