Investigations On The Crystallization Behavior And Structure-property Relationship Of Polypropylene-based Blends

Polypropylene (PP) has attracted attention widely for its good comprehensive properties, while the poor toughness largely limits its applications. Blending is one of the most widely used toughening method, polymer blends has been one of the most important method for developing new polymer materials, hence receiving more and more attention. Most of the polymer blends are incompatible or partially compatible, the phase separation would induce complex phase morphology of the blends, and the phase structure of the blends has important effect on various physical properties of polymer materials. We are proposed to investigate the crystallization behavior and the structure-property relationship of PP-based blends, and obtain theoretical and practical guidance for controllably achieving better rigid-toughness balance of PP.This thesis is focused on the PP-based polymer blends. By introducing different blending components and changing the processing methods, we have investigated the crystallization behavior and the structure-property relationship of PP-based blends, and obtained methods for preparing high-performance PP-based materials. This thesis contains six chapters.Chapter1is the introduction of the backgrounds for the whole work and some basic theories involved in our work.In Chapter2, mechanical tests demonstrated that the introduction of HDPE and further annealing treatment have significant synergistic toughening effect on PP. Characterization of microstructure evolution of PP with and without HDPE upon annealing reveals that the introduction of HDPE prominently reduces the spheralite size of PP. After annealing at130℃, a portion of chain segments in mobile amorphous fraction (MAF) are rearranged into rigid amorphous fraction (RAF) and crystalline phase to form more compacted RAF, looser assembled MAF region, and thickened lamellae, which would promote the cavitation process during deformation. For HDPE-blend PP, the rearrangement of chain segments is more prominent at the interface. As the result, more crystalline lamellae and chain segments in RAF act as crosslink points at the interface, hence enhancing the interface. Our study reveals that the dramatically improved toughness of PP blended with HDPE after annealing is largely attributed to the smaller spherulites of PP, the formation of more cavaties during deformation, and the enhanced interface in PP/HDPE blend.In Chapter3, we have investigated the dispersion and crystallization behavior of olefn block copolymer (OBC) in different PP matrix. The α-and β-PP based PP/OBC blends were prepared by introducing a-and P-nucleating agents (NA) or not introducing any NA. SEM observations revealed much finer dispersion of OBC in P-PP matrix than in α-PP matrix. The crystallization temperature of OBC in β-PP matrix is prominently lower than that in neat OBC, i.e., the crystallization of OBC is severely inhibited. By contrast, the crystallization of OBC in α-PP matrix is not inhibited. In this chapter, we found the obvious effect of the crystalline modification of PP matrix on the crystallization behavior of dispersion phase for the first time. Besides, the finer dispersion of OBC also leads to much higher toughness for β-PP/OBC blend. The mechanisms for difference in dispersion size and crystallization behavior of OBC were proposed based on the various characterizations. As the crystallization rate of PP in10OBC-b is between that in10OBC and10OBC-a, the effect of crystallization rate of PP on dispersion size of OBC can be eliminated. According to the SEM observations, the looser arranged interfibrillar region for P-PP matrix is capable of accommodating OBC hard blocks, while the more compactly arranged crystalline structure for a-PP makes it difficult for OBC hard blocks to insert into the interfibrillar region. Therefore, we inferred that the different dispersion size of OBC is mainly related with the discrepancy in crystalline structure of α-and β-PP matrix. The smaller dispersion size of OBC would probably lead to inhibited crystallization of OBC by largely reducing the number of OBC nucleating points for each micro-domain. For β-PP based PP/OBC blends, we also found that the crystallization behavior of OBC is related with the weight ratio of P-PP/OBC. Higher weight ratio values would lead to inhibited crystallization for larger proportion of OBC, vice versa. Deep understanding in the dispersion state and crystallization behavior of OBC in different PP matrix has been obtained in this work, which may be meaningful for achieving the required structure and mechanical properties of PP/OBC blends.In Chapter4, we have studied the preferable selective distribution of β-NA in PP/polycarbonate (PC) blend. It is founded that the introduction of PC induces the preferable selective distribution of P-NA in PC phase for PP/PC blends, and subsequently prohibits the nucleating effect of β-NA on PP matrix. We observed the dynamic selective distribution process, which is largely affected by the viscosity of the blending components. The increased viscosity of PC at lower blending temperature would prohibit the selective distribution of β-NA. The introduction of compatibilizer, maleic anhydride grafted polypropylene (MAPP), has succeeded in getting more β-NA capable of nucleating PP matrix in PP/PC blends, because PC domains get much smaller, correspondingly the area of interface gets larger, and far more β-NA distributes in the interface.Chapter5is the conclusion of the whole research work.On the basis of the investigations on the crystallization behavior and the structure-property relationship of PP-based blends, our work has provided theoretical and practical guidance for controllably achieving better rigid-toughness balance of PP.