Experimental observations and matching viscoelastic specific work predictions of flow-induced crystallization for molten polyethylene within two flow geometries

Abstract

Flow-induced crystallization (FIC) behavior of a high-density polyethylene melt in two entry-exit flow geometries was investigated by direct optical observation using a multi-pass rheometer and the results compared with a viscoelastic flow simulation. A set of experiments was performed at several piston speeds using a sharp and a rounded entry-exit slit and the region of onset for visible FIC was identified in both cases. During flow narrow crystal filament regions localized at the sidewalls and in a downstream “fang” region of stress accumulation were identified. A melt flow two-dimensional numerical simulation using a Lagrangian solver, FLOWSOLVE, and an 11-mode Pom-Pom model satisfactorily matched experimental pressure difference and birefringence fringe distribution for the flow. An algorithm to calculate the specific work accumulated by each fluid element in the complex flow field was implemented within FLOWSOLVE and a method was proposed to estimate the critical specific work for the onset of visible oriented FIC. The concept of specific work applied to the numerical simulations was capable of successfully predicting the experimental regions where FIC occurred.