Investigation of the Dielectric Properties of PVDF, SKF32, PVA, and PS525 Thermoplastics in the Microwave Frequency Range

Abstract

Experimental studies aimed at determining the frequency dependence of the permittivity of thermoplastics in the microwave frequency range are of considerable interest due to their use as matrices for particle-filled polymer composites. The aim of this study was to determine the processing parameters required to produce monolithic specimens of several widely used thermoplastics and to measure the frequency dependence of their complex dielectric permittivity over the frequency range from 100 MHz to 5 GHz. The investigated materials included polyvinylidene fluoride (PVDF), poly(trifluorochloroethylene-vinylidene fluoride) fluoroelastomer (SKF32), polyvinyl alcohol (PVA), and polystyrene (PS525). The specimens were fabricated by hot pressing using an experimental setup with individually optimized holding temperature, applied pressure, and holding time to obtain monolithic products free from visually observable porosity and internal defects. The frequency spectra of the complex dielectric permittivity were measured at room temperature using the Nicholson–Ross–Weir method with a vector network analyzer. The experiments revealed that the hot-pressing conditions differed significantly among the investigated thermoplastics. During the fabrication of polyvinyl alcohol specimens, yellowing was observed, presumably associated with the initial stages of thermal degradation of the material. Within the frequency range of 100 MHz to 5 GHz, the real part of the dielectric permittivity (ε′) ranged from 2.57 to 3.31 for polyvinylidene fluoride, from 2.40 to 3.16 for the fluoroelastomer, and from 3.61 to 4.63 for polyvinyl alcohol. For polystyrene, the value of ε′ remained approximately constant at 2.64 throughout the investigated frequency range. All polymers except polystyrene exhibited pronounced frequency dispersion of the complex dielectric permittivity as well as dielectric losses in the range of 0.05–0.20. In addition, the calculated reflection coefficient from a metal-backed plate ranged from −4 to −2.5 dB for thermoplastics exhibiting significant dielectric losses. The obtained frequency dependences expand the available data on the dielectric characteristics of widely used thermoplastics in the microwave frequency range and may be useful for the development of polymer composite materials with tailored dielectric properties for applications in electronics, microwave engineering, and electromagnetic wave-absorbing materials.