The AFM measurements on the skin region of SBR W504h were performed within approximately 30 µm from the directly exposed surface, whereas the measurements on the core region were performed in the central part of the specimen (around 1 mm from the exposed surface). The results are shown in Figure , which displays a plot of cantilever force as a function of indentation depth obtained from the force-distance measurements of the two samples. These indentation curves are indicative of different mechanical properties of the skin and core regions of the weathered SBR. As can be seen, an increase in applied force causes a larger indentation in the sample. To produce a certain indentation, a significantly different force is needed for all three specimens due to their different mechanical response. Although the correlation between force and indentation depth is nonlinear as described by various nanomechanical models and depends on the cantilever's tip radius and shape, the different slopes of indentation curves can be considered as reflecting the differences in local stiffness of the material. Here, to exclude any effects arising from different tip radii or shapes, and for direct comparison of the data, the same cantilever has been used for the indentation measurements on all three samples. As apparent from Figure , the slope of the indentation curve for the skin region is larger than that for the core region of SBR W504h.