Lorentz-type negative permittivity of flexible ethylene–vinyl acetate/copper composites below the percolation threshold

Abstract

Polymer composites for flexible electronics and wearable devices are evolving to harness tunable negative permittivity, a key attribute for metamaterial design. However, while metal fillers boost negative permittivity, they can overshoot desired values and compromise the material’s flexibility, making the optimal filler balance a critical unresolved challenge in achieving integrated functionality. In this study, we address a critical research gap by demonstrating that weak negative permittivity can be achieved in polymer composite films using low metal content, below the percolation threshold, while retaining the polymer’s high elasticity and thermal stability. Ethylene–vinyl acetate (EVA)/copper (Cu) composite films were fabricated at Cu loadings of (0, 10, 20, 30, and 40 wt.% of EVA) through a multilayer casting-assisted hot pressing process. Characterization using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) confirmed the effective preparation of the composites. Additionally, thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) revealed that incorporating small amounts of Cu preserves EVA’s thermal resistance and flexibility. Dielectric properties were examined across a broad frequency range: from 0.1 Hz to 10 MHz using a broadband dielectric spectrometer, and from 20 MHz to 3 GHz using an impedance analyzer. While all samples maintained low dielectric losses and weak frequency dependence at low frequencies, the 40 wt.% (≈6 vol.%) Cu-loaded sample exhibited weak negative permittivity (up to –12 at 2 GHz) that was well captured by the Lorentz model. The findings of this research revealed that the synthesized flexible composites exhibit advanced electromagnetic properties.