Study On Micro-morphology And Mechanical- Electrical Characteristics Of Polyethylene/nanomontmorillonite Composites By Simulation And Experiments

Due to excellent mechanical properties,electrical performance and processing properties,polyethylene(PE)has been widely used in insulation structure of high voltage electrical equipment.The ceaseless expansion of power grids and the continuously improvement of voltage level are happened during the rapid development of economy,so that the high voltage power equipment insulation under high temperature,high field strength of mechanical and electrical performance put forward higher requirements.It is has been evidenced by numerous studies that dielectric properties of polymer doped with nanoparticles have been improved in different degrees.However,influencing factors of nanoparticles are complex,and research results are mainly based on the conclusion of experimental data,so the inner mechanism of dielectric properties improvement is still not very clear,and there is the dispute in key issues.Therefore,it is necessary to further systematically study the influence mechanism of polymer performance.In simulation,the non-polar PE model that is as the research object in the paper is modeled by using the simulation software Materials Studio.The molecular chain segments' movement of amorphous micro-morphology and effects on mechanical properties and breakdown characteristics are discussed in PE under the different temperature,DC pre-electrical field and doping of nano montmorillonite(O-MMT)modified.And experimentally,PE/MMT composites are prepared by melt blending method,simulation results are underpinned by experimental findings that are obtained by means of X-ray diffraction,fourier transform infrared spectrometer,scanning electron microscope,dynamic thermal mechanical analyzer and test of breakdown performance,and so on.It is indicated from simulated results of PE micro-morphology that the phenomena of enhanced disorder movement and loose diffusion of PE molecular are observed due to thermal stress based on thermal exchange principle during raising temperature.These phenomena lead to the intermolecular electrostatic decreasing and the free volume increasing gradually.And then,depending on the principle of energy transfer of electrons in high field,chemical bonds are not damaged by the synergistic influence of high-energy carriers and electrical stress in PE under the condition of 1000 k V/mm DC effective pre-electrical field.At this point,the phenomenon of regional orderly movement of molecular chains is observed.The phenomenon leads to the intermolecular electrostatic increasing decreasing and the free volume decreasing gradually.Meanwhile,under the interaction of temperature and DC pre-electrical field,amorphous micromorphology characteristics of PE show stronger dependence on electrical field,regardless of the molecular chain movement,interaction energy and fractional free volume.The influence mechanism of temperature and electrical field on micro-morphology of non-polar PE is demonstrated under different temperatures and DC pre-electrical fields.The layer spacing of O-MMT approximately equals to 2.4nm testing by Xray diffraction in experiment,which is comparable to that of the simulation model.Test results of scanning electron microscope and Fourier transform infrared spectrometer show that layers of O-MMT are uniformly dispersed in PE compared with montmorillonite unmodified(YT-MMT),and the simulation viewpoint is confirmed by experiments,which the interface interaction area is formed through hydrogen bonding when PE moleculars are inserted into MMT layers,so that MMT layers are dispersed homogeneously in PE/MMT composites.The mechanism of interface bonding between nano MMT layers and PE moleculars is demonstrated.The effect of temperature,pre-electrical field and O-MMT addition on mechanical properties is researched by simulation analyzing micro-morphology change of PE.Simulating results show that mechanical property parameters are reduced with temperature increasing.Nevertheless,the effect of DC preelectrical field is slightly differing from that of temperature.When the DC effective pre-electrical field is 1000 k V/mm,Young's Modulus,Bulk Modulus and Shear Modulus of PE are improved compared to without external electrical field.And then mechanical property parameters of PE/MMT composites are improved obviously when the mass fraction of O-MMT is 4.0%.The calculation model of free volume breakdown theory is modified based on collision ionization theory,Maxwell mechanical model and Doolittle empirical formula,combined with the relevance between fractional free volume and breakdown characteristic.The simulation results of breakdown strength in PE and PE/MMT composite models show that temperature makes the breakdown field of PE decreased and that DC pre-electrical field makes breakdown field increased.When the DC effective pre-electrical field is 1000 k V/mm,breakdown strength of PE is increased by 40% compared to without external pre-electrical field.The effect of micro-morphology evolution rule on breakdown characteristics is analysed,and thus the relationship between micro-morphology and breakdown characteristic is represented.And then breakdown performance of PE/MMT composites is increased by 26% when the mass fraction of O-MMT is 4.0%.The effect of O-MMT content in composites on thickness of interface interaction area is analysed based on the strength of the hydrogen bond between MMT layer and alkane chains,and the influence of interface thickness on breakdown performance in PE/MMT composites is discussed.The consistency of calculation results and experimental results presents that the modified model could be applied to analyse breakdown characteristics in polymer quantification.