Rubbers (or elastomers) are polymers whose properties are affected by cross-linking between the individual chains. They have a fairly low cross-link density, with links at random intervals, usually of between 500 and 1000 monomers. These few cross-links are sufficient to prevent the unrestricted flow of whole molecules past neighbouring ones, but the chain segments between cross-links are free to move (provided the temperature is above the glass transition temperature, Tg). In particular, the segments can uncoil and recoil.
Statistical theory is able to provide mathematical relationships between the density of cross-links and measurable physical properties, such as the stiffness. These relationships can be used to predict the extension under a particular load, for example a balloon being inflated or a bungee jumper, or a measured property can be used to calculate the extent of cross-linking.
It is commonly known that most things get bigger when heated. Probably the best known example of a material that does not always expand when heated is water, but this only contracts on heating as the temperature rises from 0°C to 4°C. Outside this range it behaves as any other material. The explanation for the anomalous behaviour lies in a re-arrangement of the molecular structure. The underlying tendency for inter-atomic bond lengths to increase with rising temperature, which is due to the asymmetrical shape of the energy-spacing relationship, is common to all materials.
Although rubber under normal conditions expands like other materials as it is heated, when under tension it behaves differently, contracting in the loading direction, rather than expanding, as it is heated. The explanation of this behaviour lies in the crucial contribution of entropy to the elasticity of rubber, which will be covered later in this package. It will also become clear why the stiffness of rubber is so much lower than other materials. Basically, this is because rubbers deform elastically by uncoiling of long, convoluted molecules, rather than by stretching of individual inter-atomic bonds.