Poly(amic acid)s and their ionic salt solutions: Synthesis, characterization and stability study - Polyimides and Other High Temperature Polymers - page 19

Figure 20. Shear rate dependence of bulk viscosity for both series of BTDA and BPDA based Co-
PAAs.
applied shear and thus offer less resistance to shear [26]. Figures 20, 21 and 22
show the comparison of shear rate dependence of bulk viscosity of both BPDA
and BTDA based series of Co-PAA and Co-PDPAA with DEEM and DMEM. All
the curves show similar trends, and a sharp shear thinning behaviour is observed
early on, in the experiment at shear rates from 3 to 15 sec -1 . than from about 20 to
38 sec -1 indicating that the bulk viscosity remained nearly stable.
The decrease in viscosity with increasing shear rate could be interpreted in
terms of stretching and disentangling of polymer chains in the viscous mass. At
low shear rates, the polymer chain entanglements impede shear flow and, there-
fore, viscosity is high. As the shear rate increases, this allows polymer chains to
orient and slip through each other in the direction of flow and disentangle from
one another and the viscosity decreases. In other words, the decrease of bulk vis-
cosity with increasing shear rate was attributed to the gradual progressive disen-
tangling of polymer chains due to shearing of the solution.
Within the range of measurements, all shear rate dependent curves of bulk vis-
cosity for both series of BPDA and BTDA based Co-PDPAAs with DEEM as
well as DMEM showed two regions with quite different curvatures. When the
shear rate was less than 20 sec -1 , the bulk viscosity decreased more with the in-
crease in shear rate. Then, the bulk viscosity remained stable from 20 to 38 sec -1 .
However, interestingly, for both BPDA and BTDA based series of Co-PDPAAs
with DMEM there was a sharper drop in viscosity in the shear rate range of 3 to
20 sec -1 .