TRANSIENT FLOW-INDUCED CRYSTALLIZATION OF A POLYETHYLENE MELT

Extensional, flow-induced crystallization (FIC) of a high-density poly ethylene (HDPE) melt has been studied using a four-roll mill flow cell . Simultaneous measurement of the birefringence and scattering dichroi sm are used to quantify the crystallization process during and followi ng transient flow deformation in planar extensional flow. Suspension o f the HDPE phase as a droplet in a linear low-density polyethylene car rier phase prevents die blockage on crystallization and allows measure ment of the flow kinematics. Initial crystallization rates following a transient flow deformation show a stress-strain dependence. Crystalli zation induction times during flow correlate with the extension rate d uring the transient flow deformation. Measurement of the HDPE melt ste ady and oscillatory flow rheological properties, along with measuremen ts of time constants following step-shear and extensional strains, all ow determination of the viscoelastic properties which enhance FIC. Par ameters obtained from these experiments are used in a phenomenological model for FIC which allows qualitative and semiquantitative analysis of the data trends, particularly the relaxation behavior of the birefr ingence during flow cessation/crystallization. (C) 1997 John Wiley & S ons, Inc.