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Shear-Induced Precursor Structures in Isotactic Polypropylene Melt by In-Situ Rheo-SAXS and -WAXD Studies:
In-situ rheo-SAXS and -WAXD studies were carried out to investigate the nature of shear-induced precursor structures in isotactic polypropylene (iPP) melt at 165 oC, near its nominal melting point. Immediately upon the cessation of shear, SAXS patterns clearly showed an evolution of oriented structures in hundreds of angstroms, while the corresponding WAXD patterns did not exhibit any crystal reflections. SAXS patterns at later times showed that the shish-kebab morphology was developed and the kebabs possessed only a small amount of crystallinity (3%). The combined SAXS and WAXD results indicate that at the early stages of crystallization, a scaffold (network) of oriented structures is formed. These structures contain (1) primary nuclei (through homogeneous nucleation) that may be crystalline or mesomorphic but having linear connectivity along the flow direction, which form the shish entity, and (2) shish-induced layered crystalline lamellae (kebabs) oriented perpendicularly to the flow direction that have poor lateral connectivity. The results verified, quantitatively for the first time, the well-known concept that minor amounts of linear nuclei induce multiple secondary nucleation sites for the growth of a large quantity of lamellae that grow radially outward from the central core. A mechanistic pathway for the early stages of crystallization in polymer melts under flow is proposed.
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