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 change in crystalline morphology were about four orders of magnitude longer than the longest bulk relaxation time (reptation time). Because the appearance of spherulites indicates that films became equilibrated and reached a state of an isotropic melt before recrystallization, we may conclude that oriented PE chains in ultrathin films possessed a long-term memory of the preparation-induced chain stretching. Considering that the morphology and relaxation kinetics of PE films depended appreciably on the substrate properties and film thickness, we conclude that the formation of asymmetric leaf-shaped crystalline structures was also affected by the interaction of PE chains with the substrate and spatial confinement. The present results may shed new light on slow relaxation and reorganization processes encountered when long-chain polymers became oriented during sample processing.