Effects of chemical defects on anisotropic dielectric response of polyethylene

Shogo Fukushima,Fuyuki Shimojo,Priya Vashishta,Subodh Tiwari,Hiroyuki Kumazoe,Aiichiro Nakano,Rajiv K. Kalia

doi:10.1063/1.5093566

Shogo Fukushima, Fuyuki Shimojo + Show 5 more

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Dielectric polymers such as polyethylene (PE) have a wide range of energy and electronic applications. While recent studies have shown significant effects of chemical defects on the electronic structure of PE, those on the dielectric properties remain elusive. Here, first-principles quantum-mechanical calculations show anisotropic dielectric constants of PE, which are sensitive to the type of defects. Specifically, addition of iodine defects increases the high-frequency dielectric constant. Addition of hydroxyl or carboxyl group, on the other hand, causes noticeable anisotropic changes in the static dielectric constant, which is well elucidated through the rotation and concerted motions of chemical groups. The sensitivity of these defects may be exploited to rationally alter the behavior of PE.

Highlights

PE has unusual electronic structures such as negative electron affinity in its crystalline form.11 various processing methods for PE introduce defects such as amorphous region, polymer chain bending, chain end and chemical defects.6 Recent studies have shown important effects of chemical8 and morphological12 defects on the electronic structures of PE.13 These defects may play crucial roles in determining dielectric properties of PE crystal
Energy of a metastable state induced by finite electric field within periodic boundary condition is given by EE[{ψi}] = E0[{ψi}] − ΩE ⋅ P[{ψi}], (1)
Where ∆PE is the change in polarization due to finite electric field for fixed atomic positions and E is the applied electric field

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Introduction

PE has unusual electronic structures such as negative electron affinity in its crystalline form.11 various processing methods for PE introduce defects such as amorphous region, polymer chain bending, chain end and chemical defects.6 Recent studies have shown important effects of chemical8 and morphological12 defects on the electronic structures (e.g., energy levels of electron and hole trap states) of PE.13 These defects may play crucial roles in determining dielectric properties of PE crystal. Static dielectric constant is computed from the high-frequency dielectric constant and change in polarization constant due to atomic relaxation as ε0 = ε∞ + ∆ε. To understand the effects of defects on anisotropic dielectric response, we compare high-frequency and static dielectric constants for all four systems (i.e., PE, PE-I, PE-COOH and PE-OH), where the dielectric constants are computed for three crystallographic directions: [100], [010] and [001].

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