Page 19 - Handbook of Plastics Technologies

INTRODUCTION TO POLYMERS AND PLASTICS

INTRODUCTION TO POLYMERS AND PLASTICS 1.5

a T above room temperature, while a rubber has a T below room temperature. T is most
g
g
g
clearly defined by evaluating the classic relationship of elastic modulus to temperature for
polymers as presented in Fig. 1.5.

FIGURE 1.5 Relationship between elastic modulus and temperature.
At low temperatures, the material can best be described as a glassy solid. It has a high
modulus, and behavior in this state is characterized ideally as a purely elastic solid. In this
temperature regime, materials most closely obey Hooke’s law:
σ = Eε (1.3)

where σ is the stress being applied, and ε is the strain. Young’s modulus, E, is the propor-
tionality constant relating stress and strain.
In the leathery region, the modulus is reduced by up to three orders of magnitude from
the glassy modulus for amorphous polymers. The temperature at which the polymer be-
havior changes from glassy to leathery is known as the glass transition temperature, T .
g
The rubbery plateau has a relatively stable modulus until further temperature increases in-
duce rubbery flow. Motion at this point does not involve entire molecules but, in this re-
gion, deformations begin to become nonrecoverable as permanent set takes place. As
temperature is further increased, the onset of liquid flow eventually takes place. There is
little elastic recovery in this region, and the flow involves entire molecules slipping past
each other. This region models ideal viscous materials, which obey Newton’s law:
σ = ηε ˙ (1.4)

In the case of a thermosetting material, the rubbery plateau is extended until degradation
and no liquid flow will occur.

1.3.2 Crystallization and Melting Behavior (T )
m
In its solid form, a polymer can exhibit different morphologies, depending on the structure
of the polymer chain as well as the processing conditions. The polymer may exist in a ran-

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