Abstract
AbstractThe role of chain entanglements in determining the stress‐strain properties of polybutadiene networks has been investigated. The number of entanglements was varied by changing the primary molecular weight and chemical crosslink concentration. Networks essentially free of entanglements were prepared by endgroup coupling of carboxy terminated polybutadiene of 5500 molecular weight. Conventional sulfur or peroxide vulcanizates obeyed the Mooney‐Rivlin stress‐strain relation, the constant C2 which represents the deviation from simple kinetic theory diminishing with the time allowed for approaching elastic equilibrium. The constant C2 was found to increase with both the total physical crosslinking and the entanglement contribution to this quantity. The endgroup vulcanizates obeyed the simple kinetic theory of rubber elasticity and approached elastic equilibrium much more rapidly than sulfur or peroxide vulcanizates. The evidence presented suggests that the apparent deviation of elastomeric vulcanizates from the kinetic theory of rubber elasticity resulting in the appearance of the Mooney‐Rivlin C2 term in the stress‐strain relation arises from a slow relaxation process involving the entanglement crosslinks.
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