| High-Impact Polypropylene(HIPP)is widely used in our daily life,mainly due to its excellent mechanical properties,and its excellent mechanical properties are derived from the complex internal micro-morphology and chemical composition.The core phase of rubber particles in HIPP has been widely recognized as composed of crystalline PE.While recent literature has reported that the hard core phase in rubber particles can be composed mainly of crystalline PP,we know little about this new structure and several important questions remain unresolved,such as how this structure is formed,what role the copolymer components play in this process,and whether the PP hard core is more advantageous than the PE hard core in the mechanical properties of the alloy.These questions are crucial for improving the properties of the next generation HIPP.Based on Atomic Force Microscopy-Infrared(AFM-IR)and Local Thermal Analysis(LTA)techniques,this paper studied two kinds of HIPP samples with essentially the same rubber content and different ethylene content.And the distribution of block copolymers in the core-shell structure of HIPP was studied in combination with the properties of different grades obtained by TREF technology.The reasons for their different mechanical properties of the two samples were explained.First,we tested the macroscopic chemical composition of the two samples by FT-IR and used SEM and AFM to characterize the microscopic morphology of the samples.Then the chemical composition of each phase region of the core-shell structure in the HIPP was analyzed by AFM-IR technique.The PE content of each phase region was calculated quantitatively by the standard curve established in the previous literature.It was found that the main components of the hard core of the two samples are PP,and the content was approximately the same,while the PE content of the HIPP-2 in the rubber phase is significantly higher than that in the HIPP-1.Then,we propose two models for the distribution of block copolymers in the core-shell structure of the two samples in combination with properties of different chains.For HIPP-1,similar to the conjecture model proposed in previous literature,the hard core is mainly composed of short block copolymers(EsP),the middle is rubber phase(EPR),and the outer layer is surrounded by a shell composed of long block copolymers(EbP).Whereas for the HIPP-2,hypothesis model differs from previously reported,the hard core phase is composed of block copolymers with crystalline PP,while the shell layer has no evidence of its presence.Combining the experimental results with the theoretical model of two HIPP toughening modification reported in previous literature,this study provides experimental evidence for the first time that the PP core structure in HIPP core-shell particles can form a suitable copolymer structure and composition,which contributes to the stiffness and toughness equilibrium of the HIPP.Next,we performed Local Thermal Analysis(LTA)tests on the samples.The results show that there are both PE and PP crystalline phases in the HIPP-2 core phase,which verified the reliability of the proposed model.Moreover,the proposed model explains the difference of toughness energy between the two samples from the point of view of block copolymer distribution.To sum up,the distribution of block copolymers in HIPP in core-shell structure was studied by means of microscopy,AFM-IR,LTA combined with the properties of each chain segment of the sample.In HIPP-1,the hard core phase consists of EsP,where ethylene can crystallize,and the ethylene-propylene block copolymer(EbP)distributes outside the rubber particles to form a shell,showing a typical HIPP morphology.The core in HIPP-2 comprises a substantial amount of PP homopolymer in addition to EsP and EbP,which leads to PP crystallinity in the core.This is an unusual structure that contributes to HIPP rigid-toughness balance,which is one of the reasons why HIPP-2 has higher flexural modulus(1100 MPa)than HIPP-1(953MPa). |
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