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Engineering Advanced Morphologies for Structurally Reinforced Polyolefins

Abstract
The primary objective of this research is to develop new methods to enhance the mechanical properties of isotactic polypropylene (iPP). Two complementary methods were developed to produce reinforced iPP-nanographite nanocomposites. In the first method, nanocomposites were prepared through an in-situ metallocene-catalyzed polymerization technique. In the second method, a new compounding strategy was used to prepare iPP-nanographite nanocomposites with improved spatial size distribution of nanoparticle agglomerates. Finally, a new process referred to as Melt-Mastication (MM) was developed as a means to improve the mechanical properties of pure iPP through generating unique and beneficial crystal morphologies. Reinforced iPP-nanographite nanocomposites were prepared through an in-situ polymerization technique and compared to analogous composites prepared by conventional melt processing. In-situ preparation of iPP-nanogrpahite nanocomposites was accomplished via single site metallocene catalyzed polymerization of propylene within a toluene dispersion of xGnP nanoparticles. Mechanical analysis showed iPP-nanographite nanocomposites demonstrated improved stiffness and strength relative to neat iPP. The results are discussed with regard to the thermal and morphological properties. A new polymer processing method referred to as “Melt-Mastication” (MM) was developed as a means to augment the crystal morphology of iPP and thereby enhance the thermal and physical properties. Melt-Mastication is a low temperature mixing technique that subjects an iPP melt to flow induced crystallization within a chaotic flow field. Thermal calorimetry and SAXS showed that MM substantially increases the lamellar crystal thickness and crystallinity of iPP, resulting in a 50% improvement to yield strength, 55% improvement to elastic modulus, and improved temperature stability. The property improvements were attributed to a unique hierarchical organization of lamellar crystals produced by MM, distinct from conventionally prepared iPP materials. Finally, Melt-Mastication was repurposed as a compounding method for preparation of iPP-nanographite nanocomposites with enhanced nanographite dispersion. Due to flow induced crystallization, the process viscosity increases significantly during Melt-Mastication, which produces higher mixing torque and therefore shear resulting in the fragmentation of nanoparticle agglomerates. The spatial size distribution of nanographite agglomerates was evaluated via a quantitative stereological technique, and a model for agglomeration in shear flow is proposed.
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