Abstract
Abrupt contraction geometry dies have been fitted to recirculation instruments capable of small scale melt processing, and to a 38 mm single screw extruder. Flow induced birefringence has been used to determine the stresses in the polymer melts within each extrusion die. Stresses are determined at the intersection of isochromatic and isoclinic fringes. Stress birefringent studies have been conducted on blends of 95% deuterated and 5% monodisperse polystyrenes of molecular weights approximately 100 k and 200 k. Centreline principal stress profiles of branched and linear polymers are compared for each geometry. The principal stress difference (PSD) profiles of the polystyrenes are nominally similar to those of the LDPE evaluated. In comparing branched and linear polyolefins, the LDPE exhibits a much greater number of fringes and, correspondingly higher values of first normal stress difference, at entry and exit flows to the abrupt contraction geometry compared to the HDPE melt studied. Velocity fields have been quantified for the different flow conditions utilising the laser sheet lighting technique coupled with seeding the flow with glass tracer particles. The acceleration towards the slit entry is clearly apparent and profiles are seen similar, in form at least, to those for Newtonian free jets.
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