Polymers are routinely placed between solid walls to provide lubrication or adhesion. Their function in these roles depends critically on the degree of dissipation within the polymer film and at the film/wall interface as the film shears or ruptures. Good lubrication is achieved by minimizing frictional dissipation while dissipation increases the strength of adhesive bonds.
Gent and Schultz suggested a direct link between frictional losses and the adhesive performance of polymers. This correspondence has been supported by recent experiments and by some of the molecular-dynamics simulations to be described. However we find that the correspondence breaks down when the molecular motion producing dissipation occurs at different locations during shear and rupture. In the following sections, we discuss the types of rate-dependent dissipation observed in thin films and the different factors that control whether dissipation occurs within the polymer or at the wall/film interface. The results suggest an origin for interesting memory effects observed in surface-force-apparatus (SFA) experiments on thin films and expose the atomic-scale processes that produce dissipation during internal rupture of a thin film.