On the formation of S-profiled lamellae in polyethylene and the genesis of banded spherulites

A study of the habits of individual melt-grown polyethylene crystals has shown how banded spherulites form in this polymer and the essential cause to be the relief of fold surface stress. An initially planar lamella develops a characteristic S-profile over time extending out from central regions. This is a consequence of reducing surface stresses by improving fold packing whose spatial requirements are best met by inclined as opposed to the initial perpendicular fold surfaces. The axis of the S-profile is inclined to b, an inclination which introduces the essential asymmetry into the system, and the lamella slightly twisted around the axis of the S. The twist is amplified by the formation of isochiral giant screw dislocations, systematically to one side of the S for a given radius, giving diagonal linkages and each developing lamella a comparatively large increment of twist. The detailed morphology is readily accounted for on this basis as is the restriction of banding to polymers with (eventually) inclined fold surfaces.