Molecular modeling and electron transport in polyethylene

Abstract

Polyethylene is commonly used as an insulator for AC power cables. However it is known to undergo chemical and physical change which can lead to dielectric breakdown. Despite almost eighty years of experimental characterization of its electrical properties, very little is known about the details of the electrical behaviour of this material at the molecular level. An understanding of the mechanisms of charge trapping and transport could help in the development of materials with better insulating properties required for the next generation of high voltage AC and DC cables. Molecular simulation techniques provide a unique tool with which to study dielectric processes at the atomic and electronic level. Here we summarise simulation methodologies which have been used to study the properties of PE at the molecular level, elucidating the role of morphology in the trapping of excess electrons. We find that polyethylene has localised states due to conformational trapping extending below the mobility edge (above which the excess electrons are delocalised), at -0.1±0.1eV with respect to the vacuum level. These trap states with localisation lengths between 0.3 and 1.2nm have energies as low as -0.4±0.1eV in the amorphous and interfacial regions of polyethylene with more positive values in lamella structures. Crystalline regions have a mobility edge at +0.46 ±0.1eV, so we would expect transport by electrons excited above the mobility edge to delocalised states to be predominantly through amorphous regions if they percolate the sample.

Functional design of epoxy-based networks: tailoring advanced dielectrics for next-generation energy systems

“…In case of polyethylene, where charge transport dynamics through disordered molecular conformations has been studied in details, the presence of localized regions of free volume has been shown to be an important factor in electronic conduction. [40][41][42] Finally, the presence of GHE modifies the viscosity of the reacting system prior to vitrification; while the measured viscosity at 20 °C of the neat epoxy was 0.266±(4x10 -4 ) Pa.s, this figure was reduced to 0.101±(4x10 -4 ) Pa.s for 30GHE. As such, the presence of GHE would be expected to increase somewhat the efficiency of cross-linking, thereby affecting the residual unreacted end groups that remain within the network, another factor that has been shown to be important in determining the electrical behaviour of epoxy resins.…”

“…Second, the introduction of the GHE modifier produces a network system containing alkyl branches which, based upon the observed reduction in T g , leads to increased free volume. In case of polyethylene, where charge transport dynamics through disordered molecular conformations has been studied in details, the presence of localized regions of free volume has been shown to be an important factor in electronic conduction [40][41][42]. Finally, the presence of GHE modifies the viscosity of the reacting system prior to vitrification; while the measured viscosity at 20 °C of the neat epoxy was 0.266 ± (4 × 10 −4 ) Pa.s, this figure was reduced to 0.101 ± (4 × 10 −4 ) Pa.s for 30GHE.…”

A model and simulation of fast space charge pulses in polymers

“…Direct measurements and simulations show that polyethylene has multi-levels of traps (identified as chemical and physical in their nature) which limit the charge transport [20,21]. The physical traps are formed by the disorder of the chain structure and local morphology [21,22]. Those traps together control the apparent charge mobility [20][21][22].…”

“…The physical traps are formed by the disorder of the chain structure and local morphology [21,22]. Those traps together control the apparent charge mobility [20][21][22]. The electric field assisted chain movement is regarded as opening traps down to a certain depth.…”