Dielectric properties of polymers at microwave frequencies: a review

A review of the dielectric loss spectra of polymers at microwave frequencies has been carried out. While the main focus of attention is the frequency range from 100 MHz to 100 GHz, loss spectra outside this region are also reviewed because variations in temperature can cause a shift of dielectric loss into or out of the microwave range. A large volume of data for low loss polymers (polyethylene, polypropylene and poly(tetrafluoroethylene)), which are used in the communications industry, was available for review. Other polymers, for which data were available and which have significant loss at microwave frequencies are: polystyrene, poly(vinylchloride), poly(vinylidene chloride), poly(vinylidene fluoride), poly(methyl-methacrylate), poly(methyl acrylate), poly(oxymethylene), poly(ethylene oxide), poly(propylene oxide), polyacetylene, and poly(sulphur nitride). Also, the microwave dielectric properties of engineering thermoplastics such as poly(phenylene oxide), polycarbonate and polysulphane have been reviewed. The origins of microwave dielectric loss in polymers are categorized as: (a) dipolar absorption dispersions in both crystalline and amorphous polymers; (b) dipolar losses due to impurities, additives or fillers in a polymer material; (c) microwave absorption in conducting polymers (polyacetylene and poly(sulphur nitride)) for which the current carriers are electrons; and (d) photon-phonon absorption spectra corresponding to the density of states in amorphous regions of a polymer material.