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Shear-Thinning in Polymer Melts

Polymer melts are considered complex fluids because the details of the atomistic structure of the polymers impact their flow (or rheological) properties, such as the viscosity. In simple (or Newtonian) fluids, the viscosity is constant, no matter how fast the fluid is flowing. In complex (or non-Newtonian) fluids, the viscosity is not constant. Shear-thinning is the term given to the rheological phenomenon observed when the viscosity decreases with increased shear rate. Shear flow occurs when a fluid is sheared, such as typical pressure driven flow down a pipe. On this page, we provide visualizations of the atomic-level origin of shear-thinning behavior. We also show the analogous structure for tension-thinning, which occurs in extensional flow, when a material is stretched, such as when one pulls a rubber band.

These materials are all dense fluids. In the visualizations below, only a few polymer chains are shown so that one is better able to visualize the conformation of the polymers. Visualizations of the same snapshots that include all chains are available here. The chains are given different colors in order to distinguish them from each other. All of these polymers are short linear polyethylene (C128H258). Only the carbon atoms are shown in the visualization.

Interactive Structures


C128H258 at equilibrium at 450 K.
At equilibrium, the chain are coiled and entangled with each other. This makes the polymer hard to flow at low shear rates. (It has a high viscosity.)

C128H258 under planar Couette flow at a dimensionless shear rate of 0.8 and 450 K.
Under a high shear rate, the chains both extend and align with the flow direction. This makes the polymer flow more easily. (It has a lower viscosity.) Some chains are not aligned as they are caught in mid-tumble, a rotational motion due to the vorticity in shear flow.

C128H258 under planar elongational flow at a dimensionless elongation rate of 0.2 and 450 K.
Under a high extension rate, the chains both extend and align with the flow direction. This makes the polymer flow more easily. (It has a lower viscosity.) There is no tumbling here because extensional flow does not have any vorticity.

Color Legend: All atoms are colored by chain.

To see the snapshots containing all chains, click: Visualizations of Shear-Thinning in Polymer (All Chains)

References

These structures were created by former graduate students at the University of Tennessee, Dr. Chunggi Baig and Dr. Jun Mo Kim, coadvised by by Dr. David Keffer (MSE) and Dr. Brian Edwards (CBE). References to several relevant papers are provided below.

Baig, C., Edwards, B.J., Keffer, D.J., Cochran, H.D., "Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular dynamics simulations", J. Chem. Phys., 122 2005 article # 184906. doi: 10.1063/1.1897373.

Ionescu, T. C., Baig, C., Edwards, B. J., Keffer, D. J., Habenschuss, A., "Structure formation under steady-state isothermal planar elongational flow of n-eicosane: A comparison between simulation and experiment", Phys. Rev. Lett 96(3).2006 article # 037802; reprinted in Virtual Journal of Biological Physics Research 11(3) 2006. doi: 10.1103/PhysRevLett.96.037802.

Baig, C., Edwards, B.J., Keffer, D.J., Cochran, H.D., Harmandaris, V.A., "Rheological and structural studies of linear polyethylene melts under planar elongational flow using nonequilibrium molecular dynamics simulations", J. Chem. Phys., 124 2006 article # 084902. doi: 10.1063/1.2174006.

Baig, C., Edwards, B.J., Keffer, D.J., "A molecular dynamics study of the stress-optical behavior of a linear short-chain polyethylene melt under shear", Rheologica Acta 46 2007 p. 1171-1186. doi: 10.1007/s00397-007-0199-2.

Kim, J.M., Keffer, D.J., Edwards, B.J., "Visualization of Conformational Changes of Linear Alkanes & Short Polyethylenes under Shear and Elongational Flows", J. Mol. Graph. Mod. 26(7) 2008 pp. 1046-1056. doi: 10.1016/j.jmgm.2007.09.001.

Kim, J.M., Edwards, B.J., Khomami, B., Keffer, D.J., "Single-chain dynamics of linear polyethylene liquids under shear flow", Phys. Lett. A 373(7) 2009 pp. 769-772. doi: 10.1016/j.physleta.2008.12.062.

Kim, J.M., Edwards, B.J., Keffer, D.J., Khomami, B., "Dynamics of individual molecules of linear polyethylene liquids under shear: Atomistic simulation and comparison with a free-draining bead-rod chain", J. Rheol., 54(2) 2010 pp. 283-310. doi: 10.1122/1.3314298.

posted: July 2104.
updated: July 2014.