Polymers come in many forms, including plastics, rubber, and fibers. Plastics are stiffer than rubber yet have reduced low-temperature properties. Generally, a plastic differs from a rubbery material due to the location of its glass transition temperature (Tg). A plastic has a Tg above room temperature, while a rubber has a Tg below room temperature. Tg is most clearly defined by evaluating the classic relationship of elastic modulus to temperature for polymers as presented in Fig. 1.5.
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: (1.3)
where s is the stress being applied, and e is the strain. Young’s modulus, E, is the proportionality 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 behavior changes from glassy to leathery is known as the glass transition temperature, Tg. The rubbery plateau has a relatively stable modulus until further temperature increases induce rubbery flow. Motion at this point does not involve entire molecules but, in this region, 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: (1.4)
In the case of a thermosetting material, the rubbery plateau is extended until degradation and no liquid flow will occur.
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: (1.3) 
where s is the stress being applied, and e is the strain. Young’s modulus, E, is the proportionality 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 behavior changes from glassy to leathery is known as the glass transition temperature, Tg. The rubbery plateau has a relatively stable modulus until further temperature increases induce rubbery flow. Motion at this point does not involve entire molecules but, in this region, 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: (1.4) 
In the case of a thermosetting material, the rubbery plateau is extended until degradation and no liquid flow will occur.
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