In AFM indentation, the tip of the AFM cantilever is brought into contact with the sample surface using a z-piezoscanner and pressed into the surface. The corresponding applied force is measured as a function of tip displacement, thus generating a force-distance curve. After the approach cycle, the tip is retracted back to its rest position. The indentation depth can be calculated as the z-piezoscanner displacement minus the cantilever deflection. For all the measurements, a constant tip speed of 2 μm/s was set. More than 50 indentation measurements were performed at different spots of the core region as well as of the skin region across the cut surface of the weathered sample. A similar number of measurements was also carried out on an unexposed sample. The suitable spots for the measurements were selected by first imaging the surface of the sample in the tapping mode of AFM operation. Selected spots were lying on locally flat areas and far from any grooves or adjacent walls/structures. Samples for AFM measurements were prepared by microtomy using liquid nitrogen. Slices (size ≈ 2 × 2 mm 2 ) of the samples were obtained by cutting a thin cross-section from the center of the sample bar (80 x 10 x 2 mm 3 ). The obtained slices were then placed on freshly cleaned silicon (Si/SiO2) substrates. Measurements on the skin region were performed within about 30 μm from the directly exposed face, whereas the measurements on the core region were performed at the central part of the slice.