Hyunchul Jung

Properties of crosslinked polyurethanes synthesized from 4,4′-diphenylmethane diisocyanate and polyester polyol

Polyurethanes were synthesized using the high functional 4,4′-diphenylmethane diisocyanate (MDI), polyester polyol, and 1,4-butane diol. The synthesized polyurethanes were analyzed using differential scanning calorimeter (DSC), dynamic mechanical thermal analysis (DMTA), Fourier transform infrared (FTIR) spectrometer, and swelling measurement using N,N′-dimethylformamide. From the result of thermal analysis by DSC and DMTA, single Tgs were observed in the polyurethane samples at all the formulated compositions. From this result, it is suggested that the polyurethanes synthesized in this study have crosslinked structure rather than the phase-separated segmented structure because of the high functionality (f = 2.9) of the MDI. By annealing the polyurethane samples using DSC, the Tgs were increased by 4.7∼16.0°C at the various annealing temperatures. From the results of FTIR and swelling measurement of polyurethanes, it is suggested that the increase of Tg of the polyurethanes by annealing is not due to increase of the hydrogen bond strength but mainly due to the increase of the crosslink density.

Extremely high color rendering white light from surface passivated carbon dots and Zn-doped AgInS2 nanocrystals

In this study, highly luminescent carbon dots with diameters of 3–5 nm were synthesized via the carbonization of citric acid, and the effects of surface passivation and carbonization temperature on the optical properties were investigated. A red-shift in the emission wavelength was observed with increasing excitation wavelength, and oleylamine-capped carbon dots showed the highest quantum yield of approximately 43% when excited at 380 nm, whereas the reaction temperature had no influence on the emission wavelength and morphology of the dots. Additionally, I–III–VI AgInS2 nanocrystals (NCs) were prepared by thermal decomposition at a low temperature, and the emission wavelength was tuned by adjusting the growth temperature or introducing Zn ions, which enhanced the quantum yield up to 50%. For carbon dot application, a white LED was fabricated by combining a 380 nm UV LED with the carbon dots and Zn-doped AgInS2 NCs. White light from this LED exhibited an excellent color rendering index of greater than 95 with a warm color temperature, and demonstrated its potential for use in solid state lighting.

Fabrication of high color rendering index white LED using Cd-free wavelength tunable Zn doped CuInS2 nanocrystals.

Highly luminescent Cd-free Zn doped CuInS(2) nanocrystals (ZCIS NCs) were synthesized, and their properties were evaluated using X-ray diffraction, Raman, UV, and photoluminescence. The crystal structure of the ZCIS NCs was similar to the zinc blende, and the lattice constant decreased with increasing Zn concentration. By incorporation of Zn, the emission wavelength was tuned from 536 to 637 nm with concomitant enhancement of the quantum yield up to 45%. A white light emitting diodes, integrating dual ZCIS NCs (λ(em) = 567, and 617 nm) and a 460 nm InGaN LED, exhibited a high color rendering index of 84.1 with a warm color temperature of 4256.2K. The CIE-1931 chromaticity coordinates were slightly shifted from (0.3626, 0.3378) at 20 mA to (0.3480, 0.3206) at 50 mA.

Composite membranes based on sulfonated poly(ether ether ketone) and SiO 2 for a vanadium redox flow battery

Organic-inorganic composite membranes were prepared with sulfonated poly(ether ether ketone) (SPEEK) and different amounts of silica to improve chemical stability and vanadium hindrance for a vanadium redox flow battery. The durability of the prepared composite membrane was verified using a self-made dummy cell system and fully charged vanadium cathode half-cell electrolyte, which contained oxidative vanadium ions (VO2+). The prepared composite membranes, with covalent crosslinking between the organic polymer and inorganic particles, resulted in reduced vanadium permeability and enhanced chemical stability. Ion exchange capacity, water uptake, proton conductivity, and vanadium permeability decreased with increasing silica content. Selectivity was defined to consider both permeability and proton conductivity and resulted in a membrane that exhibited both high proton conductivity and low ion permeability simultaneously. The prepared 1 wt% silica composite membrane showed 133-fold higher selectivity compared with that of a Nafion112 membrane. After the stability test, the composite membrane showed little change compared to the membrane before the stability test, which confirmed the commercial prospect of SPEEK/SiO2 composite membrane for a vanadium redox flow battery.

Thermal Properties of Melt-Blended Poly(ether ether ketone) and Poly(ether imide)

The thermal properties of blends of poly(ether ether ketone) (PEEK) and poly(ether imide) (PEI) prepared by screw extrusion were investigated by differential scanning calorimetry. From the thermal analysis of amorphous PEEK–PEI blends which were obtained by quenching in liquid nitrogen, a single glass transition temperature (Tg) and negative excess heat capacities of mixing were observed with the blend composition. These results indicate that there is a favorable interaction between the PEEK and PEI in the blends and that there is miscibility between the two components. From the Lu and Weiss equation and a modified equation from this work, the polymer–polymer interaction parameter (χ12) of the amorphous PEEK–PEI blends was calculated and found to range from −0.058 to −0.196 for the extruded blends with the compositions. The χ12 values calculated from this work appear to be lower than the χ12 values calculated from the Lu and Weiss equation. The χ12 values calculated from the Tg method both ways decreased with increase of the PEI weight fraction.

Effect of amine double-functionalization on CO2 adsorption behaviors of silica gel-supported adsorbents

Amine double-functionalized adsorbents were fabricated using silica gel as supports and their capabilities for CO2 capture were examined. Aminopropyltrimethoxysilane (1N-APS), and N1-(3-trimethoxysilylpropyl)diethylenetriamine (3N-APS) were used as grafted amine compounds, and tetraethylenepentamine and polyethyleneimine were used as impregnated species. The influence of double-functionalization method on the CO2 adsorption performance and textural properties of adsorbents was investigated. The adsorption capacity, the amine efficiency, and the thermal stability of double-functionalized sorbents depend strongly upon molecular variables associated with two different functional states (i.e., chemically grafted and physically impregnated amines). The temperature dependence of adsorption isotherms reveals that the CO2 adsorption behavior in the double-functionalized adsorbents follow the diffusion limitation model proposed by Xu et al. (Energy Fuels 16:1463–1469, 2002) where the CO2 adsorption is helped by the diffusion of impregnated amines. It is also found that the adsorption isotherm in the double-functionalized sorbent system with a proper choice for grafted and impregnated amines is nearly independent of temperature, which may offer a novel means to fabricate practically useful sorbents that can be used in a wide range of temperature without loss of CO2 adsorption capacity.

Properties of flame-retarding blends of polycarbonate and poly(acrylonitrile-butadiene-styrene)

Abstract Limiting oxygen index (LOI) value, glass transition temperature (Tg), thermal degradation profile, morphology, and tensile strength and elongation at break of the flame-retarding polycarbonate (PC)-poly(acrylonitrile- butadiene-styrene) (ABS) (7/3) blends were studied. The flame retardants used were huntite-hydromagnetite compounds (HHM), triphenyl phosphate (TPP), zinc stannate, and antimony trioxide. The LOI values were increased with the increase of TPP content, but they did not change significantly when the HHM was used as a flame retardant Glass transition temperatures of the flame-retarding PC-ABS (7/3) blends were decreased significantly when TPP was added to the blends. This is mainly due to the plasticizing effect of the TPP on the PC-ABS (7/3) blends. In the flame-retarding PC-ABS (7/3) blends, the LOI value, tensile strength, and elongation at break were measured and found to be 24.6 - 31.6%, 41.4 - 50.3 MPa, and 2.16 - 33.7%, respectively. These LOI values and mechanical properties were achieved when the TPP, zinc stannate, and antimony trioxide were used as flame retardants, and when dodecylbenzenesulfonic acid sodium salt and dimethylsulfone were used as additives into the PC-ABS (7/3) blends.

Spray pyrolysis prepared yellow to red color tunable Sr 1-x Ca x Se:Eu 2+ phosphors for white LED

The spherical and submicron size of Sr₁-xCaxSe:Eu²⁺ phosphors were successfully prepared by ultrasonic spray pyrolysis. The phosphors adopted a cubic structure, and the replacement of Sr²⁺ with Ca²⁺ decreased the lattice parameter. The Sr₁-xCaxSe:Eu²⁺ showed broad and strong excitation under 420-460 nm blue light, and the emission band could be tuned from 565 to 607 nm by increasing the Ca²⁺ ratio in the host lattice. In addition, the doping of Zn²⁺ into Sr²⁺ or Ca²⁺ enhanced the emission intensity with a small red shift due to the change in crystal field strength and nephelauxetic effects. The warm and high CRI of white LED was achieved using blue LED pumped with blending phosphors of 612 nm emitting Ca₀.₉₈Zn₀.₀₂Se:Eu²⁺ and 565 nm emitting YAG. The correlated color temperatures and CRI were 4719.2K, and 86.3, respectively, and an acceptable color variation was also observed at operating currents ranging from 20 to 70 mA.

Characterization of surface modified ZnCuInS2 nanocrystals and its application to white light-emitting diodes.

Red-emitting ZnCuInS2 semiconductor nanocrystals (NCs) were synthesized and surface modification was performed on the NCs with oleylamine, trioctylphosphine, and 3-mercaptopropionic acid by the ligand exchange strategy. UV-visible spectroscopy, photoluminescence (PL) spectroscopy, time-resolved PL analysis, and attenuated total reflection Fourier transform infrared spectroscopy were used to verify the surface modification of NCs. Additionally, white light-emitting diodes (LEDs) were fabricated using surface modified red-emitting ZnCuInS2 NCs as red phosphor to compensate for the deficiency of red emission in white LEDs, consisting of blue LEDs as excitation sources and YAG:Ce as yellow phosphor.

Fabrication of highly stable silica coated ZnCuInS nanocrystals monolayer via layer by layer deposition for LED application

In this study, non-toxic and highly stable silica coated ZnCuInS NCs were synthesized by a reverse microemulsion method. The single NCs were uniformly encapsulated in a silica shell with a diameter of ~30 nm. Although hydrolyzed TEOS caused a QY reduction, and a 12.5 nm red shift occurred after silica coating, the photo and thermal stabilities were extremely improved. For LED application, the silica coated ZnCuInS NCs phosphor layer was arrayed on the InGaN LED surface by layer-by-layer deposition utilizing electrostatic attraction. When the ZnCuInS/SiO2 NCs single monolayer was fabricated, 6.73% high color conversion efficiency was achieved.