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

Gotsis Alexandros, Baird Donald G.

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Summary

In an earlier paper capillary rheometer results were presented for two liquid crystalline copolyesters consisting of 60 mole % and 80 mole % p‐hydroxybenzoic acid (pHBA) and polyethyleneterephthalate (PET). In this paper obtained by means of cone‐and‐plate (CP) and parallel plate (PP) attachments of a Rheometrics Mechanical Spectrometer are compared with the capillary data. In particular, it is observed that over the range of conditions studied, viscosity data obtained in the CP and PP agree with those obtained by means of the capillary rheometer. Furthermore, 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 for shear rates as low as 5×10−2 sec−1, 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 shear conditions but N1 for the 80 mole % pHBA/PET system 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 % pHBA/PET system were considerably higher than the measured values while the agreement between the theory and measured values was quite good in the case of the 80 mole % system.

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