Molecular constitutive equations for a class of branched polymers: The pom-pom polymer
作者:
T. C. B. McLeish,
R. G. Larson,
期刊:
Journal of Rheology
(AIP Available online 1998)
卷期:
Volume 42,
issue 1
页码: 81-110
ISSN:0148-6055
年代: 1998
DOI:10.1122/1.550933
出版商: The Society of Rheology
关键词: Branched polymers;Pom–pom model;Constitutive equations;Entanglements;Branched polymers;Pom–pom model;Constitutive equations;Entanglements
数据来源: AIP
摘要:
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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