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991.
Jong Hak Kim Byoung Ryul Min Jongok Won Yong Soo Kang 《Journal of Polymer Science.Polymer Physics》2006,44(8):1168-1178
When polymer–silver salt complex membranes were exposed to UV irradiation, the separation performances of both the permeance and selectivity for propylene–propane decreased, which was primarily attributed to the reduction of the silver ions in the membranes to silver nanoparticles. Here, the effect of the polymer matrix on the formation of silver nanoparticles in the polymer–silver salt complex membranes was investigated. This effect was assessed for the complexes of two kinds of silver salts (AgBF4 and AgCF3SO3) with several polymeric ligands containing three different carbonyl groups, including poly(vinyl pyrrolidone) (PVP) with an amide group, poly(vinyl methyl ketone) (PVMK) with a ketone group, and poly(methyl methacrylate) (PMMA) with an ester group. UV–vis spectra and transmission electron microscopy (TEM) images clearly indicated that the reduction rate of the silver ions has the following order in the various polymer matrices: PVP > PVMK > PMMA, whereas the size and the distribution of the nanoparticles exhibited the reverse order. The tendency to form silver nanoparticles was explained in terms of the differences between the comparative strengths of the interactions of the silver ions with the different carbonyl oxygens in the matrices, as well as that of the silver ions with counteranions, which was characterized by X‐ray photoelectron spectroscopy (XPS) and FT‐Raman spectroscopy. It was concluded that when the concentration of free silver ions was low due to weak polymer–silver ion and strong silver ion–anion interactions, as found with PMMA, the reduction rate of silver ions to silver nanoparticles was slow. Therefore, the PMMA–silver complex membranes were less sensitive to decreases in separation performance upon UV irradiation than compared to the PVP membranes. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1168–1178, 2006 相似文献
992.
Seng Kue Lee Chang Won Park Jong Gun Lee Kyung-Tae Kang Koushi Nishida Yoshio Shimbo Yoichi Takanishi Hideo Takezoe 《Liquid crystals》2005,32(10):1205-1212
New chiral banana-shaped liquid crystals with chiral 3-(alkoxy)propoxy terminal groups (Pn-O-PIMB5*-4O, n = 7, 8, 9 and 10) were synthesized and their mesomorphic properties and phase structures investigated by means of electro-optic measurements, polarizing optical microscopy, differential scanning calorimetry, and second harmonic generation measurements. Most of these chiral bent-core mesogens (n = 7-9) showed the antiferroelectric B2 phase, whereas P10-O-PIMB5*-4O exhibited the B7 phase. Comparing with the previously reported homologue Pn-O-PIMB(n-2)*, we conclude that the terminal chain structure, particularly the position of chiral centres, plays an important role in the emergence of particular phase structures. 相似文献
993.
994.
Won Seok Chung Ohmyoung Kwon Du Seon Choi Seungho Park Young Ki Choi Joon Sik Lee 《Superlattices and Microstructures》2004,35(3-6):325
This paper suggests and demonstrates a novel flow measurement technique: tunable AC thermal anemometry, that allows simple integration, robust measurement and extremely high accuracy. The principle and simple theoretical analysis of the technique are presented. To find the optimal condition at which the phase lag becomes most sensitive to the flow speed change, the phase lag was measured scanning the heating frequency from 1 to 100 Hz, while the flow speed of ethanol was increased stepwise from 0 to 40 mm/s. The sensitivity of the phase lag depended on the heating frequency and the flow speed. It was possible to measure the average flow speed of 0.7 mm/s with the resolution of 0.1 mm/s at 4 Hz. 相似文献
995.
996.
Hoon Hyun Jae Song Cho Byung Soo Kim Jung Won Lee Moon Suk Kim Gilson Khang Kinam Park Hai Bang Lee 《Journal of polymer science. Part A, Polymer chemistry》2008,46(6):2084-2096
In this article, we describe the synthesis of PEG‐b‐polyester star block copolymers via ring‐opening polymerization (ROP) of ester monomers initiated at the hydroxyl end group of the core poly(ethylene glycol) (PEG) using HCl Et2O as a monomer activator. The ROP of ε‐caprolactone (CL), trimethylene carbonate (TMC), or 1,4‐dioxan‐2‐one (DO) was performed to synthesize PEG‐b‐polyester star block copolymers with one, two, four, and eight arms. The PEG‐b‐polyester star block copolymers were obtained in quantitative yield, had molecular weights close to the theoretical values calculated from the molar ratio of ester monomers to PEG, and exhibited monomodal GPC curves. The crystallinity of the PEG‐b‐polyester star block copolymers was determined by differential scanning calorimetry and X‐ray diffraction. Copolymers with a higher arm number had a higher tendency toward crystallization. The crystallinity of the PEG‐b‐polyester star block copolymers also depended on the nature of the polyester block. The CMCs of the PEG‐b‐PCL star block copolymers, determined from fluorescence measurements, increased with increasing arm number. The CMCs of the four‐arm star block copolymers with different polyester segments increased in the order 4a‐PEG‐b‐PCL < 4a‐PEG‐b‐PDO < 4a‐PEG‐b‐PLGA < 4a‐PEG‐b‐PTMC, suggesting a relationship between CMC and star block copolymer crystallinity. The partition equilibrium constant, Kv, which is an indicator of the hydrophobicity of the micelles of the PEG‐polyester star block copolymers in aqueous media, increased with decreasing arm number and increasing crystallinity. A key aspect of the present work is that we successfully prepared PEG‐b‐polyester star block copolymers by a metal‐free method. Thus, unlike copolymers synthesized by ROP using a metal as the monomer activator, our copolymers do not contain traces of metals and hence are more suitable for biomedical applications. Moreover, we confirmed that the PEG‐b‐polyester star block copolymers form micelles and hence may be potential hydrophobic drug delivery vehicles. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 2084–2096, 2008 相似文献
997.
Jong Hak Kim Byoung Ryul Min Chang Kon Kim Jongok Won Yong Soo Kang 《Journal of Polymer Science.Polymer Physics》2004,42(2):232-237
The difference between the polymer matrices of poly(2‐ethyl‐2‐oxazoline) (POZ) and poly(N‐vinyl pyrrolidone) (PVP) does not have a significant effect on the facilitated propylene transport and propylene solubility in 1:1 polymer/silver salt complex membranes, according to our previous work. In this article, its origin is investigated in terms of both microstructures of silver polymer electrolytes and the coordinative interaction of silver ion with polymer and with the counteranion. Initially different microstructures of POZ and PVP become similar to each other upon dissolving a large amount of silver salt, as evidenced by propane transport properties, specific volume, and Bragg d‐spacing. The dissolution of the silver salt in the polymer solvent strongly depends on the coordinative interaction between silver ion and carbonyl oxygen of POZ and PVP. Thus, the structural similarity upon dissolving silver salts in POZ and PVP is primarily determined by the coordinative interaction between silver ion and carbonyl oxygen, which was confirmed by theoretical structure calculation based on density functional theory and by IR and Raman spectroscopy. Therefore, facilitated olefin transport for silver polymer electrolyte membranes does not strongly depend on the polymeric matrix at high silver concentrations. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 232–237, 2004 相似文献
998.
999.
1000.
We report the effect of the external electric field on the photoluminescence (PL) properties of ZnO films grown by a pulsed laser deposition method. The PL quenching of bound excitons under the electric field was attributed to a decrease in the capture cross section of the radiative centers. In addition, the change in the surface/grain boundaries charge induced a degradation of the 3.33 eV emission line over the whole sample, which remained even after voltage removal. Besides the PL degradation, this emission at 3.33 eV demonstrates the change in the thermal quenching process, where the activation energy of exciton detachment corresponds to its binding energy. All behaviors were restored to the initial state by application of the voltage with opposite polarity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim) 相似文献