Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 94

The slope of the plots as well as the activation energy decreases systematically with
decreasing the annealing temperature.
For reasons explained earlier, the use of the lowest feasible annealing tempera-
ture yields better estimates for the activation energy of the ʲ -relaxation. Follow-
ing this principle, we have found [ 35 ] that considering their respective confidence
intervals, all E ʲ values estimated from the annealing peaks are consistent with the
correlation (Eq. 3.32). Furthermore, the obtained estimates of E ʲ appear to compare
quite well against the literature values (Fig. 3.29 ). Our estimates typically involve
~ 10-20 % of uncertainty. It is seen that most of the estimates deviate by less than
20 % from the literature values, especially if one disregards a few extreme literature
values, which may be outliers.
It is obvious that the activation energies derived from the annealing peaks cor-
relate fairly well with the activation energies of the ʲ -relaxation. Nevertheless, the
respective assignment must be made with care. Note that annealing is conducted
around 0.8 T g , i.e., in the temperature region where one typically finds the absorp-
tion peaks due to the ʲ -relaxation when using traditional methods of dielectric and
mechanical spectroscopy [ 90 ]. However, the recovery process gives rise to the an-
nealing peaks, whose peak temperatures are found about 20-30 ﾰC above the an-
nealing temperature. In this temperature range, the ʲ -relaxation is not necessarily
dominant so that the activation energy estimated from the annealing peaks may
have a sizeable contribution from higher temperature and higher activation energy
relaxation processes (see Fig. 3.12 ).

160

IM Gl Mt

PVP PMMA

140

PET PS

120

100

80

60

40

50 60 70 80 90

Annealing peak E / kJ mol -1

Fig. 3.29 Correlation of the E ʲ estimates from the annealing peaks with the literature values.
Solid line represents exact equality; dash and dash-dot lines represent 10 and 20 % deviations,
respectively. IM indomethacin, GI glucose, Mt maltitol, PVP poly(vinylpyrrolidone), PMMA
poly(methyl methacrylate), PET poly(ethylene terephthalate), PS polystyrene. (Reproduced from
Vyazovkin and Dranca [ 35 ] with permission of Elsevier)