Elastic properties of thermoplastics

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. 

We already saw in section 2.6 that Young s modulus is approximately proportional to the melting temperature and thus to the binding energy. For amorphous polymers, the relevant temperature is the glass transition temperature because this is the temperature where the bonds melt. The rather low values of the glass temperature (listed in table 1.3) thus also explain why Young s modulus of polymers is smaller than for the other material classes. 

The strong decrease of Young s modulus at the glass temperature (see figure 8.10(a)) will be discussed in the next section. More interesting in the 

As the activation energies of different relaxation processes differ, their relaxation time also differs. This is the reason why the simple spring-and-dashpot model from section 8.2.1 cannot be used to make quantitative predictions. This would require coupling several such elements [97] with relaxation times chosen to fit their respective processes. 

We already saw in section 8.1.1 that the activation energy of some relaxation processes is so low that it can be overcome by thermal activation already at temperatures as low as a few kelvin. At room temperature, their relaxation 

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