Up to this point, the treatment has emphasized metallic materials because metals are most widely used for their mechanical properties. This chapter covers the differences between the properties of polymers and ceramics on the one hand and metals on the other.
Elasticity of Polymers
Elastic moduli of thermoplastic polymers are much lower and much more temperature sensitive than those of metals. Figure 12.1 illustrates schematically the temperature dependence of the elastic moduli of several types of polymers. The temperature dependence is greatest near the glass-transition temperature and near the melting point. The crosslinked polymer cannot melt without breaking the covalent bonds in the crosslinks. The stiffness of a polymer at room temperature depends on whether its glass-transition temperature is above or below the room temperature. Below the glass-transition temperature, the elastic moduli are much higher than above it. Figure 12.2 indicates that the modulus of polystyrene changes by a factor of more than 103 between 85°C and 115°C.
If a random linear polymer is cooled very slowly, it may crystallize. Otherwise, it will transform to a rigid glass at its glass-transition temperature, Tg. Figure 12.3 is a plot of how the volume may change. If it crystallizes, there is an abrupt volume change. If it does not crystallize, there is a change of slope at Tg. Other properties change as the polymer is cooled below Tg. It toughness and ductility sharply decrease and its Young's modulus greatly increases.