JKR studies of acrylic elastomer adhesion to glassy polymer substrates

Adhesion between a model acrylic elastomer and a glassy polymeric substrate has been investigated by the JKR technique. Hemispherical lenses of lightly cross-linked poly(n-butyl acrylate) (PNBA) were subjected to loading/unloading cycles on flat poly(methyl methacrylate) (PMMA) substrates. Significant adhesion hysteresis is observed at all accessible rates of unloading. The amount of hysteresis is seen to increase systematically with unloading rate and is characterized by the measured energy release rate (G). The presence of free PNBA chains, whether in the elastomer or applied directly to the interface, significantly reduces adhesion of the elastomer to the glassy substrate. Unloading profiles of the crack growth rate dependence of G yield a threshold value (G(0) = 70 ± 30 mJ/m²) which is close to the expected thermodynamic work of adhesion (W) between PNBA and PMMA. However, fits of the JKR model to loading data result in values of G which are significantly lower than W. We also examine the underlying assumptions of JKR analysis of fracture and test them by independent measurements of lens displacement (δ). The total energy dissipation in a hysteresis cycle is shown to be predicted accurately by JKR theory. Discrepancies between measured and predicted δ profiles are consistent with the effects of the finite thickness of the lens, which is not taken into account by JKR theory.