article

Abstract

The rheological properties of two LC copolyesters consisting of 60 and 80 mole% p-hydroxybenzoic acid (pHBA) and polyethylene terephthalate (PET) are presented, obtained by means of cone-and-plate (CP) and parallel plate (PP) attachments of a Rheometrics Mechanical Spectrometer. It is observed that over the range of conditions studied, viscosity data obtained in the CP and the PP agree with those obtained by means of the capillary rheometer. There is no dependence of the viscosity on geometric or dimensional factors as is observed for low molecular weight liquid crystals under some conditions. Although the viscosity is still shear rate dependent at the lowest shear rates, it is shown by stress relaxation experiments following the cessation of steady shear flow that no significant yield stresses exist. The primary normal stress difference (N1) for the 60 mole% pHBA/PET copolyester is always positive under steady state conditions but for the 80 mole% system it can be negative for certain temperatures and shear rates. Calculated values of the die swell using Tanner's theory and cone-and-plate data for the 60 mole% system were considerably higher than the measured values, while the agreement between the theory and the measured values was quite good in the case of the 80 mole% system.

J. Rheol. 29, 539-556, 1985

Rheological properties of liquid crystalline copolyester melts: Comparison of capillary and rotary rheometer results