Adhesion at the Solid–Elastomer Interface: Influence of the Interfacial Chains

We have studied the adhesion between an elastomeric lens of cross-linked poly(dimethylsiloxane) (PDMS) brought into contact with a silicon wafer covered by a grafted layer of monodisperse PDMS. The influence on the adhesion of the molecular weight and the surface density of the grafted chains was examined by modifying them independently. The mechanical test we used was the JKR method, which measures the variation of the radius of the contact area, a, between the wafer and the lens as a function of P, the load applied. This technique gives the dependence of the interfacial adhesion energy, or strain energy release rate, G, on the crack propagation speed, V. We modified the apparatus commonly used to operate at constant, imposed displacement in an effort to obtain data at the lowest possible loads and deformations. Recording of a and P as a function of time enabled the determination of G as a function of V down to speeds as low as 5 nm/s. Our results show that G still depends on V, even in the lowest range of speeds. If we refer to the low-velocity values of strain energy release rate as adhesion energies, G₀, we find that G₀ is affected strongly by the grafted layer and apparently governed by the ability of the chains from the grafted layer to penetrate the network. The adhesion energies we measure are invariably higher than that expected from accounting for only the surface energy of PDMS. Layers grafted from the melt state over a range of molecular weights, where the grafted chains have their unperturbed dimensions, show less adhesion enhancement than do layers of the same thickness produced by grafting high molecular weight polymers from a range of solution concentrations, which results in compressed configurations of the grafted chains when the solution grafted layers are dried. There appears to be a thickness for the grafted layers which is optimum in generating adhesion enhancement between the lens and the wafer. By modifying the nature of the network, we have shown that the structure of the elastomer plays an important role in determining the adhesion energy.