CRYSTALLIZATION, MELTING AND MORPHOLOGY OF SYNDIOTACTIC POLYPROPYLENEFRACTIONS .2. LINEAR CRYSTAL-GROWTH RATE AND CRYSTAL MORPHOLOGY

Linear crystal growth rates of two narrow molecular weight fractions o f syndiotactic polypropylene having the same syndiotacticity have been measured using polarized light microscopy over a temperature range of > 20 degrees C. It has been found that a regime III to regime II tran sition at a supercooling of similar to 50 degrees C exists. Structure analysis via electron diffraction (ED) experiments indicates that no c hange of growth planes has been found during this regime transition. N evertheless, a gradual change of the crystal perfection due to a chain packing change from a crystal incorporated with isochiral packing def ects to a majority of cell III structure in this supercooling range ha s been observed. The validity of the nucleation theory applied to s-PP is discussed. For the crystal morphological study, single crystals wi th rectangularly faceted lamellae can be grown at high crystallization temperatures (low supercooling) in thin s-PP film samples as observed via transmission electron microscopy. Similar to the results reported by Lovinger and Lotz, the ED patterns show that the long axis of the single lamellar crystal is the b-axis. On decreasing the crystallizati on temperature, spherulites are developed. Cracks on the lamellar crys tals have been observed, and they are always perpendicular to the b-ax is. This phenomenon has been explained by invoking the observation tha t the coefficient of thermal expansion along the b-axis is about one o rder of magnitude larger than that along the a-axis, as measured via w ide-angle X-ray diffraction experiments. However, at high crystallizat ion temperatures, the cracks are found less frequently. This is due to the pure cell III crystal packing that forms at these temperatures le ading to the incorporation of fewer isochiral packing defects which pr omote crack initiation.