Study of the Mullins Effect in Carbon Black-Filled Styrene–Butadiene Rubber by Atomic Force Microscopy Nanomechanics

For a long time, it has been difficult to reach a consensus on the physical origin of the “Mullins effect” because of the limited capabilities for visualizing microscopic structures. This work investigates the microscopic mechanism of the Mullins effect in carbon black (CB)-filled styrene–butadiene rubber (SBR) after application of cyclic uniaxial tension. We used atomic force microscopy (AFM) nanomechanics to characterize the nanoscale mechanical properties of CB/SBR vulcanizate after the application of different tensile loadings. The stress-softening behavior of CB/SBR was directly observed at the nanoscale for the first time. The rubber matrix and interfacial region exhibit different softening behaviors at different elongations, which provides insight into the microscopic mechanism of the Mullins effect. Moreover, we found that recovery from the Mullins effect starts with the recovery of the interfacial region, while the rubber matrix takes longer to recover its strength. This approach could provide new research perspectives for the mechanisms that induce the Mullins effect.