Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 200

Fig. 4.29 E ʱ dependencies
for thermal degradation of
virgin PS and PS-clay nano-
composites containing differ-
ent percentages of clay. The
1 % clay system is exfoliated,
and 3 and 5 % are intercalated
systems. (Reproduced from
Vyazovkin et al. [ 81 ] with
permission of ACS)
may change because of a change in the reaction mechanism and, thus, a change in
the reaction model. The degradation mechanism changes can be caused by various
factors, one of which is the introduction of nanoparticles [ 85 - 87 ]. For example, the
PS-clay (5 wt .%) nanocomposite prepared by ultrasound-assisted solution mixing
has demonstrated a significant increase in the degradation temperature relative to
virgin PS [ 88 ]. Nevertheless, the isoconversional activation energy for the thermal
decomposition of the nanocomposite has been found to be markedly lower than that
for virgin PS. Further analysis has revealed that the process in the nanocomposite is
described by a lower value of the preexponential factor and different reaction model
[ 90 ]. Apparently, these two are the reasons for the enhanced thermal stability of this
particular nanocomposite.
At any rate, performing kinetic analysis beyond estimating the E ʱ dependence
can be beneficial in figuring out what components of the kinetic triplet are linked
to the observed changes in the thermal stability of a polymer. For instance, isotactic
PS (iPS) is more thermally stable than regular atactic PS (aPS) [ 89 ]. Isoconver-
sional treatment of TGA data on the thermal degradation of these two polymers
yields the E ʱ dependencies depicted in Fig. 4.30 . The E ʱ values for both polymers
are practically independent of ʱ . On average, the activation energies for iPS are
about 10 kJ mol − 1 larger than those for aPS. Obviously, larger activation energy
is consistent with larger thermal stability observed experimentally. By using the
compensation effect, as discussed in Sect. 2.2, one can also evaluate the conversion
dependence of the preexponential factor. The resulting dependencies are presented
in Fig. 4.31 . Clearly, for the thermal degradation of iPS, the ln A ʱ values are great-
er than for the thermal degradation of aPS. However, larger preexponential factor
means a faster process and, therefore, lower thermal stability. That is, an increase
in the preexponential factor diminishes the process deceleration associated with an
increase in the activation energy.
No less important is to figure out how the reaction model for iPS can affect the
thermal stability of this polymer relative to that of aPS. However, this particular
task faces an important general issue of the applicability of the commonly used