EXTENSIONAL FLOW-INDUCED CRYSTALLIZATION OF A POLYETHYLENE MELT

Measurements of flow-induced orientation and crystallization have been made on a high-density polyethylene melt (HDPE) undergoing a planar e xtensional flow in a four-roll mill. The HDPE was suspended as a cylin drical droplet at the flow stagnation point in a linear low density po lyethylene (LLDPE) carrier phase. Deformation and crystallization of t he HDPE droplet phase were monitored using video imaging in conjunctio n with measurement of the birefringence and dichroism to quantify the in-situ transformation kinetics. Planar deformation rates along the sy mmetry axis of the molten HDPE phase were on the order of 0.03 s-1. Me asurements of the initial transformation rate following flow cessation at 131.5-degrees-C and 133.2-degrees-C show a dependence on initial a morphous phase orientation and the total Hencky strain achieved during flow. The flow-induced crystallization rate is enhanced over the quie scent transformation rate by orders of magnitude, however, the depende nce on temperature is less dramatic than expected for a nucleation-con trolled growth mechanism. Analysis demonstrates that the melting point elevation model cannot account either qualitatively or quantitatively for the phenomena observed, suggesting that alternative explanations for the strong orientation dependence of the transformation rate are n eeded.