Stress softening in natural rubber vulcanizates. Part IV. Unfilled vulcanizates

Abstract

AbstractStress softening (Mullins effect) is observed in gum natural rubber vulcanizates. The magnitude of this softening is similar to that in carbon black‐filled vulcanizates. The amount of stress softening is slightly greater in vulcanizates cured to produce predominantly polysulfide crosslinks than in those containing monosulfide or carbon–carbon crosslinks. The total recovery of stress softening in the vulcanizates containing monosulfide or carbon–carbon crosslinks suggests that the phenomenon is attributable to a quasi‐irreversible rearrangement of molecular networks due to localized nonaffine deformation resulting from short chains reaching the limit of their extensibility. This non‐affine deformation results in a displacement of the network junctions from their initial random state.