Materials for engineering, 3rd Edition - (Malestrom)

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Materials for engineering
2
3
4
Stress
1
Strain
5.4 Schematic stress–strain curves for various polymers (see text
for key).
Curve 2 is typical of polystyrene below its T g , showing near linear elastic
behaviour up to brittle fracture. Curve 3 is typified by (amorphous)
polycarbonate, which yields in a ductile manner, exhibiting tough behaviour.
We will now consider the mechanical properties in more detail. We have
so far assumed that the mechanical properties of polymers are independent
of the rate of testing, which is not the case because the properties are governed
by the mobility of the constituent polymer chains. This mobility is in turn
dependent on the inherent stiffness of the chain, the degree of chain
entanglement, the extent of cross-linking and the degree of crystallinity.
5.4.1 Stiffness
Consider an individual molecule in an amorphous polymer. When a stress is
applied, deformation can take place by two processes, bond stretching and
bond angle opening, and rotation of segments of chain about the chain
backbone. Below the glass transition temperature, the former are the main
deformation mechanisms. As the temperature increases above T g , however,
individual backbone bonds are able to rotate (Fig. 5.5) and, since the carbon–
carbon bonds are at an angle of 109° 28′ to one another, this ability to rotate
can bring about enormous shape changes in the polymer chain, which becomes
randomly kinked. Additionally, individual chains can slide locally relative to
each other, with other regions remaining elastically deformed. On unloading,
these elastic regions pull the polymer back to its original shape. This viscous
process takes time and the polymer will exhibit leathery properties whose
response can be modelled empirically by ‘spring-and-dashpot’ combinations
as described in Chapter 2.