Relaxed glasses

The relaxatioa temperature appears to iacrease with increa sing HFP coateat. Relaxatioa iavolves 5—13 of the chaia carboa atoms. Besides a and y relaxations, one other dielectric relaxation was observed below —150° C, which did not vary ia temperature or ia magnitude with comonomer content or copolymer density (55). The a relaxation (also called Glass 1) is a high temperature transition (157°C) andy relaxation (Glass 11) (internal friction maxima) occurs between —5 and 29°C. 

The kinetic character of the glass transition and the resulting non-equilibrium character of the glassy state are responsible for the phenomena of structural relaxation, glass transition hysteresis, and physical aging (Kovacs, 1963 Struik, 1978). 

Ta Main relaxation (glass transition or melting) temperature. 

Fig. 5. Isobaric volume v versus T, illustrating volume hysteresis effect. The equilibrium curve in the liquid well above Tg is unique. At a constant cooling rate g, the V falls out of equilibrium below Tg. When the sample is then annealed at constant T, the volume becomes densified and may reach a relaxed glass state, depending on the temperature. The lower portion of the solid curve represents the behavior after heating at constant q. Note c remains under its liquid value to T> Tg. This plot is similar to that found for amorphous Selenium in Ref. 71 and for a polymeric system in Ref. 79.

Included in this section is a description of the various transformations taking place in amorphous alloys, such as structural relaxation, glass transition and amorphous-to-crystalline transition. The main techniques employed to study these transitions are described in more detail and further conclusions are given that can be drawn from the results of these studies. 

By using dynamic mechanical analysis (DMA), tensile, compression, bending and torsion tests can be carried out under static or dynamic conditions. Elastic moduli can also be obtained by DMA. With DMA, measurements of constant stress or constant strain can also be made. Thus thermal expansion coefficients, stress relaxation and creep can be investigated by DMA. Here we only give an example of the measurement of the thermal expansion coefficient. In Figure 4.96, the thermal expansion curves of Fe-Co-Si-B amorphous alloy are plotted. Curves 1 and 2 are the measured results for a relaxed glass and as-quenched glass. 

Figure 4.96 Therrrval expansion curve of Fe-Co-Si-B alloy [106], I, Relaxated glass II, as quenched glass...

Polycaproiactam (nylon 6). This polymer has a crystallinity 50 and shows u scries of relaxations as the specimen is heated. The major relaxations are at 50 C (glass-to-rubber relaxation of the amorphous fraction), and crystal moiling point at 220 C. Other less intense relaxations of the amorphous fraction occur below 0 C. The difference between the temperature characteristics of amorphous and crystalline polymers illustrated in Fig. 4.21 is most marked and entirely characteristic (compare also Figs. 4.12 and 4.11). Glas.sy polymers (4.N.4) have one dominant relaxation (glass-to-rubber) and, sometimes, a smaller secondary relaxation (as in PMMA, PVC, and polycarbonate ). Crystalline polymers usually have several relaxations. 

Due to the cooperative character of molecular movements associated with this relaxation, the linearity of the semilog plot v versus 1/T is not fulfilled (Fried 2007). The activation energy of a-relaxation (glass transition) can be determined by using the equation (10.5) only when it is ntilized in a limited frequency range. 

An alternative method to observe dielectric properties is termed thermal stimulated currents (TSC). This method involves polarization of a sample at high temperature (relative to Tg) and quenching to a temperature where depolarization is kineticaUy prevented in the time scale of the experiment. The temperature is then increased and the depolarization current is measured, yielding peak values associated with polymer transitions analogous to t", E" and tan S values obtained by conventional dielectric and dynamic mechanical measurements. The TSC spectra can reveal secondary relaxations, glass transitions and liquid or crystalline phase transitions and hquid crystalhne phase transitions. TSC has been applied to PBT/PC and PA6/ABS blends to study the intermixing of the components of the respective blends [58]. The TSC method is described in several references [59-61]. 

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