Coarse-Grained Model for Prediction of Hole Mobility in Polyethylene

Abstract

Electrical conductivity measurements of polyethylene indicate that the semicrystalline structure and morphology influence the conductivity. To include this effect in atomistic charge transport simulations, models that explicitly or implicitly take morphology into account are required. In the literature, charge transport simulations of amorphous polyethylene have been successfully performed using short oligomers to represent the polymer. However, a more realistic representation of the polymer structure is desired, requiring the development of fast and efficient charge transport algorithms that can handle large molecular systems through coarse-graining. Here, such a model for charge transport simulations in polyethylene is presented. Quantum chemistry calculations were used to define six segmentation rules on how to divide a polymer chain into shorter segments representing localized molecular orbitals. Applying the rules to amorphous systems yields distributions of segments with mode and median segment lengths relatively close to the persistence length of polyethylene. In an initial test, the segments of an amorphous polyethylene were used as hopping sites in kinetic Monte Carlo (KMC) simulations, which yielded simulated hole mobilities that were within the experimental range. The activation energy of the simulated system was lower compared to the experimental values reported in the literature. A conclusion may be that the experimental result can only be explained by a model containing chemical defects that generate deep traps.

Analytical Model for the Molecular Ionization Energy in an External Electric Field

“…Distance R max is plotted against electric field E in Figure , and we note that the distance for most cases is considerably longer than the intermolecular distances in condensed phases. Because free electrons are very reactive, it is likely that electron transfer processes between molecules take place rather than a full dissociation process, for example, as has been modeled for polymers. , …”

“…Because free electrons are very reactive, it is likely that electron transfer processes between molecules take place rather than a full dissociation process, for example, as has been modeled for polymers. 29,30 The contribution in eq 8 from the difference in the dipole moment between the cation and the molecule, Δμ, is illustrated in Figure 3. For example, a value of Δμ of ∼15 D is required to give a contribution of −1 eV to the ionization energy at an electric field of 30 MV/cm.…”

“…Because free electrons are very reactive, it is likely that electron transfer processes between molecules take place rather than a full dissociation process, for example, as has been modeled for polymers. 29 , 30 …”