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

The self-nucleation behavior of β-form isotactic polypropylene (β-iPP) crystals in ultrathin films has been probed by atomic force microscopy (AFM) and optical microscopy (OM). The hexagonal β-iPP single crystals can be obtained via a self-seeding method at much higher crystallization temperatures (e.g., 156 °C). We found that the average number nucleation density (N) of β-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 °C, since α-iPP grows about 20–2400% faster than β-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 α-iPP and β-iPP were grown by homoepitaxy on their own crystals, and no trace of the epitaxial overgrowth of α-iPP lamellae on the entire growth front of β-iPP was observed, that is, a high-temperature βα growth transition appears to be absent, which probably indicates that the free energy barrier for secondary αα nucleation and ββ nucleation is much lower than that of βα nucleation in ultrathin films.