The Influence of Chain Stiffness on the Flow Behavior of Polymers in the Entrance of a Capillary

Abstract

A glass capillary attached to an Instron Rheometer has been used to study the flow behavior of a flexible, a semi-rigid, and a rigid chain polymer in a capillary. The flexible chain polymer exhibits the “wine-glass” pattern in the entrance region and undergoes a flow instability in the entrance associated with the onset of elastic fracture at a wall shear stress (τw) of 104 N/m2. The semi-rigid polymer does not exhibit the “wineglass” entrance flow pattern. However, a flow instability arises in the capillary at a wall shear stress of 102 to 103 N/m2. The rigid chain polymer exhibits behavior in the capillary entrance similar to that expected for an inelastic fluid even though large normal stresses are measured. No flow instabilities were observed for values of τw up to 2x104 N/m2, Die swell decreased with increasing chain stiffness.