COMPETITION BETWEEN TEXTURAL TRANSITIONS AND PRESSURE EFFECTS ON THE VISCOSITY OF THERMOTROPIC LIQUID-CRYSTAL POLYMERS

Steady measurements in capillary and slit dies and transient experimen ts in a cone-plate system are presented for two thermotropic copolyest ers, X7G (R) and Vectra A950, and a thermotropic copolyesteramide, Vec tra B950. The wide shear rate range covered, from 10(-2) to 10(4) s(-1 ), allows us to observe the three regions defined by Onogi and Asada ( 1). However, the copolyesters on one hand, and the copolyesteramide on the other, show different theological behavior that can be summarized as follows: For X7G (R) and Vectra A950: i. higher viscosities in sli t than in capillary die flow; ii. upward-concave pressure profiles in slit die flow; iii. overshoot peaks in transient experiments at a shea r strain gamma(M) = 2. For Vectra B950: i. lower viscosities in slit t han in capillary die flows; ii. linear or slightly convex pressure pro files in slit die flow; iii. overshoot peaks in transient experiments at a shear strain of gamma(M) = 180. These seemingly contradictory res ults are explained by the competition between the texture changes alon g the flow (a transition from a worm texture to a flow-aligned region) and the pressure effect on viscosity.