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

Abstract

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, monotonic flow curves have been associated with extrudates that transcend from smooth to being volume distorted. Alternatively, nonmonotonic flow curves have been associated with a sequence of extrudate appearances ranging from smooth via surface distortions and “spurt” to volume distortions. New experiments however indicate that monotonic flow curves can also be associated with “spurtlike” distorted extrudates. For several long-chain branched polymers, it is reported that while the average pressure increases monotonically with increasing flow rate, the extrudate distortions transition through an unanticipated regime where the extrudate consists of alternating smooth and volume-distorted zones. Its origin is conjectured rel...