13. Recovery of a deformed plastic body

Given a sufficiently high stress any polymer will deform, even if the temperature is far below the glass transition temperature. A good example is yielding of the polymer at the yield stress.

Once the stress is removed after yielding the polymer seems to remain deformed. In reality, the polymer will very slowly return to its original dimensions that it had before the deformation. This is called recovery.

Below the glass temperature a polymer can be deformed but will not forget its original shape.
Yielding causes the spherical molecules to be deformed into an ellipsoid shape.

Due to the yielding all polymer molecules are deformed from a spherical into an ellipsoid shape. This induces a rubber stress in the molecules. The rubber stress will cause Kuhn segments in the molecules to rotate in such a way that the spherical shape returns in due time.

At temperatures around the glass transition temperature recovery lasts a few hours. It is a slow process. The time for recovery will increase very rapidly at lower temperatures. At a sufficiently low temperature recovery is not observable on human time scale.

Summary

  • Recovery is the process of a deformed polymer that slowly returns to its original shape.
  • The driving force for recovery is the rubber stress, induced by the deformation.
  • The time scale of the recovery process is long: a couple of hours at a temperature close to the glass transition temperature.

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