Structural–Mechanical AFM Study of Surface Defects in Natural Rubber Vulcanizates

Structural–mechanical properties of surface defects in NR vulcanizates in undeformed and stretched states including static cracks are studied by atomic force microscopy. Elastoplastic microdefects (cracks and strands) are observed on the undeformed surface at certain sample cutting conditions. In the stretched materials, matrix detachment is found at the poles of inclusions but the propagation of such longitudinal nanocracks is hampered by inclusions and cross-linking heterogeneities. The elastic modulus within these detachments is 2 times higher the matrix. The properties of other defects–transverse cracks in the stretched rubber depend on filler concentration and distance from the crack tip. Such cracks are covered by polymer strands (thickness ∼20 nm) and a network of nanofibrils (∼1 nm). The measured modulus near the crack tip reaches 1 GPa and reduces to 200 MPa at the end. The observed features are related to the strain induced crystallization and high local extension of NR.