Dielectric behaviour of styrene‐methyl methacrylate copolymer foams at microwave frequencies

Ambient dielectric effects were studied by the short‐circuited waveguide method at 9.375 GHz microwave frequency in foams from copolymers of styrene and methyl methacrylate of varying compositions, generated under identical conditions of solvent‐nonsolvent ratio, impregnation time and steaming period. The results were correlated with foam‐density, porosity and composition and were interpreted in terms of dipole‐dipole interactions and the applicability of Wiener's inequalities, logarithmic law of Lichtenecker and Rother with an empirical factor of 0.64 in the index based on the additive law of mixtures and the Böttcher‐Bruggeman's relation. Scanning electron micrography on copper‐coated fractured surfaces of the samples was done in order to examine the degree of foamability, cell‐size distribution and the mechanism of failure. Microhardness, compressive and flexural strength were also measured. Loss characteristics were similarly ascribed to copolymer foamability and dipoleorientability. A prudent choice in the monomer reactivity ratio might therefore yield an optimum reinforcement and a proper dielectric requirement suitable for electronic and radar device constructions.