Steady and transient rheological behavior of mesophase pitches

Abstract

The steady and transient rheological behavior of three mesophase pitches (two of them obtained from catalytic polymerization of naphthalene and methyl naphthalene, respectively, and one from supercritical extraction of petroleum pitch) was investigated. Steady shear flow curves were generated and showed that all pitches exhibited a shear thinning behavior at low rates of shear (region I flow). For higher shear rates, the viscosity became constant (region II flow). Similarly to observations on liquid-crystal polymers (LCPs), this behavior was attributed to the polydomain structure that mesophase pitches exhibit. Reflective polarized light microscopy on quenched samples showed that the mesophase pitches’ domain size decreased with increasing rates of shear. This domain shrinkage was successfully predicted using a model developed for LCPs [Reference 1984]. The transient rheological behavior associated with inception and cessation of steady shear flow also was studied for the three mesophase pitches. The results showed that, upon start-up of flow, the shear stress exhibited an overshoot, and its magnitude greatly depended on the shear history of the mesophase pitches. This behavior, already reported for LCPs, is probably a result of the polydomain structure of mesophase pitches.