Glass, generally transition

In Chapter 6 we have seen that tan 8, a measure of the relative energy dissipation, depends on temperature and frequency, and that it shows maxima at transitions. A strong maximum occurs at the glass - rubber transition, weaker maxima at secondary transitions. In general tan <5is higher when the E(T) curve is steeper. 

The temperature dependence of the relaxation modulus at 500 seconds of polycarbonate (7), polystyrene (8), and their blends (75/25, 50/50, and 25/75) was obtained from stress-relaxation experiments (Figure 4, full lines). In the modulus-temperature curves of the blends, two transition regions are generally observed in the vicinity of the glass-rubber transitions of the pure components. The inflection temperatures Ti in these transition domains are reported in Table I they are almost independent on composition. The presence of these two well-separated transitions is a confirmation of the two-phase structure of the blends, deduced from microscopic observations. 

It has to be mentioned that the effect of dilution is in the terminal zone (long times) greater than in the glass-rubber transition zone (short times), which is specified by log ac. This becomes also clear from Fig. 16.11 the values of log ac in Figs. 16.12-16.14 were taken in the transition zone. It is beyond the scope of this book to go into more detail into this subject. For more details of the time-concentration superposition principle the reader is referred to the monograph by Ferry (General references, 1980, Chap. 17). 

It will be clear that the mechanical properties of polyketones at ambient temperatures are sensitive for these ageing and these moisture absorption effects especially due to the presence of the glass-rubber transition in that temperature region. The influence of both effects is in general opposite to each other the stiffness increases due to ageing and decreases due to moisture absorption. Moisture absorption effects are time and object dimensions dependent whereas ageing effects are only time dependent. 

Figure 6 shows the master curves for the PS films with M of 4.9k and 140k drawn by horizontal and vertical shifts of each curve shown in Fig. 5 at the reference temperatures of 267 and 333 K, respectively [26]. The master curves obtained from the dependence of lateral force on the scanning rate were very similar to the lateral force-temperature curves, as shown in Fig. 3. Hence, it seems plausible as a general concept that the scanning rate dependence of the lateral force exhibits a peak in a glass-rubber transition. Also, it is clear that the time-temperature superposition principle, which is characteristic of bulk viscoelastic materials [35], can be applied to the surface relaxation process as well. Assuming that Uj has a functional form of Arrhenius type [36, 37], the apparent activation energy for the aa-relaxati(Mi process, A//, is given by ...

Since the coloration of glasses by transition metal and rare earth ions results from ligand field effects, several general trends can be predicted. First, a change in oxidation state results in a change in the number of 3d or 4f electrons, resulting in a different number of possible electronic transitions for otherwise identical conditions. Since each possible electronic transition represents an absorption with a different energy, a difference in oxidation state will result in a different absorption spectrum. 

In any case, the general transition behavior is quite consistent with the notion of separation into two phases one, crystalline PEO the other, glassy PS. Further, the separation must be essentially complete, for the glass phase obviously exists in the presence of the molten crystalline phase (7 > 7 ). (It should be pointed out that a case in which 7 > has never been observed for homopolymers.) The particular finding here, of course, reflects the existence of two phases, each behaving nearly independently with respect to transition temperatures. 

This chapter will review the relationships among synthetic detail, morphology, and resulting mechanical behavior. While effects on the glass-rubber transition and modulus will be emphasized, aspects of toughness and impact resistance will be touched upon and applications discussed. In order to generalize this critique, polymer I is defined as the first synthesized polymer, and polymer II as the second synthesized polymer. Even when the order of synthesis is immaterial, as in mechanical blends, this notation will prove useful. Since the basis of this chapter lies in the two-phased nature of these materials, it is appropriate to examine first the fundamental reasons underlying phase separation. 

Figure 2.6. Idealized glass-rubber transition behavior of incompatible two-polymer combinations. In general, both glass-rubber transitions will be observed. ...

The temperature dependence of the glass—rubber transition follows the Vogel—Fulcher equation, which is essentially a generalization of the WLF equation ... 

Polymers will be elastic at temperatures that are above the glass-transition temperature and below the liquiflcation temperature. Elasticity is generally improved by the light cross linking of chains. This increases the liquiflcation temperature. It also keeps the material from being permanently deformed when stretched, which is due to chains sliding past one another. Computational techniques can be used to predict the glass-transition and liquiflcation temperatures as described below. 

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