Distinct variations occur in the samples when exposed to the aging elements in the weatherometer as seen from the evolution of cross-link density in Figure . The biggest change can be seen in SBR, where degradation generally occurs through cyclic cross-linking, which leads to an increase in stiffness (embrittlement). The corresponding coupling distribution width compliments the formation of different chain lengths between cross-links. As established earlier, the regions closer to the surface undergo stiffening due to direct exposure to irradiation, oxygen, water, and heat. Like in the T2 measurements, the constrained fractions also show up in the InDQ signal. This can be seen as a step-up in the InDQ curve at short evolution time up to 100 µs (probing lower values is not feasible due to the pulse sequence length). The build-up curve in Figure represents this feature for SBR weathered up to 504 h.