Real time in situ X-ray diffraction studies on the melting memory effect in the crystallization of β-isotactic polypropylene

The melting memory effect during the crystallization and heating of semi-crystalline polymers was clearly demonstrated using β-isotactic polypropylene (β-iPP). Differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron radiation source were employed to investigate the role of the newly formed α-form crystals via phase transformation from the metastable β-form during the melting process, and to elucidate the memory effect of these new α-form crystals during the crystallization process. The evolution of the memory effect in β-iPP during the crystallization and melting processes is ideally based on the existence of locally ordered α-form in the melt. We monitored the role of this local order by preparing the melt state using a range of hold temperatures and hold times. It was found that the final melt temperature and hold time greatly affect the crystallization behavior during cooling and the phase transformation behavior during heating. Lower hold temperatures and shorter hold times lead to samples rich in α-modification, whereas longer hold times generate samples rich in β-modification during crystallization. At higher hold temperatures even a short hold time is sufficient to destroy the local order in the melt, and the resulting sample exhibits more β-modification. The results are explained on the basis of the existence of local order in the amorphous melt along with external nucleating agent during the crystallization process.