Browsing by Author "Wei, KH"

Now showing 1 - 20 of 62

A study on blends of liquid crystalline copolyesters with polycarbonate .2. Transesterification control
(1996-01-31)
Wei, KH
;
Su, KF
;
交大名義發表
;
材料科學與工程學系
;
National Chiao Tung University
The inhibited and catalyzed ester exchange (transesterification) during melt blending of poly(bisphenol-A carbonate) (PC) and liquid crystalline poly(oxybenzoate-co-ethylene terephthalate) (POB-PET 40/60; P46) was investigated with differential scanning calorimetry. It was found that the ester exchange between P46 and PC was effectively inhibited for a 20% P46 blend at 240 degrees C, as further confirmed by nuclear magnetic spectroscopy. When the blending temperature and P46 concentration increased, only the transesterification between the PET segment in P46 and PC took place under inhibition. The morphology of the blends was analyzed with scanning electron microscopy and displayed a disconnected interface between P46 and PC under inhibition. Conversely, the transesterification took place between the POB segment in P46 and PC when a catalyst was added. (C) 1996 John Wiley & Sons, Inc.
Binding effect of surface-modified cadmium sulfide on the microstructure of PS-b-PEO block copolymers
(2004-06-15)
Jeng, US
;
Sun, YS
;
Lee, HY
;
Hsu, CH
;
Liang, KS
CdS nanoparticles with surfaces modified by mercaptoethanol have been synthesized for greater solubility in dimethylformamide (DMF) and the ability to preferentially bind to the poly(ethylene oxide) (PEO) blocks of polystyrene-block-poly(ethylene oxide) (PS-b-PEO) copolymer. The size of the surface-modified CdS nanoparticles (M-CdS) in DMF can be characterized by the Schultz distribution of a mean radius of 18.4 Angstrom and a polydispersity of 0.5, using small-angle X-ray scattering (SAXS). These M-CdS nanoparticles can further disperse in the PS-b-PEO, with a slightly larger mean radius than that observed in DMF. The uptake of M-CdS nanoparticles for the PEO blocks of the PS-b-PEO saturates around 7% in volume fraction. The binding of the nanoparticles promotes a segregation of the PEO/nanoparticles and leads to spherical PEO/M-CdS microdomains in the composite film with a greatly enhanced thermal stability. The structural information obtained from X-ray scattering agrees well with the transmission electron microscopy images and differential scanning calorimetry results.
Bulk and surface properties of layered silicates/fluorinated polyimide nanocomposites
(2002-11-15)
Jiang, LY
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Intercalated layered silicates/fluorinated polyimide nanocomposites with good overall properties are synthesized from organics-modified-montmorillonite and poly(amic acid), as confirmed by x-ray and transmission electron microscopy results. Both the bulk and surface (nanometer-domain) mechanical properties of synthesized silicates/fluorinated polyimide nanocomposites increased substantially as compared to that of pure polyimide. In particular, the Young's modulus and surface hardness of the fluorinated polyimide containing 5 wt % 2NH(2)-mont are 69% and 100% larger than that of pure polyimide, respectively. Additionally, the reduced elastic modulus by nanoindentation of the nanocomposites is 74% larger than that of the pure polyimide. The barrier properties of the nanocomposites, such as thermal expansion and water absorption retardation, are enhanced. (C) 2002 American Institute of Physics.
CdS nanoparticles induce a morphological transformation of poly(styrene-b-4-vinylpyridine) from hexagonally packed cylinders to a lamellar structure
(2005-07-26)
Yeh, SW
;
Wei, KH
;
Sun, YS
;
Jeng, US
;
Liang, KS
Mercaptoacetic acid-modified CdS nanoparticles disperse selectively in the poly(4-vinylpyridine) domains of poly(styrene-b-4-vinylpyridine) (S4VP) block copolymer as a result of the formation of hydrogen bonds. This process in turn induces the morphological transformation of S4VP from a hexagonally packed cylinder structure into a lamellar structure, as evidenced from transmission electron microscopy, atomic force microscopy, and small-angle X-ray scattering studies. This transformation is consistent with that suggested by a theoretical prediction of the phase diagram of the mixed diblock copolymer/nanoparticle system.
Curing kinetics and viscosity change of a two-part epoxy resin during mold filling in resin-transfer molding process
(2000-09-06)
Lee, CL
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The curing kinetics and the resulting viscosity change of a two-part epoxy/amine resin during the mold-filling process of resin-transfer molding (RTM) of composites was investigated. The curing kinetics of the epoxy/amine resin was analyzed in both the dynamic and the isothermal modes with differential scanning calorimetry (DSC). The dynamic viscosity of the resin at the same temperature as in the mold-filling process was measured. The curing kinetics of the resin was described by a modified Kamal kinetic model, accounting for the autocatalytic and the diffusion-control effect. An empirical model correlated the resin viscosity with temperature and the degree of cure was obtained. Predictions of the rate of reaction and the resulting viscosity change by the modified Kamal model and by the empirical model agreed well with the experimental data, respectively, over the temperature range 50-80 degrees C and up to the degree of cure alpha = 0.4, which are suitable for the mold-filling stage in the RTM process. (C) 2000 John Wiley & Sons, Inc.
Effect of incorporated CdS nanoparticles on the crystallinity and morphology of poly (styrene-b-ethylene oxide) diblock copolymers
(2005-05-15)
Yeh, SW
;
Wu, TL
;
Wei, KH
;
Sun, YS
;
Liang, KS
Surface-modified CdS nanoparticles selectively dispersed in hexagonally packed poly(ethylene oxide) (PEO) cylinders of poly(styrene-b-ethylene oxide) (PSEO) block copolymers were prepared. The photoluminescence and ultraviolet-visible characteristics of the presynthesized CdS nanoparticles in N,N-dimethylformamide and in PEO domains of the PSEO block copolymers were determined. Because of strong interactions between the CdS nanoparticles and PEO chains, as shown by Fourier transform infrared spectroscopy, the incorporation of the CdS nanoparticles prevented the PEO cylinders from properly crystallizing; this was confirmed by differential scanning calorimetry and wide-angle X-ray diffraction measurements. The intercylinder distance between the swollen and reduced-crystallinity CdS/PEO cylinders in turn increased, as confirmed by small-angle X-ray scattering and transmission electron microscopy. At a high CdS concentration (43 wt % or 8.3 vol % with respect to PEO), however, the hexagonally packed cylindrical nanostructure of the PSEO diblock copolymers was destroyed. (c) 2005 Wiley Periodicals, Inc.
Effect of material and process variables on the performance of resin-transfer-molded epoxy fabric composites
(2000-09-06)
Lee, CL
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The effects of material and process variables on glass fabric-reinforced epoxy composites by the resin-transfer molding (RTM) process were studied. It was found that the molded aged resin with 55% fiber exhibited twice the mold-filling time and caused a 7-15% deterioration in the interlaminar shear strength (ILSS) and in the flexural strength of the composites as compared to those of the composites molded with fresh resin. At a 55% fiber volume fraction, composites molded with aged resin resulted in a 35% longer filling time and a 4-12% decreased ILSS and flexural strength as compared to those of the composites at a 44% fiber volume fraction. Moldings with a perimeter inlet exhibited a 65% shorter mold-filling time, 28% reduced void content, and 6% improved flexural strength as compared to those of the composites molded with the center inlet. (C) 2000 John Wiley & Sons, Inc.
Effect of montmorillonite on thermal and moisture absorption properties of polyimide of different chemical structures
(2001-08-15)
Tyan, HL
;
Wu, CY
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The thermal properties and the moisture absorption of three types of polyimide/montmorillonite nanocomposite were investigated: 3,3 ' ,4,4 ' -biphenyl tetracarboxylic dianhydride-4,4 ' -oxydianiline (BPDA-ODA); pyromellitic dianhydride-ODA (PMDA-ODA); and 3,3 ' ,4,' -benzophenone tetracarboxylic dianhydride-ODA (BTDA-ODA). The inhibition effect on in-plane coefficients of thermal expansion (CTE) and moisture absorption of these polyimide nanocomposites by layered silicates from montmorillonite was found to decrease with the crystallinity in the pristine polyimides. The largest reduction, 30% in in-plane CTE occurred in the case of amorphous BTDA-ODA containing 5 wt % montmorillonite as compared with that of pure BTDA-ODA, while the reduction in in-plane CTE was 20% for the case of semicrystalline BPDA-ODA. The maximum reduction in moisture absorption, 43%, also took place for the case of 3/97 ODA-Mont/BTDA-ODA as compared with that of pure BTDA-ODA, whereas the semicrystalline 1/99 PPD-Mont/BPDA-ODA showed a 30% reduction as compared with that of pure BPDA-ODA. (C) 2001 John Wiley & Sons, Inc.
Effect of reactivity of organics-modified montmorillonite on the thermal and mechanical properties of montmorillonite/polyimide nanocomposites
(2001-01-01)
Tyan, HL
;
Leu, CM
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Montmorillonite/polyimide (3,3',4,4'-benzophenone tetracarboxylic dianhydride-4,4'-oxydianiline, BTDA-ODA) nanocomposites displaying different morphological, thermal, and mechanical characteristics were synthesized by involving mono-, di-, and trifunctional group swelling-agent-modified montmorillonite and poly(amic acid). It was found that the dispersion of silicate layers in BTDA-ODA shifted from intercalated to exfoliated structures when the number of functional groups of the swelling agent increased to three as evidenced from X-ray diffraction and transmission electron microscopy studies. The improved morphology of the nanocomposites resulted in their enhanced mechanical and thermal properties. In particular, a 70% increase in Young's modulus, a 50% increase in the maximum stress, and a 30% reduction in the coefficients of thermal expansion for the case of BTDA-ODA nanocomposites containing 5 wt % trifunctional group swelling-agent-modified montmorillonite as compared to that of pure BTDA-ODA.
Effect of surface-hydroxylated CdS nanoparticles on the morphological transformation of polystyrene-block-poly(ethylene oxide) thin films
(2004-10-06)
Yeh, SW
;
Chang, YT
;
Chou, CH
;
Wei, KH
;
材料科學與工程學系
Polystyrene-block-poly(ethylene oxide) (SEO) block copolymer thin films, in which CdS clusters have been sequestered into the PEO domains of the SEO block copolymers, are found to induce the morphological transformation of PEO from cylinders to spheres, as shown by using atomic force microscopy (AFM), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). This transformation is caused by the presence of hydrogen-bonding interactions between surface-hydroxylated CdS and PEO, as confirmed by nuclear magnetic resonance (NMR) studies.
Effect of the orientation of CdSe nanorods on the electron mobility of CdSe/P4VP nanodomains self-assembled within a poly(styrene-b-(4)-vinyipyridine) diblock copolymer thin film
(2006-03-01)
Li, CP
;
Yeh, SW
;
Chang, HC
;
Huang, JY
;
Wei, KH
Enhanced collective electron transport by CdSe quantum dots confined in the poly(4vinylpyridine) nanodomains of a poly(styrene-b-4-vinylpyridine) diblock copolymer thin film
(2006)
Li, CP
;
Wei, KH
;
Huang, JY
;
材料科學與工程學系
;
光電工程學系
Enhanced electroluminescence of poly(2-methoxy-5-(2 '-ethylhexyloxy)-1, 4-phenylene vinylene) films in the presence of TiO2 nanocrystals
(2006-06-01)
Weng, CC
;
Chou, CH
;
Wei, KH
;
Huang, JY
;
材料科學與工程學系
We demonstrated that in a nanocrystalline TiO2/poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) composite, electroluminescence of this composite is enhanced via addition of TiO2 nanoneedles. The TiO2 nanoneedles enhance the partial crystallization of MEH-PPV around TiO2, which in turn causes a decrease in the hole barrier height and an increase in hole mobility. The I-V measurement was established on an indium tin oxide/MEH-PPV:TiO2/Al device to identify the electrical properties of the composites.
Enhanced luminance and thermal properties of poly(phenylenevinylene) copolymer presenting side-chain-tethered silsesquioxane units
(2005-11-01)
Chou, CH
;
Hsu, SL
;
Yeh, SW
;
Wang, HS
;
Wei, KH
We have synthesized, using the Gilch polymerization method, a new series of high-brightness, soluble copolymers (POSS-PPV-co-MEHPPV) of poly(p-phenylenevinylene) (PPV) containing side-chain-tethered polyhedral oligomeric silsesquioxane (POSS) pendent units and poly(2-methoxy-5-[2-ethylhexyloxy]-1,4-phenylenevinylene) (MEHPPV). This particular molecular architecture of POSS-PPV-co-MEHPPV copolymers possesses not only a larger quantum yield (0.85 vs 0.19) but also higher degradation and glass transition temperatures relative to those of pure MEHPPV. The maximum brightness of a double-layered-configured light-emitting diode (ITO/PEDOT/emissive polymer/Ca/Al) incorporating a copolymer of MEHPPV and 10 mol % PPV-POSS was 5 times as large as that of a similar light-emitting diode incorporating pure MEHPPV (2196 vs 473 cd/m(2)).
Enhancement of imidization of poly(amic acid) through forming poly(amic acid)/organoclay nanocomposites
(1999-08-01)
Tyan, HL
;
Liu, YC
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The imidization kinetics of nanocomposites of poly(amic acid) and organoclay were analyzed with in situ Fourier Transform Infrared spectra at several temperatures. The poly(amic acid) studied consisted of pyromellitic dianhydride and 4,4'-oxydianiline. It was found that by dispersing a small amount of organoclay in nanometer scale in the poly(amic acid), both the imidization temperature and the imidization time of the poly(amic acid) can be reduced dramatically. In specific, when two parts of exfoliated silicate layers of organoclay was dispersed in the poly(amic acid), the imidization temperature was lowered by 50 degrees C (250 degrees C versus 300 degrees C) for achieving a complete imidization. Additionally, the imidization time of the poly(amic acid) at 250 degrees C can be reduced to 15 min when the amount of organoclay increased to seven parts. Using a first-order reaction to model the imidization kinetics of poly(amic acid)/organoclay, a 20% drop in the activation energy for imidization of PMDA-ODA/organoclay (7 phr) was obtained as compared to that for imidization of neat PMDA-ODA. (C) 1999 Elsevier Science Ltd. All rights reserved.
Ester-amide exchange in blends of liquid-crystalline copolyester and polyamide 6
(2000-08-15)
Ho, JC
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The kinetics of the ester-amide exchange in solution blends of the random liquid-crystalline polyester copoly(oxybenzoate-terephthalate) (P64) and polyamide 6 (PA6) were studied with carbon nuclear magnetic resonance. With second-order reversible reactions assumed, the activation energy of the ester-amide interchange in 30/70, 50/50, and 70/30 P64/PA6 blends were all about 24.0 kcal/mol. The pre-exponential factors for the ester-amide exchange in 30/70, 50/50, and 70/30 P64/PA6 blends were 2.01 x 10(11), 2.59 x 10(10), and 2.74 x 10(12) min(-1), respectively. (C) 2000 John Wiley & Sons, Inc.
High-tensile-property layered silicates/polyurethane nanocomposites by using reactive silicates as pseudo chain extenders
(2001-12-18)
Tien, YI
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Tethered layered silicates/polyurethane nanocomposites displaying high tensile strength and high elongation to break were synthesized by using reactive swelling-agent-modified silicates as pseudo chain extenders for a polyurethane prepolymer. The dispersion of layered silicates in polyurethane was found to be transformed from an intercalated to an exfoliated structure when the number of hydroxyl groups of the swelling agent increased as evidenced from the transmission electron microscopy and wide-angle X-ray diffraction analyses. The improved morphology of the nanocomposites resulted in an ultrahigh efficiency in enhancing the mechanical properties of polyurethane, despite variations in the molecular weight and in the extent of hydrogen bonding in the hard segment phase. In particular, a 34% increase in Young's modulus, a 1.7-fold increase in the tensile strength, and a 1.3-fold increase in the elongation to break occurred in the nanocomposite of polyurethane containing only 1 wt % trihydroxyl-group swelling-agent-modified silicates as compared to those of pristine polyurethane.
Hydrogen bonding and mechanical properties in segmented montmorillonite/polyurethane nanocomposites of different hard segment ratios
(2001-03-01)
Tien, YI
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Hydrogen bonding in the hard segments of the synthesized montmorillonite/polyurethane nanocomposites of various hard segment ratios was found to decrease with the increasing amount of montmorillonite regardless of the hard segment ratios, but reached plateau values at 5 wt% montmorillonite concentration. The maximal reductions of the hydrogen bonding in the polyurethane nanocomposites ranged from 20 to 37%, depending on the hard segment ratios as compared to that in the pure polyurethane. The maximal strength and the elongation at break of the polyurethane nanocomposites increased dramatically as compared to that of pristine polyurethane, and the maximal values occurred at 1 wt% montmorillonite concentration. (C) 2001 Elsevier Science Ltd. All rights reserved.
Induced gamma -> alpha crystal transformation in blends of polyamide 6 and liquid crystalline copolyester
(2000-07-11)
Ho, JC
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
The equilibrium melting temperatures and the induced gamma --> ct crystal transformation in blends of random liquid crystalline copoly(oxybenzoate-ethylene terephthalate) at molar ratio 60/40 (P64) and polyamide 6 (PA6) under different thermal treatments were investigated with differential scanning calorimetry and wide-angle X-ray diffraction, respectively. It was found that the equilibrium melting temperatures of P64/PA6 blends depended strongly on the amount of P64, and the presence of a small amount of P64 in PA6, around 5%, induced additional crystallization of the or-form crystal in the quenched PA6. Moreover, the onset temperature for the gamma --> alpha crystal transformation in the 5/95 P64/PA6 blend was reduced by 30 degrees C as compared to that in the pure PA6 case (160 degrees C vs 130 degrees C).
Layered silicates/fluorinated polyimide nanocomposites for advanced dielectric materials applications
(2002-03-18)
Jiang, LY
;
Leu, CM
;
Wei, KH
;
材料科學與工程學系
;
Department of Materials Science and Engineering
Structured silicates/fluorinated polyimide nanocomposites display excellent water-absorption retardation behavior. This leads to lower leakage-current densities and lower dielectric constants under higher humidity conditions than those of the pristine fluorinated polyimides. Part of the chemical structure of the lavered silicate used in this work is shown in the Figure.