On the use of shear rate or shear stress as the indipendent variable in polymer melt rheology

Autori: P. Lomellini, D. Ferri
Presentato al: Southern Europe Conference on Rheology
Anno: 1999
Volume: 1
Issue: Unico
Casa Editrice: SIR-Cuen Napoli
Lingua: English


Abstract
As a matter of fact the rheological functions of polymer melts are almost always plotted as a function of shear rate and quite rarely against the shear stress. The reasons of that are still rather vague and obscure, perhaps more psycological than physical. From the scientist point view one can speculate that the shear rate is the rate at which the material is “probed” and thus can be thought as the reciprocal of a kind of observation time; this time can be compared with the average relaxation time of the polymeric liquid. The engineer may add the argument that the shear rate is related to the volume flow rate for instance across a die and this is interesting whenever one thinks in terms of productivity rate.
However the use of shear rate as the independent variable has some limitations especially if one wants to study the effect of temperature or structural parameters (average molecular weight, molecular weight distribution, presence of a dispersed phase, lubricant content, etc.) on the rheology of polymers. Some examples of this are shown in the next section.


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