Variations of tensile stress and birefringence at a fixed extension for vulcanized rubber samples were investigated.
The spectroscopic measurements gave significant birefringence values even when the samples were turbid. Stretched natural rubber samples with high crystallinity were not broken as long as 1 h in the highly stretched state, though very high degree of chain extension was detected. On the other hand, other samples with low or no crystallinity were broken much earlier, despite the lower degree of chain extension. These results suggested the reinforcement effects of strain-induced crystals, also during the stress-relaxation process.