Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 79

Fig. 3.14 Conversion against
temperature curves for the
glass transition of amorphous
indomethacin at four heating
rates

2.0

1.5

1.0

5 o C min -1
10 o C min -1
15 o C min -1
25 o C min -1

0.5

0.0

10 20 30 40 50 60 70 80 90

T / o C

is equal to the rate of the following heating [ 42 , 43 ]. A suitable temperature range of
measurements is typically from T g + 40 to T g − 40.
For measurements performed on heating (Fig. 3.14 ), the isoconversional calcula-
tions need to use only the ascending part of the ʱ versus T curves up to ʱ = 1. The
resulting dependence of the activation energy of conversion is shown in Fig. 3.15 .
The observed decreasing shape of the E ʱ versus ʱ dependence is typical for the
glass transition and has been observed in variety of other systems, including many
polymers [ 44 ]. The decrease in E is easy to understand in terms of the cooperative
molecular mobility discussed earlier (Fig. 3.12 ). The glassy phase is characterized
by a small amount of free volume that permits only local noncooperatve motion
(e.g., the ʲ -process) that dominates well below T g . As temperature rises approach-

Fig. 3.15 Conversion
dependence of the activa-
tion energy evaluated from
the data shown in Fig. 3.14 .
(Adapted from Vyazovkin
and Dranca [ 41 ] with permis-
sion of ACS)

350

300

250

200

150

0.0 .2 0.4 .6 0.8 .0

α