Localised flow-induced crystallisation of a polyethylene melt

The sensitivity of flow-induced crystallisation (FIC) to the nature of flow type is demonstrated using a high-density polyethylene (HDPE) for two different flow geometries. A contraction-expansion slit geometry was used to create a mixed, but primarily simple shear flow, while a cross-slot geometry provided a region within the flow of high extension. Flow-induced birefringence was captured at a melt processing temperature of 155 degrees C to identify the principal stress difference within the two flows and determine regions of higher stress within the HDPE. The experiments were then repeated at 125 degrees C, and FIC was identified using bright-field observation. Crystallisation was observed within the regions that previously exhibited high stress levels. It was found that lower deformation rates in pure shear were required when compared with simple shear to create the crystal filaments.