Seulgi Kim

Optimization of Emulsion Polymerization for Submicron-Sized Polymer Colloids towards Tunable Synthetic Opals

Submicron-sized polymeric colloidal particles can self assemble into 3-dimensional (3D) opal structure which is a useful template for photonic crystal. Narrowly dispersed polymer microspheres can be synthesized by emulsion polymerization in water using water-soluble radical initiator. In this report, we demonstrate a facile and reproducible emulsion polymerization method to prepare various polymeric microspheres within 200 - 400 nm size ranges which can be utilized as colloidal photonic crystal template. By controlling the amount of monomer and surfactant, monodisperse polymer colloids of polystyrene (PS) and acrylates with various sizes were successfully prepared without complicated synthetic procedures. Such polymer colloids self-assembled into 3D opal structure exhibiting bright colors by reflection of visible light. The colloidal particles and the resulting opal structures were rigorously characterized, and the wavelength of the structural color from the colloidal crystal was confirmed to have quantitative relationship with the size of constituting colloidal particles as predicted by Bragg equation. The tunability of the structural color was achieved not only by varying the particle size but also by infiltration of the colloidal crystal with liquids having different refractive indices.

Mechanism of surface modification of a porous-coated Ti-6Al-4V implant fabricated by electrical resistance sintering

A porous-coated Ti-6Al-4V implant was fabricated by electrical resistance sintering, using 480 μF capacitance and 1.5 kJ input energy. X-ray photoelectron spectroscopy (XPS) was used to study the surface characteristics of the implant material before and after sintering. There were substantial differences in the content of O and N between as-received atomized Ti-6Al-4V powders and the sintered prototype implant, which indicates that electrical resistance sintering alters the surface composition of Ti-6Al-4V. Whereas the surface of atomized Ti-6Al-4V powders was primarily TiO2, the surface of the implant consisted of a complex of titanium oxides as well as small amounts of titanium carbide and nitride. It is proposed that the electrical resistance sintering process consists of five stages: stage I – electronic breakdown of oxide film and heat accumulation at the metal-oxide interface; stage II – physical breakdown of oxide film; stage III – neck formation and neck growth; stage IV – oxidation, nitriding, and carburizing; and stage V – heat dissipation. The fourth stage, during which the alloy repassivates, is responsible for the altered surface composition of the implant.

Novel solid-phase parallel synthesis of N-substituted-2-aminobenzo [d]thiazole derivatives via cyclization reactions of 2-iodophenyl thiourea intermediate resin.

A novel solid-phase methodology has been developed for the synthesis of N-alkyl, N-acyl, and N-sulfonyl-2-aminobenzo[d]thiazole derivatives. The key step in this procedure involves the preparation of polymer-bound 2-aminobenzo[d]thiazole resins 5 by cyclization reaction of 2-iodophenyl thiourea resin 3. The resin-bound 2-iodophenyl thiourea 3 is produced by addition of 2-iodophenyl isothiocyanate 2 to the amine-terminated linker amide resin 1. These core skeleton 2-aminobenzo[d]thiazole resins 5 undergo functionalization reactions with various electrophiles, such as alkyl halides, acid chlorides, and sulfonyl chlorides to generate N-alkyl, N-acyl, and N-sulfonyl-2-aminobenzo[d]thiazole resins 6, 7, and 8, respectively. Finally, N-alkyl, N-acyl, and N-sulfonyl-2-aminobenzo[d]thiazole derivatives 9, 10, and 11 are then generated in good yields and purities by cleavage of the respective resins 6, 7, and 8 using trifluoroacetic acid (TFA) in dichloromethane (DCM).

Hierarchical opal grating films prepared by slide coating of colloidal dispersions in binary liquid media

There are active researches on well ordered opal films due to their possible applications to various photonic devices. A recently developed slide coating method is capable of rapid fabrication of large area opal films from aqueous colloidal dispersion. In the current study, the slide coating of polystyrene colloidal dispersions in water/i-propanol (IPA) binary media is investigated. Under high IPA content in a dispersing medium, resulting opal film showed a deterioration of long range order, as well as a decreased film thickness due to dilution effect. From the binary liquid, the dried opal films exhibited the unprecedented topological groove patterns with varying periodic distances as a function of alcohol contents in the media. The groove patterns were consisted of the hierarchical structures of the terraced opal layers with periodic thickness variations. The origin of the groove patterns was attributed to a shear-induced periodic instability of colloidal concentration within a thin channel during the coating process which was directly converted to a groove patterns in a resulting opal film due to rapid evaporation of liquid. The groove periods of opal films were in the range of 50-500 μm, and the thickness differences between peak and valley of the groove were significantly large enough to be optically distinguishable, such that the coated films can be utilized as the optical grating film to disperse infra-red light. Utilizing a lowered hydrophilicity of water/IPA dispersant, an opal film could be successfully coated on a flexible Mylar film without significant dewetting problem.

Allosteric inhibition site of transglutaminase 2 is unveiled in the N terminus

Previously we have demonstrated transglutaminase 2 (TGase 2) inhibition abrogated renal cell carcinoma (RCC) using GK921 (3-(phenylethynyl)-2-(2-(pyridin-2-yl)ethoxy)pyrido[3,2-b]pyrazine), although the mechanism of TGase 2 inhibition remains unsolved. Recently, we found that the increase of TGase 2 expression is required for p53 depletion in RCC by transporting the TGase 2 (1–139 a.a)–p53 complex to the autophagosome, through TGase 2 (472–687 a.a) binding p62. In this study, mass analysis revealed that GK921 bound to the N terminus of TGase 2 (81–116 a.a), which stabilized p53 by blocking TGase 2 binding. This suggests that RCC survival can be stopped by p53-induced cell death through blocking the p53–TGase 2 complex formation using GK921. Although GK921 does not bind to the active site of TGase 2, GK921 binding to the N terminus of TGase 2 also inactivated TGase 2 activity through acceleration of non-covalent self-polymerization of TGase 2 via conformational change. This suggests that TGase 2 has an allosteric binding site (81–116 a.a) which changes the conformation of TGase 2 enough to accelerate inactivation through self-polymer formation.

Flexible polymer opal films prepared by slide coating from alcoholic media

In this study, we investigated the fabrication of a colloidal photonic crystal film on flexible substrate, which can be used as a scaffold material for various photonic applications. A thin polyethylene terephthalate (PET) film was treated by oxygen plasma and then used as a flexible substrate. The surface characterizations revealed that the surface roughness increased, and that the treated PET film was rendered hydrophilic. The contact angles of the films with water/isopropyl alcohol (IPA) mixtures at five different weight ratios (25%, 40%, 50%, 60%, and 75% of IPA) prepared on both treated and non-treated PET films were measured. The results indicated that water/IPA wets on oxygen plasma-treated PET film. The colloidal dispersions were prepared in five different water/IPA mixtures, and slide coating was carried out on both PET films aided by hot air to promote opal formation. Serious dewetting of opal film was observed on the non-treated PET film, except that with a 75% IPA. However, the treated PET film enabled the formation of high-quality opal film, regardless of IPA content in colloidal dispersion. The optical properties and surface morphologies of opal films on flexible PET film were confirmed by reflectance spectra and scanning electron microscopy images of the respective films.

Molecular weight effect of partially sulfonated PS- b -PDMS diblock copolymers as proton exchange membrane for direct methanol fuel cell

Partially sulfonated polystyrene-b-poly(dimethylsiloxane) (sPS-b-PDMS) block copolymer membranes were prepared using three different polymers with various molecular weights and block ratios. Eleven different products were obtained with controlled degree of sulfonation (DS) of PS block ranging from 22% to 48%, and they were cast into proton exchange membranes (PEMs). Each PEM was rigorously characterized to see the effect of molecular weight and PS content to PEM for direct methanol fuel cells (DMFC). The first set of sPS-b-PDMS with the highest molecular weight and 50% PS block ratio generally showed high mechanical strength and selectivity (proton conductivity/ methanol permeability), and the second set with medium molecular weight and 60% PS ratio showed high proton conductivity. The last PEM with the lowest molecular weight and 80% PS ratio exhibited a poor tensile strength which is not suitable for PEM application. When the membrane/electrode assemblies (MEAs) were fabricated using the sPS-b-PDMS PEMs with moderately high DS (~40%), one from the second set (moderate molecular weight, 60% PS content) showed the best power performance (90 mW/cm2 at 70 °C) in active mode DMFC operation, which was found to be 20% higher than that of Nafion 115 MEA.