STEADY AND TRANSIENT RHEOLOGICAL BEHAVIOR OF MESOPHASE PITCHES

The steady and transient rheological behavior of three mesophase pitch es (two of them obtained from catalytic polymerization of naphthalene and methyl naphthalene, respectively, and one from supercritical extra ction of petroleum pitch) was investigated. Steady shear flow curves w ere generated and showed that all pitches exhibited a shear thinning b ehavior at low rates of shear (region I flow). For higher shear rates, the viscosity became constant (region II flow). Similarly to observat ions on liquid-crystal polymers (LCPs), this behavior was attributed t o the polydomain structure that mesophase pitches exhibit. Reflective polarized light microscopy on quenched samples showed that the mesopha se pitches' domain size decreased with increasing rates of shear. This domain shrinkage was successfully predicted using a model developed f or LCPs [Marrucci (1984)]. The transient rheological behavior associat ed with inception and cessation of steady shear flow also was studied for the three mesophase pitches. The results showed that, upon start-u p of flow, the shear stress exhibited an overshoot, and its magnitude greatly depended on the shear history of the mesophase pitches. This b ehavior, already reported for LCPs, is probably a result of the polydo main structure of mesophase pitches. (C) 1998 The Society of Rheology.