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
