Wei Xie

Western Kentucky University, Bowling Green, KY, United States

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Publications (18)29.36 Total impact

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    ABSTRACT: Organophilic montmorillonite was prepared using ion-exchange method between sodium ions in clay layers and stearyltrimethyl ammonium chloride in the various solvents, including deionized water, ethanol, acetone, and toluene. The montmorillonite has the largest d001 spacing, as determined by X-ray diffraction in toluene, than the other solvents considered. Ethanol can completely wash out the overexchanged stearyltrimethyl ammonium chloride among layers of montmorillonite. However, deionized water is the preferred ion-exchange solvent. The thermal stability of organophilic montmorillonite was investigated by high-resolution thermogravimetric analysis (TGA). Polystyrene–montmorillonite nanocomposites were obtained by suspension free radical polymerization of styrene in the dispersed organophilic montmorillonite. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed that montmorillonite had been exfoliated. 5.0 wt % of clay in the synthesized nanocomposite was found to be the optimum content that improved both thermal and mechanical properties over those of pure polystyrene under the experimental conditions applied. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 101–109, 2004
    Journal of Applied Polymer Science 01/2004; 91(1):101 - 109. · 1.40 Impact Factor
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    ABSTRACT: Polystyrene-Organo Montmorillonite (PS-MMT) nanocomposites were prepared by suspension free radical polymerization of styrene in the dispersed organophilic montmorillonite. The results of X-ray diffraction (XRD) and Transmission Electron Microscopy (TEM) indicated that exfoliated nanocomposites were achieved. The effect of organic modifiers (surfactants) on the properties of the synthesized nanocomposites was studied. It is found that polystyrene-MMT nanocomposite with 5.0 wt% of organo-MMT gave the greatest improvement in thermal stability, and polystyrene-MMT nanocomposites with 7.5 wt% of organo-MMT showed the greatest improvement in mechanical properties, compared with that of pure polystyrene (PS) in our experimental conditions. The alkyl chain length of surfactant used in fabricating organo-MMT affects the synthesized PS nanocomposites: the longer the alkyl chain length that the surfactant possesses, the higher the glass transition temperature of the PS nanocomposite, However, the organoclay in the nanocomposites seems to play a dual role: (a) as nanofiller leading to the increase of storage modulus and (b) as plasticizer leading to the decrease of storage modulus. This results in a lower storage modulus of PS-TMOMMT and PS-TMTMMT nanocomposites than that of PS-TMDMMT and PS-TMCMMT nanocomposites. Further study is needed to confirm the above hypothesis.
    Polymer Engineering and Science 12/2002; 43(1):214 - 222. · 1.24 Impact Factor
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    ABSTRACT: Organically modified layered silicates (OLS) with high thermal stability are critical for synthesis and processing of polymer layered silicate nanocomposites (PLSN). In the current study, the non-oxidative thermal degradation chemistry of alkyl and aryl quaternary phosphonium-modified montmorillonites (P-MMT) was examined using TGA combined with pyrolysis/GC-MS. The morphology evolution at elevated temperature was investigated using in-situ high-temperature X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The onset decomposition temperature via TGA of these P-MMTs ranged from 190 to 230 °C. The initial degradation of the alkyl P-MMTs follows potentially two reaction pathways − β-elimination [Eβ] and nucleophilic displacement at phosphorus [SN(P)] − reflecting the multiple environments of the surfactant in the silicate. Aryl P-MMT decomposition proceeds via either a reductive elimination through a five-coordinate intermediate or radical generation through homologous cleavage of the P−phenyl bond. Overall, the interlayer environment of the montmorillonite has a more severe effect on stability of the phosphonium surfactant than previously reported for ammonium-modified montmorillonite (N-MMT). Nonetheless, the overall thermal stability of P-MMT is higher than that of N-MMT. These observations indicate that, in addition to their conventional purpose as stabilizers, phosphonium salts offer unique opportunities for melting processing polymer layered silicate nanocomposites.
    Chemistry of Materials - CHEM MATER. 10/2002; 14(11).
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    ABSTRACT: One of the fundamental aspects of the carbon fiber-reinforced, high-temperature polyimide composite AFR700B/T650-35, namely, the curing chemistry involved in the polyimide formation, was studied in real time with thermogravimetry/Fourier transform infrared (FTIR)/mass spectrometry (MS) evolved-gas analysis techniques. The off-gas reaction products identified by FTIR and MS essentially confirmed the literature polyimide curing mechanisms. However, the FTIR/MS data obtained could also accommodate a reversed curing chemistry in which the elimination of water from amide ester formation occurred first and was followed by the release of methanol from subsequent imidization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2213–2224, 2002
    Journal of Applied Polymer Science 03/2002; 83(10):2213 - 2224. · 1.40 Impact Factor
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    ABSTRACT: Recently, polymer–clay hybrid materials have received considerable attention from both a fundamental research and application point of view.1–3 This organic–inorganic hybrid, which contains a nanoscale dispersion of the layered silicates, is a material with greatly improved physical and mechanical characteristics. These nanocomposites are synthesized through in situ polymerization or direct intercalation of the organically modified layered silicate (OLS) into the polymer matrix. Thus, understanding the relationship between the molecular structure and the thermal stability (decomposition temperature, rate, and the degradation products) of the OLS is critical. In this study, modern thermal analysis techniques combined with infrared spectroscopy and mass spectrometry (TGA-FTIR-MS) were used to obtain information on the thermal stability and degradation products of organic modified clay. Furthermore, the thermal and mechanical properties of clay-filled PMMA nanocomposites were determined by using TGA and DSC. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1702–1710, 2002
    Journal of Applied Polymer Science 02/2002; 83(8):1702 - 1710. · 1.40 Impact Factor
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    ABSTRACT: The oxidative stability of the carbon fiber-reinforced composite of polyimide was examined, in real time, using the evolved gas analysis techniques. Off-gas degradation products suggested the onset temperature for chain scissions to be fairly low at about 190–220°C. Based on the off-gas products present and the trend of their release, the composite degradation mechanism appeared to be similar between 190 and 371°C, thereby marking 371°C to be the highest accelerated aging temperature for its long-term lifetime prediction. Beyond 371°C, different degradation mechanisms would apply. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1219–1227, 2002
    Journal of Applied Polymer Science 02/2002; 83(6):1219 - 1227. · 1.40 Impact Factor
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    ABSTRACT: The thermal stability of organically modified layered silicate (OLS) plays a key role in the synthesis and processing of polymer-layered silicate (PLS) nanocomposites. The nonoxidative thermal degradation of montmorillonite and alkyl quaternary ammonium-modified montmorillonite were examined using conventional and high-resolution TGA combined with Fourier transform infrared spectroscopy and mass spectrometry (TG−FTIR−MS) and pyrolysis/GC−MS. The onset temperature of decomposition of these OLSs was approximately 155 °C via TGA and 180 °C via TGA−MS, where TGA−MS enables the differentiation of water desorbtion from true organic decomposition. Analysis of products (GC−MS) indicates that the initial degradation of the surfactant in the OLS follows a Hoffmann elimination reaction and that the architecture (trimethyl or dimethyl), chain length, surfactant mixture, exchanged ratio, or preconditioning (washing) does not alter the initial onset temperatures. Catalytic sites on the aluminosilicate layer reduce thermal stability of a fraction of the surfactants by an average of 15−25 °C relative to the parent alkyl quaternary ammonium salt. Finally, the release of organic compounds from the OLS is staged and is associated with retardation of product transfer arising from the morphology of the OLS. These observations have implications to understanding the factors impacting the interfacial strength between polymer and silicate and the subsequent impact on mechanical properties as well as clarifying the role (advantageous or detrimental) of the decomposition products in the fundamental thermodynamic and kinetic aspects of polymer melt intercalation.
    Chemistry of Materials 08/2001; 13(9). · 8.24 Impact Factor
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    ABSTRACT: Micro-thermal analysis (TA) is an exciting new innovation in the field of materials characterization. It combines the visualization power of atomic force microscopy (AFM) with the characterization capabilities of TA. Micro-TA can be used to characterize materials and surfaces, and to visualize the spatial distribution of phases, components, and contaminants in samples such as polymer blends. When two or more polymers are combined (incompatible blend), the microstructure and morphology have a direct impact on the materials final mechanical and chemical properties. Knowledge of the size and distribution of these phases can lead to a better understanding, and ultimately optimization of mechanical properties.In this study, micro-TA is used to examine the uniformity of high-density polyethylene and polystyrene blend with composition 70/30 and 50/50 (by weight) using different blend methods, including physical blending, blending in an extruder, and blending on a Haake mixer. Since the compatibilizer are commonly used in the polymer industry, the effect of compatibilization of block copolymers on the uniformity of a 70/30 (by weight) blend of high-density polyethylene (HDPE) and polystyrene (PS) is also investigated by micro-TA technique and the results are compared with that from other techniques, including scanning electron microscopy (SEM) and Raman spectroscopy. The compatibilizers investigated are several block copolymers, including poly(styrene-b-ethylene), S-b-E; poly(styrene-b-ethylene/propene), S-b-EP; and poly(styrene-b-ethylene/butene-b-styrene). The results from micro-TA shows the similar trend that from SEM and Raman. The results shows that S-b-E block copolymer is the most effective compatibilizer among the compatibilizer tested, which decreases the domain size of the dispersed polystyrene phase while improving its dispersion in the polyethylene matrix.
    Thermochimica Acta - THERMOCHIM ACTA. 01/2001; 367:135-142.
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    ABSTRACT: Polymer/organically modified layered silicate (PLS) nanocomposites are a new class of filled polymers with ultrafine phase dimensions. They offer an outstanding combination of stiffness, strength and weight that is difficult to attain separately from the individual components. Additionally, the nanoscopic phase distribution as well as synergism between polymer and the layered silicate result in additional properties, such as flame retardency, enhanced barrier properties and ablation resistance, which are not observed in either component. These nanocomposites are synthesized by blending the organically modified layered silicate (OLS) into the polymer melt. Thus, understanding the relationship between the molecular structure and the thermal stability (decomposition temperature, rate, and the degradation products) of the organic modification of the layered silicate is critical. During this study, modern thermal analysis techniques combined with infrared spectroscopy and mass spectrometry (TGA–FTIR–MS) were used to obtain information on the thermal stability and degradation products. The effect of chemical variation (alkyl chain length, number of alkyls, and unsaturation) of organic modifiers on the thermal stability of the organically exchanged montmorillonite are discussed. A range of interesting results is observed, however, not all are currently understandable.
    Thermochimica Acta. 01/2001;
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    ABSTRACT: Deposit formation is considered as an important problem challenging the improvement of coal-combustion performance. Deposits usually result from the thermal decomposition of coal-borne mineral matter followed by impact and adhesion along the hot gas pathway. Several ash deposit samples collected from the heat exchange tube in the Atmospheric Fluidized Bed Combustor (AFBC) at Western Kentucky University, were used in the present study. The SDT-MS and XRD techniques were employed to examine the sample composition and the mechanism responsible for the deposit formation. This work especially concentrated on the investigation of the chlorine containing species and the behavior of chlorine during the heat treatment. It was proved that the alkali or alkali earth chloride would vaporize significantly under the tested condition, and these compounds were likely responsible for the deposit formation.
    Thermochimica Acta 08/2000; 357(1):231-238. · 1.99 Impact Factor
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    ABSTRACT: The major advantage of fluidized bed combustion (FBC) of fuels is the ability to absorb SO2 and HCl when limestone is used in the combustor. The combustion of high chlorine coal does generate some concerns about the possibility of chlorine-related corrosion of boiler components at high temperatures. It is believed that molten alkali salts may cause severe corrosion when they deposit on the surfaces of heat exchange tubes in a boiler. A better understanding of the effect of temperature on the deposition of alkali chloride will help find a way to convert them to harmless salts before they condense on the components of an FBC system. Furthermore, the knowledge of the distribution of sulfur and chlorine in ash under different conditions is necessary not only to understand the principle and efficiency of desulfurization by limestone, but also to evaluate the role of limestone in the capture of HCl. In this study three different fuels with different chlorine, sulfur, and alkali contents were burned in a 0.1 MWth bench scale FBC system. The effects of coal types, temperature, position, and exposure time on the composition of ash deposits were investigated by ICP-AES and XRD spectroscopy. The major compound in the ash deposits is CaSO4. The experimental results show that the operating temperature has a major effect on the condensation of alkali chloride. The absorption of HCl is favored at the lower temperatures.
    Energy & Fuels - ENERG FUEL. 07/2000; 14(5).
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    ABSTRACT: A laboratory scale fluidized bed reactor and a bench scale 0.1 MWth fluidized bed combustor were used to study the effect of operating conditions on the formation of Polycyclic Aromatic Hydrocarbons (PAHs) in fly ash from fluidized bed combustion systems. A high volatile bituminous coal was chosen to investigate PAH emissions during the entire pyrolysis to oxygen-rich combustion process. During the experiments, the fluidized bed reactor was operated at temperatures between 700 °C and 900 °C, while the excess air ratio was varied from 0 to 1.3. An extraction and GC/MS analysis of PAHs was used in this study. Approximately 40 different PAHs were identified during the tests, of which only a few are specified by the U.S. EPA. The experimental results indicate the majority of the PAHs in the solid phase (bed and fly ash) are derived from the breakdown reactions during the processes of combustion and/or pyrolysis in a Fluidized Bed Combustion (FBC) system, although FBC systems have an efficient solid−gas mixing process and relatively long residence time. The total amount of PAHs in the fly ash was much higher than that in the raw coal and in the gas phase. Three- and four-ring aromatic compounds were the major PAHs from pyrolysis conditions, while naphthalene (two-rings) is the dominant compound in bed ash collected from oxygen-rich combustion conditions. Only naphthalene was detected in the bed ash in the FBC system. High-speed secondary air (air staging) injected into the freeboard of the FBC system is an effective method for minimizing PAH emissions, along with the other benefits including minimizing NOx and SOx emissions.
    Environmental Science and Technology 04/2000; 34(11). · 5.48 Impact Factor
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    ABSTRACT: This project was designed to evaluate the combustion performance of and emissions from a fluidized bed combustor during the combustion of mixtures of high sulfur and/or high chlorine coals and municipal solid waste (MSW). The effect of sulfur dioxide on the formation of molecular chlorine during co-combustion of fuels was examined in this study. Sulfur dioxide was shown to be an effective inhibitor for the formation of molecular chlorine through the Deacon Reaction and, subsequently, the formation of chlorinated organics. Theoretically, co-firing high sulfur coals with MSW will decrease the possibility of polychlorodibenzodioxin/furan (PCDD/F) formation during the combustion process. A mixture of coal and PVC pellets was burned in a 0.1 MWth bench-scale fluidized bed system at WKU and no detectable amounts of chlorinated organics were found in the flue gas and bed ash. The results from this study indicated the practical effects of using coal as a combustion support fuel when burning MSW.
    Energy & Fuels 03/2000; 14(3). · 2.85 Impact Factor
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    ABSTRACT: Two 1000-h burns were conducted with the 12-in. (0.3 m) laboratory AFBC system at Western Kentucky University. Operating conditions similar to those used at the 160-MW AFBC system at the TVA Shawnee Steam Plant located near Paducah, KY, were used. A low-chlorine (0.012% Cl and 3.0% S) western Kentucky No. 9 coal and a high-chlorine (0.28% Cl and 2.4% S) Illinois No. 6 coal were used in this study. Four different metal alloys [carbon steel C1020 (0.18% C and 0.05% Cr), 304 SS (18.39% Cr and 8.11% Ni), 309 SS (23.28% Cr and 13.41% Ni), and 347 SS (18.03% Cr and 9.79% Ni)] were exposed uncooled in the freeboard area at the entrance to the convective pass, where the metal temperature was approximately 900 K. A two-phase investigation was carried out in order to study the fate of chlorine during coal combustion in an AFBC system and to study the susceptibility of boiler components to corrode in combustion gases containing hydrogen chloride. As determined from emission and ash studies, the temperature in an AFBC system plays a key role in the retention of chloride, which is more favorable at low operating temperatures. A small amount of scale failure was observed on the other three samples in both test runs. On the basis of the SEM-EDS mapping results, there was no localized chloride distribution observed on the surface of the coupons, either in the scale failure area nor on the rest of the metal part. Some trace amount of chloride was found but was evenly distributed on the surface of the coupons. There was no concentration of chloride on the spot of scale failure. The scale failure might be due to sulfur attack and/or the effect of erosion. Further study with higher chlorine-content coals for more conclusive information is needed.
    Energy & Fuels - ENERG FUEL. 02/1999; 13(3).
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    ABSTRACT: Four ash samples which were deposited on four different locations in the atmospheric fluidized bed combustor, FBC, system at the TVA Shawnee Steam Plant near Paducah, KY, were employed in the present study. The SDT/MS and XRD techniques were applied to idenfity the sample composition and the mechanism that form the deposits. The major compound in the deposits from convection pass inlet, superheater and multiclone inlet was CaSO4, and the predominant minor compound was CaO in the samples. Also, CaS was found in the deposits from superheater by XRD analysis. In the case of recycle feed pipes, Ca(OH)2 and CaCO3 were the major components, and CaSO4 and CaO the predominant minor compound in the deposits. The combined SDT/MS with XRD results are useful techniques for identification of unknown ash samples.
    Thermochimica Acta 01/1998; · 1.99 Impact Factor
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    ABSTRACT: Polyimides are attractive for high temperature applications as the matrix resins, due to their excellent thermo-oxidative stability and mechanical properties. As composite materials for aerospace applications, they possess certain unique characteristics such as toughness, resistance to temperature and solvents as well as high tensile strength and modulus. A series of novel PMR polyimides based on substituted benzidines are examined and compared to the state-of-the-art PMR-15. The assigned glass transition temperatures and the mechanism for the thermal decomposition of four specific PMR polyimides are obtained, using thermal mechanical analysis and TGA/FTIR/MS techniques. Their thermal decomposition steps are proposed, identified and also compared to each other.
    Thermochimica Acta s 367–368:143–153. · 1.99 Impact Factor
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