Dielectric behavior of polystyrene foam at microwave frequency

Abstract

Abstract Polystyrene foams have been generated and fabricated into differently shaped structures', by change of steaming period under constant impregnation time and solvent‐nonsolvent composition. Optical photomicrographs of samples both plain and wax‐copper‐coated reveal uniform appearance, distinct grain‐boundaries, and random cell size distribution. Dielectric measurements have been made on test specimens cut according to wave‐guide size at 9.375 GHz X‐band microwave frequency by short‐circuited wave‐guide method of Smith and Hippel modified by Dakin and Works. Dielectric constants are linear, on direct and semi‐log scales in bulk‐density and volume‐fraction, obeying Weiner's inequalities. Formulae of Landau‐Lifshitz, Beer, Maxwell‐Wagner, Odelevsky, etc. have been tried. Data fit best with the logarithmic law of Lichtenecker and Rother. Specific polarization is also a true function of density. Dielectric constant vs bulk‐density plots of foams resemble dielectric‐constant vs fractional‐density plots based on the theoretical derivation by Smith for polystyrene compacts, signifying that compacts containing closely‐spaced oblong‐spherical particles arc physically similar to foams having spherical gas inclusions in plastic structures. Tan δ lying in the range 0.002–0.0038 results from conformational polarization (β‐relaxation at room‐temperature for wide‐angle torsional oscillations of side‐groups with co‐operative motion from wriggling chains). It is therefore possible for low‐loss foam dielectrics suitable for micro‐wave applications to be made by this method.