Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 199

Fig. 4.28 TGA curve show-
ing the effect of switching of
gaseous atmospheres between
air and nitrogen on the ther-
mal degradation of PMMA
at 200 ﾰC. (Reproduced from
Peterson et al. [ 71 ] with
permission of ACS)

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or, in other words, acceleration of the degradation rate. A switch from nitrogen to air
obviously causes the opposite effect. The effect is explained [ 71 ] by the formation
of thermally stable radical species that suppress the process of depolymerization.
Isoconversional analysis of the polymer degradation kinetics is frequently lim-
ited to estimating a dependence of the effective activation energy on conversion. By
comparing the values of the activation energy, one can obtain quick insights into the
origins of thermal stability changes due to modification of polymeric materials. For
example, cross-linking of a polymer can increase significantly its thermal stability
that is reflected in an increase of the activation energy [ 74 ]. The enhancement of
thermal stability can also be accomplished by addition nanoparticles, i.e., by con-
verting polymers to nanocomposites [ 75 - 79 ].
For example, relative to virgin PS, the thermal and thermo-oxidative degrada-
tion of PS-clay nanocomposites can occur at tens of degrees higher temperature
[ 80 ]. Slowing down of the respective degradation kinetics reveals itself in markedly
increasing effective activation energy of the process. An example of this effect is
shown in Fig. 4.29 that compares the isoconversional activation energies for vir-
gin PS and several PS-clay nanocomposites [ 81 ]. The nanocomposite containing
1 wt.% of clay is PS brush on exfoliated clay that was synthesized by surface-
initiated polymerization. The nanocomposites containing 3 and 5 wt. % of clay are
intercalated clay materials that were prepared by in situ polymerization of styrene
in the presence of organically modified clay. It is seen that the activation energy of
thermal degradation is greater in any of the composites than in virgin PS, although
the exfoliated system may be somewhat more effective in raising the activation
energy than the intercalated one.
Although the activation energy of thermal degradation is a major reason that
determines thermal stability, it is not the only reason. A change in thermal stability
or, more generally, a change in the reaction rate may be associated with a change
in the preexponential factor. In fact, a change in the preexponential factor appears
to be the primary cause of deceleration [ 82 ] as well as acceleration [ 10 , 83 , 84 ] of
chemical reactions when they are confined to nanopores. Also, thermal stability