Abstract：Modification of polymers by nano-doping, regulation of the dispersion state of nanofillers in polymers, in order to improve the macroscopic properties of polymers have become hot research topics in the field of insulation. The crosslinking process of polymers also changes the state of nanoparticle dispersion and affects the bonding of the phase interface between nanoparticles and polymer matrix, resulting in differences in macroscopic properties. Montmorillonite (MMT) with its large length-to-thickness ratio can form a stable and solid interfacial zone with the polymer matrix, bind the movement of the molecular chain and block the charge, which can effectively improve the mechanical and electrical properties. Therefore, organic montmorillonite (OMMT) with a mass fraction of 0.5% was selected to modify PE, and cross-linked polyethylene/organic montmorillonite (XLPE/OMMT) nanocomposite with different crosslinking degrees were prepared by regulating the crosslinking time. The X-ray diffraction (XRD) test was used to determine the intercalation dispersion state of montmorillonite and the changes in the aggregated state structure of the specimens, and the scanning electron microscope (SEM) results were combined to analyze the changes of the crystalline morphology. The effects of the crosslinking degree on the tensile properties of XLPE/OMMT were analyzed in terms of plastic deformation and elastic deformation. The crosslinking degree of the dielectric properties of XLPE/OMMT was analyzed by using the conductance-temperature characteristics, dielectric constant and dielectric loss tangent test of the sample and the two-parameter Weibull distribution of power frequency breakdown field strength, combined with the crosslinking mesh structure and the interfacial properties of OMMT. The results of XRD show that OMMT reaches the exfoliated state in the XLPE/OMMT under the action of PE-g-MAH, and the crystallinity of the specimen decreases continuously with the increase of crosslinking time. The results from the gel content test show that the crosslinking degree of XLPE/OMMT increases and then decreases with the increase of crosslinking time, and reaches the maximum at 15~20min. It is concluded that XLPE/OMMT is in the state of "undercrosslinking" before crosslinking for 15min, "positive crosslinking" at 15~20min, and "over-crosslinking" after 20min. It is confirmed by SEM that the crosslinked network inhibits the crystal growth and the crystal size decreases with the increase of crosslinking time. Under the positive crosslinked state, the crystal size distribution is most uniform. The joint action of OMMT and crosslinked bonds improves the tensile strength, and the perfect crosslinked network increases the elastic modulus and enhances the toughness. The electrical conductivity of the specimens increased with temperature. The crosslinked network and the homogeneously distributed interfacial region increase the potential barrier and improve the activation energy. The increase of crosslinking degree forms a stronger intermolecular force, which hinders the turning polarization and decreases the dielectric constant and dielectric loss angle tangent; the addition of OMMT and the perfection of crosslinking structure together improve the breakdown field strength; however, excessive crosslinking increases the microscopic defects and decreases the breakdown field strength. The following conclusions can be drawn from the analysis of the experimental results: (1) The crosslinking degree of nanocomposites affects the uniformity of crystal size and crystallinity. However, the deterioration of dispersion of OMMT leads to an increase of crystal size difference during over-crosslinking. (2) The combination of crosslinked bonds, three-dimensional mesh structure and OMMT effectively improves the tensile strength and enhances the toughness and elastic modulus of the material. (3) The electrical conductivity of nanocomposites is closely related to its crosslinking degree and the interfacial force formed by OMMT and the matrix. In the positive crosslinking state, the combined effects of the crosslinked bonds and interfacial results in the decrease of dielectric constant and dielectric loss tangent of the composites. The integrity of the microstructure of the composite promoted by proper crosslinking and the barrier effect of OMMT improve the electrical strength of the composites.
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2023, Vol. 38 
