Adhesion of soft materials depends on interfacial strength

In the conventional description, peeling of an adhesive from a solid substrate is characterized in terms of adhesion energies at adhesive-substrate interfaces. However, this energy based identification faced considerable difficulties in explaining why the adhesion energy, also known as peel strength /gamma p , is many orders of magnitude greater than Dupré's thermodynamic work of adhesion, sigma0. In this study we present experimental evidence to demonstrate that polymer adhesion is governed by interfacial strength formed between the adhesive and substrate whose surface energy /gamma s plays no explicit role in controlling the adhesion strength /sigma adh . On different substrates with widely varying surface energies, by carrying out both tensile-adhesion tests to demonstrate the dependence of /sigma adh on applied rate and temperature and conventional peeling tests to show the effects of peeling speed v p and temperature on /gamma p , we propose that /gamma p explicitly depends on /sigma adh through a characteristic length scale P instead of /gamma s and /gamma 0 . Here P is the distance of the adhesive layer away from the peeling front, beyond which the adhesive undergoes little deformation. Since P is a much larger length scale than a molecular scale and /sigma adh directly depends on polymer-substrate interfacial interactions rather than substrate surface energy, peel strength /gamma p has no relationship with /gamma 0 and its dependence on temperature and peeling speed stems from the time and temperature dependencies of interfacial strength /sigma adh . As activated processes, interfacial debonding on shorter time scales and lower temperature required higher interfacial stress. Consequently, /sigma adh is rate and temperature dependent.