Rubbery region, polymers

As the temperature is increased there is available sufficient energy to melt the crystalline polymer, the Tm, and before this for the amorphous polymer sufficient energy so that in both cases ready wholesale movement of polymer chains occurs. The entire polymer now behaves as a viscous liquid such as molasses. For the cross-linked material wholesale mobility is not possible, so it remains in the rubbery region until the temperature is sufficient to degrade the material. 

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. 

A fully crystalline polymer would follow curve b no glass transition occurs and the modulus drops sharply at Tm, either to zero or to a tail of the rubbery region (ai). 

We chose the temperature of 600 K for our simulations and initially choose a density of 0.9g/cm so that we were performing the simulations in the rubbery region of the polymer. Examination of the poling results (Figure 8) reveals that at this density and temperature, and a dilute concentration (5% by weight), the system does not behave as the non-interacting rigid gas model predicts. The predicted order for this system has a value of 0.60. The system s calculated order parameter... 

The choice of the process depends, inter alia, on the requirements of the end product with injection molding, narrower tolerances can be achieved than with thermoforming or rotomolding, for example. The nature of the polymer also plays a role it depends, for instance, on the stability of the polymer at the high temperatures in a process. The length of the rubbery region on the temperature scale determines the ease of vacuum forming too brittle a polymer is difficult to machine, for example. 

An important factor with thermoforming is that the polymer should show a pronounced rubbery region on the temperature scale. For this reason, amorphous polymers such as PVC, PS, poly(methyl methacrylate) (PMMA), polycarbonate (PC), acrylonitrile butadiene styrene (ABS), etc. are well suited for thermo forming. With semicrystalline polymers, the rubbery region is largely masked by the crystallinity (Fig. 23.19). With PE and polypropylene (PP), thermoforming is, therefore, a critical operation, in which the processing conditions should be very carefully controlled. 

Figure 4.3 Specific volume-temperature curves for a semicrystalline polymer. (A) Liquid region (B) viscous liquid with some elastic response (C) rubbery region (D) glassy region (E) crystallites in a rubbery matrix (F) crystallites in a glassy matrix. 

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