Rheostructural studies on a synthetic mesophase pitch during transient shear flow

The steady, shear viscosities of a synthetic mesophase pitch (Mitsubishi AR-HP) obtained from rate-sweep experiments at 0.1–10 s−1 exhibited shear-thinning (region I) and plateau responses (region II), but displayed a hysteresis during the decreasing rate sweep. Transient tests revealed that the shear stress (and consequently the shear viscosity) displayed a local maximum and a minimum before approaching a steady state. Following steady flow at 1 s−1, a reversal of flow direction or a very short interruption in flow did not lead to the maxima or minima in the transient shear stress, but the maxima and the minima reappeared in the transient stress after a rest time of ∼1000 s. An experimental protocol was developed to preserve the rheological samples, and their microstructure was characterized in three orthogonal planes for the initial and final states. The initial microstructure was found to have a weak, but preferred, orientation of mesophase layers in the radial direction of the rheometer cone-plate (due to the initial squeezing flow). The initial microstructure changed to a flow-aligned fibrous structure after shearing in the viscometric flow.