The diagrams below illustrate the effect of cross linking on the mechanical response of the elastomer. In (a) the material is unvulcanized and when the sample is put in tension the molecules both uncoil and slide past each other. When the force is removed the molecules recoil but their slip remains and the material does not return to its initial length. In (b) the crosslinks in the vulcanized material prevent the sliding of the molecules but not the uncoiling under tension. When the tensile force is removed from this material it returns to its initial size. The stress/strain diagram shows the viscoelastic response of the unvulcanized sample and the non-linear-elastic response of the vulcanized material.

From: Askeland, "The Science of Engineering Materials," PWS (1994) and
Callister, "Materials Science and Engineering," Wiley (1997)