proceedings article

Abstract

The transient extensional viscosity of the nematic melt of the thermotropic liquid crystalline co-polyester Vectra A950 was characterised in uniaxial extensional flow in a commercial constant strain rate rheometer. Measurements were carried out in injection moulded samples to examine the influence of the presence of initial orientation and structure in the samples and compared with those of compression moulded samples, as well as with samples where the skin had been removed and only the core remained. The extensional viscosity of Vectra A950 is strain hardening, similar to some flexible chain polymers. The melt strength, however, is lower than the melt strength of e.g. LDPE, the sample breaking at 2.5 to 3.5 (Hencky) strain units. If the viscosity at a certain value of strain is plotted vs. the strain rate, a shear thinning curve is obtained with no indication of a plateau at low rates. This is similar to the shear viscosity and it is probably due to the contribution of the domain structure at low rates. The power-law indices are higher than the corresponding one of the shear viscosity. Injection moulded samples show higher viscosity than compressed samples, even though the contribution of the highly oriented skin is not dominant. The differences are attributed to the structure present in the bulk of the injection moulded part.

Progress and Trends in Rheology 5, 222-224, 1998

Elongational viscosity of a thermotropic liquid crystalline polymer