Molecular instabilities in capillary flow of polymer melts: Interfacial stick‐slip transition, wall slip and extrudate distortion

Abstract

This article summarizes our recent experimental studies of an important class of polymer melt rheological phenomena including flow instabilities, stick-slip transitions, interfacial melt/wall slip, gross extrudate distortion and sharkskin like surface roughening of extrudates. Our experimental results indicate that most aspects of the perplexing flow behavior of strongly entangled melts in polymer extrusion can be depicted in terms of melt/wall interfacial interactions at a molecular level. For high surface energy walls, a transition from no-slip to slip hydrodynamic boundary condition (HBC) occurs through molecular disentanglement (coil-to-stretch) of adsorbed chains from unbound chains. For weakly adsorbing walls, interfacial slip takes place through a second molecular mechanism involving a stress-induced polymer desorption process. Since the coil-stretch transition of adsorbed chains is controlled by interfacial shear stress, the HBC can be unstable throughout the die wall under the condition of a constant flow rate, where the wall stress may fluctuate in time. Unstable HBC due to the interfacial molecular instabilities is shown to be the origin of a variety of global and local flow instabilities including the oscillating flow and the sharkskin like distortion of extrudate surface. With a set of new experiments to directly probe the interfacial molecular processes, we will be able to support the unified understanding of intriguing capillary melt flow characteristics summarized in this article.

Slipping fluids: a unified transient network model

“…They showed that the critical shear stress for HDPE extrusion increased with increase in temperature, which is in agreement with Brochard±de Gennes model [20]. Wang and Drda [3] argue that if sudden desorption was the governing mechanism for slip, then the critical stress should decrease with an increase in temperature.…”

“…2 can be used to predict the flow curve, i.e. wall shear stress (( w ) vs. apparent shear rate a 4Qa%R 3 , where the flow rate can be obtained by integrating Eq. (29).…”

“…9. Comparison of model prediction for apparent shear rate vs. wall shear stress with polyethylene melt experimental data [3]. Flow curve for D 1.04 mm is fitted using model parameters F f 75, F g 100, f 1265, g 12.65 in the bulk region, and F f 60 000, at a critical shear stress of about 0.3 MPa.…”

“…Interestingly, Wang and Drda [3] also reported a second critical stress, at which the flow rate again increased discontinuously. The flow curves after this second critical stress do not show diameter dependence, unlike those after the first critical stress.…”

“…Pressure-drop oscillations and rough extrudate surface (melt fracture) are well known phenomena that occur in controlled flow extrusion. These phenomena have been extensively studied [22,3] and reviewed [23±26, 58,82]. Appendix C summarizes the main experimental reports on wall slip in melts.…”