Knots make polymers weaker
May 7, 1999
When a length of string is knotted its weakest point is just outside the entrance to the knot. Now Michael Klein and colleagues from the University of Pennsylvania and DuPont's research labs in Delaware have shown that this same behaviour occurs on the molecular scale. Klein and co- workers have calculated how the strength of a polymer strand is influenced by a trefoil knot (Nature 399 46). The findings have been backed up by experimental results that are due to be published in Nature. The work is of practical importance because plastics and other polymer- based materials are widely used in many applications where the strength of the material is important.
Polymers are long chain-like molecules made up of smaller molecules called monomers. It is well known that polymer chains often tie themselves in knots, but it was not clear how this influenced the mechanical properties of the polymer. Klein and colleagues modelled what happens when a knotted polymer chain consisting of 144 carbon atoms was stretched. They found that as the ends of the chain were pulled apart, the knot gradually got tighter and tighter - reducing the number of carbon atoms in the knot - until the polymer chain ruptured next to the knot. According to their calculations, a polyethylene polymer breaks once the number of carbon atoms in the knot is reduced to 23.