Bong Jun Park

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.

Smart monodisperse polystyrene/polyaniline core–shell structured hybrid microspheres fabricated by a controlled releasing technique and their electro-responsive characteristics

We synthesized core/shell structured monodisperse semi-conducting polystyrene/polyaniline (PS/PANI) composite microspheres, and studied their electro-responsive electrorheological (ER) performance under an applied electric field strength when dispersed in silicone oil. An over-layer of the conducting PANI is formed by a controlled releasing process of the aniline pre-adsorbed on the PS seed latex without any surface modification, in which the PS seed microspheres were readily prepared by a dispersion polymerization method. The PS microspheres are observed to be regularly encapsulated with PANI from the morphology test. The ER behaviors of the hybrid microspheres are characterized using a rotational rheometer equipped with a high voltage supplier. The yield stress and flow response as a function of shear rate under certain electric field strengths are investigated using the universal yield stress equation and the Cho–Choi–Jhon model of rheological equation of state, respectively. The relaxation time and achievable polarizability of the fluid from its dielectric analysis were also found to be well correlated with the ER performance of the ER fluid.

Effect of Magnetic Nanoparticle Additive on Characteristics of Magnetorheological Fluid

Magnetorheological (MR) fluids, suspension of magnetic pure carbonyl iron (CI) in non magnetic carrier, were prepared with and without magnetic CI nanoparticle additive in this study. Initially, the magnetic CI nanoparticle additive was synthesized in a rather simple process of decomposition of penta carbonyl iron (Fe(CO)5) using oleyl amine and kerosene. Magnetic property and morphology of the synthesized magnetic CI nanoparticles were confirmed via vibration sample magnetometer (VSM) and transmission electron microscopy (TEM), respectively. MR fluids, prepared as a mixture of pure CI and CI nanoparticle additive of different weight ratio in carrier fluid, was investigated under different external magnetic field strengths via a rotational rheometer. Sedimentation of the MR fluid was characterized by an optical analyzer, Turbiscan. Their flow behaviors at a steady shear mode were examined with and without magnetic CI nanoparticle additive under magnetic field strength. The MR fluids with magnetic CI nanoparticles added demonstrated slightly higher yield behaviors, suggesting that pure CI and CI nanoparticle additive were being oriented in the magnetic field direction under an applied magnetic field and with much strengthened structure.

Polyaniline nanoparticle–carbon nanotube hybrid network vapour sensors with switchable chemo-electrical polarity

Chemo-resistive sensors were prepared from monodisperse poly(aniline) nanoparticles (PaniNP) synthesized via oxidative dispersion polymerization. Poly(styrene sulfonic acid) (PSSA) was used as the stabilizer and dopant agent. PaniNP transducers were assembled by spraying layer by layer a solution containing different concentrations of PaniNP and multi-wall carbon nanotubes (MWNT) onto interdigitated electrodes. This process led to stable sensors with reproducible responses upon chemical cycling. Chemo-electrical properties of these sensors have been investigated in sequential flows of pure nitrogen and nitrogen saturated with a set of volatile organic compounds (VOC). Interestingly the sensing mode of PaniNP transducers (the NVC or PVC effect) can be switched simply by increasing PaniNP content or by the addition of only 0.5% of MWNT to reach a resistance lower than 150 Omega. Due to their original conducting architecture well imaged by atomic force microscopy (AFM), i.e. a double percolated conductive network, PaniNP-MWNT hybrids present both higher sensitivity and selectivity than other formulations, demonstrating a positive synergy. Mechanisms are proposed to describe the original chemo-electrical behaviours of PaniNP-based sensors and explain the origin of their selectivity and sensing principle. These features make them attractive to be integrated in e-noses.

Emulsion Polymerized Polystyrene/Montmorillonite Nanocomposite and its Viscoelastic Characteristics

A novel polystyrene (PS)/clay nanocomposite was synthesized using a simple emulsion polymerization method in the presence of sodium ion exchanged montmorillonite (Na‐MMT). Prior to the radical polymerization procedure with potassium persulfate (KPS) as an initiator, the hydrophobic styrene monomer was intercalated into hydrophilic clay layers using sodium dodecyl sulfate (SDS) as a surfactant. The FTIR spectra of the products showed the characteristic absorbance peaks of both the synthesized PS and Na‐MMT. The x‐ray diffraction patterns of the products exhibited an increase in the d 001‐spacing, pointing to the intercalation of the PS chains into the intergalleries of the Na‐MMT. The enhancement of the thermal properties of the nanocomposite materials, such as the increase in the glass transition temperature of the PS, was investigated by differential scanning calorimetry (DSC). Furthermore, based on the viscoelastic properties of the products examined using a rotational rheometer with a parallel plate geometry, the nanocomposites were found to exhibit more rapid shear thinning and increased storage (G′) and loss (G″) moduli with increasing clay content.

Magnetorheological carbonyl iron particles doubly wrapped with polymer and carbon nanotube

To improve magnetorheological (MR) characteristics of carbonyl iron (CI) microparticles, we introduce two-step CI coating procedures with poly(methyl methacrylate) (PMMA) and multiwalled carbon nanotube (MWNT) for the first time. Core(CI)-shell(PMMA) (CI-PM) structured composite magnetic microbeads were synthesized by dispersion polymerization first, showing improved dispersion stability and sedimentation quality. However, its magnetic strength was decreased compared with that of pristine CI-based MR fluid. To enhance its magnetic strength, the core-shell type CI-PM particles were wrapped with MWNT via ultrasonication. Rheological properties of their MR fluid were measured by a rotational rheometer with a magnetic field supplier. The CI-PMMA-MWNT (CI-PM-NT) particles were found to show better flow and dynamic rheological properties compared to those from CI-PM particles while density of the CI-PM-NT particles was not much different from that of CI-PM, implying that sedimentation stability of the CI-PM-NT ...

Preparation and rheological characteristics of ethylene-vinyl acetate copolymer/organoclay nanocomposites

Ethylene‐vinyl acetate copolymer (EVA) with 40 wt.% vinyl acetate content (EVA40)/organoclay nanocomposites were prepared using a melt intercalation method with several different clay concentrations (2.5, 5.0, 7.5, and 10.0 wt.%). X‐ray diffraction confirmed the formation of exfoliated nanocomposite in all cases with disappearance of the characteristic peak corresponding to the d‐spacing of the pristine organoclay. Transmission electron microscopy studies also showed an exfoliated morphology of the nanocomposites. Morphology and thermal properties of the nanocomposites were further examined by means of scanning electron microscopy (SEM) and thermo gravimetric analysis (TGA), respectively. Rheological properties of the EVA40/organoclay nanocomposites were investigated using a rotational rheometer with parallel‐plate geometry in both steady shear and dynamic modes, demonstrating remarkable differences with the clay contents in comparison to that of pure EVA40 copolymer.

Core-shell typed polymer coated-carbonyl iron suspensions and their magnetorheology

In order to resolve aggregation and sedimentation problem of carbonyl iron (CI) for magnetorheological (MR) fluids, polymer coated composite magnetic particles (CIPMMA) were synthesized via a dispersion polymerization method using CI dispersions in methyl methacrylate monomer, and then adopted the product as a dispersed phase of MR fluids. Especially, to improve mechanical property of the coated poly(methyl methacrylate) (PMMA) onto CI microspheres, a cross-linking method was adopted in this study. Flow and viscoelastic properties of the MR fluids were analyzed via a rotational rheometer equipped with a magnetic field supplier using a parallel plate measuring system.

Miniemulsion fabricated Fe3O4/poly(methyl methacrylate) composite particles and their magnetorheological characteristics

In order to improve drawbacks such as sedimentation of magnetic particles and abrasion of the magnetorheological (MR) fluid, we have fabricated Fe3O4/poly(methyl methacrylate) (PMMA) composite particles via a double miniemulsion method and studied their MR properties. Morphology and chemical composition of the synthesized Fe3O4/PMMA composite particles were investigated by transmission electron microscopy and Fourier transform infrared, respectively. The density of the Fe3O4/PMMA composite particles was measured to be lower than that of the as-synthesized Fe3O4 particles, indicative of an improvement of the composite particles to stay dispersed. Rheological characteristics of the Fe3O4/PMMA based MR fluid dispersed in a nonmagnetic carrier fluid were measured by both static and dynamic tests using a rotational rheometer under an external magnetic field. Shear stress, yield stress, and storage modulus from the rheological measurements were obtained to be increased with applied magnetic field strengths.

Effect of Polymer Encapsulation on Electrophoretic Property of Organic Pigment

Organic pigment has been considered as one of the most suitable materials for the colored electrophoretic ink particles in a microcapsule type electrophoretic display, in which the electrophoretic display is consisted with the electrophoretic ink particles dispersed in the low dielectric medium. In this study, the effect of polymer encapsulation on electrophoretic properties of organic pigment nanoparticles in low dielectric medium was investigated. Blue colored organic pigments were encapsulated with poly(methyl methacrylate) via two step dispersion polymerization of methyl methacrylate with the presence of the organic pigments. Vinyl imidazole was used as a co-monomer to enhance the electrophoretic properties of the pigments. The modified particle surface was characterized by measuring zeta potential in low dielectric medium.