Stress-strain properties of rubber
The stress-strain curve is typical of an elongated crystalline rubber, and its main component is the entropy change caused by the system becoming ordered. As the molecule is gradually straightened, the isolation effect of the branch chains on the molecular chain disappears, and the intermolecular attraction becomes significant, thus helping to resist further deformation, so the rubber will show high tensile strength when fully stretched. The stress of rubber under constant strain is a function of temperature. The rubber stress will increase proportionally with the increase of temperature. This dependence of rubber stress on temperature is known as the Joule effect and illustrates the fundamental difference between the elasticity of metal and rubber. In metals, each atom is held in a strict lattice by interatomic forces, and the work done to deform the metal is used to change the distance between the atoms, causing a change in internal energy. Therefore, its elasticity is called "energy elasticity", and its elastic deformation range is much smaller than that of "entropy elasticity" in rubber, which is mainly due to the change of entropy in the system. In the general range of use, the stress-strain curve of rubber is nonlinear, so the elastic behavior of rubber cannot be determined simply by Young's modulus.
