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.