Effect of formation conditions on the supermolecular organization and mechanical properties of polyethylene film

Films formed by extruding medium-pressure polvethylene through a flat-slot have a spherulitic supermolecular organization whose principal parameters (size and shape of the spherulites) are determined by the extrusion conditions. Thus, as the draw-down ratio increases, the radius of the spherulites decreases, and their degree of flattening relative to the direction of extension increases. Stretching these films leads to a transition from a spherulitic to an orientational supermolecular order, whose period is genetically related to the diameter of the starting spherulites. Films containing flattened spherulites have a yield point anisotropy opposite in sign to the degree of flattening. The mechanical anisotropy, like the degree of flattening, increases with increase in the draw-down ratio. The probable cause of flattened spherulite formation is the draw-down process, whose mechanical field may retard the radial growth of the spherulites in the take-off direction.