AbstractLinear polyethylene was extruded from a capillary rheometer with the driving piston operated at fixed speed and at fixed pressure. Apparent viscosity and melt density were measured in both extrusion modes. Apparent density decreased at shear rates approaching the melt fracture region in fixed piston‐speed operation. Flow of other polymer melts was essentially incompressible in fixed piston‐speed operation, and all polymers exhibited incompressible flow in fixed‐pressure extrusion. The oscillating portion of the flow curve of linear polyethylene reflects alternating periods in which the polymer exits faster and slower than the rate at which the advancing piston clears the rheometer reservoir. Linear polyethylene behaves differently from most other polymers in fixed piston‐speed extrusion and during melt fracture because of the existence of a more extensive entanglement network in the melt. It is suggested that melt fracture in general results from a tensile failure of the entanglement network, which may occur at the die inlet and in the
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