Most of us have played with lumps of “silly putty,” the strange substance shown in the photographs on page 34. Given a bit of time, this material flows like a viscous liquid. Forced to respond quickly, it bounces like rubber. We can trace this “viscoelastic” behavior, which shows up in all polymer melts, to the knotting of the chains of “monomers” that make up the polymers. Shearing forces tend to undo certain knots, but this takes a finite time τ. In a time greater than τ the original knots fade out, and the melt flows. Over shorter times the original knots are all present, and the melt behaves like an elastic network.
A theory based on the snake‐like motion by which chains of monomers move in the melt is enhancing our understanding of rheology, diffusion, polymer–polymer welding, chemical kinetics and biotechnology.