Alte und neue Wege in der Rheometrie der Polymer-Schmelzen
Polymer melts are nonlinear, pseudoplastic, viscoelastic, rubberlike liquids. Because of this complex rheological behaviour experimental investigations with conventional methods seem to be insufficient. New test modes are necessary, in shear as well as in elongation, in order to obtain results whic...
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| Format: | Article |
| Language: | deu |
| Published: |
Swiss Chemical Society
1984-02-01
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| Series: | CHIMIA |
| Online Access: | https://www.chimia.ch/chimia/article/view/9606 |
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| Summary: | Polymer melts are nonlinear, pseudoplastic, viscoelastic, rubberlike liquids. Because of this complex rheological behaviour experimental investigations with conventional methods seem to be insufficient. New test modes are necessary, in shear as well as in elongation, in order to obtain results which serve as guidelines for the formulation of the ’correct’ constitutive equation which is the central problem of polymer melt rheology. In this paper, a review on conventional and new test methods is given.
Part I deals mainly with shear flows. Capillary rheometry is the standard method for the determination of the viscosity function up to high shear rates. In capillary flow, the melt elasticity leads to extrudate swell, and the melt fracture phenomenon terminates the range of stable, laminar flow. For small strains, polymer melts are linear viscoelastic, and the standard tests are stress relaxation, constant strain rate, oscillations, and creep (constant stress). In simple planar shear and in viscometric flows of incompressible liquids the shear stress p21 and two normal stress differences, N1 and N2, represent the rheological behaviour. With the cone-and-plate rotational rheometer p21 is determined from the torque, N1 from the thrust which tends to open the cone-and-plate gap, and N1 and N2 from the radial pressure distribution. For polymer melts, at the onset of the shear test not only p21 and N1 show remarkable maxima, but also the shear recovery. These maxima indicate a ’break-down’ of the physical structure of the melt due to deformation. The combined influences of deformation and time can be factorized for the shear modulus obtained from stress relaxation tests with large shear strains.
For more general shear flows, a new sandwich-type biaxial shear rheometer is described in which arbitrary shear flows in two perpendicular directions can be performed. The first results show that after a rotation of the shear flow direction an anisotropy of the shear stress components follows due to the primary molecular orientation which needs time for readjustment to the new shear direction.
In part II methods of elongational rheometry are described. In simple elongation small samples provide rheological information even if only a small amount of polymer is available. The sample, glued to metal discs which act as clamps, is floating on silicone oil. The difficulties arising during the elongation from the necking of the sample near the clamps can be overcome by means of a correction of the servo drive control. The sample is cut into short cut-offs at the end of the test. From the weight of the cut-offs the total elongation follows and the deviation from the ideal test conditions. A second elongational rheometer is of the rotary clamp type with the advantage that very large total strains can be achieved in simple elongation. In constant strain rate tests, polymer melts start with a linear viscoelastic response followed by a remarkable strain-hardening. This is in contrast to the shear-thinning in shear-flow. For LDPE up to Hencky strains ε = 7 (corresponding to a stretch of λ = 1100) no rheologically steady state of flow is achieved. The recovery results illustrate that the same polymer melt responds more like a viscous liquid or a rubberlike elastic solid, depending on the strain rate or the duration of the test.
A re-designed rotary clamp is the mechanical basis for a more general rheometer by means of which the behaviour of polymer melts in equibiaxial, planar and other multiaxial elongations can be investigated. A very efficient classification for these types of flow was developed, and by means of a new definition of elongational viscosities the nonlinear behaviour can be described easily because reference is made to the linear viscoelastic limiting case. Elongational tests of very different modes were performed with always the same polyisobutylene batch at room temperature. Planar tests are especially interesting because of two different viscosities resulting, one measured in the direction of elongation, the second measured in the perpendicular direction in which the acting stress keeps the sample dimension constant. By a recent modification of this rheometer, multiaxial elongational tests can be performed during which the main axes of the strain rate tensor can be rotated. Such tests allow one, similar to the rotation of the main axes in shear flow, to investigate the rheological consequences of a molecular orientation developed during a primary deformation period.
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| ISSN: | 0009-4293 2673-2424 |