Structure Evolution Of Impact-Resistant Polypropylene Copolymer And Its Structure-Property Relationship

This thesis is on the characterization of structure and property of impact-resistant polypropylene copolymer (IPC) as well as their relationships. Isotactic polypropylene (iPP) is widely used owning to its superior mechanical properties. However, the poor impact resistance of iPP and its sensitivity to notch, particularly at low temperature disenable it to be used as engineering plastic. Incorporating a discrete rubbery phase is known as an effective way to toughen iPP. The invention of reactor alloying technology is a milestone for the development of toughening iPP by adding the rubbery phase, and the productions produced with this technology is called high impact-resistant polypropylene copolymer. It is generally accepted that the excellent rigidity-toughness balance and superior properties of IPC originate from its complex compositional heterogeneity and unique heterogeneous morphology. However, a complete understanding of the multi-scale phase morphology in IPC and its evolution towards the equilibrium state, as well as the structure-property relationship, has not been achieved yet. In practical thermoplastic process, IPC has to be heated to molten state and then be cooled to room temperature. The multi-scale phase structure and the consequential final performance of IPC are strongly influenced by the thermal history. Therefore, it is meaningful and necessary to investigate the evolution of phase morphology and further correlate the final mechanical properties of IPC with various phase morphologies.By investigating the multi-structure, the evolution process of the condensed state as well as corresponding changes in mechanical properties of IPC, we are proposed to get new understanding on its structure-property relationships, and possible way to further improve the performances of IPC. This thesis contains five chapters.Chapter 1 is the introduction of the backgrounds for the whole work and some basic theories used in our work.In Chapter 2, influence ofα-andβ-nucleation on brittle-ductile transition temperature (BDTT) of IPCs and their phase morphologies are comparatively investigated. Impact test showed that the BDTT ofβ-IPC is approximately 24℃lower than that of a-IPC. Structural characterizations reveal that dispersion of the noncrystalline ethylene-propylene random copolymer-rich (EPR-rich) phase is finer inβ-IPC in comparison with that in a-IPC. For the reason of faster crystallization rate of (3-IPC and looser lamellar arrangement, more portions of EPR-rich components is included in the interlamellar region in comparison with that of a-IPC. It also leads to improved mobility for the amorphous polypropylene chains, as is confirmed by dynamical mechanical analysis (DMA). In conclusion, we propose that the finer distribution of EPR-rich phase, which might result from faster growth rate of theβ-crystal and looser lamellar arrangement ofβ-spherulite, should be responsible for the improved impact-resistance and lower BDTT in (3-IPC samples.Chapter 3 reports an abnormal morphology evolution at a relative low temperature (room temperature or lower) after IPC has crystallized. The evolution phase is darker in color seen by optical microscopy, and we call it "black phase". It shows difference in evolution rate in a-IPC andβ-IPC. By characterization of Raman spectroscopy, it is found that EPR enriched in the "black phase". Further comparison by controlling the LLPS or crystallization rate shows that the longer the LLPS experiences or the slower the crystallization rate, which allows a deeper coarsening for the non-crystalline phase, the earlier for the appearing of the "black phase". The mechanical for this phase evolution is still under consideration.In Chapter 4, propylene/silica nanocomposites are studied based on the earlier studies on the toughening of polypropylene. We report the structures and properties changes following different annealing time of iPP/hydrophilic silica and iPP/hydrophobic silica nanocomposites. It is found that the properties such as tensile property, Tg and rheology as well as dispersion of the nanoparticles are improved in iPP/hydrophilic silica composite while they show the opposite trend in the iPP/hydrophobic silica composite. This work is not completed yet, and further characterization on this work is still undertaken.Chapter 5 is the conclution of the whole research work.