Molecular structure and properties of salt cross-linked polyethylene under external electric field based on density functional theory
Authors: Li Ya-Sha  , Xie Yun-Long  , Huang Tai-Huan  , Xu Cheng  , Liu Guo-Cheng  ,   College of Electrical Engineering and New Energy, China Three Gorge University, Yichang 443002, China
Published: December 2020

Cross-linked polyethylene is the main power cable insulation material and is widely used in high voltage cables. In order to study the effect of external electric field on the molecular structure of salt cross-linked polyethylene, in this paper we use the basis set of def2-TZVP for Zn atom, uses the basis set of 6-31(d) for C, H, O atoms, and uses the Minnesota density functional (M06-2X) to optimize the molecular structure of salt cross-linked polyethylene, then we obtain the stable structure of its ground state. On this basis, the molecular structure, total energy, kinetic energy, potential energy, dipole moment and polarizability changes of salt cross-linked polyethylene under the action of different external electric fields (from 0 to 0.020 a.u.) are studied by the same method. The influence of external electric field on energy level, energy gap, orbital distribution and composition of frontier orbit are studied. And the effect of external electric field on bond level, breaking bond and infrared spectrum of atoms are also discussed. The research results show that as the external electric field intensity increases, the cross-linked polyethylene molecule is gradually transformed from the spatial network structure into a linear structure, and the total energy and kinetic energy of the molecule are reduced, but its potential energy, dipole moment and polarizability are gradually increased. The highest occupied molecular orbital energy level increases with the increase of external electric field intensity. The lowest unoccupied molecular orbital energy level starts to decrease continuously from the electric field intensity of 0.011 a.u. (1 a.u. = 5:1421011 V/m), the energy gap decreases continuously, and the critical breakdown field intensity is 11.16 GV/m. With the external electric field increasing dramatically, the highest occupied molecular orbital is obviously converged at chain end in the direction of inverse electric field. Its orbital composition is more than 60%, contributed by the C atom of methyl group in the polyethylene terminal. The molecular polyethylene chain end of the inverse electric field direction exhibits an electrophilic reactivity, and C atoms are more likely to lose electrons. The Mayer bond order value of the CC bond decreases gradually, which leads the CC bonds to break more easily, and thus forming the methyl carbon negative ions. The lowest unoccupied molecular orbital moves along the electric field direction and is converged at the other end of polyethylene chain, nearly 80% of its orbital composition is contributed by the methyl of polyethylene chain end. The molecule shows a nucleophilic reactivity at the polyethylene end along the electric field direction, methyl is easier to obtain the electrons. The Mayer bond order value of the CH bond decreases gradually, and it brings about the CH bond more likely to break into H positive ions. The infrared absorption peaks of polyethylene chains are mainly concentrated in the high frequency region. With the increase of electric field intensity, the red shift occurs and the bond energy of polyethylene chain decreases. The infrared absorption peak of the cross-linked salt bridge is mainly concentrated in the low frequency area. Although there are both red shift and blue shift, the effect of red shift is more obvious, and the energy of the whole salt bridge decreases. From the variation of molecular potential energy, energy gap and Mayer bond order value, it is found that the stability of salt cross-linked polyethylene molecular system decreases with the increase of external electric field intensity.