Elasticity

The phenomenon of elasticity of rubber and other elastomers is a result of a tendency of large and very flexible amorphous polymeric chains to form random, thermodynamically favorable, conformations [18]. If a certain amount of crosslinking is also present, then these random conformations occur between the crosslinks. In a vulcanized (crosslinked) rubber elastomer, the crosslinks may occur at every five hundred to a thousand carbon atoms. The distance between the ends in such polymers is much shorter than when these elastomers are fully stretched. Deformation or stretching of rubber straightens out the various conformations in the molecules. They tend to return to the original state, however, when the forces of deformation are removed. So each segment behaves in a manner that resembles a spring. Some elastomeric materials are capable of high elongation and yet still capable of returning to the original conformation. Some soft rubbers, for instance, can be extended as high as 800% and even higher with full recovery. There is a preference for trans conformation, a planar zigzag at high extension. Rigidity of the chains, however, or crystallinity would hinder extension and, particularly, the recovery. High viscosity and a glassy state would do the same. The high degree of elasticity of rubbers is due in part to the effects of thermal motions upon the long polymeric chains. These motions tend to restrict vibrational and rotational motions about the single bonds in the main chain. Such restrictive forces in the lateral direction, however, are much weaker than are the primary valence forces in the longitudinal direction. Greater amplitudes of motion also take place perpendicular to the chains rather than in the direction of the chains. These increased motions in the perpendicular direction result in repulsive forces between extended or parallel chains. Such forces cause them to draw together after stretching. So, the stretched rubber molecules retract due to longitudinal tension until the irregular arrangement of molecules is achieved again. This more random conformation is actually a higher entropy state. When unstretched rubber is heated it increases in dimension with an increase in the temperature, as one might expect. At higher temperatures, however, rubbers, upon elongation, have a higher tendency to contract. This can be written as follows:

In summary, polymeric materials exhibit rubber elasticity if they satisfy three requirements: (a) the polymer must be composed of long-chain molecules, (b) the secondary bond forces between molecules must be weak, and (c) there must be some occasional interlocking of the molecules along the chain lengths to form three-dimensional networks. Should the interlocking arrangements be absent, then elastomers lack memory, or have a fading memory and are not capable of completely reversible elastic deformations.

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مواضيع ذات صلة

The Crystalline State

Rheology and Viscoelasticity of Polymeric Materials

Thermodynamics of Elasticity

The Glass Transition and the Glassy State

Induction Forces in Polymers

Effects of Dipole Interactions

Structure and Property Relationship in Organic Polymers

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