Introduction to the mechanical properties of polymers

It must be recognised from the beginning that the mechanical properties of polymers are highly dependent on temperature and on the time-scale of any deformation; polymers are viscoelastic and exhibit some of the properties of both viscous liquids and elastic solids. This is a result of various relaxation processes, as described in sections 5.7.2 and 5.7.3, and examples of these processes are given in sections 5.7.4 and 5.7.5.

At low temperatures or high frequencies a polymer may be glass-like, with a value of Young's modulus in the region 109–1010 Pa, and it will break or yield at strains greater than a few per cent. At high temperatures or low frequencies it may be rubber-like, with a modulus in the region 105–106 Pa, and it may withstand large extensions of order 100% or more with no permanent deformation. Figure 6.1 shows schematically how Young's modulus of a polymer varies with temperature in the simplest case. At still higher temperatures the polymer may undergo permanent deformation under load and behave like a highly viscous liquid.

In an intermediate temperature range, called the glass-transition range, the polymer is neither glassy nor rubber-like; it has an intermediate modulus and has viscoelastic properties. This means that, under constant load, it undergoes creep, i.e. the shape gradually changes with time, whereas at constant strain it undergoes stress-relaxation, i.e. the stress required to maintain the strain at a constant value gradually falls.