Pure and doped crystalline polyethylene under Earth’s conditions: A density functional theory study

Polyethylene is a low-cost plastic widely used in daily applications, but its resistance to chemical processes contributes to prolonged degradation. The accumulation of plastic waste harms humans, animals, and the environment. In this study, we performed first-principles calculations based on Density Functional Theory to explore the structural and electronic properties of crystalline polyethylene under varying temperature and pressure conditions, corresponding to different layers of the Earth’s surface and interior. These calculations were performed with and without doping using naturally abundant molecules, H2, N2, and O2. As pressure and temperature increased toward the Earth’s core, the lattice notably shrank with decreasing lattice constants. This compression was accompanied by the spreading of the electronic bands and the narrowing of the bandgap, eventually transforming the material into a conductor near the Earth’s core. Doping the material had different effects on its atomic and electronic structures. While H2 had a slight impact on the lattice constants and the band structure, N2 and O2 perturbed them more significantly. Notably, doping with N2 under ambient conditions reduced the bandgap from 6.6 (for an insulator) to 3.4 eV, transforming it into a semiconductor. We hope that our findings will contribute to the development of materials with enhanced degradation or modified properties for alternative applications.