Mechanism of βα recrystallization in isotactic polypropylene

βα recrystallization refers to a process of melting of the β crystallites followed by a recrystallization into α phase in isotactic polypropylene (iPP). It is normally observed only after cooling down the β-nucleated iPP (β-iPP) from the crystallization temperature to below 100 °C before heating. In this work, several β-iPP with different molecular weights were employed to investigate the βα recrystallization in the heating process starting at the crystallization temperature (Tc). It turned out that the non-cooled β-iPP with a molar mass of about 12 kg/mol (β-iPP12K) showed a βα recrystallization during slowing heating. The extent of the βα recrystallization gradually reduced with the increase of the Tc of the β-iPP12K. This peculiar behavior of β-iPP12K can be assigned to the existence of less stable β-crystallites which are susceptible to βα recrystallization. The β-iPP12K preferred to crystallize into thin β-crystallites with low stability which could be melted at low temperatures to form “amorphous self-nuclei” during slow heating. These local ordered amorphous phase would serve as nucleation-sites for α growth in a favorable temperature range. With respect to the β-iPP with high molar mass, although the “amorphous self-nuclei” could be induced at a certain temperature, this temperature was already beyond the appropriate temperature for α growth.