Effect of interfacial bonding on the strength of adhesion of elastomers. III. Interlinking by molecular entanglements

Polydimethylsiloxane (PDMS) networks have been formed by end‐linking linear PDMS molecules. When a second layer is cast on top of a fully gelled lower layer, the new molecules diffuse into the surface of the lower layer and form molecular loops (“entanglements”) in the course of end‐linking with themselves. The two layers are then joined only by these macromolecular loops. Measurements have been made of the work required to separate such layers under threshold conditions, i.e., at low rates, high temperatures, and, in some cases, in the swollen state. Values of the work of detachement have been found to be 15–25 J/m2, generally about one‐half of the work of fracture of the layers themselves, and consistent with the inferred density of interlinking molecular loops at the interface. Values of the work of detachment were higher for higher densities of interlinking, roughly in proportion, and for interlinking molecular strands of higher molecular weight, in accordance with the theory of Lake and Thomas. In the absence of interlinking the work of detachment was extremely small, only about 70 mJ/m2.