Rubber Surface Change and Static Charging under Periodic Stress

Abstract

Rubber materials play an important role in robotics, due to their sensing and actuating abilities, that are exploited in soft smart materials endowed with shape-adaptive and electroadhesive properties. The application of an electric field produces non-linear deformation that has been extensively modelled, but is not understood at the molecular level. The symmetric effect (the production of an electric field due to rubber deformation) was recently discovered and explained as follows: rubber surface chemical composition and adsorptive properties change during rubber deformation, allowing the surface to exchange charge with the atmosphere. The present work describes the complex surface morphology and microchemistry of tubing made from vulcanized natural rubber, showing that it is rough and made from two domain types: stiffer elevations containing Br or Al (depending on the sample used) and O, that rise above an elastic base that is exempt of elements other than C and H. The surface area fraction occupied by the elastic base is higher in the strained rubber than when it is relaxed. Electrostatic potential on rubber surfaces was measured as a function of the stretching frequency, using Kelvin electrodes and showing frequency-dependent potential variation. This is explained considering charge exchange between the atmosphere and rubber surface, mediated by water vapor adsorbed in the stretched rubber and trapped when it relaxes.