The ability of rubber to deform and then recover its shape when stress is removed is one of the most fundamental phenomena in polymer science. It is also vital to the functions of elastic proteins and muscles.
For a material such as rubber to be elastic, three molecular conditions must be met: the chains must be joined together into a network structure; they must be flexible enough to change their spatial arrangements and extension dramatically in response to an imposed strain; and they must have high energy to produce these changes.
The first condition can be observed in the way that a typical rubber band will stretch when pulled on, obeying the simple relation known as Hooke’s law: a 10 percent increase in the pulling force makes the rubber band 10 percent longer. However, the rubber will not continue to stretch at this rate. It will reach a limit at which the stretching (strain) becomes permanent, a state that is called plastic deformation.
It is not possible to determine the exact point at which this occurs using a traditional mechanical test that measures tensile strength and strain; it is, therefore, determined by computer simulation. Specifically, these simulations predict that, when stretched to very low limits of extension, a number of the chains will transition from their thermal equilibrium end-to-end distances by changing the direction of their rotation around bonds and increasing their bond lengths. This produces a new elasticity force that opposes the applied strain, reducing the total tensile force on the sample and resulting in a plateau in the simulated stress.
