Elastic properties of elastomers and duromers

Elastomers and duromers are characterised by additional covalent cross-links between the chain molecules. In the energy-elastic regime, these additional bonds do not influence the elastic properties significantly Young s modulus only increases slightly. 

Contrary to metals and ceramics, the elastic strains in elastomers can become very large and attain values of several hundred percent. The reason is that the molecules are straightened during deformation, but the cross-links prevent the molecules from shding past each other and thus inhibit plastic deformation. Upon unloading, entropy-elasticity completely restores the initial arrangement of the molecules. This behaviour is called hyperelasticity. 

During deformation of hyperelastic materials, large strains of 100% or more can occur. The material behaviour is strongly non-linear. Therefore, the theory of large deformations has to be used to describe the material behaviour (see section 3.1). 

If the cross-linking density of a polymer is increased further, the entropy-elastic behaviour vanishes nearly completely because the large number of crosslinks prevent the straightening of the molecules. For this reason, duromers show only a small decrease of Young s modulus with temperature (see figure 8.10(d)) caused by relaxation processes. They are energy elastic even above the glass temperature. 

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