As is shown in Figure , with weathering the rigid fraction in NR increases, the addition being the oxidized surface, which stabilizes at about 4 % between 504 and 988 h. On the contrary, in SBR a continuous increase in rigid fraction up to about 7 % occurs due to oxidation of the surface and the underlying layers. The larger amount of rigid components in SBR compared to NR corroborates with the observation made by DMTA towards high temperatures. The second fitting fraction, labeled as the mobile 1 fraction (fm1), corresponds to the component that mainly represents the cross-linked bulk. The third fraction composed of defects is represented as the mobile 2 fraction (fm2). These two fractions can be better characterized by MQ measurements and hence, are deferred to section 4.4.1. Nevertheless, for completeness, the fitting yielded a negligible amount of defect fraction in all SBR samples. NR weathered up to 988 h contained up to about 9% defects with a 𝑇 2,𝑚2 𝑒𝑓𝑓 of about 12 µs. The corresponding shape parameter βm2 was force-fitted to a limiting minimum value of 0.8 (stretched exponential), meaning that the mobile 1 and 2 fractions cannot be reliably separated in all cases. 𝑇 2,𝑚1 𝑒𝑓𝑓 (also, in Figure ) corresponding to the majority bulk remains relatively unchanged in NR, whereas a gradual decrease is observed in SBR due to an increase in cross-link density with the weathering duration. An increasing fr and decreasing 𝑇 2,𝑚1 𝑒𝑓𝑓 in SBR suggests a presence of a gradient of stiffness due to weathering. In NR, the skin and core could be somewhat distinct. These aspects would become clearer when discussing the cross-link densities in section 4.4.