Moreover, the slopes of the curves for the weathered SBR are also remarkably large compared with the unexposed sample. The marked increase in stiffness of the core compared to the unexposed sample indicates a region that has undergone higher cross-linking due to weathering and corroborates with the shift in Tg observed in the DMTA trends. The second Tg observed in the DMTA plot of SBR W504h is, thus, resulting from the even stiffer skin region, as confirmed here by AFM. As such, from the AFM results and the concepts of diffusion of oxygen, it can