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

The melting memory effect during the crystallization and heating of semi-crystalline polymers was clearly demonstrated using beta -isotactic polypropylene (beta -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 alpha -form crystals via phase transformation from the metastable beta -form during the melting process, and to elucidate the memory effect of these new alpha -form. crystals during the crystallization process. The evolution of the memory effect in beta -iPP during the crystallization and melting processes is ideally based on the existence of locally ordered alpha -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 alpha -modification, whereas longer hold times generate samples rich in beta -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 beta -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. (C) 2001 Elsevier Science Ltd. All rights reserved.X1187sciescopu

Memory effect of locally ordered alpha-phase in the melting and phase transformation behavior of beta-isotactic polypropylene

Structural changes in beta-isotactic polypropylene (beta-iPP) during the heating were studied by means of differential scanning calorimetry and real-time in situ X-ray diffraction using a synchrotron source. Crystalline phase transformation and the memory effect caused by residual nuclei of alpha-iPP were observed during the heating of beta-iPP. The memory effect observed in beta-iPP during heating and crystallization is believed to be due to the existence of locally ordered alpha-from in the melt. The effect of local alpha-form order was probed by studying the behavior under heating of samples with a range of thermal histories. Samples were heated above the equilibrium melting temperature of iPP to remove all residual local order and the memory effect associated with this local order. The samples crystallized isothermally at different temperatures exhibited a significantly different melting and phase transformation behavior during heating. beta-iPP is found to be an excellent material for the study of polymorphism, phase transformations, and characteristic memory effects in semicrystalline polymers. (C) 2003 Elsevier Science Ltd. All rights reserved.X1157sciescopu