Extensional Flow Behavior - Introduction

So-called sandwich rheometers have sometimes been used in the study of rubber elasticity and melt viscoelasticity [141]. In a sandwich rheometer, twin sample plaques are placed in gaps formed by a central steel plate and two outer plates that are part of the same frame. These instruments are difficult to load and clean, and there is no direct control of the gap. Sliding plate melt rheometers were developed to make measurements of nonlinear viscoelastic behavior under conditions under which cone-plate flow is unstable, ie. in large, rapid deformations [ 142 ]. The sample is placed between two rectangular plates, one of which translates relative to the other, generating, in principle, an ideal rectilinear simple shear deformation. Creep tests can be carried out either by use of a feedback loop that generates a plate displacement that gives rise to a constant stress, or by use of a pneumatic drive, as in Laun s sandwich rheometer [143]. 

Many years of experience with sliding plate rheometers have revealed several phenomena that limit their utility under certain conditions. First, the normal stress differences create a pressure gradient in the sample that tends to pump melt in from the ends of the sample and out toward the edges. [ 147-149]. This flow can be prevented by the use of fluorocarbon side- rails. More serious limitations are imposed by slip, cavitation and rupture, which interrupt experiments at sufficiently high strains and strain rates. At the same time, however, sliding plate rheometers have been found to be useful tools for the study of melt slip [150]. Elastomers are particularly resistant to shearing deformations and even the use of deep grooves in the plates does not ensure their adherence [146]. 

Most experimental studies of melt behavior involve shearing flows, and we saw in Chapter 5 that linear viscoelastic behavior is a rich source of information about molecular structure. However, no matter how many material functions we determine in shear, outside the regime of linear viscoelasticity such information cannot be used to predict behavior in other types of deformation, ie., for any other flow kinematics. A class of flows that is of particular importance in commercial processing is extensional flow. In this type of flow, material elements are stretched very rapidly along streamlines. Nonlinear behavior in extensional deformations provides information about structural features of molecules that are not revealed by shear data. 

In particular, long-chain branching is known to have an important effect on the response of a melt to a stretching flow that is not revealed in shear flows. 

From the point of view of tube models, the two key elements of nonlinear behavior are tube orientation and tube or chain stretch. The former nonlinearity can be probed using shear flow, but shear flows are not effective in generating significant chain stretch. As we have seen, chain stretch in shear is strongly suppressed by the mechanism of convective constraint release (CCR) up to extremely high shear rates. The CCR mechanism of relaxation is qualitatively much less important in extensional flows than in shear flows, because in the former molecules on neighboring streamlines move at the same velocity. Thus, extensional flows are of particular importance in the study of nonlinear viscoelasticity. 

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Since homogenous melts are covered in a later account of pressure build-up and power input in the extruder (Chapter 7), this chapter confines itself to the flow behavior of homogenous unfilled polymer melts and on the introduction of the most important rheological parameters such as viscosity, shear thinning, elasticity, and extensional viscosity. The influence of these rheological properties on simple pressure- and drag flows is demonstrated, while the influence of rheological parameters on pressure build-up and power input in the extruder is described in more detail in Chapter 7. 

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