A recent development in extensional rheometry of molten polymers is based on the use of a pair of gears as a ‘rotational clamp’. In a tensile tester for polymer melts, the rotational clamp provides the draw-down of the melt being extruded out of the die of a melt indexer. The result is a quick characterization of extensibility and tensile strength. In a new extensional rheometer, two rotational clamps are used which extend homogeneously a molten polymer rod floating on a silicone oil bath. For constant rotational speeds of the clamps, the (Hencky) strain rate is constant. The creep test with constant tensile stress can be performed in the same rheometer making use of a servo motor of quick response with an appropriate servo control. A further modification of the servo control is described by means of which any uniaxial stress or strain history can be imposed on the melt under test, with stress σ>0 the only constraint. In this rheometer, the extensional behavior of a LDPE melt was measured at 150°C under a variety of mechanical conditions. The data presented demonstrate the versatility of the apparatus.
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J. Meißner (1972) studied this question.