FUNDAMENTALS OF POLYMER MELT RHEOLOGY

Rheology is the study of flow by definition, and polymer melt riieology is basically concerned with the description of the deformation of polymer melts under the influence of applied stresses. Molten thermc lastics are viscoelastic materials in the sense that their response to deformation lies in varying extent between that of viscous liquids and elastic solids. In purely viscous liquids, the mechanical energy is di pated into the systmns in the form of beat and cannot be recovered by releasing the stresses. Ideal solids, on the other hand, deform elastically such that the deformation is reversible and the energy of deformation is fully recoverable when the stresses are released. 

Aparticular thermoplastic melt behaves as a viscous liquid or an elastic solid during ptocesang ( )mations dqiending on the relationdi between the time scale of deformation to which it is subjected and the time required for the time-depmident mechanism to reqmnd. The ratio of characteristic time to the scale of deformation is defined as the Deborah number De = by Reiner [1,2], where is the characteristic time and X, is the time scale of deformation. The characteristic time, X , for any material can always be defined as the time required for the material to reach 63.2% or 1 — (1/e) of its ultimate retarded elastic response to a step change. If De 1.0, elastic effects are dominant, whereas if De 0.S, viscous effects prevail. For any values of Deborah numbers other than these two extremes, the materials depict viscoelastic behavior. 

Thermoplastic melts display the ability to recofl by virtue of their viscoelastic nature. However, they do not return completdy to their original state when 

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