Reinforcing fillers are widely used to improve the modulus, toughness and abrasion resistance of rubber

vulcanizates. Below is a stress$-$strain plot for an SBR vulcanizate containing carbon black. Stretching the

reinforced compound to $=2$ produces a typical $$ vs$.$ profile for an elastomer. After allowing the

material to relax for a few seconds $)$ before stretching again, the recorded stress is less than the

original test until the original elongation, $=2,$ is reached. After this point, the typical $$ vs$.$ profile is re$-$

established.

Suggest a mechanism for this stress$-$softening behavior, and account for the observation that upon

leaving the compound to rest for many hours between stretching cycles, this behavior is not observed.