Leathery region, polymers

In the rubbery region, which is just above (in terms of temperature) the leathery region, polymer chains have high mobility and may assume many different conformations, such as compact coils, by bond rotation and without much disentanglement. When these rubbery polymers are elongated rapidly, they snap back in a reversible process when the tension is removed. This elasticity can be preserved over long periods of time if occasional cross-links are present, as in vulcanized soft rubber, but the process is not reversible for linear polymers when the stress is applied over long periods of time. 

Since at temperatures below the Tg the chains of an amorphous polymer are randomly distributed and immobile, the polymers are typically transparent. These glassy polymers behave like a spring and when subjected to stress, can store energy in a reversible process. However, when the polymers are at temperatures slightly above the Tg, i.e., in the leathery region, unless crosslinks are present, stress produces an irreversible deformation. 

Region II (b to c) is the glass transition region. Typically, the modulus drops a factor of about a thousand in a 20-30°C range. The behavior of polymers in this region is best described as leathery. For static or quasistatic measurements, such as illustrated in Fig. 2.24, the glass transition temperature, Tg, is often taken at the maximum rate of turndown of the modulus at the elbow, i.e., where dPE/dT is at a maximum. 

As shown in Fig. G.2, the temperature at which the polymer behavior changes from glassy to leathery is known as the Tg. The rubbery plateau has a relatively stable modulus until, as the temperature is further increased, a rubbery flow begins. 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, eventually the onset of liquid flow takes place. There is little elastic recovery in this region, and the flow involves entire molecules slipping past each other. 

Leathery or transition region Here the polymer chain segments undergo short-range diffusional motion. A typical diffusion time from one site to another is on the order of 10 s. 

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