Flow behavior of polymer melt

Even at a phenomenological level, the flow behavior of polymers differs significandy from that of low-viscous liquids such as water. The flow behavior of polymer melts is markedly dependent on the applied shear stress o, the shear rate y, and on time. Elastic effects and normal stresses also occur in practice, and their effects can either help or hinder the processes underway. 

Polymer chains anchored on solid surfaces play a key role on the flow behavior of polymer melts. An important practical example is that of constant speed extrusion processes where various flow instabilities (called sharkskin , periodic deformation or melt fracture) have been observed to develop above given shear stress thresholds. The origin of these anomalies has long remained poorly understood [123-138]. It is now well admitted that these anomalies are related to the appearance of flow with slip at the wall. It is reasonable to think that the onset of wall slip is related to the strength of the interactions between the solid surface and the melt, and thus should be sensitive to the presence of polymer chains attached to the surface. 

Takahashi H (1990) Flow behavior of polymer melts under extremely high shear rates and flow analysis of injection molding. J Rheol (Jpn) 18 172-9. 

FIGURE 13.5 Idealized flow behavior of polymer melts and solutions indicating Newtonian... 

Before going into detail on the flow behavior of polymer melts, it may be useful to describe and define some of the basic terminology used in fluid flow. 

Accurate description of flow of the polymer melt through the die requires knowledge of the viscoelastic behavior of the polymer melt. The polymer melt can no longer be considered a purely viscous fluid because elastic effects in the die region can be significant. Unfortunately, there are no simple constitutive equations that adequately describe the flow behavior of polymer melt over a wide range of flow conditions. Thus, a simple die flow analysis is generally very approximate, while more accurate die flow analyses tend to be quite complicated. 

The flow behavior of polymer melts and solutions (Figs. 3-8 and 3-10) shows at low and high shear rates what appear to be Newtonian viscosities with a transition regime in between. Develop a model based on the molecular structure of a polymer melt or solution to describe these phenomena. 

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