Rubber elasticity
Elastomers have a unique set of properties: immediate deformation and recovery, fully reversible deformation, and exceptional deformability. The vulcanization process of latex from the rubber tree by sulphur was independently discovered by the American Goodyear in 1839 and the British citizen Hancock in 1843 but it was not until about a century later that the chemistry and physics behind the discovery was revealed. The sparse covalent network that was formed when sulphur linked some of the double bonds of adjacent cis-Polyisoprene polymer molecules together rendered a single molecule with the following features: high molar mass in-between cross-links yet dense enough to restrict flow, high conformational mobility and very weak and intermolecular attraction. Without going into detail about the thermodynamics behind the rubber elasticity, the reader still needs to grasp the concept of order and disorder and that every spontaneous process moves from order to disorder.
Degree of disorder is defined as entropy. When a rubber elastic material is stressed it readily deforms due to the high molecular mobility and the weak intermolecular attraction. In an ideal elastomer there is actually no difference in intermolecular attraction in the unstressed and deformed state. The deformation stretches the polymers and orients them along the stress direction. With the orientation they become less disordered, or expressed in thermodynamic terms – lower entropy.
As soon as the stress is released the elastomer returns to as disordered a state as possible. Hence, it immediately retracts. The large deformability comes from the scarce cross-linking density that permits a high degree of deformation before the polymers in-between the cross-links become totally stretched out. Most rubbers are in fact quite close to the ideal state.
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