13,168 research outputs found
Preparation of optically active beta-amino acids from microbial polyester polyhydroxyalkanoates
No full textAn efficient method for the preparation of optically active ethyl P-aminobutyrate from the biopolymer, poly-(R)-(-)3-hydroxybutyrate (PHB) obtained from bacterial cells has been established using chemical transformations: simple recovery of PHB from bacterial cells followed by acidic alcoholysis, tosylation, nucleophilic substitution by azide, and an indium mediated reduction.This work was supported by the First Presidential Young
Investigator’s Award to S. Y. Lee. S. H. Park was supported by
the Brain Korea 21 Project from the Korean Ministry of
Education
Novel clay treatment and preparation of poly(ethylene terephthalate)/clay nanocomposite by in-situ polymerization
No full textNovel clay treatment was proposed for poly(ethylene terephthalate) (PET)/clay nanocomposite and the clay dispersed trimellitate ester oligomers were prepared by esterification with 1, 2, 4-Benzenetricarboxylic anhydride (TMA, trimellitic anhydride) and montmorillonite (MMT) dispersed ethylene glycol (EG). The structure, interlayer distance and thermal stability of the clay dispersed trimellitate ester oligomers were investigated by NMR, WAXD, SAXS and TGA techniques
Temperature-sensitive pluronic/poly(ethylenimine) nanocapsules for thermally triggered disruption of intracellular endosomal compartment
No full textPluronic hydrogel nanoparticles cross-linked with poly(ethylenimine) (PEI) were synthesized by a modified emulsification/solvent evaporation method. Pluronic F-127 preactivated at the terminal group with p-nitrophenyl chloroformate was dissolved in dichloromethane, and the organic solution was emulsified in deionized water containing PEI by sonication. Primary amine groups of PEI in the aqueous phase were conjugated and/or crosslinked with activated Pluronic F-127 in the vicinity of the water/dichloromethane interface, resulting in the formation of shell-cross-linked Pluronic/PEI nanocapsules. Pluronic/PEI nanocapsules exhibited a volume transition behavior over a temperature range of 24-33 degrees C. The thermally reversible swelling/deswelling of Pluronic/PEI nanocapsules was caused by temperature-dependent hydrophobic interaction of cross-linked and/or grafted Pluronic polymer chains in the nanocapsules. Pluronic/PEI nanocapsules were utilized to break up intracellular endosomal compartments by swelling-induced destabilization of the endosomal membrane triggered by a cold-shock treatment.the Ministry of Health and
Welfare, Korea
A new bandwidth extension technique using inductive buffer peaking for high data rate optical receiver circuit
No full textThree-dimensional self-assembly by ice crystallization
No full textThree-dimensional self-assembly of colloidal particles by ice crystallization is observed in a water based-colloidal suspension. When water containing polystyrene beads freezes and is crystalized into ice, the polystyrene beads are extruded outward from the ice regions. Consequently, the concentration of polystyrene beads increases rapidly and they are assembled together into a regular structure. As ice crystallization proceeds, a color appears abruptly. This indicates that the polystyrene beads have been assembled three dimensionally. The generated three-dimensional structure is confirmed by scanning electron microscopy images and the existance of a photonic band gap measured by an ultraviolet-visible spectrophotometer. The sample fabricated by this method is compared with a sample made by conventional vertical deposition. (C) 2002 American Institute of Physics
On improving time-delay control under certain hard nonlinearities
No full textTime-delay control (TDC), owing to its simplicity and unusual performance against parameter variations and disturbances, has been noted and recognized as a promising technique. We have observed, however, that in the presence of so-called hard nonlinearity such as saturation or static friction, TDC reveals some problems commonly found in other methods like PID control or disturbance observer. Specifically, when the system has a saturation limit, TDC shows the windup phenomenon; when static friction and Stribeck effect are dominant, TDC reveals the stick-slip phenomenon. In this paper, we have reported these phenomena associated with TDC, presented their causes, and proposed their remedies. Specifically, through analysis, simulation, and experiment, we have shown that both the phenomena cause serious degradations in control performance; that they result from a common cause, the inherent integral effect in TDC; and that the proposed compensators are effective enough to handle the phenomena. In addition to their effectiveness, the proposed compensators have unusual simplicity and efficiency that matches the positive attributes of the original TDC. (C) 2002 Elsevier Science Ltd. All rights reserved
Effect of evaporation temperature on the quality of colloidal crystals at the water-air interface
No full textThree-dimensional assemblies of 230 nm polystyrene (PS) colloidal particles were prepared on the suspension surface by evaporating the water in which the particles were suspended. The assembled colloidal particles were then transferred from the water surface onto a glass substrate by simple evaporation and sedimentation. In this study, we analyzed the structures of the colloidal assemblies formed at evaporation temperatures of 30, 40, 60, and 90 degreesC. At 30 degreesC, the rate of particle sedimentation is faster than the rate of crystallization on the water surface. Consequently, the PS particles randomly stack on the glass substrate before forming nuclei on the water surface. At higher evaporation temperatures, on the other hand, the rate of crystallization on the water surface exceeds the sedimentation rate, leading to an improvement in the quality of the resulting colloidal crystal. However, crystalline quality diminishes at evaporation temperatures greater than 60 degreesC because the high crystal growth rate leads to the formation of defects. As a result, there exists an optimum evaporation temperature that yields the highest quality crystals. Importantly, this novel process enables the rapid (within I h) fabrication of large-scale three-dimensional colloidal crystals
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