Rubbery region

NR with standard recipe with 10 phr CB (NR 10) was prepared as the sample. The compound recipe is shown in Table 21.2. The sectioned surface by cryo-microtome was observed by AFM. The cantilever used in this smdy was made of Si3N4. The adhesion between probe tip and sample makes the situation complicated and it becomes impossible to apply mathematical analysis with the assumption of Hertzian contact in order to estimate Young s modulus from force-distance curve. Thus, aU the experiments were performed in distilled water. The selection of cantilever is another important factor to discuss the quantitative value of Young s modulus. The spring constant of 0.12 N m (nominal) was used, which was appropriate to deform at rubbery regions. The FV technique was employed as explained in Section 21.3.3. The maximum load was defined as the load corresponding to the set-point deflection. 

Chain scission is the predominant process at lower dose levels (<5000 Mrads), yielding a decrease in elastic modulus at ambient temperature. Additional crosslinking at high doses (>5000 Mrads) results in an increase in elastic modulus at ambient temperature and in the rubbery region above Tg. 

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. 

All three transitions are shown by a semi-crystalline thermoplastic with a chain length long enough to extend the rubbery region to above the melting point. 

It is possible that there are in fact two secondary types of transitions one, the transition between the glassy and leathery states, called the Tg and an additional transition occurring between the leathery and rubbery regions. 

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. 

The chemical structure of ABS suggests that ABS would allow to reach high permeation fluxes because of its rubbery regions and high separation factors due to the glassy matrix. 

As v varies in the opposite way as G, it increases with T (almost discontinuously at secondary transitions), and reaches a value of the order of 0.45 0.01 at Tg — 20 K. Then, it undergoes a rapid increase and attains a value very close to 0.50 in the rubbery region. The shape of temperature variations of v is represented in Fig. 11.6. 

Existence of a time-temperature equivalence that takes a different mathematical form in the glassy state (Arrhenius) and in the glass transition and rubbery regions (WLF). 

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). 

The rubbery region is wholly or partially masked by the crystallinity the elastic response of the melt is, therefore, much less pronounced. 

Figure 5.6. Effect of chain length on rubbery region.

---