Self-Nucleation of beta-Form Isotactic Polypropylene Lamellar Crystals in Thin Films

The self-nucleation behavior of beta-form isotactic polypropylene (beta-iPP) crystals in ultrathin films has been probed by atomic force microscopy (AFM) and optical microscopy (OM). The hexagonal beta-iPP single crystals can be obtained via a self-seeding method at much higher crystallization temperatures (e.g., 156 degrees C). We found that the average number nucleation density (N) of beta-iPP lamellar crystals deceases with increasing self-nucleation temperature (T-s) and heating rate (V-h). Upon heating the polymorphic structures induced by shear flow to a temperature between 143 and 156 degrees C, since alpha-iPP grows about 20-2400% faster than beta-iPP, the former overtakes and engulfs the growth front of the later when crystallized for a sufficiently long time. However, the morphological observations show that both alpha-iPP and beta-iPP were grown by homoepitaxy on their own crystals, and no trace of the epitaxial overgrowth of alpha-iPP lamellae on the entire growth front of beta-iPP was observed, that is, a high-temperature beta alpha growth transition appears to be absent, which probably indicates that the free energy barrier for secondary alpha alpha nucleation and beta beta nucleation is much lower than that of beta alpha nucleation in ultrathin films.