Microwave dielectric and mechanical behavior of foams from polystyrene and poly(methyl methacrylate) blends

Abstract

AbstractFoams have been generated from blends of polystyrene and poly(methyl methacrylate) under identical conditions of solvent–nonsolvent ratio, impregnation time, and heating period. Scanning electron micrographs of copper‐coated fractured surfaces of the samples reveal a distinct transition in their structures from a more or less homogeneous cell‐size distribution with prominent grain boundaries to complete lack of foaming and cellularity through random dispersions of spherical poly(methyl methacrylate) inclusions in polystyrene‐foam matrix. Dielectric measurements have been made on test specimens at 9.375 GHz X‐band microwave frequency by the method of Smith and Hippel modified by Dakin and Works. Percent porosity is the least at 20 wt% of polystyrene while foam density linearly decreases with percent porosity. Dielectric constant is linear in foam density on direct and semilog scales (passing through unity and zero, respectively), in blend composition and also in percent porosity. Dielectric constant increases with increase of foam density or weight percent of polymethyl‐methacrylate and decreases with increase of percent porosity. The dielectric constant is, however, nonlinear in volume fraction of polystyrene on both direct and semilog scales, obeying Weiner's inequalities. The logarithmic law of Lichtenecker and Rother with an empirical factor 0.8276 in the index fits the data best. Specific polarization is minimal at 60 wt% of polymethyl methacrylate. The calculated Bottcher‐Bruggleman plot based on the interacting spherical particle model is found to be of limited applicability. This is explained on the basis of long‐range intra‐ and intermolecular dipole–dipole interactions. The pattern of the change of dielectric loss tangent (tan δ), lying in the range 0.150–0.045 and attaining a minimum at 20 wt% polystyrene, has similarly been explained in terms of α and β relaxations due to conformational rearrangements and steric hindrances of rotations. The compressive strength measured as a function of composition shows that the reinforcement depends on an optimization between the degree of cellularity and packing.