Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 171

Fig. 4.6 The E ʱ dependencies obtained for polymerization of ʵ-caprolactone by isoconversional
methods of Friedman, Kissinger-Akahira-Sunose ( KAS ), and Ozawa and Fynn and Wall ( OFW ).
The initiators used are: ( a ) Ti(IV) n -propoxide, ( b ) Ti(IV) n -butoxide, ( c ) Ti(IV) tert -butoxide, ( d )
Ti(IV)2-ethylhexoxide. (Reproduced from Meelua et al. [ 18 ] with permission of Springer)
isoconversional method to DSC data yields a certain type of the E ʱ versus ʱ depen-
dencies that can be interpreted in terms of the reaction mechanisms discussed in the
previous section. Let us consider some examples of the dependencies reported in
the literature.
In the simplest case, the effective activation energy can be found practically in-
variable throughout polymerization. An example of such behavior has been reported
[ 18 ] for ring-opening polymerization of ε-caprolactone initiated by a series of tita-
nium (IV) alkoxides. As seen in Fig. 4.6 , the application of isoconversional methods
results in obtaining the E ʱ values that remain nearly constant as ε-caprolactone is
converted to poly(ʵ-caprolactone). It is noteworthy that the activation energies dif-
fer by more than 30 kJ mol − 1 depending on the initiator. Apparently, the observed
difference in the E ʱ values is due to the difference in the activation energy of ini-
tiation ( E i ) in Eq. 4.11. Surprisingly, the dependence of the activation energy on
conversion does not show any changes associated with a transition from a kinetic
to diffusion regime. However, it should not be entirely unexpected if one considers
the temperature of polymerization relative to the temperatures of the glass transition
and melting of poly(ʵ-caprolactone). Polymerization of ε-caprolactone commences
above 150ﾰC [ 18 ]. This temperature is significantly higher than T g = −64 °C and