High-Performance Magnetorheological Suspensions of Pickering-Emulsion-Polymerized Polystyrene/FeO Particles with Enhanced Stability.

The magnetorheological (MR) performance of suspensions based on core-shell-structured foamed polystyrene (PSF)/FeO particles was investigated by using a vibrating sample magnetometer and a rotational rheometer. Core-shell-structured polystyrene (PS)/FeO was synthesized by using the Pickering-emulsion polymerization method in which FeO nanoparticles were added as a solid surfactant. Foaming the PS core in PS/FeO particles was carried out by using a supercritical carbon dioxide (scCO) fluid. The density was measured by a pycnometer. The densities of PS/FeO and PSF/FeO particles were significantly lowered from that of the pure FeO particle after Pickering-emulsion polymerization and foaming treatment. All tested suspensions displayed similar MR behaviors but different yield strengths. The important parameter that determined the MR performance was not the particle density but rather the surface density of FeO on the PS core surface. The morphology was observed by scanning electron microscopy and transmission electron microscopy. Most FeO particles stayed on the surface of PS/FeO particles, making the surface topology bumpy and rough, which decreased the particle sedimentation velocity. Finally, Turbiscan apparatus was used to examine the sedimentation properties of different particle suspensions. The suspensions of PS/FeO and PSF/FeO showed remarkably improved stability against sedimentation, much better than the bare FeO particle suspension because of the reduced density mismatch between the nanoparticles and the carrier medium as well as the surface topology change.