A Unifying Theory of Polymer Liquids
Polymer liquids, also known as polymer melts, have a unique flow behavior because of the microscopic dynamics of their constituent molecular chains. Multiple theories have been developed over the last half-century to explain how these chains move, but these theories, each individually tailored to specific chain flexibilities, are mutually incompatible. Now, Robert Hoy of the University of South Florida, Tampa, and Martin Kröger of the Swiss Federal Institute of Technology (ETH) in Zurich have developed a unified analytic theory that predicts the behavior of molecular chains in polymer melts from first principles.
Hoy and Kröger model molecules in polymer melts as long chains of beads connected by springs. Varying the stiffness of the chains, they simulate the molecular dynamics over a range of flexibilities. This flexibility governs how molecules interact—determining, for example, the likelihood of them becoming entangled, and the extent to which molecules can move without impinging on their neighbors. Existing theories that consider only limited flexibility ranges differ in how they describe these interactions and therefore make conflicting predictions about more general cases.
With their unifying model, Hoy and Kröger obtain analytic expressions that predict the molecules’ freedom of movement, the average number of entangled molecules in a given volume of liquid, and the overall properties of the polymer melt. Their new model combines the earlier, disparate theories, reproducing each of them in their specific flexibility regimes and filling in the gaps that separated them.
The researchers say that their model could reduce the computational power required to simulate polymer melts and may ultimately help design plastics with optimized properties, such as high mechanical strength and melting temperature.
This research is published in Physical Review Letters.
–Sophia Chen
Sophia Chen is a freelance science writer based in Tucson, Arizona.