Melt recrystallization behavior of carbon-coated melt-drawn oriented isotactic polypropylene thin films

The melt recrystallization of vacuum carbon evaporated melt-drawn iPP thin films at varying melting temperature, melting time and recrystallization temperature was studied by means of transmission electron microscopy combined with electron diffraction. It was found that the carbon-coated melt-drawn highly oriented iPP thin films melt-recrystallized always in an oriented way with well ordered crystalline lamellae perpendicular to the drawing direction during sample preparation. This is related to the strong fixing effect of the vacuum evaporated carbon film on the oriented iPP chain stems on the surface layer. Since these fixed iPP chain stems are originally included in the alpha-iPP crystal lattice, they serve as alpha-iPP nuclei during the recrystallization process and induce highly oriented iPP lamellae composed of only alpha-iPP crystals. This indicates that the frequently reported orientation-induced beta-iPP crystallization is a nucleation controlled process. Moreover, by comparing the original oriented structure of melt-drawn iPP films with the recrystallized ones, it is concluded that the characteristic wide angle lamellar branching is severely suppressed in elongation-induced crystallization, while takes place evidently during melt-recrystallization.