Effects of substrate modification on the interfacial adhesion of acrylic elastomers

Axisymmetric adhesion tests are used to probe the adhesion between a carboxylated, poly(n-butyl acrylate) (PNBA) elastomer and a variety of substrates. The elastomer is in the form of a hemispherical cap, which is pressed against a flat substrate to give a circular contact area. A standard fracture mechanics approach is used to relate the applied load and the radius of the contact area to the energy release rate, g which can be viewed as an adhesion energy. For a given substrate, g is a function of the crack velocity, v, defined as the time derivative of the contact radius. The contact pressure does not appear to play a role in the development of the adhesive forces, and the specific relationship between g and v depends on the substrate. In all cases this relationship can be adequately described by the empirical expression, g(v) = g(o)(l + (v/v*)(0.6)). Values of g(o) are within a factor of about 2 of the expected thermodynamic work of adhesion, but values of v* vary from 5.5 to 215 nm/s, depending on the detailed nature of the substrate. Decreases in the adhesion energy, as quantified by a decrease in g(o) and/or an increase in v* are determined primarily by the segmental mobility of the substrate molecules. Comparison of results for free and grafted PNBA chains indicates that translational diffusion of molecules at the substrate is not required in order to substantially reduce the adhesion energy.