article

Abstract

The primary normal stress difference, N1, was determined in both steady state and transient shear flow experiments for thermotropic copolyesters of 60 and 80 mole% p-hydroxybenzoic acid (pHBA) and polyethylene terephthalate (PET). N1 was observed to be negative for the 80 mole% system at low shear rates and temperatures below 332 0C. At higher shear rates or higher temperatures N1 was positive. The values of N1 for the 60 mole% pHBA/PET system were always positive over the same range of shear rates and temperatures. Although it is not certain as to the origin of the negative values, a probable cause may be small density changes upon the inception of flow as well as the existence of a solid phase or texture development in the melt. It is shown, however, that it is thermodynamically admissible for liquid crystalline fluids to exhibit negative values for N1.

Rheologica Acta 25, 575-286, 1986

Primary normal stress difference of two liquid crystalline copolyesters