Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 195

It is usually postulated [ 63 ] that decomposition of the macromolecular peroxide
is a rate-limiting step in the overall kinetics of thermo-oxidative degradation of
polymers. The single oxygen-oxygen bond has a moderate dissociation enthalpy of
~150 [1] kJ mol − 1 that can provide an estimate for the activation energy of mono-
molecular decomposition:

(4.55)

MMOOH MMO →⋅ +⋅ OH .

However, the bimolecular decomposition reaction

2 MMOOH MMO MMOOHO →⋅ + ⋅ +

(4.56) 2

encounters an even lower energy barrier, which is the reason why the activa-
tion energy of decomposition of organic peroxides is usually found to be around
100 kJ mol − 1 [ 62 , 64 ]. Therefore, despite the numerous steps involved in thermo-
oxidative degradation, oxidative conditions have a “simplifying effect” [ 62 ] on deg-
radation, and the process is commonly found to have the activation energy close to
100 kJ mol − 1 for many polymers.

4.3.2 Isoconversional Treatment

During thermal and thermo-oxidative degradation, polymer chains break forming
lower molecular weight products. The volatile products escape from a degrading
polymer that results in the mass loss of the polymer sample. For this reason, the
polymer degradation kinetics is convenient to follow by means of thermogravimet-
ric analysis (TGA).
The application of an isoconversional method to the thermal degradation of
many vinyl polymers yields a somewhat common type of the E ʱ dependence. Some
of such typical dependencies are presented in Fig. 4.24 for degradation of PE, PP,
PS, and polyvinylpyrrolidone (PVP) [ 65 , 66 ]. It is seen that the earliest stages of
degradation ( ʱ ₒ 0) are characterized by lower activation energies. However, as
degradation progresses, the activation energy rises and tends to level off at around
200-250 kJ mol − 1 .
The lower energy stages of degradation are associated with initiation of the pro-
cess on the defects of polymer structure or so-called weak links [ 62 , 67 , 68 ]. These
include branch points as well as peroxide and hydroperoxide groups that are formed
during polymerization in the presence of oxygen. Head-to-head groups and unsatu-
rated vinylidene end groups that result from bimolecular radical termination are
also examples of such weak links.
The presence of weak links significantly lowers the thermal stability of a poly-
mer. For example, the thermal stability of PS increases markedly if the polymer
does not contain the head-to-head links [ 69 ]. Another example is PMMA, whose
thermal degradation is initiated at vinylidene end groups formed during regular