Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 197

Fig. 4.26 Activation energies
as a function of conversion
estimated by an isoconver-
sional method from the mass
loss data for thermal degrada-
tion of PMMA having differ-
ent end groups. (Data from
Hu and Chen [ 70 ])

300$5
300$&22+
300$2+
300$& W
300$$,%1

α

present in the degradation of PMMA-R 12 . Also, the degradation temperature for the
latter has increased by ~ 90 ﾰC relative to the former. It should be noted that the first
step in the degradation of PMMA is associated with the presence of weak links (see
Peterson [ 71 ] and references therein). Obviously, the introduction of more stable
end groups significantly increases the thermal stability of PMMA. The increase in
stability is reflected in the values of the effective activation energy estimated by an
isoconversional method. As seen in Fig. 4.26 , the identity of the end group affects
dramatically the activation energy of the initial stages of the thermal degradation of
PMMA. If for regular PMMA-AIBN E ʱ ₒ0 is ~ 150 kJ mol − 1 , for PMMA-R 12 it is
over 200 kJ mol − 1 .
Note that the effect of weak links is limited to the initial stages of polymer deg-
radation (Figs. 4.24 and 4.26 ). Once all weak links broke giving way to initiation,
further initiation is only possible by breaking regular bonds that have higher energy.
As a result, the activation energy rises to the plateau value that can be estimated by
using Eq. 4.51. As noted earlier, E p − E t /2 in Eq. 4.11 is about 10-20 kJ mol − 1 so
that in the steady state the effective activation energy of polymerization is about
10-20 kJ mol − 1 larger than E i /2. Following the same reasoning, we can obtain an
estimate for the effective activation energy of degradation in accord with Eq. 4.51.
Instead of E p , Eq. 4.51 has E dp . The two values differ by the enthalpy of polymer-
ization. For vinyl polymers, the heat of polymerization is roughly 65 8 kJ/mol
of monomer [ 6 ]. With regard of this, E dp − E t /2 in Eq. 4.51 should be about 75-
85 kJ mol − 1 . Then the effective activation energy of polymer degradation in the
steady state should be 75-85 kJ mol − 1 greater than half the activation energy of ini-
tiation, which can estimated as the enthalpy of dissociation a carbon-carbon bond.
Depending on the molecular structure, this enthalpy varies around 280-340 kJ mol − 1
[ 62 ]. Then half of this value (i.e., 140-170 kJ mol − 1 ) plus 75-85 kJ mol − 1 would
give us an estimate for the activation energy of degradation somewhere in the range
of 210-260 kJ mol − 1 . This estimate agrees fairly well with the plateau E ʱ values
found for the actual thermal degradation of different polymers (Figs. 4.24 and 4.26 ).