Fei Fei Fang

Magnetorheology: materials and application

As one of the most important field-responsive intelligent and smart soft matter materials, magnetorheological (MR) fluids, consisting of magneto-responsive magnetizable particles suspended in non-magnetic fluids, have drawn a lot of attentions in both academia and industry as their physical and rheological characteristics can be controlled with external magnetic field strength. In this highlight, preparation methods and MR properties of various magnetic composites with soft magnetic particles and polymers are reviewed. In addition, some industrial applications, such as a MR dampers and a MR polishing, are briefly summarized.

Core-shell structured semiconducting PMMA/polyaniline snowman-like anisotropic microparticles and their electrorheology.

Core-shell structured semiconducting snowman-like particles were synthesized, and their electrorheological (ER) characteristics under an applied electric field were examined. Monodispersed snowman-like poly(methyl methacrylate) (PMMA) particles were fabricated previously using a seed emulsion polymerization procedure. These anisotropic particle-based ER fluids, which were tested using a rotational rheometer, exhibited unusual ER properties in the flow curves at various electric field strengths when analyzed using the Cho-Choi-Jhon model. The dielectric spectra, as supporting data for the ER effect, were measured using a LCR meter. The relaxation time of the ER fluid was relatively shorter than typical ER fluids.

Core-shell structured carbonyl iron microspheres prepared via dual-step functionality coatings and their magnetorheological response.

The dispersion stability of soft magnetic carbonyl iron (CI)-based magnetorheological (MR) fluids was improved by applying a unique functional coating composed of a conducting polyaniline layer and a multiwalled carbon nanotube nest to the surfaces of the CI particles via conventional dispersion polymerization, followed by facile solvent casting. The coating morphology and thickness were analyzed by SEM and TEM imaging. Chemical composition of the polyaniline layer was detected by Raman spectroscope, which also confirmed the coating performance successfully. The influence of the functional coating on the magnetic properties was investigated by measuring the MR performance and sedimentation properties using a vibrating sample magnetometer, rotational rheometer, and Turbiscan apparatus. Improved dispersion characteristics of the MR fluid were observed.

Sequential Coating of Magnetic Carbonyliron Particles with Polystyrene and Multiwalled Carbon Nanotubes and Its Effect on Their Magnetorheology

A two-step process for the sequential coating of magnetic carbonyliron (CI) particles with polystyrene (PS) and multiwalled carbon nanotubes (MWCNTs) was used to improve the sedimentation stability of micrometer-sized magnetic CI particles for magnetorheological (MR) applications under an applied magnetic field. The CI particles were initially coated with nanosized PS beads using an in situ dispersion polymerization method and then wrapped with a dense MWCNT nest through a solvent-casting method in a water/oil emulsion system. The morphology, MR performance, and sedimentation stability of the synthesized magnetic composite particles were examined. The composite particles showed enhanced MR characteristics and dispersion stability.

Silica nanoparticle decorated polyaniline nanofiber and its electrorheological response

We fabricated polyaniline nanofibers (PANIFs) with an average diameter of 350 nm through an interfacial polymerization by using dodecylbenzylsulfate acid as a dopant, and then examined their electrorheological (ER) characteristics. Silica modification was carried out in ethanol/water solution via a modified Stober method, and successful location of the silica nanoparticles on the surface of PANIF was confirmed by both scanning electron microscope (SEM) and transmission electron microscope (TEM) images. Thermogravimetric analysis (TGA) showed enhanced thermostability of the modified nanofibers. ER tests including flow curve and amplitude/frequency sweeps, which were performed by a rotational rheometer, demonstrated that the nanosilica modified PANIF exhibits higher ER effect than pure PANIF. In addition, the resultant flow curves are fitted well by Cho–Choi–Jhon model rather than Bingham model, while the yield stresses of both ER fluids versus electric field strengths are fitted well into a single line by the universal yield stress equation.

Well controlled core/shell type polymeric microspheres coated with conducting polyaniline : fabrication and electrorheology

We report the fabrication and electrorheology of encapsulated core/shell structured microspherical particles with crosslinked poly(methyl methacrylate) (PMMA) as a core and conducting polyaniline (PANI) as a shell. Monodisperse PMMA particles were synthesized by dispersion polymerization initially, and then PANI was coated on the surface modified PMMA core via a chemical oxidation method for the application of electrorheological (ER) materials under an applied electric field strength. For the surface modification, the pristine PMMA microspheres were treated with ethylene glycol dimethacrylate, glycidyl methacrylate and oxydianiline in sequence as a crosslinking, swelling and grafting agent, respectively. Through this way the well controlled core/shell structure with a thick PANI layer was achieved. ER performance of these obtained microspheres dispersed in a silicone oil was investigated, in which their yield stress was analyzed based on the universal equation as a function of applied electric strength. Interesting shear stress curves observed under various electric field strengths were also found to be fitted well with the Cho–Choi–Jhon model of the rheological equation of state.

Fabrication of Carbonyl Iron Embedded Polycarbonate Composite Particles and Magnetorheological Characterization

Carbonyl iron (CI)-based magnetorheological (MR) fluid exhibits serious dispersion defect due to large density mismatch between CI particles and continuous medium, which obviously restricts further MR application. Thus, various modifications on CI particles have been explored, among which embedding CI particles within polymer matrix attracted much attention because the density of fabricated polymer/CI particles got considerably low, consequently enhancing the dispersion stability. In this paper, a more thermal stable polymer, polycarbonate (PC) with high strength, toughness, and heat resistance was adopted as polymer matrix, in which CI particles were randomly embedded via a facile solvent casting method. The embedding structure and density were confirmed by SEM/OM images and pycnometer, respectively. Finally, MR characterization was investigated.

Novel Magnetic Composite Particles of Carbonyl Iron Embedded in Polystyrene and Their Magnetorheological Characteristics

Magnetic carbonyl iron (CI) particles have attracted great attention due to their high saturation magnetization and appropriate particle size for magnetorheological (MR) materials. However, hydrodynamic instability of the CI particle suspension such as sedimentation which was attributed to the large density mismatch between the dispersed CI particles and the medium has affected its predominant role for MR applications. In this study, we fabricated novel magnetic CI/polystyrene (CI/PS) composite via a facile solvent casting method and then adopted as dispersed phase of MR fluid. The CI particles were found to be embedded within PS matrix, preventing them contact each other directly, consequently improving stability of the CI suspension in the MR fluid.

Iron oxide/MCM-41 mesoporous nanocomposites and their magnetorheology

Mesoporous nanocomposite materials of magnetic iron oxide-containing MCM-41 (IO/MCM-41) were prepared by simple thermal oxidation of Fe-containing MCM-41 initially prepared by a direct synthesis route using Fe3+ salt. The magnetic saturation of the fabricated nanocomposite materials was measured using a vibrating sample magnetometer, while surface morphology and inner framework of the composite materials were studied using a field emission scanning electron microscope and a transmission electron microscope to confirm their mesoporous nanocomposite formation. The fabricated magnetic materials were then adopted as a magnetorheological (MR) fluid, where the IO/MCM-41 magnetic nanocomposites were dispersed in a nonmagnetic medium oil in addition to as an additive for carbonyl iron-based MR fluid. Their MR properties of flow curve along with yield stress and viscoelastic properties under applied magnetic fields were investigated using a rotational rheometer.

Electroactive response of mesoporous silica and its nanocomposites with conducting polymers

Electroactive response of suspensions of mesoporous silica and its nanocomposites with conducting polyaniline and copolyaniline inside its channels were examined under an applied electric field, mainly focusing on their rheological characteristics. Initially these conducting polymer/mesoporous silica nanocomposites were synthesized and their physical properties were studied by scanning electron microscopy, transmission electron microscopy and N2-adsorption isotherm. Both mesoporous silica and its nanocomposites were then adopted as an electrorheological (ER) material, being dispersed in silicone oil. Typical ER behaviors of shear stress curve as a function of both applied electric field and shear rate were observed for these ER systems. In the absence of an electric field, the suspensions behaved almost like Newtonian fluids. However, when an electric field was applied to the suspension, their shear stresses increased with a shear rate, demonstrating a yield stress. Comparing with ER fluids based on mesoporous silica and polyaniline, the polyaniline/mesoporous silica based ER fluid showed enhanced ER performance due to the anisotropic characteristics of electrical properties of the polyaniline/mesoporous silica particles. In addition, it was found that the shear stress model (Cho-Choi-Jhon model) gave a good representation of the flow curves of these nanocomposite systems.