PerspectiveMaterials Science

Predicting the Flow of Real Polymers

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Science  30 Sep 2011:
Vol. 333, Issue 6051, pp. 1834-1835
DOI: 10.1126/science.1211863

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One of the earliest mass-produced polymers, low-density polyethylene (LDPE), is still widely used in products ranging from plastic wrapping to plastic bags because of its low cost as well as the flow properties of its melted state when it is pressed, stretched, or pulled. LDPE contains molecules with both a wide range of sizes and long side branches, and even branches that have branches, yielding irregular treelike structures (1). Each branch can have a wide range of molecular weights and can be positioned arbitrarily along other branches, so the number of distinct species within an LDPE melt is unimaginably large. If the molecular species in such a melt cannot even be fully inventoried, how can one hope to model the flow properties—the rheology—of the melt, which depends on how these branches tangle? On page 1871 of this issue, Read et al. (2) developed a model that accounts for the flow properties by making simplifying assumptions about the distribution of branches and how they transfer force through the molecule.