On Morphology and Multiple Melting in Polypropylene

A series of polypropylenes, including two modem high crystallinity materials (HCPP) were subjected to a stepwise crystallization procedure as a guide to their properties. The results were complicated by the development of double melting endotherms at the highest crystallization temperature. Both HCPP, when grown at the temperature of 145degreesC, give a double melting peak, but the proportions of the two peaks varied according to the density of nucleation. By partially melting a specimen between the two peaks, it was possible to assign the higher peak to radial dominant lamellae and the lower peak to mostly tangential subsidiary lamellae filling the space in between. Uniformly cross-hatched lamellae at the center of spherulites melt along with the lower melting population in the outer regions of the spherulites. Even if differences in crystallization temperature are eliminated, the properties of nucleated as opposed to nonnucleated PP may therefore be influenced by differences between the center and periphery of spherulites, with "central" properties much more in evidence in nucleated material. The development of these double endotherms is related to morphological constraint, rather than molecular fractionation. Their form is strongly influenced by cross-hatching, but the phenomenon is still found at 160degreesC where cross-hatching does not form.