Polymer melts with long-chain side branches and more than one junction point, such as commercial low density polyethylene (LDPE), have extensional rheology characterized by extreme strain hardening, while the shear rheology is very shear thinning, much like that of unbranched polymers. Working with the tube model for entangled polymer melts, we propose a molecular constitutive equation for an idealized polymer architecture, which, like LDPE, has multiple branch points per molecule. The idealized molecule, called a “pom-pom,” has a single backbone with multiple branches emerging from each end. Because these branches are entangled with the surrounding molecules, the backbone can readily be stretched in an extensional flow, producing strain hardening. In start-up of shear, however, the backbone stretches only temporarily, and eventually collapses as the molecule is aligned, producing strain softening. Here we develop a differential/integral constitutive equation for this architecture, and show that it predicts rheology in both shear and extension that is qualitatively like that of LDPE, much more so than is possible with, for example, the K-BKZ integral constitutive equation.
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Research Article|
January 01 1998
Molecular constitutive equations for a class of branched polymers: The pom-pom polymer
T. C. B. McLeish;
T. C. B. McLeish
IRC in Polymer Science and Technology, Department of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, United Kingdom
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R. G. Larson
R. G. Larson
Department of Chemical Engineering, 2300 Hayward, University of Michigan, Ann Arbor, Michigan 48109-2136
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J. Rheol. 42, 81–110 (1998)
Article history
Received:
May 27 1997
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A companion article has been published:
Letter to the Editor: A note added to “Molecular constitutive equations for a class of branched polymers: The pom–pom polymer” [J. Rheol. 42, 81 (1998)]
Citation
T. C. B. McLeish, R. G. Larson; Molecular constitutive equations for a class of branched polymers: The pom-pom polymer. J. Rheol. 1 January 1998; 42 (1): 81–110. https://doi.org/10.1122/1.550933
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