Secondary glass-rubber relaxation

Fig. 6. Transition map for PPG showing the primary glass-rubber relaxation (a process) and the secondary relaxation (J3 process)...

The fact that the relaxation times associated with the glass-rubber transition and the secondary relaxations are independent of molecular weight suggests... 

Figure 8, Transition map for PIB. The hypersonic result corir siderahly extends the available frequency range. The primary (P) glass-rubber relaxation line and secondary (S) relaxation line are from Ref. 22.

Usually the primary (P) glass-rubber relaxation cannot be resolved from the secondary relaxation at hypersonic frequencies. However, this is not always the case. The Brillouin frequencies Awd) and tan 8 for polypropylene glycol (PPG) (13) are plotted versus temperature in Figure 9. Two tempratures of maximum loss are observed. The higher temperature loss at 100 °G and a frequency of 4.40 GHz correlates very well with the primary glass-rubber relaxation line determined by dielectric relaxation at gigahertz frequencies (13), The lower temperature loss at 50°G and a frequency of 5.43 GHz correlates with an extension of the secondary transition line. The transition map is shown in Figure 10. 

Gladfelter and co-workers probed polymer viscoelastic relaxations with temperature-controlled FFM (272,273). The dependence of the mean value of the friction force on temperature was reported to be correlated to the glass-to-rubber transition and/or secondary relaxation mechanisms in films of PMMA, poly(ethylene terephthalate) (PET), and PS. Viscoelastic mechanical losses were thought to be the dominant contribution to friction, which were found to obey the time-temperature superposition principle. Interestingly, the surface Tg s and values for the activation energy measured for the /3-relaxation at the surface were lower than those of the bulk material. 

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. 

It is tempting to relate the temperature at which the ductile-brittle transition takes place to either the glass transition or secondary transitions (Section 5.2.6) occurring within the polymer. In some polymers such as natural rubber or polystyrene Tb and Tg occur at approximately the same temperature. Many other polymers are ductile below the glass transition temperature (i.e. Tb < Tg). In this case it is sometimes possible to relate T to the occurrence of secondary low-temperature relaxations. However, more extensive investigations have shown that there is no general correlation between the brittle-ductile transition and molecular relaxations. This may not be too unexpected since these relaxations are detected at low strains whereas Tb is measured at high strains and depends upon factors such as the presence of notches which do not affect molecular relaxations. 

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