Fraction of effective sulfur crosslinking in polybutadiene rubber vulcanizates

Shear modulus measurements as a function of temperature for the high cis‐ and high trans‐isomers of 1,4‐polybutadiene, vulcanized with sulfur in amounts between 2 and 44% of the recipe, show the crosslinking efficiency of sulfur in these systems to be slightly lower than in SBR‐1000, but higher than in natural rubber. Two distinct modes of mechanical behavior are noted with these stereo regular rubbers. There is pronounced crystalline behavior at low degrees of vulcanization. After crystallinity has been destroyed by increased crosslinking, both polymers behave as typical crosslinked amorphous materials. Additional evidence is found to support the conclusion that polymer structure, as well as chemical reactivity, plays an important role in the chemistry of vulcanization.