A two-fluid model for inconsistency of observations in capillary rheometry of thermotropic copolyesters

Abstract

Capillary rheometry of liquid-crystalline polymers is described in terms of the stratified laminar flow of two fluids. The fluid entering the capillary is assumed to have a low viscosity because it has been oriented, except in a thin peripheral shear layer, by the converging entry flow. In the course of its transit through the die the orientation is lost as the fluid is sheared. It is assumed that the oriented fluid is ‘‘transformed’’ to a fluid with power-law viscosity behavior when it has been subjected to some critical value of total shear strain. The interface between the two fluids migrates radially towards the center of the capillary as the flow progresses. With physically reasonable magnitudes for the model parameters the model can account for the experimentally observed concave-upward nonlinearity of Bagley plots and for die diameter dependence of nominal viscosity. Arguments on the effects of deformation on texture and viscosity are proposed to account for other observations of shapes of Bagley plots and of diameter dependence of viscosity.