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Figure 19. Potential reaction mechanisms of PAA, Co-PAA and Co-PDPAA degradation at RT.
Out of above three potential poly(amic acid) degradation reactions shown in
Figure 19, the first two are ruled out as per the following reasoning: the first reac-
tion is ruled out, since in our study the acid number was shown to decrease rather
than increase (Figures 13-17) which would have been had it undergone hydrolysis
reaction as per the first reaction. Large scale imidization is also less likely as its
rate constant is two or three orders of magnitude less than that for the depoly-
merization reaction as well as that for amic acid hydrolysis. Hence the second re-
action is also ruled out. Therefore the only feasible reaction is the anhydride for-
mation, the third reaction.
3.3. Rheological study of PAA, Co-PAA and Co-PDPAA
It is known from a laboratory study that the rheological properties of polymer so-
lutions depend on the bulk viscosity of the solution [25]. In our study, the bulk
viscosity was not extremely high but was sufficiently high to be determined by
the Brookfield viscometer. We have studied the rheology (shear rate and tempera-
ture dependence of viscous flow) and calculated the activation energy of the vis-
cous flow behaviour of the PAA, Co-PAA and Co-PDPAA.
3.3.1. Bulk viscosity as a function of shear rate at constant temperature
A shear rate dependence of bulk viscosity was found in most polymer solutions.
In a dilute solution, the randomly extended chains can deform and orient with the
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