Hyung-il Lee
University of Ulsan
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Publication
Featured researches published by Hyung-il Lee.
Journal of Macromolecular Science, Part B | 2012
Sang Hyop Choi; Dong Hoon Kim; Anjanapura V. Raghu; Kakarla Raghava Reddy; Hyung-il Lee; Koo Sik Yoon; Han Mo Jeong; Byung Kyu Kim
Nanocomposites of waterborne polyurethane (WPU) reinforced with functionalized graphene sheets (FGSs) were effectively prepared by casting from a colloidal dispersion of FGS and WPU, and the morphology and physical properties were examined. The finer aqueous FGS dispersions or WPU with smaller particles yielded nanocomposites with enhanced electrical conductivity and thermal resistance due to finely dispersed FGS. The FGS nucleated the crystallization of the polycaprolactone (PCL) segments in WPU and improved its modulus. However, FGS inhibited crystal growth and deteriorated the tensile properties at high deformation, i.e., tensile strength and elongation at break, because the interaction between FGS and WPU hindered the chain rearrangement of WPU in the nanocomposite.
Journal of Macromolecular Science, Part B | 2014
Su Jin Han; Hyung-il Lee; Han Mo Jeong; Byung Kyu Kim; A. V. Raghu; Kakarla Raghava Reddy
Graphene, prepared by the thermal reduction of graphite oxide (GO), was modified with stearic acid to enhance its lipophilicity. A novel method, using the intrinsic epoxy groups on the graphene, was utilized for reaction with stearic acid to minimize the negative impact of the normal functionalization method on the π-electronic system of graphene. Gravimetric analysis, thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectroscopy, and X-ray photoelectron spectroscopy (XPS) showed that the stearic acid was effectively attached to the graphene. In addition, Raman spectroscopy and electric conductivity of the graphene showed that this novel modification method, utilizing intrinsic defects, did not damage the π-electronic system of the sp2 bonded carbons. The dispersion of graphene in a low density polyethylene (LDPE) matrix was enhanced; consequently, the reinforcing effect in tensile testing was improved by the lipophilic modification. The crystallization behavior observed by differential scanning calorimetry (DSC) showed that the crystallization of LDPE was hindered by dispersed graphene, more evidently when dispersed uniformly.
Materials | 2014
Jin Ho Park; Trung Dung Dao; Hyung-il Lee; Han Mo Jeong; Byung Kyu Kim
Shape memory behavior of crystalline shape memory polyurethane (SPU) reinforced with graphene, which utilizes melting temperature as a shape recovery temperature, was examined with various external actuating stimuli such as direct heating, resistive heating, and infrared (IR) heating. Compatibility of graphene with crystalline SPU was adjusted by altering the structure of the hard segment of the SPU, by changing the structure of the graphene, and by changing the preparation method of the graphene/SPU composite. The SPU made of aromatic 4,4′-diphenylmethane diisocyanate (MSPU) exhibited better compatibility with graphene, having an aromatic structure, compared to that made of the aliphatic hexamethylene diisocyanate. The finely dispersed graphene effectively reinforced MSPU, improved shape recovery of MSPU, and served effectively as a filler, triggering shape recovery by resistive or IR heating. Compatibility was enhanced when the graphene was modified with methanol. This improved shape recovery by direct heating, but worsened the conductivity of the composite, and consequently the efficiency of resistive heating for shape recovery also declined. Graphene modified with methanol was more effective than pristine graphene in terms of shape recovery by IR heating.
ACS Applied Materials & Interfaces | 2009
Metin Sitti; Brian Cusick; Burak Aksak; Alper Nese; Hyung-il Lee; Hongchen Dong; Tomasz Kowalewski; Krzysztof Matyjaszewski
This work reports on repeatable adhesive materials prepared by controlled grafting of dangling hetero chains from polymer elastomers. The dangling chain elastomer system was prepared by grafting poly(n-butyl acrylate) (PBA) chains from prefunctionalized polydimethylsiloxane (PDMS) elastomer networks using atom transfer radical polymerization. To study the effects of chain growth and network strain as they relate to network adhesion mechanics, various lengths of PBA chains with degree of polymerizations (DP) of 65, 281, 508, and 1200 were incorporated into the PDMS matrix. PBA chains with a DP value of 281 grafted from a flat PDMS substrate showed the highest (approximately 3.5-fold) enhancement of nano- and macroscale adhesion relative to a flat raw (ungrafted and not prefunctionalized) PDMS substrate. Moreover, to study the effect of PBA dangling chains on adhesion in fibrillar elastomer structures inspired by gecko foot hairs, a dip-transfer fabrication method was used to graft PBA chains with a DP value of 296 from the tip endings of mushroom-shaped PDMS micropillars. A PBA chain covered micropillar array showed macroscale adhesion enhancement up to approximately 7 times relative to the flat ungrafted prefunctionalized PDMS control substrate, showing additional nonoptimized approximately 2-fold adhesion enhancement due to fibrillar structuring and mushroom-shaped tip ending. These dangling hetero chains on elastomer micro-/nanofibrillar structures may provide a novel fabrication platform for multilength scale, repeatable, and high-strength fibrillar adhesives inspired by gecko foot hairs.
Polymer Chemistry | 2014
Vivek Mishra; Seo-Hyun Jung; Han Mo Jeong; Hyung-il Lee
A series of thermoresponsive ureido-derivatized polymers were synthesized via atom transfer radical polymerization (ATRP) and a post-modification process. All of the polymers were designed to exhibit an upper critical solution temperature (UCST) in aqueous solution, which was mainly dependent on (i) the presence of ureido moieties, (ii) the chain length of the side groups, and (iii) the degree of quaternization (DQ) on a triazole ring. Side group modification of PHEMA was carried out by DCC coupling with 4-pentynoic acid to prepare PHEMA-alkyne (PHA). 2-Azidoethylurea and 3-azidopropylurea were combined with the PHA backbone via click reaction, resulting in triazole-ring containing ureido-derivatized polymers. Quaternization reactions with methyl iodide were conducted on the triazole ring of each polymer to further control the UCSTs. Results showed that the UCST could be precisely tuned by the level of DQ.
Journal of Colloid and Interface Science | 2014
Trung Dung Dao; Hyung-il Lee; Han Mo Jeong
A graphene was coated with a thin alumina layer to prepare a novel nanosheet which had high thermal conductivity but low electrical conductivity. The nanosheet with minimal aggregation was prepared effectively by first coating it with aluminum tri-sec-butoxide in anhydrous dimethylformamide, followed by rapid calcination in an inert atmosphere after the hydrolysis of the alkoxide. The morphology observed by scanning electron microscopy and elemental mapping by energy-dispersive X-ray spectrometry showed that the alumina layer coated on the graphene surface was uniform and ultra-thin. Thermogravimetry demonstrated that the uniformly coated alumina protective layer substantially improved the thermal stability of the graphene and that the electrically-insulative alumina layer effectively reduced the electrical conductivity of the graphene. The enhanced polar nature of surface as well as the increased surface roughness due to the coated alumina improved the dispersion of the graphene in the polar acrylic rubber matrix and the interaction at the interface. This led to an effective improvement of the thermal conductivity but marginal increase in electrical conductivity by the filler. Tensile modulus increased drastically to as high as 470% for the composite reinforced with the 5 phr (about 2.5 vol%) loading of the alumina-coated graphene.
Journal of Biological Chemistry | 2012
Kang-Mu Lee; Yongjin Park; Wasimul Bari; Mi Young Yoon; Junhyeok Go; Sang Cheol Kim; Hyung-il Lee; Sang Sun Yoon
Background: The human intestine, in which Vibrio cholerae exerts its virulence, is an anaerobic environment. Results: When grown anaerobically with trimethylamine N-oxide (TMAO), V. cholerae exhibited enhanced growth and cholera toxin (CT) production was remarkably induced. Conclusion: Anaerobic TMAO respiration may serve as a signal to increase V. cholerae virulence. Significance: A novel growth condition that induces CT production is uncovered. Vibrio cholerae is a Gram-negative bacterium that causes cholera. Although the pathogenesis caused by this deadly pathogen takes place in the intestine, commonly thought to be anaerobic, anaerobiosis-induced virulence regulations are not fully elucidated. Anerobic growth of the V. cholerae strain, N16961, was promoted when trimethylamine N-oxide (TMAO) was used as an alternative electron acceptor. Strikingly, cholera toxin (CT) production was markedly induced during anaerobic TMAO respiration. N16961 mutants unable to metabolize TMAO were incapable of producing CT, suggesting a mechanistic link between anaerobic TMAO respiration and CT production. TMAO reductase is transported to the periplasm via the twin arginine transport (TAT) system. A similar defect in both anaerobic TMAO respiration and CT production was also observed in a N16961 TAT mutant. In contrast, the abilities to grow on TMAO and to produce CT were not affected in a mutant of the general secretion pathway. This suggests that V. cholerae may utilize the TAT system to secrete CT during TMAO respiration. During anaerobic growth with TMAO, N16961 cells exhibit green fluorescence when stained with 2′,7′-dichlorofluorescein diacetate, a specific dye for reactive oxygen species (ROS). Furthermore, CT production was decreased in the presence of an ROS scavenger suggesting a positive role of ROS in regulating CT production. When TMAO was co-administered to infant mice infected with N16961, the mice exhibited more severe pathogenic symptoms. Together, our results reveal a novel anaerobic growth condition that stimulates V. cholerae to produce its major virulence factor.
Polymer Chemistry | 2013
Jae Min Bak; Kyung-Bin Kim; Ji-Eun Lee; Yongjin Park; Sang Sun Yoon; Han Mo Jeong; Hyung-il Lee
We report the unique thermoresponsive properties of fluorinated polyacrylamides, poly[N-(2,2-difluoroethyl)acrylamide] (P2F). The solubility of fluorinated polyacrylamides in water can be easily controlled by changing the number of fluorine atoms in N-ethyl groups. Moreover, we demonstrate that fluorinated polyacrylamides are less cytotoxic than poly(N-isopropylacrylamide) (PNIPAM).
Macromolecular Rapid Communications | 2014
Vivek Mishra; Seo-Hyun Jung; Jong Mok Park; Han Mo Jeong; Hyung-il Lee
The purpose of this study is to develop novel triazole-containing hydrogels (TGs) as drug carrier and to investigate the sustained drug release accomplished by their time-dependent swelling behavior. The synthetic pathway of TGs includes: (1) DCC-coupling on hydroxyethyl methacrylate (HEMA) to prepare HEMA-alkyne (HA), (2) click-coupling to prepare a triazole-ring-containing monomer (TM), and (3) the synthesis of a series of TGs. The aggregation between triazole rings is found to be responsible for drug release controllability. Rhodamine 6G is studied as a model anticancer drug for release experiments. The effects of pH and temperature on the properties of sustained drug release are also studied.
Polymer Chemistry | 2014
A. Balamurugan; Hyung-il Lee
A water-soluble polymeric probe for selective sensing of cysteine and homocysteine by colorimetric titration was designed and synthesized. Most interestingly, temperature-tunable sensitivity of cysteine detection was achieved. The polymer selectivity toward various amino acids was investigated, and the highest selectivity (25–30-fold higher) was achieved for cysteine and homocysteine compared to other amino acids. The temperature-dependent cysteine sensing studies were performed to demonstrate temperature-tunable sensitivity towards cysteine. The sensing ability decreased with increasing temperature from 25 to 55 °C due to hydrophobic aldehyde moieties being hidden in the folded chains of the polymer.