Determination of trap energy in polyethylene with different aging status by molecular dynamics and density function theory

In this work, the thermal pyrolysis process and produced chemical defects of crystalline polyethylene, taking the effects of oxygen into account, are explored by molecular dynamics simulation based on ReaxFF force field. The trap energies and sites of various types of chemical defects are calculated by density function theory (DFT), in which hybrid functional XC HSE06 is adopted. The results indicate that double bond C=C, conjugate double bond C=C-C=C, C=C=C and carbonyl C=O are the main products during thermal aging, and these defects are easier to formed at the breaking points of carbon backbone, which is different from the geometries generally used in the previous DFT computations. For electrons, chemical defects C=O and C-C=C-C at breaking points (BP) produce relative deep traps 1.810 and 1.054 eV, and C=C at BP shows shallower traps with energy of 0.445 eV. For holes, C=C-C=C at BP and C=C in the backbone generate deep traps 1.614 and 1.119 eV, and C=C at BP presents shallower traps with energy of 0.689 eV.