Mechanical behaviour of polymers

There is a multitude of polymers with widely differing properties, making them suitable for different applications like rubber tyres, crash helmets, food packagings, or plastic bags. This multitude is due to the fact that polymers consist of organic chain molecules whose structure can be controlled within wide margins (see also chapter 1). 

As already discussed in section 1.4, we distinguish amorphous and semicrystalline thermoplastics, elastomers and duromers. All explanations given in the following relate to amorphous thermoplastics, unless noted otherwise. The peculiarities of the other groups are discussed separately. 

The mechanical properties of amorphous thermoplastics are mainly determined by the intermolecular bonds between the chains, not by the covalent bonds within them. These intermolecular bonds are, depending on the chemical composition, van der Waals, dipole, or hydrogen bonds. The different strengths of these bonds cause the wide spectrum of mechanical properties. In addition, the geometry of the molecules is important because they have to move against each other, especially so in plastic deformation. 

The bonds between the chains are weaker than covalent or metallic bonds and may be overcome by thermal activation even at room temperature. Thus, as we will see in detail in section 8.1, polymers are in their high-temperature regime even at room temperature. Their deformation is therefore time-dependent, and it is not always easy to distinguish elastic and plastic deformations. The mechanical properties of polymers are the subject of sections 8.2 to 8.4. Methods to improve the mechanical properties of polymers are discussed subsequently. The chapter closes with a brief discussion of the sensibility of polymers against environmental influences. 

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Before discussing tire complex mechanical behaviour of polymers, consider a simple system whose mechanical response is characterized by a single relaxation time x, due to tire transition between two states. For such a system, tire dynamical shear compliance is [42]... 

In the last section we considered tire mechanical behaviour of polymers in tire linear regime where tire response is proportional to tire applied stress or strain. This section deals witli tire nonlinear behaviour of polymers under large defonnation. Microscopically, tire transition into tire nonlinear regime is associated with a change of tire polymer stmcture under mechanical loading. 

The search for quantitative structure-property relationships for the control and prediction of the mechanical behaviour of polymers has occupied a central role in the development of polymer science and engineering. Mechanical performance factors such as creep resistance, fatigue life, toughness and the stability of properties with time, stress and temperature have become subjects of major activity. Within this context microhardness emerges as a property which is sensitive to structural changes. 

Minervino, M., Gighotti, M., Lafarie-Frenot, M.C., Grandidier, J.C. The effect of thermooxidation on the mechanical behaviour of polymer epoxy materials. Polym. Testing 32, 1020-1028 (2013)... 

This model is a very useful way to describe the physical and mechanical behaviour of polymers with low crystallinity. It is important to note that the model disregards some important parameters ... 

It is important to be able to predict and understand the resultant properties of a blend and its morphology from the properties of the constituent polymers. Predicting the mechanical behaviour of polymer blends and composites with respect to composition, structure (morphology of the blend) and properties of the components covering a wide temperature range are particularly important. ... 

An understanding of the mechanical behaviour of polymer blend materials becomes vital for creating innovative and economical designs for various components. The mechanical properties of polymer blends depend markedly on... 

Dutta, P. K. (1998) Thermo-mechanical behaviour of polymer composites , in Advanced Multilayered and Fibre-Reinforced Composites, Y. M. Haddad ed., London Kluwer Academic Publishers, pp. 541-54. 

Kruger, T. (1991) Mechanical behaviour of polymer modified cement mortars under complex stress states, in Proc. Int. Symp. Concrete Polymer Composites, H. Schorn and M. Middel, eds, Bochum, Germany pp. 135-46. 

Copolymerisation is another approach to improve the mechanical behaviour of polymers, especially their ductility. In copolymerisation, different monomers are used to form the chain molecules. There are several possibilities to arrange the monomers, shown in figure 8.26. The monomers can alternate alternating copolymerisation) or can be arranged irregularly random copolymerisation). In block copolymers, there are longer chain segments of one type... 

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