Morphology and mechanical properties of polypropylene/poly (propylene-1-octene) in-reactor alloys prepared by Metallocene/Ziegler-Natta hybrid catalyst

摘要

A series of isotactic polypropylene/poly(propylene-1-octene) (iPP/PPOc) in-reactor alloys were synthesized by a one-step polymerization process, using Metallocene/Ziegler-Natta hybrid catalyst. The alloys were characterized by FT-IR, DSC, optical microscopy and SEM. The results suggested that the spherical morphology was maintained during one-step polymerization process, which provided a potential application for one-step polyolefin in-reactor alloys. A characteristic "shell-core" structure of the nascent alloy particles was observed for the first time. This phenomenon may be due to the difference between the homopolymerization and copolymerization rate at different active centers. It was also found that the majority of the elastomers in the matrix were homogeneously distributed in the alloys. The introduction of the relatively long 1-octene branches could effectively reduce the crystal size and the crystallinity of the obtained iPP/PPOc alloys and made it possible to vary their rigidity and elasticity in a wide range. The crystallization kinetics of the alloys with pure iPP was also investigated. With the increase of elastomer content, an increase of nucleation density (the nuclei number per unit area) and the decrease of crystal perfection could be clearly observed. In comparison with pure PP, the overall crystallization rates and the growth rates of the spherulites of the alloys decreased obviously. These results indicated that the growth rate of the spherulites was the decisive step for the overall crystallization rate in this case, which can be explained on the basis of dilution effect and obstruction effect on the mobility of PP chains in the propylene-octene copolymer. Investigation of the mechanical properties indicated that notched Izod impact strength of iPP/PPOc alloys have obviously increased in comparison with that of pure iPP. The improvement of impact strength can be mainly attributed to the increase of random copolymer content. Based on the understanding of microstructure and phase morphology, the correlation between morphological structure and mechanical properties has been established.