&Bullet; physics 14, p56
According to the results of new neutron scattering experiments, polymer molecules in plastics move in ways that are not covered by frequently used models.
Melt a plastic and its components, known as polymers, wobble around. Experts usually describe polymer movement using what is known as the pipe model, in which plastics are thought of as a tangle of polymer strands – think of a bowl full of worms. The model assumes that each strand moves independently within a virtual pipe. Michaela Zamponi from the Jülich Research Center in Germany and colleagues have now observed a strand movement in a polymer melt that contradicts the idea of an independent movement  . The results build on other studies questioning the tube model (see Viewpoint: Tube Model Under Stress) and could help researchers refine theoretical concepts of polymer behavior.
Using neutron scattering spectroscopy, the team examined a polymer melt with a few short chains of polyethylene in a sea of long strands of the same material. Since the long polyethylene strands contain a heavier hydrogen isotope than the short chains, the neutrons scatter differently from the two types of strands, so that the researchers can follow the movement of the short strands.
While monitoring the center of mass movement of the short strands, they observed two behavioral regimes. At short translation distances, the movement of the short strands slowed as they grew apart. At longer distances, the speed at which the short strands were moving slowed and instead slowed down the rate of diffusion when the center of gravity of the strands reached a size on the order of the diameter of the virtual pipe.
They also found that the movements of short strands at short intervals were associated with those of neighboring strands, which deviates from a standard assumption made by the tubular model. This cooperative movement can result from interactions between the segments that go beyond simple local friction.
Sophia Chen is a freelance science writer based in Columbus, Ohio.
- M. Zamponi et al., “Cooperative chain dynamics of tracer chains in highly entangled polyethylene melts” Phys. Rev. Lett.126187801 (2021).