On fold surface ordering and re-ordering during the crystallization of polyethylene from the melt

Abstract

Novel observations of polyethylene crystallizing from the melt as row structures on linear nuclei show that at lower temperatures, when there is banded crystallization, lamellae form first with disordered fold surfaces. Banding itself is associated with the re-ordering of those fold surfaces towards their preferred inclined condition. Lamellae which form initially are close-packed and normal to the row but, at greater radial distance, are separated and develop S-profiles behind the growth front, in conjunction with isothermal lamellar thickening. At the growth front, molecules still add to lamellae whose normals are parallel to the chain axis. A different situation pertains at temperatures >∼127°C where the first lamellae to form do so epitaxially with inclined fold surfaces in two orientations symmetrically inclined to the axis of the row. When lamellae widen, this morphology changes to one of planar lamellae normal to the row, which retain inclined fold surfaces. The difference between growth above and below ∼127°C is due to the development of fold interactions over time, towards the ordering of {201} fold surfaces. For the lower temperatures the rate of radial advance is faster than that of fold-surface ordering with the change, from fold surfaces being initially perpendicular then re-ordered, to being inclined from the start, expected when the relative rates reverse. Banding does not occur when molecules add to lamellae with inclined fold surfaces, only when fold re-ordering occurs within existing lamellae consistent with this process being the cause of banded growth in polyethylene.