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Structural Evolution And Performance Improvement Of Polymer Multiphase Systems Promoted By Water Molecules In Extruder And Their Mechanism

Posted on:2020-06-04Degree:DoctorType:Dissertation
Country:ChinaCandidate:M WuFull Text:PDF
GTID:1361330620958608Subject:Mechanical engineering
Abstract/Summary:PDF Full Text Request
During polymer melt mixing extrusion,it is important to realize good exfoliation and dispersion of fillers especially nanofillers in polymer matrices for obtaining high-performance polymeric?nano?composites.Water-assisted mixing extrusion?WAME?exhibits the advantages of both common melt mixing extrusion and solution mixing,so polymer blends and polymeric?nano?composites are prepared by WAME equipment in this dissertation.By investigating the effects of injected high-pressure water on the microstructure/phase morphology,rheological properties and macroscopic properties of the polymer multiphase system and the relationship among them,the mechanisms of high-pressure water during extrusion to promote the exfoliation and dispersion of?nano?fillers and the phase morphology evolution of immiscible blends are revealed.Poly?vinylidene fluoride?/halloysite nanotubes?PVDF/HNTs?nanocomposites are prepared by using extrusion with and without water injection.The results show that WAME facilitates to promote the better dispersion of the HNTs in the PVDF matrix,which is mainly attributed to the formation of hydrogen bonds among the fluorine or hydrogen atoms of PVDF molecular chains,the water molecules and the siloxane groups of HNTs.The addition of the HNTs especially their better dispersion induced by injected water is beneficial to increasing the crystallinities and the fractions of?-phase crystal?F???s?in the PVDF/HNTs nanocomposites?the F???reaches 0.689 with adding 10 wt%HNTs?.Better HNTs dispersion and higher crystallinities and F???s improve the Young's moduli,tensile strengths and elongations at break for the PVDF/HNTs nanocomposites.It is deduced from morphology observations on the fractured surfaces that cracks initially form at the vicinity of HNTs aggregates and the thinner brittle fracture edge region for the nanocomposites fabricated without and with water injection,respectively.This is attributed to the important roles of the PVDF crystallization behavior and HNTs dispersion on the formation of the voids,wedges and ridges,which are the origins for forming cracks and easily formed at region with more HNTs aggregates or lower crystallinity.The improved dispersion of the HNTs results in lower elongational viscosities and higher draw ratios at break of the PVDF/HNTs nanocomposites prepared with water injection.Poly?methyl methacrylate?/expanded graphite?PMMA/EG?composites are prepared using WAME.The results show that the EG exhibits better exfoliation and dispersion in the composites prepared by WAME than that in those without water injection,which facilitate to form more EG layer networks and strengthen the PMMA-EG interfacial interaction,and so improve the thermal conductivity?0.57 W/mK with adding 7 wt%EG?and tensile properties of the composites.The mechanism for promoted EG exfoliation and dispersion in the PMMA matrix during WAME is analyzed by the synergistic effect of the high-pressure water and the shear force provided by the extruder screw.The effects of high-pressure water on the phase morphology of the immiscible PMMA/polystyrene?PS?blends prepared by WAME are investigated.The results show that high-pressure water facilitates to improve the compatibility of the PMMA/PS blends,reducing the dispersed PS phase size and its distribution width and enhancing the anti-creep and tensile properties of the blends.The results of the high-pressure rheological test show that the plasticizing effect of water molecules is stronger than the thickening effect of the pressure for the PMMA/PS blend melt.The PMMA/PS?60/40 w/w?/graphene oxide?GO?nanocomposites are prepared by using WAME.The results show that the synergistic effect of the injected water and the screw shear force not only favors the better dispersion of the GO,but also promotes the GO reduction to obtain the reduced GO?rGO,the value of C/O increases from 2.87 for GO to7.94?and the rGO to locate at the interface of PMMA and PS phases.The rGO at the interface is effective in compatibilizing the immiscible PMMA/PS blend and stabilizing the fine co-continuous phase structure via forming the hydrogen bonds and?-?stackings with PMMA and PS,respectively;moreover,it facilitates to form massive thermal conductive pathways.The nanocomposites with higher reduction degree of the GO exhibit higher thermal conductivity and stability?when adding 0.8 wt%GO,the thermal conductivity and the temperature of heat-resistance index reaches 0.74 W/mK and 179.3C,respectively?.Using the nanocomposite with0.8 wt%GO as a thermal interface material of LED lamp,the lamp surface termperature is reduced by about 10C.The PMMA/PS?60/40 w/w?/GO nanocomposites prepared by WAME are foamed by using batch foaming,and the effect of GO content on the cell structure and performance of the foamed nanocomposites is investigated.The results show that with the increase of GO content,the cell density increases in the PMMA phase,and the average cell diameter reduces and the cell density first increases and then decreases in the PS phase.Smaller cell in the PS phase improves the compression properties of the foamed nanocomposite.When adding higher contents of the GO,the foamed nanocomposites exhibit higher conductivity(for example,the conductivity reaches 1.210-4 S/m with adding 0.8 wt%GO)than the unfoamed nanocomposite,which is mainly ascribed to the fact that the small cell in the continuous PS phase is favorable for the rGO sheets to form more interconnected conductive networks.The foamed PMMA/PS/rGO nanocomposite exhibits better infrared absorption performance and heat-insulating effect,which can be used as the heat-insulating material of the building.
Keywords/Search Tags:Water-assisted mixing extrusion, polymer multiphase system, microstructure/phase morphology, property, water molecule action mechanism
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