Hyung-Seok Lim

Monodisperse conducting colloidal dipoles with symmetric dimer structure for enhancing electrorheology properties

This study introduces an electrorheological (ER) approach that allows us to obtain remarkably enhanced ER properties by using monodisperse colloidal dimer particles. Two sets of colloidal particles, which are spheres and symmetric dimers, were synthesized employing the seeded polymerization technique. The aspect ratio of dimer particles was ~1.43. Then, the surface of the particles was coated with polyaniline by using the chemically oxidative polymerization method. After preparation of the particle suspensions having the same particle volume and concentration, their ER behavior was investigated with changing the electric field strength. At the same experimental condition, both shear stress and shear yield stress of the dimer particle suspension remarkably increased, compared with those of the spherical particle suspension. This attributes to the fact that the shape anisotropy of suspending particles effectively led to increase in the dipole moment under the electric field, thus resulting in formation of a well-structured colloidal chains between the electrodes.

The Effects of Particle Conductivity on the Electrorheological Properties of Functionalized MCNT-Coated Doublet-Shaped Anisotropic Microspheres

AbstractThis study describes the effect of conductivity on the electrorheological (ER) behavior of anisotropic particle suspensions. Anisotropic microparticles were prepared by two-step seeded polymerization. To control the particle conductivity, functionalized multi-wall carbon nanotubes were coated on the particle surface via a layer-by-layer (LBL) method. Particle conductivity increased with layer numbers and the yield stress of the fluids increased with increasing electric field strength. However, the yield stress decreased when suspended particle conductivity increased. Although high conductivity allowed high particle polarization ability, field-induced ER fluid viscoelastic properties decreased.

Rattle type α-Fe2O3 submicron spheres with a thin carbon layer for lithium-ion battery anodes

A facile method for the synthesis of rattle type α-Fe2O3 submicron spheres with a thin carbon layer using a hydrogel template is described in this paper. The high surface area, internal structure and thin carbon layer contributed to the improved cyclability of a lithium ion battery at high current rates.

One-Pot Template-Free Synthesis of Monodisperse Hollow Hydrogel Microspheres and their Resulting Properties

Monodisperse poly(methacrylic acid/ethyleneglycoldimethacrylate) (MAA/EGDMA) hollow microcapsules, which exhibit pH-responsive behavior, are prepared by diffusion of cationic surfactants and hydrophobic interaction. During the association of the negatively charged hydrogel microspheres and an oppositely charged surfactant (cetyltrimethylammonium bromide, CTA(+)B), the hydrophobic polymer-surfactant complexes that form are separated from the internal water; consequently, a hollow structure can be formed. Confocal laser scanning microscopy, UV spectro-scopy and zeta potential are employed to study the formation of the hollow structure during the diffusion of the cationic surfactant. The controlled release behavior of methylene blue as a model drug from the as-prepared poly(MAA/EGDMA) microcapsules with a hollow structure is investigated under different pH conditions. The hollow structure can be retained, even during repetitive pH changes.

Facile synthesis of monodisperse poly(MAA/EGDMA)/Fe3O4 hydrogel microspheres with hollow structures for drug delivery systems: the hollow structure formation mechanism and effects of various metal ions on structural changes

This study presents a facile fabrication method for monodisperse poly(methacrylic acid/ethylene glycol dimethacrylate)/Fe3O4 [poly(MAA/EGDMA)/Fe3O4] composite microcapsules with magnetic properties and hollow structures for use as a targeted drug delivery system. In aqueous solution, the iron ions diffuse into the negatively-charged spherical polymer particles by electrostatic attraction. Since the obtained polymer–metal complexes decrease the repulsive force between the negatively-charged carboxyl groups of the polymer chains, the chelating parts of the polymer particles lose their hydrophilic properties and become relatively hydrophobic. As the number of iron ions continues to increase in the polymer particles, an internal cavity begins to be formed by interaction between the hydrophobic domains and the inner polymer chains. After the reducing agent is added into the synthetic system, Fe3O4 nanoparticles formed in the shells of the polymer particles retain the hollow structure. Confocal laser scanning microscopy and zeta potential measurements are employed to confirm the formation of the hollow structure during the diffusion of the various metal ions into the polymer gel. The magnetic properties of the poly(MAA/EGDMA)/Fe3O4 composite microcapsules synthesized under diverse experimental conditions are characterized by a vibrating sample magnetometer. The controlled release behavior of the model drug doxorubicin hydrochloride from the polymer–metal hollow composite microcapsules is investigated under different pH conditions.

Carbon-doped ZnO submicron spheres functionalized with carboxylate groups and effect of dispersion stability in the colloidal system for high photocatalytic activity

In this work, we propose a facile fabrication method for C-doped zinc oxide (ZnO) submicron spheres using ionizable surface groups without an additional surface modification process after heat treatment. The synthetic method is comprised of diffusion of zinc precursors into pH-responsive polymer microspheres and a subsequent heat treatment step. After heat treatment at a certain temperature, the ionizable functional groups remaining on the surface of the C-doped ZnO submicron spheres resulted in improved dispersion stability of the spheres in the fluid colloidal system. Using the photocatalytic activity test, the relationship between dispersion stability and the photocatalytic property of the C-doped ZnO submicron spheres was investigated. As a result, the content of carbon atoms and the surface functional groups depended on the pH of the reaction medium.

Highly monodisperse magnetite/carbon composite microspheres with a mesoporous structure as high-performance lithium-ion battery anodes

In this study, we propose a fabrication method for highly monodisperse magnetite/carbon (Fe3O4/C) composite microspheres with a mesoporous structure. Highly monodisperse porous polystyrene (PS) microspheres are synthesized by traditional seeded polymerization. Textural properties of porous PS microspheres can be controlled by using different amounts of diluent. In order to carbonize and introduce ferrous (Fe2+) and ferric (Fe3+) ions into the pores of PS microspheres, the sulfonation reaction is carried out using sulfuric acid. Fe3O4 nanocrystals are formed in the pores as well as on the surface of the sulfonated porous PS microspheres by a simple wet chemical method. The obtained mesoporous structure of the Fe3O4/C composite microspheres is still retained after a heat treatment in a nitrogen atmosphere. The homogeneous distribution of Fe3O4 nanocrystals in the porous carbon matrix was analyzed through elemental mapping by preparing cross-sections using focused ion beam scanning electron microscopy milling. When the composite electrodes are tested as an anode material in a Li-ion half-cell, the mesoporous Fe3O4/C composite microspheres exhibit not only a high reversible capacity of 562 mA h g−1 after 100 cycles at 1 C but also good capacity retention at various current rates (0.1–10 C) with a high coulombic efficiency of above 99%.

A facile template-free synthesis of pH-responsive polyelectrolyte/amorphous TiO2 composite hollow microcapsules for photocatalysis

This paper presents a novel, facile method for fabricating pH-responsive inorganic/organic composite microspheres with hollow structures in the absence of a step for the removal of the core. Using polyelectrolyte (hydrogel) hollow microspheres without a need for further calcination and chemical etching, minimally cross-linked poly(methacrylic acid/ethylene glycol dimethacrylate/3-(trimethoxysilyl) propylmethacrylate), poly(MAA/EGDMA/TMSPM) [pMET] microspheres were prepared by a distillation–precipitation method. Once the pMET microspheres disperse in 2-propanol, the hollow structure is clearly observed and can be maintained more rigidly in a dry state after amorphous TiO2 (a-TiO2) is incorporated into the shell of a microsphere by a sol–gel method. The a-TiO2-incorporated pH-responsive pMET composite [pMET/TiO2] hollow microspheres were tested as a photocatalyst in acidic and neutral conditions. The photocatalytic performance of the hollow pMET/TiO2 composite microcapsules under neutral condition was better than that under acidic conditions due to their large active surface and strong adsorption ability against methylene blue (MB).

Simple fabrication and electrochemical performance of porous and double-shelled macroporous CuO nanomaterials with a thin carbon layer

In this work, different types of CuO particles with a thin carbon layer were fabricated and tested as anode materials for lithium ion batteries. The pH-responsive hydrogel microspheres are utilized as a template for morphological and structural changes of composite particles. The structure of CuO particles and a thin carbon layer have the effect to improve the electrochemical performances of the lithium ion cells.

Multiwalled carbon nanotube/SiO2 composite nanofibres prepared by electrospinning

Carbon nanotube (CNT)/SiO2 composite nanofibres were fabricated by electrospinning treated calcination under nitrogen atmosphere. To obtain CNT/SiO2 composite nanofibres, functionalised CNTs (fCNTs) were prepared by chemical oxidation and mixed with tetraethyl orthosilicate : H2O : H3PO4 : poly(vinyl alcohol) solutions. The resulting solutions were electrospun under adjusted parameters and the properties of the composite nanofibre solution were measured. The dispersity and direction of CNTs in the composite nanofibres were also monitored. By using fCNTs, the CNTs were well dispersed in CNT/SiO2 composite nanofibres by improving the dispersity of CNTs in the TEOS sol-gel/PVA solution. fCNTs can therefore be used to enhance the dispersity and direction of CNTs in SiO2 composite nanofibres prepared by electrospinning.