Large-deformation and long-time behavior of entangled melts in complex geometries

Recent particle-tracking velocimetric (PTV) observations have revealed strain localization either during startup shear beyond the stress overshoot or after a large step shear of entangled polymers [e.g., Macromolecules, \textbf{42}, 6261 (2009)]. The physical pictures leading to these decohesion events have been put forward [J. Chem. Phys. \textbf{127}, 064903(2007); J. Rheol. \textbf{53}, 1389 (2009)]. In this presentation we apply the particle-tracking velocimetric method [Macromol. Mater. Engr. \textbf{292}, 15 (2007)] to study similar strain localization phenomena originating from yielding of the entanglement network in other forms of deformation including uniaxial extension, ``squeeze flow'' and extrusion of polymers from a wide open space into a narrow opening. The striking discontinuities in the velocity profile can all be understood in terms of a shear yielding criterion. The research is funded, in part, by a grant from the National Science Foundation (CMMI-0926522)