Further study on double-melting endotherms of isotactic polypropylene

The origins of the single- and double-melting endotherms of isotactic polyp ropylene crystallized at different temperatures were studied carefully by d ifferential scanning calorimetry, wide-angle X-ray diffraction, and small-a ngle X-ray scattering. The experimental data show that spontaneous crystall ization occurs when the crystallization temperature is lower than 117 degre es C; thus the lamellae formed are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae leads to do uble endotherms. On the other hand, when the crystallization temperature is higher than 136 degrees C, two major kinds of lamellae with different thic kness are developed during the isothermal process, which also results in th e double-melting endotherms. In the intermediate temperature range the lame llae formed are perfect, and there is only a single peak in the distributio n of lamellar thickness. This explains the origin of the single-melting end otherm. (C) 2000 John Wiley & Sons, Inc.