Atomic force microscopy of stretched rubbers

Abstract

Atomic force microscopy (AFM) is a powerful tool for studying the structural-mechanical properties of polymers and polymer composites. Modern AFM methods provide not only a 3D-image of the surface but also its physical and mechanical properties. Elastomeric vulcanizates and their composites are widely used in tire industry. In this work, unfilled and filled (5, 30, 50 wt. parts of carbon black) rubbers in the tensile state, including open cracks, were investigated by the method of rapid nanoindentation. The defects in the stretched polymer matrix in the vicinity of the inclusions are shown. A binder oriented along the axis of deformation in the gaps between the filler inclusions forms strand-like structures. The stress-strain state of the stretched polymer is heterogeneous. The stiffness of the polymer in the strands exceeds that of the surrounding matrix, i.e., the strands take the main load. At the same time, weakly loaded matrix areas shielded from load by filler inclusions are present in the material. Adjacent filler aggregates retain their relative position during stretching, and the strands appear in the filler-free areas. In the notch made in the stretched rubber, the polymer is in a critically stretched state – a slightest increase in stretching will lead to further crack propagation. In this case, an overstretched area up to 10 mm wide having local defects and strands appears along the axis of the crack. The stiffness of the matrix in this region is hundreds of times greater than in the initial material, which indicates deformations many times greater than macroscopic ones.