Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 189

Fig. 4.20 Activation energy
as a function of conversion
for reaction between DGEBA
and PDA ( open circles: stoi-
chiometric ratio, solid circles:
excess of amine). (Adapted
from Sbirrazzuoli et al. [ 60 ]
with permission of Wiley)
α
other hand, for initiation by Sm triflate, the activation energy is significantly smaller
than that of propagation, and for La triflate it is about the same for initiation and
propagation.
No distinct decrease in the E ʱ values is seen in the initial stages of curing DGEBA
with 1,3-phenylene diamine [ 60 ] (PDA; Fig. 4.20 ). Apparently, for this process, the
activation energy of initiation is similar to that of propagation. This is confirmed by
performing the same reaction with the fivefold excess of amine over the stoichio-
metric amount. Under the stoichiometric conditions, the DGEBA to amine ratio is
such that the DGEBA and amine blocks alternate forming a polymer chain. When
the amount of amine exceeds significantly the stoichiometric one, the DGEBA
blocks would be mostly capped by amine blocks and, thus, would not be capable
to build the polymer chain. In this situation, the activation energy of the whole
process should be similar to the activation energy of initiation. As seen in Fig. 4.20 ,
the respective E ʱ value is practically constant (~ 55 kJ mol −1 ) throughout the whole
process.
The system with excess of amine cannot produce a cross-linked polymer so that
the reaction system cannot develop high viscosity and vitrify, which, in its turn,
prevents a transition from a kinetic to diffusion regime. The absence of this transi-
tion is the chief reason why the E ʱ values at larger conversions remain unchanged.
However, the transition occurs in the stoichiometric system (Fig. 4.20 ). In it, the
initial E ʱ values are similar to those for the nonstoichiometric system that indicates
that both systems have the same initiation process. As cross-linking progresses, the
viscosity of the reacting system rises and the effective activation energy of curing
decreases.
Dynamic rheology measurements (Fig. 4.21 ) shed light on physical changes tak-
ing place in the stoichiometric system throughout cross-linking [ 60 ]. The system
gels at about 123 ﾰC as detected by the point of intersection of the storage and loss
modulus curves. According to the DSC measurements, it happens at conversion of