| The composition, molecular chain structure, crystallization and phase morphology of different hiPP and PE copolymer were investigated by solvent fractionation, 13C NMR, DSC, TEM, PLM, AFM and SEM to establish the molecular chain structure-property relationships of this material. Some novel results are obtained.The production of high impact polypropylene copolymer (hiPP) is based on reactor granule technology. A sequential polymerization process is usually applied to produce hiPP. First, isotactic polypropylene (iPP) particles with porosity are prepared. These particles are transferred to the next reactor where the elastomeric phase (EPR) is produced within the iPP particles. The end-products exhibit excellent toughness-rigidity balance.The original homopolymerized iPP particle exhibits a multiple structure, i.e., the iPP particle ( ca. 0.6 mm in diameter) is an agglomerate of many subparticles (ca. several to hundred microns in diameter), while the subparticle is in turn formed by a great deal of primary particles (ca. 100 nm in diameter); It is found for the first time that ethylene-propylene copolymer (EPR) phases in polypropylene (iPP) particle produced in the first stage slurry polymerization exhibit a developing process from exterior to interior. During the early stage of ethylene-propylene copolymerization, with lower content of copolymerized ethylene (7.4 mol%), the EPR phases occur only in external layer of the particle, while at the late stage of the copolymerization with higher content of copolymerized ethylene (26.7 mol%), the elastomer phases distribute uniformly in the whole particle. This phenomenon is due to an effect of mass transfer resistance. The origin of mass transfer resistance is loosely agglomerate inclusions of low tacticity polypropylene within the semi-filled micropores inside the iPP particles. It is the inclusions inside the micropores that resist the diffusion of ethylene/propylene comonomers into the particle.It is found that the dispersed phase of hiPP in both the solution-cast films and the bulk exhibits a multilayered core-shell structure, i.e., inner core, intermediate layer and outer shell. The inner core is mainly composed of polyethylene and ethylene-propylene segmented copolymers with shorter sequence length, the intermediate layer is ethylene-propylene random copolymer (EPR), and the outer shell consists of ethylene-propylene block copolymers with longer sequence length. The outer shell could not only increase the stiffness of the dispersed phase, but also be considered a compatibilizing layer to enhance interfacial adhesion between the dispersed phase and the iPP matrix. It is the multiphase morphology that makes the material possess both the high rigidity and the high toughness.The study on morphological structure of melt-drawn films of polyethylene containing small amount of copolymerized component indicates that, in addition to highly oriented lamellae, the melt-drawn films of the polyethylene copolymers contain a large amount of fibrous crystals with average diameter about 12 nm, which are parallel to drawing direction. Simulation experiment (adding 1 wt% ultra-high molecular weight polyethylene into regular high density polyethylene) proves that the formation of the fibrous crystals originates from ultra-high molecular weight component in the polyethylene copolymers. However, this kind of fibrous crystal is different from classical extended chain fibrous crystal. Morphological characteristic of the fibrous crystal of the copolymer should be a crystal-bridged structure with an alternating alignment of crystalline and noncrystalline regions. A model of the crystal-bridged fibrous crystal has been proposed, which not only benefits understanding the mechanism of the oriented crystallization of polymers, but also provides a theory foundation for improving properties of the material.
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