Upon 988 h of weathering, NR undergoes a further reduction in its cross-link density. This finding is different from the low-field measurement where the Dmed seemed to have stabilized at extended exposure durations. The corresponding distribution appears to slightly narrow down upon extended weathering. This is consistent with the low-field results and could be due to the recross-linking of some weakly coupled segments that are present after 504 h of weathering, thus leading to uniformity of chain lengths between cross-links.