Study On The Crystallization And Structure-Property Relationship Of High Performance Polyolefins

Polyolefins. as represented by polypropylene (PP) and polyethylene (PE). are the most widely used commodity plastics in the world. PP and PE have been paid continuous attentions because of their diverse applications, superior properties and comparatively low costs. Polyolefin has a long history of over 60 years. Nevertheless, with the invention of new catalysts and improved polymerization technique, the development of polyolefin has been keeping a fast and steady pace these years. Many chemical companies have turned their interests back to the research and development of PE and PP. In the past few decades, the new catalysts and controllable polymerization have enabled the tailoring of specific structure in polyolefins. As a result, producing high performance polyolefins has been realized. However, it should be noted that besides the preparation techniques, a fully understanding of the relationship between chain structure, condensed structure and macroscopic property of polyolefin is also necessary and even more important for its high performance improvement.Most polyolefins are semicrystalline polymers. Hence, the research of polyolefin is closely related to various aspects of polymer crystallization, and comprehensive knowledge in polymer physics is also required to construct the correlation between structure and property of polyolefins. In the present dissertation, the chain structures, crystalline structures, phase structures of several high performance polyolefins including bimodal high density polyethylene (HDPE), impact-resistant polypropylene copolymer (IPC), short-chain branched polyethylene and?-modification PP have been well characterized. Combined with their macroscopic properties under theses specific microstructures, the new knowledge about the structure-property relationship of these polyolefins has been acquired. The innovations of our studies presented in this dissertation lie in the following aspects:(1) We proposed a new method to simultaneously improve the crystallization rate and melt intensity of bimodal HDPE, which can be used to slow down the sagging problem during pipe molding to some extent; (2) Based on the research of shear-enhanced crystallization phenomenon in IPC, new information about the composition, chain structure and phase structure of IPC material has been obtained; (3) A new method has been utilized to investigate the structure-property relationship in polyolefin by capturing and characterizing the specific condensed structure and corresponding property during annealing:(4) Innovative thermal treatment and preparation method have been used to clarify the difference in HDT property of a and?PP; (5) A new strategy, which utilized the microphase separation feature of block copolymer and the crystallization behavior of small molecules, for preparing polymer-based shape memory elastomer was proposed.By introducing a small amount of ultra-high molecular weight components into PE100 bimodal HDPE. we have simultaneously improved the crystallization rate and melt intensity of this material. The sagging problem during the extrusion of PE100 is hopefully to be retarded to some extent by our modification method. In the research of the crystallization behavior of IPC. we investigated the shear-enhanced crystallization (SEC) behavior in IPC under various shearing conditions by using nonisothermal crystallization analysis, isothermal crystallization kinetics, Hoffman-Lauritzen theory and morphological observation. It was found that the crystallization peak of IPC after sufficient melt shearing could be elevated by about 10?, which indicated a crystallization rate 10 times faster than the as-received sample. Kinetics analysis showed that the transition temperature between regime II and regime III in IPC has been shifted after shearing, and the fold surface free energy decreased from 116.5 erg/cm2 to 76.6 erg/cm2. SEC in IPC has a strong memory effect, which can only be erased under annealing temperature close to the equilibrium melting point of IPC. By correlating the composition, chain structure, phase structure and crystallization behavior of IPC, we finally revealed the underlying mechanism of SEC and its relaxation behavior. Also, we have testified the rationality of the mechanism by extra rheological experiment.In the research of microstructure and heat distortion temperature (HDT) property relationship of polyolefins. we first paid attention to the microstructure evolution of two PE with different molecular topologies during annealing treatment. By capturing and characterizing the specific condensed structure and corresponding property during annealing, we found that HDT of both linear PE and branched PE have been improved by annealing. With regard to linear PE, the elevation of HDT resulted from the elevation of crystallinity. However, for branched PE, the crystallinity is no longer the only factor influencing the HDT property. Multi-scaled structure rearrangements have been observed during the annealing of branched PE. On the one hand, annealing improved its crystallinity. On the other hand, annealing also restricted the relaxation of crystalline components. The results explained the reason of diverse change in the HDT of branched PE during annealing. Another research on HDT related to the isotactic polypropylene (iPP) with different crystalline modification. Through stepwise crystallization procedure and rheometer-based preparation method, we have successfully prepared both nearly pure a-PP and?-PP. Meanwhile, with the help of precise self-nucleation treatment, we eliminated the difference in spherical size between a-PP and?-PP. For the first time, we clearly show that, given the condition of similar crystallization kinetics, crystallinity and spherical size, the HDT of a-PP appears to be higher than that of?-PP. The conclusion clarified a long-existed misunderstanding of the HDT property of P-PP.At last, a new strategy, which utilized the microphase separation feature of block copolymer and crystallization of small molecules, for preparing polymer-based shape memory elastomer was proposed. According to the strategy, a kind of shape memory elastomer comprising SEBS and paraffin was prepared. The microphase structure, thermal property, mechanical property and shape memory behavior of SEBS/paraffin composite were characterized. Also, the principle of preparation and shape memory mechanism of SEBS/paraffin has been concluded.