The molecular structure of flowing polymer melts

The application of shear stress to polymer melts as a consequence of extrusion processes is commonly believed to result in the partial orientation of the molecules in the direction of the shear. Polymer samples examined after extrusion and solidification have shown a relatively high degree of orientation. It has long been assumed that this degree of orientation was reached during the period of shear stress. The subsequent partial reorganization of these orientated molecules has been considered to cause or at least provide a contributing factor to the phenomenon of “die swell”. An in situ study has been made of the behaviour of flowing polyethylene melts under shear stress using X-ray diffraction, i.r. reflectance and Raman spectroscopy all of which are capable of yielding molecular level structural and orientational data. The results of these examinations suggest that there is not a high degree of orientation present in the flowing melts. It is concluded therefore, that the high degree of orientation found in extruded polymer samples is not present during the extrusion process but forms after extrusion as the material cools and crystallizes.