Pressure oscillations and periodic extrudate distortions of long-chain branched polyolefins

Capillary rheometry is a much used technique for measuring pressure-flow rate behavior of polymer melts. The nature of such a "flow curve" depends on polymer architecture, die geometry, die material composition, and rheometer operating conditions. Typically, with increasing flow rates, monotonie flow curves have been associated with extradâtes that transcend from smooth to being volume distorted. Alternatively, nonmonotonic flow curves have been associated with a sequence of extradate appearances ranging from smooth via surface distortions and "spurt" to volume distortions. New experiments however indicate that monotonie flow curves can also be associated with "spurtlike" distorted extradates. For several long-chain branched polymers, it is reported that while the average pressure increases monotonically with increasing flow rate, the extradate distortions transition through an unanticipated regime where the extradate consists of alternating smooth and volume-distorted zones. Its origin is conjectured related to the specific viscoelastic flow properties of long-chain branched materials in the reservoir-die contraction region. Using a fast-response pressure transducer in the reservoir near the capillary die entry, the presence of small-amplitude pressure oscillations corresponding to the distortion period is confirmed. The critical conditions for the appearance of this phenomenon depend strongly on molecular mass and branching distribution.