Steady shear viscosity and oscillatory complex viscosity of poly(p-phenylene terephthalamide) solutions in sulfuric acid

This study investigated the steady shear viscosity eta and the oscillatory complex viscosity eta* of poly(p-phenylene terephthalamide) solutions in sulfuric acid (PPTA/H2SO4) by rheometric measurements. The influences of steady shear rate, concentration, and temperature were investigated in detail. The results indicated that in the plateau regime (eta independent of (gamma)over dot regime), eta shows a non-monotonic variation with the concentration increase, which reaches the maximum value when the solution changes from the isotropic phase into the nematic-isotropic biphasic system. At the low shear rate, the corresponding critical concentration C-eta(*) is close to the critical concentration C*, at which the birefringence emerges in solutions. In the plateau regime, the dependence of eta on temperature follows different rules when the system is in different phases. In the isotropic phase, eta decreases with the increasing temperature and obeys the Arrhenius exponential rule. Unlike the isotropic solution, eta increases with the temperature in the nematic-isotropic biphasic region and decreases linearly with the temperature in the nematic phase region. The eta(*) measured at different angular frequencies (omega) was compared with eta measured at different eta similar to(gamma)over dot For the isotropic solutions, the vertical bar eta*vertical bar similar to omega. curve coincides with the eta similar to(gamma)over dot variation in a wide shear rate range for the plateau regime, while in the nematic phase region, vertical bar eta*vertical bar is always smaller than eta. For the biphasic systems, the consistency can be seen only in a narrow shear rate range and vertical bar eta*vertical bar deviates from eta with the increasing shear rate.