Investigation of electro-optical characteristics of photo-aligned TN-LCDs on PCEMA surfaces

In this work, a novel photo-alignment material, poly(cinnamolyethyl methacrylate) (PCEMA), was synthesized by photo-dimerization. We investigated the electro-optical characteristics of twisted nematic (TN)-liquid crystal displays (LCDs) photo-aligned with linearly polarized UV light irradiation at normal direction on the PCEMA surfaces. Excellent voltagetransmittance characteristics were observed. The threshold voltage of the photo-aligned TN-LCD decreases with increasing UV irradiation time. Additionally, response times are almost the same as for a rubbing-aligned TN-LCD.     

Microphase‐separated structure of telechelic urethane acrylate anionomers and their network in various solvents

Telechelic urethane acrylate anionomer (UAA) chain showed less viscosity and polyelectrolyte behavior in water than dimethyl acetamide (DMAc) because of hydrophobic aggregation. UAA networks prepared in different solvents (water and DMAc) exhibited very different swelling behaviors in the same swelling medium, which can be interpreted as due to the very different microstructures formed in the solvents. UAA networks prepared with water (UAHG networks) had microphase‐separated hydrophilic and hydrophobic domains, whereas UAA networks synthesized with DMAc (UADG networks) had relatively homogeneous network structures. The mechanical property of the UAHG and UADG networks, measured with a dynamic mechanical analyzer, was also very sensitive to the solvent type used during the crosslinking reaction. UAHG networks with a microphase‐separated structure had a higher modulus than UADG networks. The results of the mechanical property measurements showed that water was a much better solvent for the hydrophilic hard segments of UAA chain than DMAc, even though DMAc dissolved both hydrophilic and hydrophobic segments of UAA chain. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2081–2095, 2000     

Binding of uranyl ion by 2,2′-dihydroxyazobenzene attached to a partially chloromethylated polystyrene

A 2,2′-dihydroxyazobenzene (DHAB) derivative was attached to a chloromethylated cross-linked polystyrene derivative in view of high affinity of DHAB for uranyl ion. Chloromethyl groups of the resin were converted to quaternary ammonium ions by treating with tertiary amines. Capacity of the resins for uranyl-uptake was measured, revealing that about 20 mg of uranium can be complexed to 1 g of the resins. Formation constants (K_f) for uranyl complexes of the resins were determined. In the presence of >0.1 M bicarbonate ion at pH 8.10, log K_f of about 15 was obtained. As bicarbonate concentration was lowered, K_f decreased considerably. Degrees of uranyl-uptake from rapidly flowing uranyl solutions were measured, and the results suggested that rate of uranyl-uptake may not impose a major barrier to application of the resins in uranium extraction from seawater. Uranium extraction from seawater with the resins was carried out on the east coast of Korean peninsula. The amount of uranium extracted from seawater was about 10 µg/g resin. This is not satisfactory for economical processes of uranium recovery from seawater. Results of the present study, however, suggested that modification of the DHAB-containing resins can improve uranyl-binding ability, probably leading to economical recovery of uranium from seawater. In addition, simulation of uranyl-binding processes in seawater with the laboratory procedures developed in this study was satisfactory. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3169–3177, 1999     

Growth and Optical Properties of Tm:GdVO4 Single Crystal

Thulium-doped gadolinium vanadate (Tm:GdVO₄) single crystal has been successfully grown by a modified Czochralski (CZ) technique. Effective distribution coefficient of Tm was determined to be 0.74. Absorption characterization was performed in the 800 nm region and the maximum absorption peak was found at 799 nm for p polarization. Fluorescence spectra for tuning at the maximum absorption were obtained around 1.8-2.0µm region with 100 nm bandwidth. This suggests that a Tm:GdVO₄ crystal is expected as a new promising LD pumped solid-state laser in the 2µm region.     

Self-Assembly of a Molecular Floral Lace with One-Dimensional Channels and Inclusion of Glucose

A three-dimensional network with one-dimensional channels (see picture) has been self-assembled from the nickel(II ) complex of cyclam and 1,3,5-benzenetricarboxylate in water through hydrogen-bond formation. The channels have an appropriate diameter (10.3 Å) to include D -glucose with a formation constant of K_f=(1.38±0.01)×10⁴ M ⁻¹. Under similar conditions maltose is not included.     

Partially imidized poly[(amic ester)-alt-imide]: A novel polyimide pregursor

Partial imidization of poly[(amic ester)-alt-(amic acid)], a precursor for the “strictly alternating” copolyimides, was performed via selective chemical imidization of amic acid units in the copolymer precursor. The resulting, poly[(amic ester)-alt-imide], showed superior properties such as an excellent solution stability, good processibility and improved optical transparency, which are essential for the use as a precursor for the polyimide.     

Vacuum pyrolysis study of graphite-like pyropolymers from 1,4-diphenyl-1-buten-3-yne

1,4-Diphenyl-1-buten-3-yne was metathesis polymerized over NbCl₅- or TaCl₅-based catalysts followed by pyrolysis to obtain graphite-like pyropolymers. The brown metathesis polymer, poly(1,4-diphenyl-1-buten-3-yne), has the structure of fully conjugated backbone and was annealed at 250°C to obtain partially crosslinked polymer which showed 20% increase of carbon residue on pyrolysis over 400°C than the pyrolyzed poly(1,4-diphenyl-1-buten-3-yne) without annealing. The polymers were characterized by GPC, EA, NMR, XPS, TGA, DSC, FT-IR, laser-Raman, x-ray diffraction, and SEM studies. © 1996 John Wiley & Sons, Inc.     

Comparative analysis of characteristics of Si, Mg, and undoped GaN

We have grown undoped, Si- and Mg-doped GaN epilayers using metalorganic chemical vapor deposition. The grown samples have electron Hall mobilities (carrier concentrations) of 798 cm²/V s (7×10¹⁶ cm⁻³) for undoped GaN and 287 cm²/V s (2.2×10¹⁸ cm⁻³) for Si-doped GaN. Mg-doped GaN shows a high hole concentration of 8×10¹⁷ cm⁻³ and a low resistivity of 0.8 Ω cm. When compared with undoped GaN, Si and Mg dopings increase the threading dislocation density in GaN films by one order and two orders, respectively. Besides, it was observed that the Mg doping causes an additional biaxial compressive stress of 0.095 GPa compared with both undoped and Si-doped GaN layers, which is due to the incorporation of large amount of Mg atoms (4–5×10¹⁹ cm⁻³).