Microscopic calculation of the energetics of ions in polyethylene

Abstract

Polarization energies are calculated for a point charge ion in amorphous polyethylene using local segment and non-local distributed molecular polarizabilities with material structures simulated by both general-purpose and specialist Monte Carlo software. An expression is derived for the polarization energy of a molecular system divided into submolecules that may acquire both induced dipoles and induced charges in the presence of external charges. The two descriptions of the molecular response lead to results that generally differ only because of the polarizability difference, so that less computationally demanding local polarizabilities may suffice, with results for distributed response estimated by suitable scaling. The polarization energy is correlated with the molecular packing around the ion and its position in the polymer material through fluctuations of the polymer density. The polarization energy distribution from the general-purpose software has a range of some ±0.6 eV about a maximum at −1 eV and from the specialist software has a range of some ±0.5 eV centered about a maximum at −1.5 eV, both for a density of 0.8 g cm−3. Relaxation of the sample after inserting the ion lowers the polarization energy by typically 0.5 eV and broadens the distribution. The results for polyethylene as a test system suggest that the methodology may be applied to more complicated systems, such as practical polymer electrolytes.