Electronic structure of polyethylene — Crystalline and amorphous phases of pure polyethylene and their interfaces

For understanding electron and hole transport, it is important to know the electronic structure of the material. The density of states and particularly the energy band gap are two important properties characterizing the electronic structure. With the help of density functional theory the electronic structure of can be calculated. Three different cases are studied: the crystalline phase, the amorphous phases, and crystalline-amorphous interfaces. The density of states, band gaps, and the generals shapes of the electronic states are calculated. Some improvement of the band gap of crystalline polyethylene (PE), from 6.0 to 6.7 eV, by including van der Waals forces, which generally are not treated in density functional theory. The band gap of the amorphous phase was found to be 6.2 eV, and a physically realistic interface has a band gap of 5.9 eV, which is 0.8 eV lower than for the crystalline phase. These results indicate that for semicrystalline materials, an understanding of conduction behaviour must consider three phases: crystalline, amorphous and interface regions. The consequent conjecture that the interface phase with its lowest band gap might provide the main contribution to conduction is in accordance with literature.