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

Abstract

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.