Weontae Oh

NEXAFS spectroscopy study of the surface properties of zinc glutarate and its reactivity with carbon dioxide and propylene oxide

Abstract The surface state of polycrystalline zinc glutarate (ZnGA) catalyst and its catalytic adsorption of carbon dioxide (CO 2 ) and propylene oxide (PO) were investigated by using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The outermost layer of ZnGA catalyst was found to contain more hydrocarbon units (i.e., glutarate ligand component) than the inner layers. The ZnGA catalyst was found to reversibly react with CO 2 and to readily react with PO via adsorption onto the catalyst surface and insertion into the ZnO bond. Experiments in which the catalyst was treated with CO 2 followed by PO and vice versa showed that each of these components can replace the other component on the catalyst surface. This reversible adsorption and insertion of CO 2 and PO on the ZnGA surface provides a clue to the mechanism underlying the production of alternating poly(propylene carbonate) in the ZnGA-catalyzed copolymerization of CO 2 and PO. However, in comparison to CO 2 , PO was more easily adsorbed onto the ZnGA catalyst and inserted into the ZnO bond. As a consequence, PO significantly modified the catalyst surface. This suggests that the ZnGA-catalyzed copolymerization is initiated by PO rather than CO 2 .

Optical, dielectric and thermal properties of nanoscaled films of polyalkylsilsesquioxane composites with star-shaped poly(ε-caprolactone) and their derived nanoporous analogues

Abstract Nanoscaled films of poly(methylsilsesquioxane-co-ethylenylsilsesquioxane) (PMSSQ–BTMSE) and polymethylsilsesquioxane (PMSSQ) were prepared from the respective soluble prepolymers, and their thermal, optical, and dielectric properties were characterized. The PMSSQ–BTMSE nanofilm containing an ethylenyl bridge comonomer unit BTMSE was found to be thermally more stable than the nanofilm of PMSSQ, a representative polyalkylsilsesquioxane. Further, in comparison to the PMSSQ film the PMSSQ–BTMSE film exhibits a larger refractive index and a larger dielectric constant but a smaller out-of-plane thermal expansion coefficient, when both prepolymers are cured under the same conditions. These characteristics of PMSSQ–BTMSE films are due to the ethylenyl bridge provided by the BTMSE comonomer unit, which promotes the formation of a tighter, more perfect network structure in cured PMSSQ–BTMSE films. Composite films were prepared by solution-blending the pore generator (referred to as the porogen), a star-shaped poly(e-caprolactone), with the soluble prepolymers then drying the resulting solution, and the nature of their curing reactions and extent of porogen calcination were investigated. It was found that the thermal curing and calcination processes of the composite films successfully produce PMSSQ–BTMSE and PMSSQ films containing pores that are about 400 nm in diameter. It was confirmed that the presence of these generated nanopores significantly reduces the refractive indices and the dielectric constants of the dielectric films but increases their out-of-plane thermal expansivity, depending on the initial porogen loading. The porosities of the nanoporous dielectric films were estimated from the measured refractive indices. The surface topographies of these films were also investigated, giving information about the sizes of the pores generated in the films. It was also found that prior to calcination the presence of the porogen increases the refractive index and dielectric constant of the composite dielectric films because of its inherent high polarizability, and also increases the thermal expansivity of MSSQ–BTMSE composite films but very slightly decreases the thermal expansivity of MSSQ composite films. It is demonstrated that PMSSQ–BTMSE films and the related nanometer scale nanoporous films are candidates for use as low and ultra-low dielectric interlayers in the fabrication of advanced microelectronic devices.

Residual Stress Evolution in Dielectric Thin Films Prepared from Poly(methylsilsesquioxane) Precursor

This study was supported by the Ministry of Industry & Energy and the Ministry of Science & Technology [Korean Collaborative Project for Excellence in Basic System IC Technology (98-B4-C0-00-01-00)] and in part by the Center for Integrated Molecular Systems (KOSEF).

Effect of montmorillonite on wettability and microstructure properties of zein/montmorillonite nanocomposite nanofiber mats

Zein is a hydrophobic protein produced from maize and has great potential in a number of industrial applications such as food, food coating and food packaging. The objectives of this study are to determine the effects of montmorillonite on the wettability and microstructure properties of zein/montmorillonite nanocomposite nanofiber mats fabricated by the electrospinning technique in ethyl alcohol aqueous solution. The zein/montmorillonite nanofiber mats were characterized by field-emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and contact angle measurements. This study shows that the introduction of montmorillonite resulted in the improvement of the thermal stability and hydrophilicity for the zein matrix. X-ray diffraction patterns and transmission electron microscopy micrographs suggest the coexistence of intercalated montmorillonite layers over the examined montmorillonite contents. Since montmorillonite is a hydrophilic clay, its addition can be used not only to produce nanomaterials with the already known improved properties but also to enhance the hydrophilicity of material.

X-ray scattering study of thermal nanopore templating in hybrid films of organosilicate precursor and reactive four-armed porogen

The miscibility and the mechanism for thermal nanopore templating in films prepared from spin-coating and subsequent drying of homogenous solutions of curable polymethylsilsesquioxane dielectric precursor and thermally labile, reactive triethoxysilyl-terminated four-armed poly(epsilon-caprolactone) porogen were investigated in detail by in situ two-dimensional grazing incidence small-angle x-ray scattering analysis. The dielectric precursor and porogen components in the film were fully miscible. On heating, limited aggregations of the porogen, however, took place in only a small temperature range of 100-140 degrees C as a result of phase separation induced by the competition of the curing and hybridization reactions of the dielectric precursor and porogen; higher porogen loading resulted in relatively large porogen aggregates and a greater size distribution. The developed porogen aggregates underwent thermal firing above 300 degrees C without further growth and movement, and ultimately left their individual footprints in the film as spherical nanopores.

Electrospinning Fabrication of Polyvinyl alcohol)/Waterborne Polyurethane/Silver Composite Nanofibre Mats in Aqueous Solution for Anti-bacterial Exploits

Nanofibre mats of poly(vinyl alcohol) (PVA), waterborne polyurethane (WBPU) and nanometre silver (Ag) colloids have been fabricated by an electrospinning method in aqueous solutions. Since PVA is a water soluble and biocompatible polymer, it is one of the best materials for preparation of electrospun antibacterial nanofibre mats. WBPU was used as a filler to enhance the properties of homopolymer nanofibre. Transmission electron microscopy analyses showed a uniform distribution of silver in the fibres. In anti-bacterial tests, the PVA/WBPU/Ag composite nanofibres showed excellent anti-bacterial performance, indicating practical uses as a new preservative. Moreover, the PVA/WBPU/Ag nanofibres showed improved thermal properties.

Grazing-incidence small-angle X-ray scattering studies on templating nanopores in networked polymer thin films with a multi-armed porogen

The mechanism of thermal pore generation in organosilicate thin films loaded with a six-armed star-shaped poly(e-caprolactone) porogen was quantitatively investigated by using in-situ grazing-incidence small-angle X-ray scattering and thermogravimetry. These analyses found that the blend components have a limited miscibility that depends on the composition; for porogen loadings up to only 20 wt%, molecularly miscible blend films were obtained. Even for the miscible blend films, heating the films produced a curing reaction of the precursor matrix component, leading to the phase separation of the porogen component. This phase separation was found to begin at 393 K for 10 wt% porogen loaded films and at 373 K for 20 wt% porogen loaded films, and to continue for temperatures up to 423 K. The porogen aggregates remained and were confined within the matrix film without any further growth or movement until complete thermal decomposition above 564 K.

Residual Stress Behavior in Methylsilsesquioxane-Based Dielectric Thin Films

Abstract Residual stress of methylsilsequioxane film, which was spin-coated on silicon substrate and followed by soft-baking, was measured in-situ during curing and subsequent cooling with varying processing conditions. The thickness and refractive index of the cured films were measured using ellipsometry. Their structure was also examined by X-ray diffraction.

Spatially resolved in situ measurements of the transport of organic molecules in a polycrystalline nanoporous membrane

We report spatially resolved, quantitative, in situ, nondestructive measurements of the transport of organic molecules through a polycrystalline, anisotropic, nanoporous molecular sieve membrane, with micron-scale resolution. A method based on photoacoustic spectroscopy experiments during permeation through a nanoporous membrane, combined with a physical model of photoacoustic signal generation from a heterogeneous membrane, allows extraction of concentration profiles in situ. In particular, we demonstrate the steady-state concentration profiling of the organic molecules p-xylene and n-hexane during their transport through a nanoporous zeolite silicalite membrane. The implications for elucidating structure-property relationships in membrane materials for separations, catalytic, or nanotechnology applications are discussed.

Electrospraying Fabrication and Characterization of Low Molecular Weight Poly(vinyl alcohol)/Silver Composite Nanospheres for Antibacterial Applications

Low molecular weight poly(vinyl alcohol) (LMW-PVA)/silver (Ag) composite nanospheres were prepared using an electrospraying technique by controlling the concentration of PVA solution. Since PVA is water-soluble and biocompatible, it is one of the best polymers for preparation of antibacterial materials. The composite nanospheres were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and antibacterial activity measurement. The LMW-PVA/Ag composite nanospheres had ∼350 nm average diameter and showed powerful antibacterial ability.