Rheological properties of rodlike polymers in solution. 2. Linear and nonlinear transient behavior

Rheological and rheo‐optical studies are reported for isotropic solutions of the mesogenic rodlike polymer poly(1,4‐phenylene‐2,6‐benzobisthiazole) (PBT). Several PBT samples were used with average contour lengths from 95 to 135 nm. Concentrations were varied over a range just below the concentration Cc for the formation of an ordered (nematic) state. The predictions of a single‐integral constitutive equation of the BKZ type utilizing experimental estimates of the distribution of discrete relaxation times is compared with experimental data on the steady‐state viscosity ηκ, the recoverable compliance function Rκ, and the steady‐state flow birefringence as functions of the shear rate κ, with satisfactory results. The relaxation of the shear stress and the flow birefringence on cessation of steady‐state flow at shear rate κ are also compared with the single‐integral constitutive equation, and it is found that in the nonlinear response range the data can be superposed over a wide range in κ. The overall behavior is qualitatively similar to that for flexible chains, which can also be fitted by the single‐integral constitutive equations over similar ranges of η0R0κ, with η0 and R0 the limiting values of ηk and Rκ for small κ. Of course, the dependence of η0 and R0 on concentration and molecular weight differs markedly for rodlike and flexible‐chain polymers.