基于密度泛函理论的外电场下盐交联聚乙烯分子的结构及其特性

交联聚乙烯是主要的高压电缆绝缘材料.为了研究外电场对盐交联分子结构的影响,本文对Zn原子使用def2-TZVP基组, C, H, O原子使用6-31G(d)基组,运用明尼苏达密度泛函(M06-2X)对交联聚乙烯分子进行优化得到了它的稳定结构.并研究了不同外电场(0—0.020 a.u., 1 a.u.=5.142×10~(11)V/m)作用下盐交联聚乙烯分子结构和能量变化,外电场对前线轨道的能级和成分的影响,原子之间的键级、断键和红光光谱的变化.研究结果表明:随着电场的增大,交联聚乙烯分子从空间网状结构逐渐变成线性结构,总能量降低,但势能增大,偶极矩和极化率升高,交联聚乙烯分子的稳定性随着电场的增大而降低;最高占据轨道能级持续增大,最低空轨道能级从0.011 a.u.电场开始持续降低,能隙持续降低,临界击穿场强为11.16 GV/m;沿电场方向聚乙烯链端表现出亲核反应活性,它的C—C键更容易断裂,形成甲基碳负离子,逆电场方向聚乙烯链端表现出亲电反应活性,它的C—H键更容易断裂形成H正离子;分子红外光谱高频区吸收峰明显红移,低频区吸收峰既有红移又有蓝移.

Molecular structure and properties of salt cross-linked polyethylene under external electric field based on density functional theory

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. = 5:142×10¹¹ 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 C–C bond decreases gradually, which leads the C–C 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 C–H bond decreases gradually, and it brings about the C–H 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.