Morphological study of isotactic polypropylene thin films on different substrates using grazing incidence wide-angle X-ray diffraction

Isotactic polypropylene (iPP) is one of the most commonly used polymers owing to its excellent mechanical properties and well-balanced physical properties. These properties can be achieved and controlled through a better understanding of its hierarchical structures. The crystal structures of iPP thin films of ∼120 nm thickness formed on graphite and silicon substrates were investigated via grazing incidence wide-angle X-ray diffraction (GIWAXD) with hard and tender X-rays along with grazing incidence small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy. The lamellae were found to be edge-on-oriented on the graphite substrate due to epitaxial iPP crystal growth and flat-on-oriented on the silicon substrate due to iPP crystal growth along the thermodynamically dominant orientation. GIWAXD results obtained with tender X-rays revealed that the orientation of the crystal lamellae did not change along the thickness direction and that the degree of crystallinity at the surface was lower than that in the interior of the iPP thin films for each substrate. These results indicate that crystallization at the substrate-film interface played a significant role in the formation of the crystal structure of the iPP thin film, that is, the crystals grew throughout the thin films along the substrate-induced orientation even in the case of edge-on orientation, where the growth axes of the crosshatched lamellae were not parallel to the thickness direction. The relationship between the growth axes of lamella and the growth of surface-induced orientation along the normal direction of the substrates was successfully clarified for the first time using the thin film systems. The obtained results can be extended to interface-induced crystallization inside bulk composite materials.