Papers by Author: Hyoung Joon Jin

Abstract: Polyurethane was used as adhesive due to high reactivity, high flexibility, and mechanical properties. Electrically conductive adhesives (ECAs) are an alternative to tin-lead solder in order to provide conductive paths between two electrical device components, which typically consist of a polymeric resin that contributes physical and mechanical properties, and conductive fillers. However, ECAs have low electrical conductivity and unstable network due to large contact points of the few micrometer-sized metal particles. In order to overcome these restrictions, multiwalled carbon nanotubes (MWCNTs) with high aspect ratio and smaller nanometer scale can be used as conductive fillers. In this study, ECAs were based on polyurethane filled with two kinds of fillers, raw MWCNTs and acid treated MWCNTs, respectively. Electrical conductivity was measured by using four-point probe. Morphology and dispersibility of fillers were observed by scanning electron microscopy and transmission electron microscopy.

Abstract: The multiwalled carbon nanotubes (MWCNTs) incorporated organic silk fibroin cryogels were fabricated through sol-gel process of aqueous silk fibroin solution, followed by freeze-drying. The MWCNT incorporated silk fibroin hydrogel was prepared by the regeneration of silk fibroin using an aqueous silk fibroin solution in which MWCNTs were dispersed by in-situ methods. The morphology and microstructure of the silk fibroin network structure were characterized using field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). The FESEM and TEM images show that the cryogels had three-dimensional network structures and the MWCNTs were well embedded in the network structures of regenerated silk fibroin. The porous properties of the MWCNT incorporated silk fibroin cryogels were investigated by nitrogen adsorption-desorption technique. The MWCNTs increased the proportions of micropores and mesopores in the silk fibroin cryogels when they were introduced in the appropriate amounts.

Abstract: Commercially, multi-walled carbon nanotubes (MWCNTs) production methods are based on the use of transition metal catalysts such as chemical vapor deposition (CVD). Raw MWCNTs usually contain metal catalysts and other carbon impurities. A common route to eliminate these impurities is an acid treatment. In addition, this route induces cutting of MWCNTs which can control the aspect ratio of MWCNTs. The aspect ratio controlled MWCNTs can use many applications such as the electrode material, biological imaging and sensing, etc. In this study, the aspect ratio of MWCNTs was controlled using an acid treatment with a 3:1 concentrated H2SO4/HNO3 mixture by varying the treatment time. Results show that an acid treatment can control the aspect ratio of MWCNTs. The aspect ratio controlled MWCNTs were observed by TEM and Raman spectra.

Abstract: We prepared composite films consisting of two biocompatible materials, bacterial cellulose and silk fibroin. Aqueous silk fibroin solution and bacterial cellulose excreted by Acetobacter xylinum were used to fabricate the composite films. It was verified by field emission scanning electron microscopy and X-ray diffraction that the two components were finely blended and that the silk fibroin was crystallized during the composition of the films. The silk fibroin penetrated well between the individual fibrils of the bacterial cellulose, while the water molecules inside the pellicular bacterial cellulose were evaporating. The composite films did not dissolve in water due to the crystallization of the silk fibroin in the composite films. We also observed the change in the mechanical properties of the composite films according to the water content. The composite films became more flexible and tougher when they were dipped in water, whereas they were very brittle in the dehydrated state.

Abstract: In this study, poly(ε-caprolactone) (PCL)/multiwalled carbon nanotube (MWCNT) composites with different contents of MWCNTs were successfully prepared by solution compounding, a method which could make them good competitors for commodity materials such as general purpose plastics, while allowing them to keep their complete biodegradability. For the homogeneous dispersion of the MWCNTs in the polymer matrix, oxygen-containing groups were introduced on their surface. The mechanical properties of the PCL/MWCNT composites were effectively increased due to the incorporation of the MWCNTs. The composites were characterized using scanning electron microscopy, in order to obtain information on the dispersion of the MWCNTs in the polymeric matrix. In the case of the composites containing 2.0 wt% of MWCNTs in their matrix, the strength and modulus of the composites were increased by 18.4% and 178.4%, respectively. In addition, the dispersion of the MWCNTs in the PCL matrix resulted in a substantial decrease in the electrical resistivity of the composites as the MWCNT loading was increased from 0 to 2.0 wt%.

Abstract: Poly(ε-caprolactone)/multiwalled carbon nanotube (PCL/MWCNT) composites with different MWCNT contents were successfully prepared by in situ bulk polymerization, which could make them good competitors for commodity materials such as general purpose plastics, while allowing them to completely retain their biodegradability. The mechanical properties of the PCL/MWCNT composites were effectively increased due to the incorporation of the MWCNTs. The composites were characterized using scanning electron microscopy, in order to obtain information on the dispersion of the MWCNTs in the polymeric matrix. In the case where 0.5 wt% of MWCNTs were dispersed in the matrix, the strength and modulus of the composite increased by 23% and 71%, respectively. In addition, the dispersion of the MWCNTs in the PCL matrix resulted in a substantial decrease in the electrical resistivity of the composites being observed as the MWCNTs loading was increased from 0 wt% to 0.5 wt%.

Abstract: Electrorheological (ER) particles were obtained by the adsorption of multiwalled carbon nanotubes (MWCNT) on the surface of silk fibroin microspheres. The resulting spherical polymeric microspheres consist of a silk fibroin core and an MWCNT shell, which is electrically conducting. The silk fibroin microspheres were prepared by the phase separation of the silk fibroin and poly(ethylene oxide) (PEO) blend solution, and the MWCNT dispersion was prepared by ultrasonication with cetyltrimethylammonium bromide (CTAB) surfactant. The ER particles were prepared using a simple process involving the blending of the silk fibroin microsphere suspension and aqueous MWCNT dispersion. The morphology of the ER particles was examined by field emission scanning electron microscopy (FESEM) and their electrical conductivity measured by the four-probe method was 4.8×10-4 S/cm. The prepared composite microspheres suspended in silicone oil showed typical ER characteristics, including the formation of a chain-like structure under an applied electric field (1.9 kV/mm). This phenomenon can be explained by the interfacial polarizability of the MWCNTs adsorbed on the surface of the polymeric microspheres.

Abstract: Sheets of poly(ε-caprolactone) (PCL)/carbon nanotube composites were prepared through the solvent cast method. Multiwalled carbon nanotubes (MWCNT) functionalized with hydroxyl groups were used as an initiator for the ring opening polymerization of ε-caprolactone to introduce PCL chains to the surface of MWCNT in order to ameliorate the dispersion of MWCNT in PCL matrix. Grafting of PCL chains to MWCNT was followed by FTIR and TEM observations. Effect of the incorporation of MWCNT on the mechanical properties of PCL was explored before and after the modification of the MWCNT by the PCL grafting.

Abstract: A highly concentrated solution of polystyrene (PS) in N,N-dimethyl formamide (DMF) was electrospun and the surface morphology of the electrospun PS fibers was investigated. Unlike the porous morphology observed on the surface of the fibers electrospun from the PS solution in a volatile solvent, e.g. tetrahydrofuran, a regular protuberance morphology was found on the surface of the fibers electrospun from the PS solution in the non-volatile solvent, DMF. This unique surface morphology was formed due to the presence of residual DMF solvent inside the electrospun PS fiber. Due to the large diameter of the PS fibers (~5 to ~10 μm) formed from the highly viscous PS solution, the DMF could not evaporate completely from inside them during the electrospinning process. Therefore, the extrusive force of the residual solvent inside the fiber induced the formation of a unique surface morphology. We believe that this unique surface morphology increases the surface area of the electrospun fibers, thus making it possible to control their wetting or adsorption behavior.