Changshu Kuo

Fabrication and characterization of asymmetric Janus and ternary particles.

Asymmetric Janus and ternary silica particles with an average diameter of 450 nm were fabricated by sequentially arranged particle-embedding and surface-modification processes. Thermally induced embedding of particles into polymer-fiber substrates allowed for precise control of the degree of particle submergence and the subsequent chemical modification of the hemispherical exposed particle surfaces. In addition to Janus particles with the desired surface-functionality ratios of 1:2, 1:1, and 2:1, this unique fabrication approach was also used to produce complicated and well-defined heterogeneous materials, including bifunctionalized Janus and ternary particles. The bifunctionalized Janus particles were produced with two hemispherical surfaces alternately labeled with gold and iron oxide nanoparticles, which simultaneously enabled anisotropic surface-plasmon resonance and a magnetic response. Ternary particles were also constructed, yielding submicrometer spheres with functionalized equatorial belts. The surface distributions of functional components in these spherical materials were carefully examined for uniformities in particle embedding. Statistical analyses revealed that the functional components were distributed with a uniformity of over 80% for all of the asymmetric Janus and ternary particles.

Synthesis of novel conducting elastomers as polyaniline-interpenetrated networks of fullerenol-polyurethanes

A processing technique for fabrication of conductive elastomers was demonstrated involving in situ polymerization of aniline at the near-surface region of the fullerenol crosslinked poly(urethane-ether) networks. The synthetic chemistry led to a thin layer (roughly 20 μm in thickness) of conductive polyanilines interpenetrated inside the elastomer matrix. This IPN network exhibited an appreciable conductivity along the doped polyaniline layer without deteriorating much the elasticity and tensile strength of the parent supporting elastomer. As a result, its r.t. conductivity was found to be 2.6 Scm−1 which increased to 5.4 Scm−1 at 430% elongation of the polymer film. The data represented the strain-dependent electronic properties of conducting elastomers, showing an increase of conductivity even after the elastomer being stretched above a length as much as 500% of its original size.

Novel fabrication of janus particles from the surfaces of electrospun polymer fibers

A novel synthetic approach for the efficient fabrication of Janus silica particles was demonstrated by embedment of zero-dimensional colloids on one-dimensional polymer fiber surfaces, followed by the surface modification on the exposed silica hemispheres. Electrospinning of poly(methyl methacrylate) and poly(4-vinyl pyridine) blends produced polymer fibers with high specific surface area and desired surface hydrophilicities. Fiber compositions determined the colloid adsorption density and uniformity. The colloid embedding resulted from the polymer softening was manipulated by the isothermal heat treatment. Subsequent silianization completed the amino functionalities on hemispherical surfaces of embedded silica colloids. Janus particles with uniform asymmetric chemical features were further labeled with gold nanoparticles before their recovery from fiber substrates. Fabrication of Janus particles, including colloid adsorption, temperature-driven embedding, and hemispherical surface modification, were investigated and are discussed.

Plasma-Synthesized Silver Nanoparticles on Electrospun Chitosan Nanofiber Surfaces for Antibacterial Applications

Chitosan nanofibers have been electrospun with poly(ethylene oxide) and silver nitrate, as a coelectrospinning polymer and silver nanoparticle precursor, respectively. The average diameter of the as-spun chitosan nanofibers with up to 2 wt % silver nitrate loading was approximately 130 nm, and there was no evidence of bead formation or polymer agglomeration. Argon plasma was then applied for surface etching and synthesis of silver nanoparticles via precursor decomposition. Plasma surface bombardment induced nanoparticle formation primarily on the chitosan nanofiber surfaces, and the moderate surface plasma etching further encouraged maximum exposure of silver nanoparticles. UV-vis spectra showed the surface plasmon resonance signature of silver nanoparticles. The surface-immobilized nanoparticles were visualized by TEM and were found to have average particle diameters as small as 1.5 nm. Surface analysis by infrared spectroscopy and X-ray photoelectron spectroscopy confirmed the interactions between the silver nanoparticles and chitosan molecules, as well as the effect of plasma treatment on the nanofiber surfaces. Finally, a bacteria inhibition study revealed that the antibacterial activity of the electrospun chitosan nanofibers correspondingly increased with the plasma-synthesized silver nanoparticles.

Enhancement of light scattering and photoluminescence in electrospun polymer nanofibers

Poly(methyl methacrylate) nanofibers with desired fiber diameters that ranged from 336 to 896 nm were electrospun as light scattering and propagation materials. The light scattering behavior of these samples as a function of the fiber diameter and fiber deposition thickness was examined by UV-vis spectrophotometry, which revealed the scattering bands in the absorption spectra. The scattering bands of these nanofibers were linearly proportional to the fiber diameter, which shows good agreement with a scattering model based on the Mie theory. The light scattering and prolonged light path lengths in the nanofiber scaffolds were monitored and quantified by the photoluminescence of a fluorescent dye, Coumarin 6, which was preloaded into the polymer nanofibers. The photoluminescence after proper normalization showed a second-order dependence on the dye loading per unit area, which is significantly different from the spin-coated thin-film samples following a first-order relationship. Nonlinear photoluminescence enhancements indicated prolonged light path lengths and multiple light absorptions within the fiber scaffolds as a result of light scattering. Even with relatively broad scattering band widths, the light scattering and photoluminescence of the electrospun nanofibers exhibited considerable wavelength selectivity, especially as the scattering bands overlapped with the excitation wavelengths of the fluorescence reagent.

Thermal analysis of energetic materials

Abstract Substances which undergo strongly exothermic chemical reactions may constitute hazards in various situations. The techniques of thermal analysis, in general, and differential scanning calorimetry (DSC) in particular, provide ways of examining such substances, in small quanitities under relatively safe conditions. A brief review of some developments in thermal explosion theory relevant to thermoanalytical studies is given. Extrapolation of the results obtained from thermoanalytical tests to more realistic situations has to be done with great care. The findings of some recent studies on energetic materials are reviewed.

Synthesis of polyurethane elastomers hypercross-linked by partially hydrated polyhydroxylated C60

Utilization of polyhydroxylated C60 (fullerenols) in a condensation reaction with diisocyanated oligo(tetramethylene oxide) led to the successful fabrication of elastomeric poly (urethane-ether) networks. These polymer networks exhibit interesting thermal behavior at low temperatures, improved tensile strength and elongation at ambient temperatures, and enhanced thermal mechanical stability at high temperatures, as compared with those of the parent linear polyurethane analogues; or with the conventional oligo (tetramethylene oxide)-derived polyurethane elastomers cross-linked by trihydroxylated reagents (1,I,1-trismethylol propane) or tetrahydroxylated reagents (pentaerythritol). The presence of a limited quantity of water molecules in the condensation reaction of fullerenols with diisocyanated prepolymers modified the physical properties of the resulting elastomeric products with a notable increase in tensile strength, modulus, and Ts over those of elasotmers prepared under anhydrous conditions. These water molecules contributed effectively to the increase of the number of cross-linking centers during the reaction.

Dependence of surface atomic arrangement of titanium dioxide on metallic nanowire nucleation by thermally assisted photoreduction

Through studying the optical, electrical and photocatalytic properties of anatase TiO(2) films with different preferred orientations, (101) and (004), this study clarified the relationship between the formation of metallic nanowires by thermally assisted photoreduction process and surface atomic bonding conditions of TiO(2). Experimental results show that the (101) anatase films which yielded much more Ag nanowires than the (004) oriented films and exhibited more complex superficial atomic bonding, which could be demonstrated by the Gaussian bands in photoluminescence spectra. This might lead to higher carrier concentration and mobility, as well as longer life time for photo-exited electrons and consequently a greater photocatalytic activity for reducing metallic ions. The fact that the anatase (101) surface acted as the preferred nucleation sites for Ag nanowires was supported by high resolution transmission electron microscopy lattice image of a TiO(2) nanofiber where an Ag nanowire was grown.

Effect of ZnO/TiO2 Nanorods Fabricated Using the Electrospinning Method in Y-Ba-Cu-O Single Grain Bulk Superconductors

In this study, the enhanced superconducting properties of single grain bulk Y-Ba-Cu-O (YBCO) superconductors by the addition of ZnO and TiO2 nano-rods (NRs) were investigated. The NRs were prepared using the electrospinning method, in which size is twice the coherence length of the superconductor and is intended supposed to serve as columnar defects to provide efficient pinning. The experimental results showed that there were no weak links formed by the addition of the NRs. All of the bulks exhibited a single grain structure, and the distribution profiles of the trapped fields were symmetrical. The value of the maximum trapped field (Bt, max) was enhanced to 0.12 T, which was 1.5 times higher than the undoped sample (approximately 0.08 T). In addition, JJ was also enhanced by the addition of the NRs. The self-field at 77 K was 4.6 χ 104 A cm-2 of the 0.05wt% ZnO NRsdoped and 0.1wt% TiO2 NRs-doped samples, respectively, which was higher than that of the undoped sample of 2.5 χ 104 A cm-2 . A microstructural analysis demonstrated that there was no reaction occurred between the NRs and YBCO precursors during the melting process. The size and shape of the NRs found in the matrix of the grown bulk were the same as those of the precursors. Consequently, the enhanced superconductivities were then attributed to the increased interface areas between the NRs and the matrix.

SYNTHESIS AND PROPERTIES OF [60]FULLERENE-POLYVINYLPYRIDINE CONJUGATES FOR PHOTOVOLTAIC DEVICES

A two-phase radical polymerization was applied for the synthesis of fullerene-containing poly(4-vinylpyridine), which were used as polyelectrolytes in the fabrication of photovoltaic devices. Two-phase radical polymerization resulted in the products of poly(4-vinyl pyridinated) fullerenes (PVPyF) containing a high fullerene content and poly(4-vinylpyridine) arms of short length. These polyelectrolytes with a C60 content of up to 35% by weight are soluble in organic solvents and water at a low pH. High efficient photoinduced charge transfer in samples of PVPyF and polyanionic polythiophene derivatives fabricated by the electrostatic layer-by-layer deposition technique in producing self-assembled multilayers (SAMs) was demonstrated. As a result, high response to the increase of photoinduced conductivity was observed on the 75 bilayer device of PVPyF-3+/H-PURET− upon irradiation. Photoconductivity was found to increase in more than one order of magnitude from 0.024 to 0.31 S/cm at an applied voltage of 1.0 volt. The sign of short-circuit photocurrent was consistent with electrons flowing from Al electrode towards the ITO modified substrate. †Deceased.