Information on Rubber - 3

Why is the stretching reversible?

The section above describes why elastomers can stretch, but does not explain why, when the stretching force is removed, the material returns to its original shape. This can be explained by thermodynamics, and a much simplified description is given here.

When the rubber is 'at rest' at normal temperatures the chain-like molecules are in a constant state of agitation and are highly kinked - due to thermal energy. This is a highly disordered state - described thermodynamically as being a state of high entropy. When the chains are stretched (less kinked), a higher state of order is obviously being imposed - in other words the chains are being forced into a state of lower entropy. As it is a fundamental law of thermodynamics that entropy strives for a maximum, the driving force is now back towards the disordered state, and, as soon as the stretching force is removed, the rubber will retract. Other factors, such as the structure of the carbon atom, also play a part, but will not be discussed here.

Fluid resistance

The structure of an elastomer, which we have seen comprises a network of chains, means that there are gaps between adjacent chains. Indeed the elasticity of rubber relies on substantial thermal motion of the chains, which would not be possible if the chains were closely packed. The free volume available in the rubber means that some liquids can enter the rubber and cause swelling - sometimes very large amounts of swelling. For example the ability of oil to swell natural rubber is well known.

Potential for swelling is largely controlled by a thermodynamic property known as solubility parameter which is described elsewhere. All rubbers and all liquids have specific values of solubility parameter, a knowledge of which enables designers to avoid excessive interaction between an elastomer and the fluids which it will contact in service.

Incompressibility

Another property of elastomers which distinguishes them from other solid materials is their incompressibility. For most practical purposes, other than use under very high pressures, elastomers do not change their volume significantly when deformed. A rubber band may stretch 600%, but if its volume were measured in the stretched state it would be found to be almost identical to its unstretched volume.

This has important implications for designing with elastomers as the stiffness of components can be controlled, not just by altering the stiffness of the rubber itself, but by various techniques of geometrical design in relation to the mounting. This phenomenon, known as shape factor effects, will be described in more detail in standard text books, and leads to great versatility in design. In particular it enables rubber components to be designed with different, and controlled, stiffnesses and other properties, in two or even three different directions.