Abstract
The first normal stress difference (N1) of lyotropic liquid crystalline solutions of poly(p-phenylene terephthalamide) (PPTA) in sulfuric acid was studied by rheometric measurements. For the first time, a negative value range of N1 was observed for the solutions in a high shear rate range. N1 showed a non-monotonic dependence on the shear rate when the PPTA/sulfuric-acid solution was in the nematic phase. The minimum value of the first normal stress difference (N1min) and the shear rate to achieve N1min (γ˙min) were observed to be closely correlated with temperature, concentration and molecular weight. Compared with some well-studied lyotropic LCP systems, the rheological behavior of PPTA/sulfuric-acid was more sensitively dependent on the concentration. Above observations indicate that the frustration of tumbling of average polymer orientation occurs in the high shear rate range and the influences of the factors are not exactly the same as those well-studied lyotropic LC polymer systems.
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