Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 204

Fig. 4.33 Thermal deg-
radation data for atactic
polystyrene ( aPS) and
isotactic polystyrene ( iPS) as
compared against the random
scission (L2, L3, L4) and
Avrami-Erofeev (A2, A3)
models

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We can now try to use this approach to see whether the thermal degradation
data for iPS and aPS are consistent with the random scission model. As seen in
Fig. 4.33 , the experimental f ( ʱ ) data show a maximum that means that we deal with
an autocatalytic type of kinetics (see Sect. 1.1). This type of kinetics is consistent
with the random scission models. Among the solid-state reaction models (Table 1.1,
Fig. 1.4), the kinetics of this type is represented by the Avrami-Erofeev models.
All these models are depicted in Fig. 4.33 . It is obvious that the Avrami-Erofeev
models cannot reproduce the degradation kinetics of either iPS or aPS. The problem
is not so much the significant difference in the absolute values of f ( ʱ ). This differ-
ence can always be diminished by multiplying f ( ʱ ) by some constant that can then
be compensated by dividing the preexponential factor by the same constant. The
problem is that the Avrami-Erofeev models have a maximum at significantly larger
ʱ than the actual thermal degradation data. On the other hand, the random scission
models reveal their maximum at practically the same ʱ as the actual data. The L2
model provides the best match among the random scission models. Undoubtedly,
the thermal degradation kinetics of either iPS or aPS can be described quite well by
the random scission models. This conclusion is in agreement with the earlier work
on the thermal degradation of aPS by Sanchez-Jimenez et al. [ 88 ].
The data presented in Fig. 4.33 can also help us to answer the earlier question of
how the reaction model for iPS can affect the thermal stability of this polymer rela-
tive to that of aPS. It is easy to see that the absolute values of f ( ʱ ) for iPS are smaller
than those for aPS. In other words, the reaction model for iPS indicates that the
degradation rate of this polymer is slower than that of aPS. Overall, the fact that iPS
has a somewhat larger thermal stability than aPS is associated with two components
of the kinetic triplet: the activation energy and the reaction model.