Steady state and transient rheological behavior of mesophase pitch, Part I: Experiment

Abstract

The rheology of a liquid crystalline mesophase pitch is reported. Over the range of shear rates and temperatures investigated, the steady state shear viscosity and the first normal stress difference display region 1 and region 2 flow behavior typical of many liquid crystalline systems. A kink in the transition between regions 1 and 2 is also observed. The first normal stress difference is negative at low shear rates, but transitions to positive values in the shear rate region of the kink in the viscosity. These phenomena are explained by change in the molecular orientation of the sample. The evolution of the shear stress in transient startup and relaxation experiments is measured. There is a distinct overshoot in the startup of shearing, and a rapid decay of stress upon cessation of flow. As expected, the magnitude of the overshoot depends on the rest time between shearing. The estimated elastic constant is consistent with other work.