article

Abstract

The transient extensional viscosity of the nematic melt of the thermotropic liquid crystalline co-polyester Vectra A950 was measured in uniaxial extensional flow in a commercial constant strain rate rheometer. At extension rates ranging between 0.005 and 1 s-1 the extensional viscosity of Vectra A950 does not reach steady state up to 3 to 4 Hencky strain units, where most samples break. If the viscosity at a certain value of strain is plotted vs. the strain rate, a rate-thinning curve is obtained. The extensional viscosity is much higher than 3 times the shear viscosity over a broad range of accumulated strain; while injection moulded samples always show higher viscosity than compressed samples. The theory developed by Larson and Doi (1992) to describe the flow of polydomain LCPs is adapted for its use in elongational flow. The predicted trends agree with the measured stress growth curves. The flow seems to be dominated by the effect of the texture. The variation in the viscosity measured for samples prepared by different methods is attributed to the variation in structure present in the samples and in particular to the initial domain size.

j. Rheology, 44(5), 1205-1223, 2000

The extensional viscosity of a thermotropic liquid crystalline polymer