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VISCOELASTICITY

VISCOELASTICITY. Viscoelasticity. Many materials display time dependence in their elastic response gum bread dough cheese Viscoelastic materials possess both elastic and flow characteristics. F. ∆. h. O. r. i. g. i. n. a. l. h. C. o. m. p. r. e. s. s. e. d. Creep.

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VISCOELASTICITY

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  1. VISCOELASTICITY

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

  3. F ∆ h O r i g i n a l h C o m p r e s s e d Creep • Consider a material placed under a weight. A constant stress is applied, and we measure how the strain (∆h/h) changes with time

  4. Elastic Material force removed F ∆h ∆ h Instantaneous elastic deformation Instantaneous elastic recovery O r i g i n a l h C o m p r e s s e d Time force applied

  5. Viscoelatic Material force removed F retarded deformation (creep) ∆ h Instantaneous elastic recovery ∆h O r i g i n a l h retarded recovery C o m p r e s s e d Instantaneous elastic deformation permanent deformation force applied Time

  6. Creep: deformation that occurs over period of time when a material is subjected to a constant stress (at constant temperature)

  7. Step Strain/Stress Relaxation • Induce an immediate step change in the strain, then monitor the stress on the material with time

  8. 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

  9. Molecular Basis of Creep and Stress Relaxation • Five theories • Chain scission • Bond interchange • Viscous flow • Thirion relaxation • Molecular relaxation

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

  11. 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

  12. Viscous flow: caused by linear chains that evenutally slip past one another

  13. Thirion Relaxation: reversible relaxation of cross-links or entanglements

  14. 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

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