Browsing by Author "Li, YS"

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In-situ measurement of the crystallization process of polycarbonate/epoxy resin by a photoresistor
(2000-03-01)
Li, YS
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Chang, FC
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應用化學系
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Department of Applied Chemistry
The PC crystallization process is measured by the photoresistor, by which the reciprocal relationship is obtained between the illumination of transmission light and the resistance. In the beginning of annealing the change of resistance is insignificant when the crystallization is still at the induction stage. Then the increasing number and size of spherulites scatter the incoming light and the resistance increases quickly. Finally the concentration of amorphous PC decreases and the increasing rate of resistance slows down. The X-ray pattern shows an ordered PC structure of 2 theta = 11 degrees in the induction period, then converts to amorphous (broad peak at 20 degrees) and spherulite (17.2 degrees) structure. The dissolving heat of PC spherulite, analyzed by differential scanning calorimetry, shows a linear relationship to the resistance. Thus the relative crystallinity can be obtained by measuring the resistance of the photoresistor from both primary and secondary crystallization. The experimental results show good correlation to the Avrami model and the highest crystallization rate is obtained at 80 degrees C. The thermodynamic factor controls the crystallization rate at higher annealing temperature while the kinetic factor dominates the crystallization rate at lower annealing temperatures. (C) 1999 Elsevier Science Ltd. All rights reserved.
Kinetics and curing mechanism of epoxy and boron trifluoride monoethyl amine complex system
(1999-09-15)
Li, YS
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Li, MS
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Chang, FC
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應用化學系
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Department of Applied Chemistry
The kinetics of cationic polymerization of epoxy resin has been studied. Due to multiple reaction exotherms and irregular baselines involved in this system, the ASTM E 698 method was chosen to determine the kinetic parameters of this epoxy/BF3-MEA system. The DER 332/BF3-MEA system follows the first order active chain end (ACE) reaction mechanism. The adding of the hydroxyl group into the system is prone to decrease the activation energy and shifts the curing into the activated monomer (AM) mechanism due to the stronger nucleophilicity of the hydroxyl group than the epoxide group. On the other hand, the DER 331/BF3-MEA system possesses lower activation energy because the DER 331 contains more hydroxyl groups-alpha-glycol and secondary alcohol. The former is the major contributor in lowering the activation energy. The hydroxyl group also can act as a chain transfer agent by retarding the propagation process, thus the reaction rate of the DER 332/BF3-MEA system is higher than the DER 331/BF3-MEA system. The DER 332/BF3-MEA system shows good correlation between the experimental data (FT-IR, DSC, and GPC) and the simulation curve based on ASTM E 698 before gelation, while a larger deviation is observed in the DER 331/BF3-MEA system. After gelation, the hydroxyl group also acts as an active site to react with residual epoxy monomers and obtain a more condense matrix. (C) 1999 John Wiley & Sons, Inc.
On greedy construction of connected dominating sets in wireless networks
(2005-12-01)
Li, YS
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Thai, MT
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Wang, F
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Yi, CW
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Wan, PJ
Since no fixed infrastructure and no centralized management present in wireless networks, a connected dominating set (CDS) of the graph representing the network is widely used as a virtual backbone. Constructing a minimum CDS is NP-hard. In this paper, we propose a new greedy algorithm, called S-MIS, with the help of Steiner tree that can construct a CDS within a factor of 4.8 + ln5 from the optimal solution. We also introduce the distributed version of this algorithm. We prove that the proposed algorithm is better than the current best performance ratio which is 6.8. A simulation is conducted to compare S-MIS with its variation which is rS-MIS. The simulation shows that the sizes of the CDSs generated by S-MIS and rS-MIS are almost the same. Copyright (c) 2005 John Wiley & Sons, Ltd.
Purification and characterization of neutral sphingomyelinase from Helicobacter pylori
(2000-04-25)
Chan, EC
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Chang, CC
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Li, YS
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Chang, CA
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Chiou, CC
Phospholipase activities of human gastric bacterium, Helicobacter pylori , are regarded as the pathogenic factors owing to their actions on epithelial eel membranes. In this study, we purified and characterized neutral sphingomyelinase (N-SMase) from the superficial components of H. pylori strains for the first time. N-SMase was purified 2083-fold with an overall recovery of 37%. The purification steps included acid glycine extraction, ammonium sulfate precipitation, CM-Sepharose, Mono-Q, and Sephadex G-75 column chromatography. Approximate molecular mass for the native N-SMase was around 32 kDa. When N-omega-trinitrophenylaminol auryl sphingomyelin (TNPAL-SM) was used as a substrate, the purified enzyme exhibited a K-m of 6.7 mu M and a V-max of 15.6 nmol of TNPAL-sphingosine/h/mg of protein at 37 degrees C in 50 mM phosphate-buffered saline, pH 7.4. N-SMase I-caches optimal activity at pH 7.4 and has a pi of 7.5. The enzyme activity is magnesium dependent and specifically hydrolyzed sphingomyelin and phosphatidylethanolamine. The enzyme also exhibits hemolytic activity on human erythrocytes. According to Western blot analysis, a rabbit antiserum against purified N-SMase from H. pylori cross-reacted with SMase from Bacillus cereus. Sera from individuals with H, pylori infection but not uninfected ones recognizing the purified N-SMase indicated that it was produced in vivo. In enzyme linked immunosorbent assays, the purified N-SMase used as an antigen was as effective as crude protein antigens in detecting human antibodies to H. pylori.
Synthesis and physical properties of linear and branch phenoxies
(2001-12-01)
Chen, JR
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Li, YS
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Chang, FC
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應用化學系
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Department of Applied Chemistry
Linear and branch phenoxies have been synthesized by reacting difunctional epoxy and diphenol or triphenol monomer, using phosphonium salt as the catalyst. The reaction kinetics is analyzed by the ASTM E 698 method, which is suitable for a first order reaction system. The bulky structure of the bisphenol Z moiety retards the reaction; therefore, the reaction temperature required is higher- at 200 degreesC, compared to 190 degreesC for the bisphenol A based system. The introduction of a branch structure increases both molecular weight and the polydispersity of the phenoxy. A slightly branched structure tends to increase the distance between the polymer chains and results in a lower shear viscosity for easier processing. A higher branch results in light crosslinking and decreases the mobility of the polymer. The branch structure has a negligible effect on its glass transition temperature because the average distance between branches is substantially longer than the critical mainchain length for T-1's Brownian motion. The 100/1 (bisphenol/ trisphenol) system has the lowest viscosity, but its mechanical properties and the gas permeation properties are poorer than those of the others. The bisphenol Z based phenoxy shows superior mechanical properties, higher T-g, lower coefficient of thermal expansion, and lower gas permeation properties than the corresponding bisphenol A based phenoxy because of the steric hindrance structure of the bisphenol Z moiety.
The kinetics, thermal and mechanical properties of epoxy-polycarbonate blends cured with aromatic amine
(1998-04-01)
Lu, MS
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Chen, JL
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Li, YS
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Chang, FC
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Li, MS
PC-DER331 blends are transparent and homogeneous due to the transesterification taking place during hot melting process. This transesterification reaction does not occur or occurs insignificantly during the preparation of PC-DER332 blends. PC is separated from the PC-DER332 melt mixture by slow cooling to room temperature but the mixture remains a single phase by quenching with an ice bath. Two systems of PC-epoxy blends, PC-DER331 blends and PCDER332 blends are cured by the m-phenylene diamine (MPDA) in a stoichiometric ratio. Curing kinetics have been carried out by differential scanning calorimeter (DSC). The presence of PC accelerates the curing reaction. Infrared spectroscopy (IR) indicates the occurrence of transesterification in the PC-DER331/MPDA blends during curing. The flexural modulus increases with the increase of the PC content while the notched Izod impact strength decreases with the increase of the PC content for all the blending systems. The fracture surfaces of the PC-DER331/MPDA blends are smooth, an indication of a homogeneous morphology. The fracture surfaces of the PC-DER332/MPDA blends are rough, an indication of a heterogeneous morphology.
The reactivity of epoxy/polycarbonate/BF3-MEA system
(2001-03-01)
Li, YS
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Chang, FC
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應用化學系
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Department of Applied Chemistry
The cationic reaction of an epoxy/BF3-MEA system occurs between the oxirane group and the oxonium group, The addition of polycarbonate to the system does not cause a transesterification reaction between the carbonate group and the oxonium group, thus the molecular weight of the PC is unchanged during the reaction. PC crystallization is observed in the cured system because the epoxy monomer accelerates the PC crystallization. The eposy/BF3-MEA and epoxy/PC/BF3-MEA systems all follow the first order reaction. The PC modified systems show lower activation energy, a lower pre-exponential factor and a higher reaction rate constant.