Morphological Changes of Isotactic Polypropylene Crystals Grown in Thin Films

Morphological variations of lamellae of isotactic polypropylene (iPP) grown in thin films have been examined experimentally by optical microscopy (OM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). A flower-shaped morphology of iPP crystals, composed of several petal-like lamellae radiating from a nucleus, was typically found. At crystallization temperatures (T_c) below 135 °C, initially petal-like lamellae with a flat α-iPP backbone and many regular branches were formed, which were able to induce epitaxial nucleation of γ-iPP, resulting in features similar to a dendrite growing in the plane of the slow growth direction (i.e., b-axis of α-iPP). With increasing T_c, these dendritic structures disappeared gradually, and the lamellae exhibited a faceted lath-like shape for T_c > 150 °C. Interestingly, periodic lateral splitting (the crystal splayed into a pair of branches) at the fast growth plane was observed at a critical width (W_max) which increased with T_c. In particular, the measured temperature dependence of the products of W_max²G (G represents the growth rate along the a*-axis) was found to be constant. We discuss the role of the diffusion field at the growth front and epitaxial crystallization with respect to morphological changes of iPP lamellae in thin films.