S. L. Lim

National University of Singapore, Singapore, Singapore

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Publications (9)13.48 Total impact

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    ABSTRACT: Photo-induced reduction of gold and platinum metal salt solutions was carried out using viologen graft copolymerized on low-density polyethylene (LDPE) films and viologen-containing poly(vinylidene fluoride) (PVDF-PVBV) microporous membranes. The effects of the UV irradiation time and concentration of the metal salt solutions on the metal ion reduction process and the resultant metal deposition on the polymeric substrates were investigated. The metal-polymer composites were characterized using X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy (SEM and TEM), atomic force microscopy (AFM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and UV-visible absorption spectroscopy. The amount of metal uptake, the state of the metal, and the size of the metal particles were found to be strongly dependent on the UV irradiation time and the type and concentration of the metal salt solution. The microporous structure and the high viologen content of the PVDF-PVBV membrane constitute an effective matrix for metal ion reduction and preparation of metal nanoparticles.
    Journal of Colloid and Interface Science 12/2004; 279(2):391-8. · 3.17 Impact Factor
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    ABSTRACT: Molecular modification of a fluorinated polyimide (FPI), 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride + 4,4‘-bis(4-aminophenoxy)diphenyl sulfone, via ozone pretreatment and thermally induced graft copolymerization with N-isopropylacrylamide (NIPAAm) in a N-methyl-2-pyrrolidone solution, was carried out. The resulting FPIs with grafted NIPAAm polymer side chains [FPI-g-P(NIPAAm) copolymers] were cast into microfiltration membranes by phase inversion in water at temperatures ranging from 4 to 55 °C. The surface composition of the membranes was characterized by X-ray photoelectron spectroscopy. The surface composition, mean pore size, and morphology of the membrane varied with the temperature of the aqueous coagulation bath. For the copolymer membrane cast below the lower critical solution temperature of the NIPAAm polymer (32 °C), the rate of water permeation increased substantially at a permeate temperature above 32 °C. For the flux of 2-propanol through the membrane cast above 32 °C, a reversed permeate temperature dependence was observed.
    Industrial & Engineering Chemistry Research - IND ENG CHEM RES. 07/2003; 42(16).
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    ABSTRACT: High-resolution X-ray photoelectron spectroscopic (XPS) measurements of the various intrinsic redox states of polyaniline (PANI), using a monochromatized Al—Kα source, were carried out. The presence of the imine, amine and positively charged nitrogen species corresponding to a particular intrinsic redox state and protonation level of the polymer was resolved quantitatively and unambiguously. The result confirmed the peak assignments of former XPS core-level studies using the lower resolution non-monochromatized Mg—Kα X-ray source. Thus, the high-resolution XPS using a monochromatized Al—Kα X-ray source is a truly unique tool for the convenient and quantitative analysis of the various intrinsic redox states of PANI.
    Colloid and Polymer Science 12/2000; 279(1):73-76. · 2.16 Impact Factor
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    ABSTRACT: Comparative ab initio restricted Hartree–Fock (RHF) and density functional theory (DFT) calculations were performed to investigate the geometric and electronic structures of various neutral aniline oligomers. These oligomers were selected to model polyaniline (PANI) in different intrinsic oxidation states, with an aim to study the applicability and extendibility of the theoretical methods to conjugated polymers. In general, we found that DFT calculations produce results that are in good agreement with observations from x-ray diffraction, ultraviolet-visible absorption, ultraviolet photoelectron and x-ray photoelectron spectroscopy. The DFT method has reproduced well the ∼4.0 eVπ–π* transition of leucoemeraldine base and the ∼2.0 eV Peierls gap transition of pernigraniline base. The valence band structure and the ∼1.2 eV energy separation of nitrogen core levels of emeraldine base are also correctly predicted. On the other hand, large discrepancies with experimental measurements are predicted by the RHF method. Single-point MP2 calculations show that the DFT-optimized structures are all at lower energy than the RHF-optimized ones.
    The Journal of Chemical Physics 06/2000; 112(23). · 3.12 Impact Factor
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for the in situ study of interactions between thermally evaporated Mg atoms and electroactive polyaniline (PANI) films of various intrinsic oxidation states. Quantitative changes in the N1s core-level spectra and the Mg2p core-level spectra, as well as the changes in surface chemical stoichiometry of these films throughout the Mg evaporation process were carefully monitored. Although the nitrogen sites appeared to be more attractive to the in-coming Mg atoms and the emeraldine base (EB) and nigraniline base (NA) films underwent an apparent decrease in intrinsic oxidation state ([=N-]/[-NH-] ratio) as a result of Mg evaporation, there is no direct interaction between the two species. On the other hand, the adsorbed oxygen from the bulk of the polymer film played a dominant role in the interfacial interactions between the polymer and the metal. Characterization of the delaminated indium-tin-oxide (ITO) and polymer surfaces from EB film and its camphorsulfonic acid-doped counterpart cast on ITO substrates had also been carried out to evaluate the interaction of PANI with the high work function metal.
    Journal of Materials Science Materials in Electronics 05/2000; 11(4):305-309. · 1.49 Impact Factor
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for an in situ study of the interactions between thermally evaporated magnesium atoms and polyaniline films of various intrinsic oxidation states. Quantitative changes in the N1s core-level spectra, the Mg2p core-level spectra and the surface chemical stoichiometries of these films throughout the magnesium evaporation process were carefully monitored. Although the nitrogen sites appeared to be more attractive to the incoming magnesium atoms, and the emeraldine and nigraniline films underwent an apparent decrease in the intrinsic oxidation state ([N]/[NH] ratio) as a result of magnesium evaporation, there is no direct interaction between the two species. On the other hand, the adsorbed oxygen from the bulk of the polymer film played a dominant role in the interfacial interactions between the polymer and the metal.
    Surface Science - SURFACE SCI. 01/2000; 454:995-999.
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    ABSTRACT: Modification of argon plasma-pretreated single-crystal silicon wafer surface via UV-induced graft polymerization with various functional monomers, such as acrylamide (AAm), N,N'-diamethylaminoethyl methacrylate (DMAEMA), and 2,2,2-trifluoroethyl acrylate (TFEA), was achieved. The modified Si(100) surfaces were characterized by X-ray photoelectron spectroscopy (XPS), imaging XPS, atomic force microscopy (AFM), and water contact angle measurements. The graft polymerization was affected by plasma pretreatment time and UV irradiation time. XPS results suggest that mild and brief plasma treatment is sufficient to cause surface oxidation and to generate sufficient peroxides and hydroxyl peroxides for the subsequent UV-induced graft polymerization in the presence of a vinyl monomer. Prolonged plasma treatment and the accompanying overoxidation of the silicon surface have an adverse effect on the graft polymerization. For all the cases investigated, the XPS results revealed that the grafted polymers form a thi
    chemistry of materials. 01/1999; 11(4):1061-1068.
  • S. L. Lim, K. L. Tan, E. T. Kang
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    ABSTRACT: X-ray photoelectron spectroscopy was employed for the in situ study of the interactions between thermally deposited Al atoms and polyaniline films of different intrinsic oxidation states. Quantitative changes in the N 1s core-level line shape, the evolution of the Al 2p core-level spectra, and the changes in surface stoichiometry of these films throughout the Al deposition process were carefully tracked. Although the nitrogen sites appear to be more attractive to the incoming Al atoms and the emeraldine and nigraniline films undergo an apparent decrease in intrinsic oxidation state ([=N–]/[–NH–] ratio) as a result of Al deposition, there is no direct interaction between the two species, and the adsorbed oxygen from the bulk of the polymer film plays a dominant role in the interfacial interactions between the polymer and the metal. © 1998 American Vacuum Society.
    Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/1998; 16:13-20. · 1.43 Impact Factor
  • S. L. Lim, K. L. Tan, E. T. Kang
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    ABSTRACT: Aluminum was thermally deposited onto H2SO4-protonated, HClO4-protonated and neutral emeraldine (EM) base films. The interfacial interactions of Al atoms with these EM films were studied in situ by X-ray photoelectron spectroscopy (XPS). The Al atoms interact preferentially with the dopant anions in the cases of the protonated EM films, and with the nitrogen atoms of the conjugated polymer backbone in the case of the neutral EM film. For the H2SO4-protonated EM film, interactions with Al resulted in the formation of aluminum sulfate species, whereas, for the HClO4-protonated film, the reaction resulted in the decomposition of the perchlorate dopant and the formation of aluminum chloride species. For the neutral EM film, the incoming Al resulted in a decrease in the intrinsic oxidation state of the polymer. The migration of bulk-adsorbed oxygen to the surface in response to Al deposition was observed in all of the EM films, and the formation of aluminum oxides, such as Al2O3, suggests that the adsorbed oxygen plays an important role in the interface formation.
    Synthetic Metals 01/1998; 92(3):213-222. · 2.11 Impact Factor