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Morphology Controlling And Properties Of Polyolefin Blends Obtained Via Shear Stress Field

Posted on:2005-07-28Degree:DoctorType:Dissertation
Country:ChinaCandidate:Y WangFull Text:PDF
GTID:1101360152455426Subject:Polymer Chemistry and Physics
Abstract/Summary:PDF Full Text Request
The morphology controlling of polymer blends during the process is one of thehottest research topics in polymer science and engineering at present. The aim ofmorphology controlling is to achieve the desired structure via special ways thusimprove the properties of polymer blends. In this work, polyolefin and their blendswere cooled and solidified under the shear stress condition via the introduction ofshear stress field during the injection process. The morphology, miscibility and phaseseparation, phase inversion, orientation and crystallization of polymer blends weregreatly affected under the effect of shear stress fields. The alteration of morphologywas kept by the way of solidification of melt blends. And then, the correlationbetween the morphology and the mechanical properties could be established.The main results are: 1.The effect of shear stress on anisotropic of injection-molded samples was studied.After the melt was injected into the mold, the specimen was forced to moverepeatedly in a chamber by two pistons that moved reversibly with the samefrequency as the solidification progressively occurred from the mold wall to themolding core part. During the cooling of the sample, three different structures wereformed, namely, the skin, the shear layer and the core. The further experimentssuggested that the morphologies among the three layers were different. 2. The study of the polyolefin (such as HDPE, LDPE, LLDPE and PP) and theirblends (such as HDPE/LLDPE, HDPE/LLDPE and PP/LLDPE) suggested that thetensile strength, the elastic module and the Izod notched impact strength could beimproved at the different degree, depending on the molecular architecture and phasemorphology. And the super polyolefin blends, which had higher tensile properties 5四川大学博士学位论文and higher impact properties, were obtained. 2d-WAXD was adopted to study theeffect of shear stress on orientation of the polymers and their blends. The resultsshowed that different polymer had the different orientation behavior in the sameshear condition, and the orientation of HDPE and PP was easier than that of LLDPEand LDPE. On the other hand, in the blends, the effect of dispersed phase on theorientation of matrix was different. In HDPE/LDPE blends, the degree of orientationof HDPE was decreased with the increasing of LDPE content, while in PP/LLDPEblends, the degree of orientation of PP was increased firstly and then was decreasedwith the increasing of LLDPE content. In PP/EC blends, the orientation ability of PPwas improved greatly when a few EC was added into PP. Further experimentsshowed that the tensile strength was related to the area of shear layer, which furtherproved that the orientation of polymers is a key to improve the mechanical properties.On the other hand, the impact strength was related to the shear layer and the corelayer. The skin and the core showed a typical ductile-fracture mode since the muchcraze was formed and some materials were pulled out during the fracture propagation.While for the shear layer, the fractured surface was very smooth, which is the typicalcharacter of brittle-fracture mode. 3. The effect of shear stress on crystallization and lamellar orientation was studiedin the work. The results of DSC, SEM and 2d-SAXD proved that different crystalstructures were formed in the different zones of the prepared samples. Much moreshish-kebab structure was formed in HDPE and the mechanical properties wereproved greatly. While in PP, shish structure formed in the skin, shish-kebab structureformed in the shear layer and kebab structure formed in the core zone. But inPP/LLDPE blends, PP formed shish-kebab structure in each different zones, while avery unique crystal morphology and lamellar orientation of LLDPE were obtained,with the lamellar stack oriented either perpendicular or 45-500 away from the shearflow direction. And the further study suggested that shish structure formed in th...
Keywords/Search Tags:Polyolefin blends, Shear stress, Super polyolefin blends, Orientation, Crystallization, Ductile-brittle transition, Miscibility, Phase separation, Phase inversion
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