Why do we stretch rubber

You could measure it, but not calculate it: the force that rubber exerts when you stretch it a lot. Rubber only follows the law of elasticity from the 1940s up to a certain degree of elongation. Now US physicists provide the explanation of what happens in rubber beyond the point of elongation.

Boulder (USA) / Urbana-Champaign (USA) - They include how the molecular chains slip against each other. With the new addition, the theory finally agrees with what has long been known from practice. This does not change anything for tire and rubber band manufacturers, but it does make some physicists happy.

"This has challenged the physicist community for sixty years," said Paul Goldbart, a physicist at the University of Illinois. Together with Leo Radzihovsky from the University of Colorado and Xiangjun Xing from Syracuse University, he re-evaluated the molecular structure of rubber. Rubber consists of long molecular strands, of which individual atoms are connected to one another at irregular intervals. If you pull the rubber, it behaves linearly up to about twice the stretch, according to the elasticity theory from the 1940s, which mainly contains Hooke's law of 1678: 20 percent more tensile force results in a 20 percent longer piece of rubber. Stretched rubber is more orderly than unstretched rubber: the elongated molecules are in a more orderly manner than the coiled ones, they are in a state of low entropy. But nature generally prefers higher entropy, the state of higher disorder.

If you now exceed the strain point, the relationship between force and distance is no longer linear. Goldberg's team explains this in the journal "Physical Review Letters" with the previously neglected, thermodynamic behavior of the connection points between the rubber molecules. It also takes into account the fact that rubber cannot be compressed. The molecules slip against each other, but cannot be compressed. The previous theory assumes that when the rubber is stretched, the connection points remain motionless in the same place on the molecule, even if it is stretched and moved. The US researchers, however, assume that the connection points can also move. The entropy of rubber is based not only on the molecular movements between the connection points, but also on the movement of the connection points themselves. With the help of a few mathematical tricks, according to the team, it also took into account the non-compressibility of rubber.

The new theory now describes well the behavior of rubber, which has always been observed in practice. It should apply to a wide range of soft, elastic substances, according to Goldbart and colleagues. They are currently testing their theory in various experiments.