VISCOELASTICITY

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VISCOELASTICITY

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Viscoelasticity

• Many materials display time dependence in their
  elastic response
• gum
• bread dough
• cheese
• Viscoelastic materials possess both elastic and
  flow characteristics

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Creep

• Consider a material placed under a weight. A
  constant stress is applied, and we measure how
  the strain (?h/h) changes with time

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• Elastic Material

force removed
F
?h
?
h
Instantaneous elastic deformation
Instantaneous elastic recovery
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Time
force applied
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• Viscoelatic Material

force removed
F
retarded deformation (creep)
?
h
Instantaneous elastic recovery
?h
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retarded recovery
Instantaneous elastic deformation
permanent deformation
force applied
Time
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• Creep deformation that occurs over period of
  time when a material is subjected to a constant
  stress (at constant temperature)

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Step Strain/Stress Relaxation

• Induce an immediate step change in the strain,
  then monitor the stress on the material with time

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• Elastic Material stress increases immediately
  with strain and remains constant
• Newtonian Fluid stresses increases with
  application of strain, quickly declines to zero
• Viscoelastic material stress increases
  immediately, declines gradually over time.
• viscoelatic solid-decline is gradual and levels
  off at ?e
• viscoelatic liquid, stress declines rapidly and
  goes to zero

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Molecular Basis of Creep and Stress Relaxation

• Five theories
• Chain scission
• Bond interchange
• Viscous flow
• Thirion relaxation
• Molecular relaxation

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• Chain scission degradative reactions such as
  oxidation or hydrolysis lead to cleavage of
  polymer chains. Chains help bear load while
  intact

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• Bond Interchange portions of polymers may change
  partners causing release of stress. In presence
  of stress, rearrangements tend to reform so as to
  reduce stress

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• Viscous flow caused by linear chains that
  evenutally slip past one another

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• Thirion Relaxation reversible relaxation of
  cross-links or entanglements

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• Molecular Relaxation below Tg, molecules move
  very slowly, thus the material appears as an
  amorphous solid. As temperature approaches Tg,
  chains move and relax at an observable rate on
  the lab time scale.
• For chains under stress, motions will tend to
  relieve the stress