Elasticity duromer

The elastic behaviour of polymers is mainly determined by the intermolecular bonds between the chain molecules, not by the covalent bonds within. For elastomers and duromers, the covalent bonds linking the chains are also relevant. In the following, we will start by discussing the elastic properties of thermoplastics and afterwards study the influence of cross-linking. 

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. 

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. 

A further important point is that the fracture strain of high-strength carbon fibres is about 2% although they deform only elastically. Considering that the strains in the polymer matrix locally exceeds that of the fibre (see figure 9.4), we see that the fracture strain in the matrix has to be rather large. To avoid crack formation in the matrix, its fracture strain should be about twice that of the fibre i. e., 4% to 5%. Currently available duromers do not... 

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