Exploring shear yielding and strain localization at the die entry during extrusion of entangled melts.

This work applied a particle-tracking velocimetric technique to observe the deformation field in the die entry pressure-driven extrusion, motivated by insights gained from previous studies of entangled melts in simple shear. Based on several styrene-butadiene rubbers and a polybutadiene melt, we show that shear yielding takes place to result in (shear banding-like) strain localization in the die entry. The degree of strain discontinuity is shown to grow with the level of chain entanglement. The critical pressure for shear yielding corresponds to a level of shear stress at a 45° inclined plane that is comparable to the melt plateau modulus, and therefore can be predicted based on our recent understanding of yielding and strain localization in startup shear. The unstable (i.e., time-dependent) shear strain localization in the die entry during continuous extrusion at a sufficiently high volumetric throughput or pressure results in extrudate distortion that is often also known as gross melt fracture. [ABSTRACT FROM AUTHOR]