Abstract: Melting recrystallization processes of melt sheared films of polypropylene(S28C) fractions have been investigated in situ by polarized optical microscope equipped with CCDcamera and hot stage. Actually, the morphological developments in the melting recrystallization are partially reappearance of oriented crystallization processes during melt shearing the fractions, which is due to a memory effect of oriented structure of polymer. For low molecular weight fraction, only incomplete spherulites with some orientation along shear direction are observed in the melting recrystallization processes of the sheared films. For middle molecular weight fractions, extended chain fiber crystals(or bands) are formed first at higher temperatures, and the bands can act as self nuclei(i.e ., row nuclei), resulting in epitaxial growth of chain folded lamellae(or fibril), i.e ., the formation of cylindrites, with further decrease of the crystallization temperature. For high molecular weight fraction, however, it is not possible to shear the melt film because of its high melt viscosity. When the low molecular weight fraction in which no fiber crystals or cylindrites are observed, are mixed with small amount(about 1%—2%) of the high molecular weight fraction, quite large number of cylindrites are formed during the melting recrystallization process of its sheared film, which implies that the component of high molecular weight plays an important role in the formation of cylindrites during the shear process of polypropylene.

Key words: Polypropylene fractions, Oriented crystallization process, Cylindrite