In situ optical microscope study of the shear-induced crystallization of isotactic polypropylene
Introduction
In most polymer processing operations such as injection molding, film blowing, and fiber spinning, the molten polymer chains are subjected to intensive shear and elongational flow fields. It is known that processing conditions profoundly influence the rate of solidification, the semicrystalline morphology, and ultimately material properties [1], [2], [3]. The semicrystalline morphology (shish-kebab structure) that developed under shear field [4], [5] is typically very different from what is observed under quiescent conditions. It is generally believed that polymer chains are oriented and stretched by the shear flow and can crystallize with different kinetics and consequently different morphologies. Although shear flow is often considered as a ‘weak’ flow, however, it does affect the overall crystallization kinetics [6], [7], [8], [9] and modify the final morphologies and properties [9], [10], [11], [12] of polymeric materials. In recent years, studies of flow-induced crystallization have drawn a lot of interests because it reveals the possibilities of controlling and predicting the final morphologies and properties of the semicrystalline polymers in most of the transformation processes, meanwhile, provided an opportunity to understand some fundamental issues among molecular conformations, ordering processes, as well as nucleation and crystallization kinetics [1], [13], [14], [15].
In this communication, we present some results of in situ optical microscopy studies of shear-induced crystallization of isotactic polypropylene melts under various thermal and shearing histories. It directly revealed the lamellar growth process occurred from an oriented long shish structure. These fibril like shish structure, the perpendicular lamellar growth geometry, as well as the beta-formation and its melting behavior can provide many physical insights on several still unsettled issues such as extended chain crystals, row nuclei, and smectic ordering [14], [15] questions. More detailed scattering studies are still underway in our laboratory.
Section snippets
Experimental details
The isotactic polypropylene homopolymer was provided by Yanshan Petrochemical Corp., Inc. The molecular weights of i-PP resin, as obtained from the GPC experiments, are Mn=9.62×104, Mw=4.38×105 with a polydispersity D=4.55.
A Linkam CSS-450 high-temperature shearing stage (Linkam Scientific Instruments Ltd, Tadworth, Surrey, UK), and an optical microscope (Nikon E600POL) were used in this study. The mechanical design and electronic control of the Linkam shear stage provided adequate conditions
Results and discussion
Fig. 1 shows the polarized optical micrographs of i-PP samples sheared at a rate of 0.5 s−1. Classical ‘shish-kebab’ morphologies are clearly observed. The landscape orientation of lamellae run perpendicular to the narrow backbone as the kebab structure should be. The structure is in close resemblance to the polymorphic shish-kebab morphology in PP melts sheared with fibers [6], [7], [11], [12], crystallization in fibers/polymer composites [17], [18], [19], or sample crystallization by a razor
Conclusion
We have presented in this report an in situ optical microscope study of the crystallization of i-PP under shear with the formation of shish-kebab like α-structure and the growth front induced β-form crystals. According to our results, a modified model has been proposed for the growing process of shear-induced crystallization in i-PP melt.
Although we cannot distinguish smectic liquid crystalline like order from row nuclei alignment with correlation length of the order of the strand molecular
Acknowledgements
This work was supported by three projects: KJCX2-SW-H07, 2003 CB 615600 and 20490220.
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