Shear-Induced Precursor Structures in Isotactic Polypropylene Melt by in-Situ Rheo-SAXS and Rheo-WAXD Studies

• Published in: Macromolecules Vol. 35; no. 24; pp. 9096 - 9104
• Main Authors: Somani, Rajesh H, Yang, Ling, Hsiao, Benjamin S, Agarwal, Pawan K, Fruitwala, Hitesh A, Tsou, Andy H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 19.11.2002
• Subjects: Applied sciences; Crystallization; Exact sciences and technology; MATERIALS SCIENCE; MORPHOLOGY; NATIONAL SYNCHROTRON LIGHT SOURCE; Organic polymers; Physicochemistry of polymers; POLYPROPYLENE; PRECURSOR; Properties and characterization; SMALL ANGLE SCATTERING; X-RAY DIFFRACTION; Shish kebab polymer; Melt crystallization; Molten state; Liquid state structure; Shear flow; Isotactic polymer; Propylene polymer; Time curve; Early stage; Experimental study; Isothermal crystallization; Mechanism
• DOI: 10.1021/ma020785o
• ISSN: 0024-9297

In-situ rheo-SAXS (small-angle X-ray scattering) and rheo-WAXD (wide-angle X-ray diffraction) studies were carried out to investigate the nature of shear-induced precursor structures in isotactic polypropylene (iPP) melt at 165 °C, 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. Subsequent polymer crystallization takes place in the framework of the scaffold, which is probably dominated by the lower molecular weight species. Amounts of the crystalline primary nuclei and the layered crystalline lamellae in the precursor structures were estimated. 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.