Rubber elasticity or entropy elasticity describes the resistance of any rubber or elastomer system against an externally applied deformation or strain. Rubber elasticity is related to reversible changes in entropy. The origin of these reversible changes of entropy within a rubber or elastomer matrix along the polymer chains can be described by the following aspects:
If a zig-zag elastomer molecule is extended due to any external force, the bonding angle between neighborhood molecules along the chain will be changed. If these small changes take place without the activation of friction processes, no deformation energy is required.
On the other hand, stretching means orientation takes place on a molecular basis and entropy (disorder within a system) will decrease.
If the external force is removed, thermal energy from the environment will be absorbed and the molecules return to their initial orientation; the bonding angle will return and the molecules will shrink to their original shape.
An elastic modulus (also known as modulus of elasticity) is a quantity that measures an object or substance's resistance to being deformed elastically (i.e., non-permanently) when a stress is applied to it. The elastic modulus of an object is defined as the slope of its stress–strain curve in the elastic deformation region: A stiffer material will have a higher elastic modulus. The elastic modulus is defined according to the following formula: