Crystallization Behavior And Electrical Properties Of SiO₂/MMT/LDPE Micro-nano Composites Under Stretch Processing

The electrical properties for semi-crystalline polymers such as polyethylene are closely related to their crystallinity,spherulite size and the stability of internal interface structure.However,when polyethylene is stretched,the crystal structure will be destroyed or recrystallized,which will lead to the changes of crystallinity,grain size,molecular chain conformation and other microstructure,then affect the electrical properties.When inorganic particles are added to the polyethylene matrix,the internal molecular chain arrangement and crystallization behavior will be changed.If the types for doped particles are different,there may be a synergistic effect when the particles interact with the matrix due to the differences in particle shape,size,type and number of functional groups,and the thickness of the surface layer,which will make the aggregation structure for the composite system complicated under the stretching action.Therefore,it is necessary to discuss the microstructure effect of polyethylene and its composites from the perspective of stretching,and to reveal the microscopic mechanism for changes of electrical properties in composites under stretching.Low-density polyethylene（LDPE）was chosen as basis material,nano Si O₂particles（treated with coupling agent）and KMMT particles（the montmorillonite particles treated with intercalation agent and coupling agent）were used as filler.Si O₂/LDPE,KMMT/LDPE,Si O₂-KMMT/LDPE and KMMT-Si O₂/LDPE composites were manufactured by melting blend method.Among them,the Si O₂-KMMT/LDPE and KMMT-Si O₂/LDPE composites changed the adding order of particles in the preparation process.The test method of SEM,PLM,AFM,XRD and DSC were used to characterize the crystal morphology,crystallite size,crystallinity and crystallization kinetic parameters in LDPE and its composites.The results show that the addition of Si O₂and KMMT particles can improve the crystallization behavior for matrix LDPE,increase the crystallinity and refine the grain size.The heterogeneous nucleation effect of Si O₂particles is excellent,while that of KMMT particles is relatively weak,and its unique lamellar structure will hinder the growth of spherulite.Both Si O₂-KMMT/LDPE and KMMT-Si O₂/LDPE composites can form spherulite structure with uniform size and compact structure.And the KMMT particles added later in Si O₂-KMMT/LDPE are densely distributed in the matrix and the crystal structure is relatively loose,while the small-sized Si O₂particles added later in KMMT-Si O₂/LDPE can fill the structure formed by KMMT particles,and the crystal structure is relatively compact and the crystal size is tiny.The crystallization behavior for LDPE and various composites was studied under different stretching ratios of 5%,10%and 20%,different stretching time of120h,240h and 360h,and heat stretching treatment at 50℃.The test method of PLM,DSC and XRD were used to characterize the crystallization behavior transformation in composites under different tensile loading.The results show that there exists a competition effect of crystallization structure destruction and recrystallization perfection in LDPE and each composite under tensile action.In the study on different stretching ratios,with the increase of the ratio,the crystallinity for LDPE material with loose structure gradually increases,and the crystal structure becomes compact and perfect.On the other hand,Si O₂/LDPE,KMMT/LDPE,Si O₂-KMMT/LDPE and KMMT-Si O₂/LDPE composites show obvious damage to crystal structure.Among them,KMMT/LDPE has serious crystal structure destruction when subjected to 5%and 10%stretching ratios,while Si O₂/LDPE can keep part of the grain structure from being damaged during tensile process.At 20%stretching ratio,the crystallinity for each composite material is improved,the crystal structure is compact,and the spherulite grows perfectly.Under the action of long-term stretching and heat stretching,the dynamic change process for crystal structure destruction and recrystallization perfection constantly occurs in LDPE and various composites,from the experimental results,heating can obviously promote recrystallization.Based on the transformation of crystallization behavior for each material during stretching process,different physical mechanisms for crystallization reconstruction under stretching force and heat stretching force are proposed,and the evolution models of crystallization structure for different materials under stretching action are constructed.The pulse electro-acoustic method was used to study the space-charge distribution characteristics for LDPE and composites after pre-applied field and short-circuit under different stretching ratios of 5%,10%and 20%,different stretching time of 120h,240h and 360h,and heat stretching at 50℃.By calculating the residual space charge and apparent mobility for each material in short circuit,the influence of different stretching effects on the trap state inside LDPE and each composite was compared and analyzed.The results show that the trap energy level for each material becomes shallow,and the internal space charge accumulation is reduced when materials are stretched at 20%ratio.In long-term stretching,LDPE,Si O₂/LDPE,Si O₂-KMMT/LDPE and KMMT-Si O₂/LDPE are stretched for 360h,KMMT/LDPE is stretched for 240h,with the lowest apparent mobility,residual average charge density and space charge accumulation.Under the action of heat stretching,the apparent mobility,residual average charge density and space charge accumulation for LDPE,KMMT-Si O₂/LDPE at 120h,Si O₂/LDPE at 72h and KMMT/LDPE at 96h are the lowest.Based on the experimental results for crystal morphology,trap state and space charge of each material,the model for charge transport and trap capture during stretching is proposed,and mechanization for changes of space charge characteristics in various material under different stretching effects is analyzed.The characteristics of broadband dielectric spectrum,AC breakdown and conductance current for LDPE and its composites under different stretching ratios of 5%,10%and 20%,different stretching time of 120h,240h and 360h,and heating stretching at 50℃were studied experimentally.The results show that with the increase of stretching ratio,permittivity and loss for LDPE and its composites first increase and then decrease.At 20%ratio,the dielectric constant and loss for each material decrease obviously,and the breakdown field strength increases.When LDPE and its composites are recrystallized perfectly under long-term stretching and heat stretching,the dielectric constant and loss for them are significantly reduced,the breakdown field strength is greatly improved,and the carrier mobility,trap density and conductance current are obviously reduced.Based on the experimental results for dielectric properties of various materials,the change of crystallization behavior and polymer trap theory,the trap-regulated carrier transport model is proposed when materials are recrystallized perfectly under stretching and the crystal structure is destroyed.