Mechanism of βα recrystallization in isotactic polypropylene

beta alpha recrystallization refers to a process of melting of the beta crystallites followed by a recrystallization into alpha phase in isotactic polypropylene (iPP). It is normally observed only after cooling down the beta-nucleated iPP (beta-iPP) from the crystallization temperature to below 100 degrees C before heating. In this work, several beta-iPP with different mo-lecular weights were employed to investigate the beta alpha recrystallization in the heating process starting at the crystallization temperature (Tc). It turned out that the non-cooled beta-iPP with a molar mass of about 12 kg/mol (beta-iPP12K) showed a beta alpha recrystallization during slowing heating. The extent of the beta alpha recrystallization gradually reduced with the increase of the Tc of the beta-iPP12K. This peculiar behavior of beta-iPP12K can be assigned to the existence of less stable beta-crystallites which are susceptible to beta alpha recrystallization. The beta-iPP12K preferred to crystallize into thin beta-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 alpha growth in a favorable temperature range. With respect to the beta-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 alpha growth.

Automated summary

Beta alpha recrystallization refers to the melting of beta crystallites followed by recrystallization into alpha phase in iPP. This phenomenon is typically observed after cooling down beta-nucleated iPP from the crystallization temperature to below 100 degrees C before heating. The study investigated this recrystallization process in beta-iPP with different molecular weights during heating starting at the crystallization temperature. It was found that beta-iPP12K, with a molar mass of about 12 kg/mol, exhibited beta alpha recrystallization during slow heating, which gradually reduced with an increase in the Tc of beta-iPP12K. As for beta-iPP with high molar mass, the temperature at which amorphous self-nuclei were induced was already beyond the appropriate temperature for alpha growth.