A modified bimodal fitting function can be used to separately fit these hardened surface fractions, and those that lie beneath that are comparatively less aged. The input parameters to the fitting function are a fixed coupling value Dmed,2/2π = 12 kHz, distribution value σln,2 = 0.01, and constrained fraction value obtained from the T2 analyses. Thus, SBR W504h has been assigned an input value of 2.5% for its rigid fraction and likewise for the other samples. Note that the pulse sequence is inefficient for these components, meaning that both Dmed,2 and the fraction are underestimated. We acknowledge that the value of Dmed,2 is chosen somewhat arbitrarily to get a good fit, but it still adheres a priori to coupling strength that corresponds to the tight crosslinking situation, in this case, induced by weathering. It is worth noting that when fr is set as a fitdependent parameter, the value of the fixed Dres (Dmed,2), would only marginally affect the result of the rigid fraction.