Formation of Asymmetric Leaf-Shaped Crystals in Ultrathin Films of Oriented Polyethylene Molecules Resulting from High-Temperature Relaxation and Recrystallization

We melted, annealed, and recrystallized ultrathin films of oriented polyethylene (PE) molecules prepared by melt-drawing. A large number of randomly oriented asymmetric leaf-shaped crystalline structures consisting of preferentially oriented lamellae were formed simultaneously, as observed by optical and atomic force microscopies. The structural arrangement within these leaf-shaped crystalline structures was identified by grazing incidence X-ray diffraction. These structures consisted of two distinct sections, both differing strongly from circular spherulites obtained by crystalizing an isotropic melt. Besides, regions containing groups of stacks of slightly inclined but well-aligned flat-on lamellae and regions of less orderly arranged edge-on lamellae were found. When increasing the annealing temperature and/or annealing time, a change in morphology from the leaf-shaped crystalline structures to spherulites with two symmetric “eyes” was observed. Intriguingly, the annealing times required for such a cha...