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[FAQ]

First of all, in fiber spinning applications, it is good to keep in mind that quite large shear rates are encountered in the flow in the die, while we have a relatively high strain rate in the fiber. To some extent, it is a conflicting situation. However, considering the process, one should favor the simulation of the elongational effects in the fiber. From there, if the fluid is highly strain hardening, the Oldroyd model could be a good choice, but a difficult one from the numerical point of view. The flow behavior of Giesekus and PTT models are quite similar, since both models exhibit shear-thinning, non-quadratic first normal stress difference and bounded elongational viscosity. These models differ from each other at very high strain rate, where the elongational viscosity of the Giesekus model is higher than that of the PTT model. One can go a bit beyond and consider the pompom model (referred to as DCPP model in POLYFLOW). It is characterized by shear thinning and interesting properties in elongation. This is especially true if the melt is branched. What property should be measured? Preferably, G'(Storage modulus), G”(Loss modulus), steady shear viscosity and transient elongational viscosity measured for a few strain rates. If the latter one cannot be obtained, a qualitative estimation is always good (e.g. no strain hardening, low strain hardening, high strain hardening). Whenever possible, empirical rules such as Cox-Merz or Gleissle can be applied.