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

Abstract

This study investigated the steady shear viscosity η and the oscillatory complex viscosity η ∗ 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 (η independent of $$ \overset{.}{\gamma } $$ regime), η 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 $$ {C}_{\eta}^{\ast } $$ is close to the critical concentration C ∗, at which the birefringence emerges in solutions. In the plateau regime, the dependence of η on temperature follows different rules when the system is in different phases. In the isotropic phase, η decreases with the increasing temperature and obeys the Arrhenius exponential rule. Unlike the isotropic solution, η increases with the temperature in the nematic–isotropic biphasic region and decreases linearly with the temperature in the nematic phase region. The η ∗measured at different angular frequencies (ω) was compared with η measured at different $$ \overset{.}{\gamma } $$ . For the isotropic solutions, the |η ∗|~ω curve coincides with the η~ $$ \overset{.}{\gamma } $$ variation in a wide shear rate range for the plateau regime, while in the nematic phase region, |η ∗| is always smaller than η. For the biphasic systems, the consistency can be seen only in a narrow shear rate range and |η ∗| deviates from η with the increasing shear rate.