Elasticity

To explain the elastic behavior of materials like rubber requires a different model. Rubber consists of molecules, which are clusters of atoms joined by chemical bonds. Rubber molecules are very long and thin. They are polymers, long chain-like molecules built up by repeating small units. Rubber polymers consist of hundreds or thousands of atoms joined in a line. Many of the bonds are flexible, and can rotate. The result is a fine structure of kinks along the length of the molecule. The molecule itself is so long that it tends to bend and coil randomly, like a rope dropped on the ground. A piece of rubber, such as a rubber band, is made of vast numbers of such kinked, twisting, rope-like molecules.

When rubber is pulled, the first thing that happens is that the loops and coils of the "ropes" straighten out. The rubber extends as its molecules are pulled out to their full length. Still more stress causes the kinks to straighten out. Releasing the stress allows the kinks, coils and loops to form again, and the rubber returns to its original dimensions. Materials made of long, tangled molecules stretch very easily. Their elastic modulus is very small. They are called elastomers because they are very "elastic" polymers.

The "kink" model explains a very unusual property of rubber. A stretched rubber band, when heated, will suddenly contract. It is thought that the added heat provides enough energy for the bonds to start rotating again. The kinks that had been stretched out of the material return to it, causing the length to contract.