Molecular Origins of Elastomeric Friction

Frictional properties of soft elastomers have been in question for over half of a century. Early studies on natural rubber originated for the sole purpose of tabulating properties for bulk consumer applications, such as viscoelastic adhesives, tires, and windshield wipers, to name a few. Empirical tabulation of frictional properties persisted until the early 1950’s, when Roth et al. and Thirion began experiments towards a fundamental understanding of rubbery sliding. Quantitative physical analysis began with the observation that the classic Coulombic laws obeyed consistently at rigid body interfaces fail at the interface between a rigid solid and a rubber. Even today, there remains an incomplete understanding of the molecular level parameters that control the frictional behavior of elastomeric surfaces. With this chapter, we explore the historical developments in elastomeric friction and discuss the evolution of an unresolved triborheological complexity. We work from an initial macroscopic perspective toward a microscopic one that describes dissipation process in terms of molecular phenomena at frictional contacts. Readers are urged to consider a competition between these molecular processes, where for soft matter, internal cohesion is comparable to interfacial adhesion. While cohesion may dominate adhesion, or visa versa, we develop a picture for elastomeric friction that encompasses both.