Results on IR and SBR samples To confirm our interpretation of the stress- optical diagrams, additional experiments were conducted on the samples prepared from synthetic cis-1,4-polyisoprene (IR) and copolymer of styrene and butadiene (SBR). The following results support our interpretation. 1

Recipes of rubber samples 2 a Natural Rubber, RSS #1 from Malaysia b N-cyclohexyl-2-benzothiazole sulfenamide, curing temperature with sulfur 140 ℃ Sample code NR a (part) Stearic acid (part) Active ZnO (part) CBS b (part) Sulfur (part) Total (part) Curing time (min) Network chain density ×10 4 (mol/cm 3 ) NR NR NR Sample code IR a (part) Stearic acid (part) Active ZnO (part) CBS b (part) Sulfur (part) Total (part) Curing time (min) Network chain density ×10 4 (mol/cm 3 ) IR a Synthetic cis-1,4-polyisoprene, IR2200 from JSR b N-cyclohexyl-2-benzothiazole sulfenamide, curing temperature with sulfur 140 ℃ Sample code SBR a (part) Stearic acid (part) Active ZnO (part) CBS b (part) Sulfur (part) Total (part) Curing time (min) Network chain density ×10 4 (mol/cm 3 ) SBR a Copolymer of styrene and butadiene, SBR1502 b N-cyclohexyl-2-benzothiazole sulfenamide, curing temperature with sulfur 150 ℃ Fast crystallization Slow crystallization Do not crystallize

Stress-optical diagrams of SBR Slow expansion (10mm/min) C = 3.2×10 -9 Pa -1 for SBR1502 Strain ratio at fracture: ca. 4.1 C. A. Berglund, J. Polym. Sci., Polym. Phy. Ed., 15, 2037 (1977). fracture Fast expansion (1000mm/min) and subsequent holding indicated in the legend The “hooks” sloped down to the left because of the absence of crystallinity.

4 fracture Slow crystallization of IR Because of the slower crystallization rate of IR, the “hook” is smaller than those of NR samples. The rate of stress relaxation of the IR sample is also slower.