More coupling information can be obtained by Fast-Tikhonov regularization (ftikreg) of the normalized build-up data. This procedure yields a visual representation of the distribution of Dres within the samples (Figure ). The median value of Dres (Dmed) corresponds to the value at which the normalized integral of these curves is 0.5. The inherent (in)homogeneity of spins in uncross-linked (pristine) NR and SBR remains relatively unchanged upon vulcanization (0 h) but increases due to weathering. Already upon 72 h of weathering of SBR, the formation of a small amount of rigid fraction can be observed towards the higher coupling end of the distribution. In addition to this, SBR at 504 and 988 h of weathering contains small amounts of, cross-linkinginduced, newly formed constrained fractions that are ubiquitous in the coupling distribution graphs. In the case of the blend, proportional changes can be observed. To summarize the low-field results, SBR undergoes perfection of cross-links leading to stiffening, whereas NR demonstrates both cross-linking and chain scission that lead to stiffening and softening processes, with chain scission eventually dominating. Interestingly, measurements of the blend material reveal that the NR and SBR phases are protected when they are a part of the blend in contrast to the respective single vulcanizates. 4.4.2 Phase-resolved studies of the evolution of cross-link densities MAS NMR experiments benefit from the resolution of a spectrum into defined peaks. This enables phase-specific studies in polymer mixtures. With every integer rotor period and a rotor frequency of 10 kHz, the shortest DQ evolution time achievable in the current setup is 100 µs.