Rubber elasticity

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Rubber elasticity refers to a property of crosslinked rubber, namely that it can be stretched up to a factor of 10 from its original length and returns very nearly to its original length upon release. This can be repeated many times with no apparent degradation to the rubber.^[1]

Rubber is a member of a larger class of materials called elastomers. Elastomers have played a key role in the development of new technologies in the 20th century and make a substantial contribution to the global economy.^[2]

Rubber elasticity is produced by many complex molecular processes and its complicated explanation requires a knowledge base consisting of advanced mathematics, chemistry, statistical physics, and the concept of entropy. Entropy may be thought of as a measure of the thermal energy that is stored in a molecule.

Common rubbers, such as polybutadiene and polyisoprene (also called natural rubber), are produced by a process called polymerization.^[3] The process starts off with very long molecules (polymers) that are built up sequentially by adding short molecular backbone units through chemical reactions. A rubber polymer follows a random, zigzag path in three dimensions, intermingling with many other rubber molecules. An elastomer is created by the addition of a small amount of a cross linking molecule such as sulfur.^[4][5]

When heated, the crosslinking molecule causes a reaction that chemically joins (bonds) two of the rubber molecules together at some point (a crosslink). Because each rubber polymer is very long, each one participates in many crosslinks with many other rubber molecules forming a continuous network.