The Weissenberg effect in molten polymers

The climb of viscoelastic liquids up a rotating rod, often called the Weissenberg effect, has often been observed in polymer solutions, and several theoretical analyses of this phenomenon have been published. However, no observations of rod-climbing in polymer melts have been reported. In the present work, three commercial polyethylenes were melted in a special chamber under a controlled atmosphere, and a rod was rotated in the pool of molten polymer. The behavior of the free surface was noted as a function of time and rotational speed.In the case of the resin with the lowest molecular weight and the highest density, the free surface reached a steady-state shape within seconds of changing the rotational speed. For the other two resins, the development of the shape of the free surface took place over periods of the order of one hour or more. In the cases where the starting transient was very long-lasting, the melt first climbed the rod and accumulated in a large bulge. Then, the fluid in the bulge flowed down to the bottom of the raised column and a pear-shaped body of liquid was formed whose shape continued to undergo gradual change for periods up to several hours. At high speeds, inertial effects destabilized the flow and led to asymmetry and time unsteadiness in the shape of the raised body of liquid.