Comparison of the Rheological Properties of Concentrated Solutions of a Rodlike and a Flexible Chain Polyamide

Abstract

The steady state shear rheological properties of solutions of a rodlike polyamide, poly‐p‐phenyleneterephthalamide (PPT), in 100% sulfuric acid have been compared with those of solutions of a flexible chain polyamide, nylon 6,6 in the same solvent. For solutions of similar concentration (c) and molecular weight (M), it was found that the primary normal stress difference (N1) and the viscosity (η), compared at the same shear rate (γ̇), were an order of magnitude greater for solutions of PPT. It was believed that this behavior could be accounted for through the formation of an enhanced entanglement network in the PPT solutions. Plots of the zero shear viscosity (η0) versus cM̄w, where M̄w is the weight average molecular weight, for both systems revealed that “bends” occurred in the data corresponding to a critical entanglement molecular weight (Mc) of 1180 for PPT (this corresponds to 30 main chain atoms (z)) and to 5260 (z=330) for nylon 6,6. More significantly, η0 was found to be proportional to (cM̄u)6.8 for solutions of PPT and to (cM̄w)3.4 for nylon 6,6 solutions. η versus γ̇ curves were similar in shape for both systems and could be reduced to the same master curve with the only difference being that the relaxation times or shifting factors were considerably greater for the PPT solutions. This suggested that the process of destroying entanglements may be similar for both polymers. The overlap parameter c[η], where [η] is the intrinsic viscosity, provided a much better correlation of η0 data from the two sets of solutions than did the segment contact parameter cM̄w. This suggested that the structural variable controlling the onset of entanglements may be a parameter such as the radius of gyration. Because of the inability of rodlike molecules to coil around each other, further insight into the nature of entanglements is obtained.