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Flow History – Morphology Development in Crystallizable Polymer Melts

Abstract Flow history-morphology interactions have been studied for three crystallizable polymers: high density polyethylene, poly(vinylidene fluoride), and polypropylene. Materials were extruded through an impinging channels film die which produces a mixed extensional and shearing flow field. Extrudate morphology was characterized by optical microscopy and x-ray diffraction. Stress fields during die flow were measured from flow birefringence and compared to calculations based on the power law model. Good quantitative agreement was found for the shearing stresses, however, large differences were observed between the measured and computed normal stresses along the symmetry axis of the flow where extensional gradients are also the highest. Extensional stresses on the order of 2.8·106, 1.4·106, and 3.9·105 dynes/cm2 were required to induce row-nucleated crystallization in the HDPE, PVF2, and polypropylene respectively. The respective stresses were found to decrease with increasing relaxation time in a non-linear fashion. An explanation for this behavior is presented based on the opposing tendencies of chain relaxation and oriented crystallization.