Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 107

Fig. 3.38 Converting depen-
dence of E ʱ versus ʱ ( circles )
to dependence of E ʱ versus T .
Solid line represents a depen-
dence of ʱ on the mean tem-
perature. E ʱ = − 200 kJ mol −1
corresponds to ʱ = 0.28,
which, in turn, corresponds to
the mean temperature 212 ﾰC.
(Adapted from Vyazovkin
and Sbirrazzuoli [ 107 ] with
permission of Wiley)

50 260

240

0

220

-50

200

-100

180

-150

160

-200

140

-250

120

-300

0.0 0.2 0.4 0.6 .8 1.0

α

Fig. 3.39 Dependence of the
effective activation energy
on the mean temperature.
Solid lines represent fits of
Eq. 3.60. (Adapted from
Vyazovkin and Sbirrazzuoli
[ 107 ] with permission of
Wiley)

50

0

-50

-100

-150

-200

-250

-300

420430 440450 460470 480490 500

T / K

ually to the two portions of the experimental E ʱ versus T dependence. The computa-
tion requires the values of T g and T m which are 342 and 553 K, respectively [ 113 ].
The fits result in the K g and U * values shown in Table 3.2 that also collects the
values reported in the literature [ 110 , 111 , 114 - 118 ] for isothermal crystallization of
PET. There is obviously a considerable spread in the literature values. However, the
values obtained from fitting Eq. 3.60 to the experimental E ʱ versus T dependence
appear to be reasonably consistent with the reported values, although seem to be
on the low side. At least partially, this is because Eq. 3.60 treats both K g and U * as
variables whereas most of the calculations in Table 3.2 have been done by setting U *
in Eq. 3.54 to the constant value 6.3 kJ mol −1 and fitting K g . Although 6.3 kJ mol −1
is used widely as “the universal value,” Hoffman et al. [ 103 ] have found that for a
set of polymers studied, the best-fit values of U * vary between 4 and 17 kJ mol −1 .
In addition, they have noted that increasing the value of U * results in evaluating a