The -C=Cstretching vibration occurs at 1662 cm -1 (v), whose intensity decreases with weathering. In the fingerprint region, the IR spectra are dominated by -CH3 and -CH2bending vibrations at 1452 cm -1 (vi) and 1375 cm -1 (vii), respectively, and the rocking vibration of -CH2at 724 cm -1 . As stated before, these peaks also decrease upon increasing the time of weathering, representing a likely explanation of the appearance of the skin in the Gram-Schmidt maps (Figure ). In SBR, the -C=Cstretching of the vinyl group at 1640 cm -1 (a) 2 tends to become stronger with successive weathering due to cross-linking. The aromatic -C=Cstretching vibration peak occurs at 1601 cm -1 (b) and its contribution is masked by the presence of the -C=Cvinyl stretching. It has been noted that an increase in the intensity of the -COOstretching vibration at 1541 cm -1 (c) in thermo-oxidatively aged SBR is an indication of the carboxylate formation as the main product on the material surface. However, such a conclusion cannot be drawn in this study due to the broadening of the -C=Cstretching.