THE RELEVANCE OF ENTRY FLOW MEASUREMENTS FOR THE ESTIMATION OF EXTENSIONAL VISCOSITY OF POLYMER MELTS

The extensional viscosity of some flexible chain polymers and a thermo tropic liquid crystalline polymer was measured in uniaxial extensional flow at constant extension rate. Power law functions were found for t he dependence of the extensional viscosity at constant accumulated str ain on strain rate. The stress growth curves were compared with measur ements in axisymmetric entry flow, where both elongation and shear occ ur. The comparison showed that the values of the extensional viscosity calculated from the measurements in the entry flow correspond to the ones calculated from the viscosity growth measured in uniaxial elongat ion and averaged over extensional strain equal to what is accumulated on the fluid as it flows from the barrel into the capillary.