Kookheon Char - Academia.edu (original) (raw)

Papers by Kookheon Char

Reactive and Functional Polymers, 2009

We demonstrate a simple methodology to incorporate interacting magnetic nanoparticles (mNPs) into... more We demonstrate a simple methodology to incorporate interacting magnetic nanoparticles (mNPs) into cylinder forming block copolymer templates. Poly(styrene-block-isoprene) (PS-b-PI) with PI cylinders and poly(styrene-block-4vinylpyridine) (PS-b-P4VP) with PS cylinders were used as the block copolymer templates and c-Fe 2 O 3 NPs coated with oleic acids were pre-synthesized for the interacting mNPs.

Polymer Degradation and Stability, 2002

The thermal degradation behavior of polyaniline (PANI) in PANI/Na + -montmorillonite (Na + -MMT) ... more The thermal degradation behavior of polyaniline (PANI) in PANI/Na + -montmorillonite (Na + -MMT) nanocomposites prepared by in-situ intercalative polymerization of aniline into Na + -MMT has been investigated by thermogravimetric analysis (TGA) and X-ray diffraction (XRD). The residual weight (TG curves) and its weight derivative (DTG curves) of the nanocomposites suggest that the PANI chains for PANI/Na + -MMT nanocomposites are more thermally stable than those for a simple PANI/Na + -MMT mixture. This improvement in the thermal stability for the nanocomposites is attributed to the presence of Na + -MMT nanolayers with a high aspect ratio acting as barriers, thus shielding the degradation of PANI in the nanogalleries and also hindering the diffusion of degraded PANI from the nanocomposites. The shielding effect of the nanolayers is found to be significant as the Na + -MMT content in the PANI/Na + -MMT nanocomposites is increased. The XRD patterns of the nanocomposites after TGA measurements indicate that the basal spacing (d 001 ) of the PANI/Na + -MMT nanocomposites is almost intact, implying that the thermal decomposition of the PANI chains is believed to occur mainly outside the silicate layers. #

We report the behavior of dewetting and layer inversion process of inverted poly(2-vinlypyridine)... more We report the behavior of dewetting and layer inversion process of inverted poly(2-vinlypyridine)/polystyrene (PVP/PS) bilayer thin films, which were prepared by spin coating a PVP layer on top of the PS layer bearing silicon wafer, using optical microscopy (OM) and atomic force microscopy (AFM). In comparison with previous data on the bilayer film with similar thicknesses and molecular weights, two

The stability of polystyrene (PS) films on silicon oxide surface was improved by adding a symmetr... more The stability of polystyrene (PS) films on silicon oxide surface was improved by adding a symmetric deuterium-labeled polystyrene- block-poly(2-vinylpyridine) (dPS-b-PVP). Above 3 wt% loading of dPS-b-PVP in the blend thin films, autophobic dewetting, however, has occurred. The autophobic dewetting of PS thin films was investigated by optical microscopy (OM) and atomic force microscopy (AFM). Since the PVP block interacts with

Nano Letters, 2015

Thin-film ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths below 400 nm ar... more Thin-film ultraviolet (UV) light-emitting diodes (LEDs) with emission wavelengths below 400 nm are emerging as promising light sources for various purposes, from our daily lives to industrial applications. However, current thin-film UV-emitting devices radiate not only UV light but also visible light. Here, we introduce genuine UV-emitting colloidal nanocrystal quantum dot (NQD) LEDs (QLEDs) using precisely controlled NQDs consisting of a 2.5-nm-sized CdZnS ternary core and a ZnS shell. The effective core size is further reduced during the shell growth via the atomic diffusion of interior Cd atoms to the exterior ZnS shell, compensating for the photoluminescence red shift. This design enables us to develop CdZnS@ZnS UV QLEDs with pure UV emission and minimal parasitic peaks. The irradiance is as high as 2.0-13.9 mW cm(-2) at the peak wavelengths of 377-390 nm, several orders of magnitude higher than that of other thin-film UV LEDs.

Macromolecular rapid communications, Jan 30, 2015

Many natural materials are complex composites whose mechanical properties are often outstanding c... more Many natural materials are complex composites whose mechanical properties are often outstanding considering the weak constituents from which they are assembled. Nacre, made of inorganic (CaCO3 ) and organic constituents, is a textbook example because of its strength and toughness, which are related to its hierarchical structure and its well-defined organic-inorganic interface. Emulating the construction principles of nacre using simple inorganic materials and polymers is essential for understanding how chemical composition and structure determine biomaterial functions. A hard multilayered nanocomposite is assembled based on alternating layers of TiO2 nanoparticles and a 3-hydroxy-tyramine (DOPA) substituted polymer (DOPA-polymer), strongly cemented together by chelation through infiltration of the polymer into the TiO2 mesocrystal. With a Young's modulus of 17.5 ± 2.5 GPa and a hardness of 1.1 ± 0.3 GPa the resulting material exhibits high resistance against elastic as well as p...

Macromolecular rapid communications, Jan 7, 2015

Sulfur-rich materials have recently attracted keen interest for their potentials in optical, elec... more Sulfur-rich materials have recently attracted keen interest for their potentials in optical, electrochemical, and pesticidal applications as well as their utility in dynamic covalent bond chemistry. Many sulfur-rich polymers, however, are insoluble and processing methods are therefore very limited. The synthesis and characterization of water-dispersible polymer nanoparticles (NPs) with the sulfur content exceeding 75% by weight, obtained from the interfacial polymerization between 1,2,3-trichloropropane and sodium polysulfide in water is reported here. The interfacial polymerization yields well-defined sulfur-rich NPs in the presence of surfactants, which are capable of serving a dual role as a phase transfer catalyst on top of emulsifiers. Such dual role allows for the control of the product NP size by varying its concentration. The surfactants can be easily removed by centrifugation and redispersion in water is also reported here. The resulting sulfur-rich NPs are characterized th...

In this study, we present a unique approach to form uniform monolayers of nanoparticles (NPs) dep... more In this study, we present a unique approach to form uniform monolayers of nanoparticles (NPs) deposited on a substrate with high surface coverage density up to 2x10^12 NPs/cm^2. This was achieved through the wet coating of fine NPs with a diameter less than 10 nm. The mechanism of monolayer formation was decomposed into two stages: first the deposition driven by

Ž . Ž . Multilayer electroluminescence EL devices based on the self-assembled layers of poly p-ph... more Ž . Ž . Multilayer electroluminescence EL devices based on the self-assembled layers of poly p-phenylene vinylene PPV and Ž . Ž . poly sodium 4-styrenesulfonate PSS were prepared and characterized. The increase of recombination region for the radiative decay of polaron excitons in the film device was achieved by two different fabrication methods: increase of the number of bilayers and increase of ionic strength of PSS solution using a salt additive. When simply increasing the number of bilayers, the relative device efficiency was improved proportional to the number of bilayers deposited. In contrast, the addition of NaCl salt to the PSS dipping solution resulted in higher thickness deposition but exhibited low luminescence level as well as low quantum efficiency. We believe that the decrease of the quantum efficiency, despite the increased thickness in the case of added salt in the PSS solution, is caused by the defect formation at the interface between PPVrPSS film and Al electrode. In order to minimize this Ž . Ž . problem, we inserted one insulating bilayer composed of cationic poly allylamine hydrochloride PAH and anionic Ž . Ž . poly methacrylic acid PMA in direct contact with the Al electrode. Upon this insertion, significant improvement of device efficiency was achieved and the quantum efficiency was shown to be proportional to the total film thickness even in the case of the salt addition scheme. ᮊ

Macromolecular rapid communications, Jan 11, 2015

Emission wavelength control in fluorescent nanoparticles (NPs) is crucial for their applications.... more Emission wavelength control in fluorescent nanoparticles (NPs) is crucial for their applications. In the case of inorganic quantum dots or dye-impregnated silica NPs, such a control is readily achieved by changing the size of the particles or choosing appropriate fluorescent dyes, respectively. A similar modular approach for controlling the emission wavelength of fluo-rescent polymer NPs, however, is difficult. This article reports on fluorescent polymer NPs, the synthesis of which provides a platform for a modular approach towards the preparation of fluorescent NPs of desired emission wavelength. Atom-transfer radical polymerization (ATRP) is employed to synthesize reactive ester polymers, which are then easily modified with a commercially available dye and subsequently subjected to nanoprecipitation. The resulting NPs, with low size polydispersity, show an enhanced emission quantum yield when compared with the same dye molecules in solution.

Block copolymers are well known to spontaneously form a wide range of nanostructures. In thin fil... more Block copolymers are well known to spontaneously form a wide range of nanostructures. In thin films, they are additionally affected by the presence of a substrate and a free surface. This often leads to the morphology oriented in the direction parallel to the substrate. However, for the fabrication of functional nanostructures, the perpendicular orientation is preferred. In this study, we

Nonionic poly(ethylene oxide) surfactants or triblock poly-(ethylene oxide)-poly(propylene oxide)... more Nonionic poly(ethylene oxide) surfactants or triblock poly-(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) copolymers as structure directing templates have been widely used for preparing mesoporous silicas such as SBA-15, SBA-16, and MSU-X. However, the organic modifications of the mesoporous silica framework and the efforts to improve weak hydrothermal stability of mesoporous materials have not been not so successful. In present study, we

International Journal of Nanomedicine, 2014

Drug Delivery and Translational Research, 2012

The promise of cellular therapy lies in healing damaged tissues and organs in vivo as well as gen... more The promise of cellular therapy lies in healing damaged tissues and organs in vivo as well as generating tissue constructs in vitro for subsequent transplantation. Adult stem cells are ideally suited for cellular therapies due to their pulripotency and the ease with which they can be cultured on novel functionalized substrates. Creating environments to control and successively driving their differentiation toward a lineage of choice is one of the most important challenges of current cell-based engineering strategies. In recent years, a variety of biomedical platforms have been prepared for stem cell cultures, primarily to provide efficient delivery of growth or survival factors to cells and a conducive microenvironment for their growth. Here, we demonstrate that repeating tetralayer structures composed of biocompatible poly(methacrylic acid) (PMAA)/poly(acryl amide) (PAAm)/poly(methacrylic acid) (PMAA)/poly(ethylene oxide)-block-poly(ε-caprolactone) (PEO-b-PCL) micelles arrayed in layer-by-layer (LbL) films can serve as a payload region for dexamethasone (dex) delivery to human mesenchymal stem cells (MSCs). This architecture can induce MSC differentiation into osteoblasts in a dose-dependent manner. The amount of dex loaded in the films is controlled by varying the deposition conditions and the film thickness. Furthermore, release of dex is also controlled by changing the amount of covalent crosslinking of multilayers via thermal treatments. The multilayer architecture including payload and cell-adhesion region introduced here are well suited for extended cell culture thus affording the important and protective effect of both dex release and immobilization. These films may find applications in the local delivery of immobilized therapeutics for biomedical applications, as they can be deposited on a wide range of substrates with different shapes, sizes, and composition.

The adsorption process of the spin self-assembly (SA) method applied to the present work is mainl... more The adsorption process of the spin self-assembly (SA) method applied to the present work is mainly driven by both the viscous force by fast solvent elimination and the electrostatic interactions between oppositely charged species. On the other hand, the centrifugal and air shear forces applied by the spinning process significantly enhance the desorption of weakly adsorbed polyelectrolyte chains and also induce the planarization of the adsorbed polyelectrolyte layer. By combination of four different forces mentioned above, the adsorbed film thickness per bilayer was shown to be easily controlled from about 5 ?to 40 ?by varying the spinning speed (W) and the mole concentration of polyelectrolytes. The surface of multilayer films prepared with the spinning process is quite homogeneous and smooth compared with the films prepared with the conventional layer-by-layer deposition, indicating a surface roughness of 3 5 ? as evidenced from X-ray reflectivity and atomic force microscopy (AFM).

Thin Solid Films, 2002

ABSTRACT

Polymer, 2002

Aqueous solutions of poly(ethylene oxide-b-(dl-lactic acid-co-glycolic acid)-b-ethylene oxide) wi... more Aqueous solutions of poly(ethylene oxide-b-(dl-lactic acid-co-glycolic acid)-b-ethylene oxide) with two different molecular weights (550±3.4K±550 and 750±3.5K±750) show the change in turbidity as a function of temperature. The variation in turbidity is also found to be independent of block copolymer concentration. By increasing the molecular weight of the hydrophilic PEO end block with the molecular weight of the hydrophobic PLGA middle block kept almost the same, it is found that the phase boundary is shifted to a higher temperature and only soft gels are observed. Large aggregates or clusters of micelles with sizes larger than 5000 nm are observed with dynamic light scattering at the temperatures of turbid region and the size distribution of the aggregates is also found to depend only on temperature, not on concentration. The storage moduli (G 0 ) of aqueous solutions of the PEO±PLGA±PEO do not show the abrupt change at the sol-to-(hard) gel boundary, as normally seen in the case of PEO±PPO (poly(propylene oxide))±PEO block copolymers and the behavior of G 0 as a function of temperature is qualitatively the same and independent of polymer concentration except the absolute values of G 0 , which are proportional to the copolymer concentration. This implies that micelle±micelle clusters are formed due to hydrophobic attraction between core phases. q

Polymer, 2003

In this study, we demonstrate that the adsorbed amount of respective layers in spin self-assemble... more In this study, we demonstrate that the adsorbed amount of respective layers in spin self-assembled multilayer films can be controlled and predictable using an empirical power-law equation in terms of spin speed and initial solution concentration. The amount of a pair of polyelectrolytes ...