Primary normal-stress difference for two liquid crystalline copolyesters

Abstract

The primary normal-stress difference,N ₁, was determined in both steady and transient shear flow experiments for thermotropic copolyesters of 60 and 80 mole% p-hydroxybenzoic acid (PHB) and polyethylene terephthalate (PET).N ₁ was observed to be negative for the 80 mole% system at low shear rates and temperatures below 332 °C (a tensile stress, e.g. σ₁₁ is taken here to be positive). At higher shear rates and for temperatures above 332 °C,N ₁ was found to be positive. Values ofN ₁ for the 60 mole% pHBA/PET system were always positive over the same range of shear rates. Although it is not certain as to the origin of the negative values, a probable cause is small density changes upon the inception of flow as well as the existence of a solid phase or some texture development in the melt. However, it is shown that it is thermodynamically admissible for liquid crystalline fluids to exhibit negative values ofN ₁.