No-Hyung Park

Preparation of polymer‐dispersed liquid crystal films containing a small amount of liquid crystalline polymer and their properties

Polymer-dispersed liquid crystal (PDLC) films were synthesized by the copolymerization of liquid crystalline polymer (LCP) precursor, urethane acrylate (UA), and mesogenic monomer (AI) at different conditions. The morphology of polymer matrix changed with the weight ratio of polymer/liquid crystal (LC) ratio and curing temperature, resulting in a large change in the droplet size of LC domains in the PDLC film. The components used in the synthesis of polymer matrix, that is, the weight ratio of LCP, AI, and UA, also strongly influenced the morphology of PDLC films. A small amount of LCP was copolymerized with UA and AI in the preparation of polymer matrix to improve the electrooptical properties such as the viewing angle. Added LCP also affected the morphology and the properties of PDLC. The hydrophobicity of LCP caused changes in the droplet size of LC domain in PDLC films and the anchoring energy between matrix polymer and LC droplets. As the hydrophobicity of the matrix increases, the droplet size of LC domain also increases; on the contrary, anchoring energy decreased, leading to the decrease of driving voltage.

Mechanical and surface hardness properties of ultraviolet-cured polyurethane acrylate anionomer/silica composite film

An organic-inorganic composite was prepared by mixing silica emulsion with urethane acrylate anionomer (UAA) emulsion. The silica emulsion was prepared by sol-gel process, and stabilized by a steric stabilizer, polyvinyl pyrrolidone. UAA/silica composite film was made by an ultraviolet curing method. The mechanical and rheological properties of the UAA/silica composite were greatly improved. This composite also showed thermal stability during increase of temperature. Surface hardness was improved with increasing silica content up to proper amount of silica. However, in abrasive test, because UAA/silica composite film has a more abrasive contact area than UAA film, the former was abraded much more than the latter. In comparison to UAA film, the UAA/silica composite showed much smoother abrasive surface and less roughness.

Photochromic characteristics of monodisperse microcapsules containing azobenzene derivative-doped nematic liquid crystals

Highly mono-sized dye-doped liquid crystal (LC) microcapsules were prepared by the solute codiffusion method for application in photoswitchable devices. Azobenzene derivatives, which can be photoisomerized by irradiation with UV (366 nm) and visible (433 nm) light, were used as a photoresponsive dichroic dye. The microcapsules have a spherical shape and a single dye-doped LC domain. After UV light (366 nm) irradiation, PDLC films prepared using the microcapsules attained a photostationary state within 10 min and this state maintained stability. PDLC films made by employing a dichroic dye having electron donor and acceptor groups in the molecule showed especially good photoisomerization properties. The reversible trans-cis -photoisomerization occurred sharply by irradiating alternately with visible and UV light.

Preparation of UV‐curable PEG‐modified urethane acrylate emulsions and their coating properties. II. Effect of chain length of polyoxyethylene

To prepare self-emulsifiable urethane acrylate, poly(ethylene glycol)-modified urethane acrylates (PMUA), containing polyoxyethylene chains as a terminal group were synthesized by the reaction of a residual isocyanate group with poly(ethylene glycol) (PEG). Five types of PMUA were synthesized using five types of PEG having different molecular weight. As the chain length of polyoxyethylene of PMUA increased, the thermal stability of their emulsions improved and the tensile strength of their UV-cured films were also increased. For PMUA600 prepared using PEG600, the thermal stability of the emulsion and tensile strength of UV-cured film were relatively low. However, the emulsions of PMUAs which were synthesized using PEG2000, PEG4000, and PEG6000 were stable with increasing temperature and the tensile strength of their cured films was greater than that of PMUA 600. When PMUA600 was mixed with PMUA2000, PMUA4000, and PMUA6000, the thermal stability of the emulsions of mechanical properties of their UV-cured films were improved greatly.

The effect of mono-sized liquid crystal domains on electro-optical properties in a polymer dispersed liquid crystal prepared by using monodisperse poly(methylmethacrylate)/liquid crystal microcapsules

Highly mono-sized poly(methyl methacrylate) (PMMA)/liquid crystal (LC) microcapsules having a mono-sized single LC domain were prepared by the solute codiffusion method and solvent evaporation. The size of the LC domain in the microcapsules could be controlled by the amount of LC introduced during the swelling stage. The electro-optical properties of the polymer dispersed liquid crystal (PDLC) prepared by using the microcapsules was highly improved. In particular, the threshold voltage was lowered and the switching behaviour with an applied electric field was sharpened drastically compared with PDLC prepared simply by solvent evaporation-induced phase separation.

Organic–inorganic microhybrid materials via a novel emulsion mixing method

Water-dispersible urethane acrylate anionomers (UAAs) were prepared using anionic dimethylol propionic acid and silica emulsion was prepared by a sol–gel process using poly(vinyl pyrrolidone) and tetraethoxysilane. The above two emulsions were mixed, and UAA/silica composite gels were prepared. FTIR, X-ray diffraction, SEM, and AFM were used to verify the structure and the inclusion of silica in the composite gels. To investigate the influence of silica in UAA gels on the physical properties, the tensile strength and elongation at break were measured, and the annealing effect was evaluated by the physical properties. As the silica content increased, the tensile strength of the composite gels increased; however, up to a certain amount of silica, phase separation was induced and physical properties were reduced.

Preparation of cholesteric liquid crystal microcapsules for application in colour image storage media

Cholesteric liquid crystal (CLC) microcapsules for application in image storage media can be obtained via a diffusion-controlled polymerization method (DPM). To improve the swelling of the CLC seed particle, in poly(methylmethacrylate) (PMMA), a polymerizable acrylate based on a cholesterol moiety was synthesized and copolymerized with MMA to prepare the seed particle. As a result, monodispersed and CLC core/shell-structured microcapsules may be obtained. The resulting CLC microcapsules selectively, absorbed visible light at around 660 nm, and so appeared blue in the mesophase. Polymer dispersed cholesteric liquid crystal (PDCLC) cells were prepared using the CLC microcapsules, and were used as an image storage medium in reversible writing/erasing experiments.

Synthesis of graphene nano-sheet without catalysts and substrates using fullerene and spark plasma sintering process

*Korea National University of Transportation, Civil. Environmental and Transportation Engineering, Chungju 380-702, Korea**Department of Textile Convergence of Biotechnology & Nanotechnology, Korea Institute of Industrial Technology, Ansan 426-910, Korea***National Nanotechnology Policy Center, Korea Institute of Science and Technology Information (KISTI), Seoul 130-741, Korea****Department of Bio & Environmental Engineering, Semyung University, Jecheon 390-711, Korea*****Division of Materials Science and Engineering, Hanyang University, Seoul 133-791, Korea

Phosphate removing by graphene oxide in aqueous solution

Phosphate has been removed in waste water by chemically synthesized graphene oxide. Removing efficiency of phosphate was investigated using phosphate dispersion aqueous solution, and 70 % of phosphate was removed in phosphate dispersion solution by chemically synthesized graphene oxide solution. Removing efficiency of phosphate was increased from 70 % to 80 % with assistant of iron nano-particle in chemically synthesized graphene oxide solution. Phosphate removing capacity was up to 89.37 mg/g at initial phosphate concentration of 100 mg/l and temperature of 303 K. The Freundlich was applied to describe the equilibrium isotherms and the isotherm constants were determined.

Fabrication of PLA/TiO 2 nanofibers using melt-electro-spinning

Electrospun webs have been widely investigated for applying to drug delivery system (DDS) because of their high specific surface area and high porosity. In this study, the composite webs of PLA (poly(lactic acid)) and were fabricated by melt-electro-spinning method for applying to drug delivery system. The morphologies of PLA/, webs were observed using scanning electron microscope (SEM) and field emission transmission electron microscope (FE-TEM). The crystal structures of PLA/ composite webs were confirmed by X-ray diffractometer (XRD).