Tzong-Ming Wu

Preparation and characterization of thermosensitive polymers grafted onto silica-coated iron oxide nanoparticles

In this study, multifunctional nanoparticles containing thermosensitive polymers grafted onto the surfaces of 6-nm monodisperse Fe(3)O(4) magnetic nanoparticles coated by silica were synthesized using reverse microemulsions and free radical polymerization. The magnetic properties of SiO(2)/Fe(3)O(4) nanoparticles show superparamagnetic behavior. Thermosensitive PNIPAM (poly(N-isopropylacrylamide)) was then grafted onto the surfaces of SiO(2)/Fe(3)O(4) nanoparticles, generating thermosensitive and magnetic properties of nanocomposites. The sizes of fabricated nanoparticles with core-shell structure are controlled at about 30 nm and each nanoparticle contains only one monodisperse Fe(3)O(4) core. For thermosensitivity analysis, the phase transition temperatures of multifunctional nanoparticles measured using DSC was at around 34-36 degrees C. The magnetic characteristics of these multifunctional nanoparticles were also superparamagnetic.

Synthesis and montmorillonite-intercalated behavior of dendritic surfactants

A series of novel dendrons serving as surfactants for surface modification of montmorillonite (MMT) have been synthesised via a convergent approach. The most distinguishing characteristic of the prepared dendrons is the long alkyl chain at the periphery. This would certainly bring about good solubility, high yield and easy purification. By simple acidification of the “head group” upon the dendron and then ion-exchange with Na+-MMT, the interlayer distance could be enlarged significantly between the layered silicates. The organic/inorganic ratio determined by thermogravimetric analysis (TGA) is in the range of 60%–91%. This also confirms the occurrence of dendron ion-exchange with Na+-MMT. Transmission electronic microscopy (TEM) reveals the intercalated and exfoliated morphology of the dendron-modified MMTs. Modification using the generation 2 dendritic surfactant (G2) with molecular weight (Mw) = 2887.5 has achieved the exfoliated morphology in this work.

Crystallization of poly(ethylene terephthalate–co–isophthalate)

Melt crystallization behaviors of poly(ethylene terephthalate) (PET) and poly(ethylene terephthalate-co-isophthalate) (PETI) containing 2 and 12 mol % of noncrystallizable isophthalate components were investigated. Differential scanning calorimetry (DSC) isothermal results revealed that the introduction of 2 mol % isophthalate into PET caused a change of the crystal growth process from a two-dimensional to a three-dimensional spherulitic growth. The addition of more isophthalate up to 12 mol % into the PET structure induced a change in the crystal growth from a three-dimensional to a two-dimensional crystal growth. DSC heating scans after completion of isothermal crystallization at various Tcs showed three melting endotherms for PET and four melting endotherms for PETI-2 and PETI-12. The presence of an additional melting endotherm is attributed to the melting of copolyester crystallite composed of ethylene glycol, tere-phthalate, and isophthalate (IPA) or the melting of molecular chains near IPA formed by melting the secondary crystallite Tm (I) and then recrystallizing during heating. Analyses of both Avrami and Lauritzen-Hoffman equations revealed that PETI containing 2 mol % of isophthalate had the highest Avrami exponent n, growth rate constant Go, and product of lateral and end surface free energies σσe.

Side chain dendritic polyurethanes with shape-memory effect

In this study, thermally reversible polyurethanes (PUs) with shape memory effect were developed by using hydrogen bonding to enhance physical interactions. Two different types of PUs were synthesized: (1) a linear PU whose hard segment consists of methylene di-para-phenyl isocyanate (MDI) and di(ethylene glycol) (DEG); its soft segment is made of Tone 0260 polyol, a polyester polyol; (2) a side-chain dendritic PU which replaces DEG with different generations of dendritic chain extenders. By incorporation of the dendritic structure with peripheral long alkyl chains (strong van der Waals forces), the miscibility between hard and soft segments can be significantly improved. Consequently, the hydrogen bonding reinforced hard segments (malonamide linkages) of side chain dendritic PUs result in greatly enhanced mechanical properties and shape memory effect. During cyclic shape memory tests, one of the series can effortlessly achieve complete recovery in less than 10 second without any deficiency.

Organo-clay hybrids based on dendritic molecules: preparation and characterization

Three generations of dendrons (G1, G2 and G3) with phenyl end-groups were intercalated into montmorillonite (MMT) layered silicates in dimethylformamide/water cosolvent. These dendrons synthesized via the convergent route were different in size and shape with molecular weights ranging from 930 to 5975?g?mol?1. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) indicate that the respective intercalations of MMT with G1, G2 and G3 dendrons exhibited organized characteristics, and the interlayer spacings were 38, 77 and 115?? for G1/MMT, G2/MMT and G3/MMT, respectively. Furthermore, the modified dendron/MMT hybrids could be well dispersed into organic solvents, such as DMF, due to the presence of hydrophobic dendritic molecules. Our results indicate that organic/inorganic hybrids resulting from the association of dendrons and inorganic layered silicate can be obtained not only by ionic exchange reaction, but possibly by a direct organized route as well. Moreover, the preparation of dendron/MMT hybrids was investigated at different dendron/MMT molar ratios regarding the changes of interlayer distance. When the molar ratio is within the range of 0.25?1.0 cationic exchange capacity (CEC) equivalents, the d spacings increase from 19 to 38?? for G1/MMT, 15 to 77?? for G2/MMT and 14 to 115?? for G3/MMT, revealing a conformation change of the intercalating dendrons.

Nanostructured Ag surface fabricated by femtosecond laser for surface-enhanced Raman scattering

Femtosecond laser was employed to fabricate nanostructured Ag surface for surface-enhanced Raman scattering (SERS) application. The prepared nanostructured Ag surface was characterized by field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The FESEM images demonstrate the formation of nanostructure-covered femtosecond laser-induced periodic surface structure, also termed as ripples, on the Ag surface. The AFM images indicate that the surface roughness of the produced nanostructured Ag substrate is larger than the untreated Ag substrate. The XRD and XPS of the nanostructured Ag surface fabricated by femtosecond laser show a face centered cubic phase of metallic Ag and no impurities of Ag oxide species. The application of the produced nanostructured Ag surface in SERS was investigated by using rhodamine 6G (R6G) as a reference chemical. The SERS intensity of R6G in aqueous solution at the prepared nanostructured Ag surface is 15 times greater than that of an untreated Ag substrate. The Raman intensities vary linearly with the concentrations of R6G in the range of 10(-8)-10(-4)M. The present methodology demonstrates that the nanostructured Ag surface fabricated by femtosecond laser is potential for qualification and quantification of low concentration molecules.

P-side up AlGaInP-based light emitting diodes with dot-patterned GaAs contact layers.

High-brightness p-side up AlGaInP-based red light emitting diodes (LEDs) with dot-patterned GaAs contact layer and surface rough structure are presented in this article. Initial LED structure of p-GaP/AlGaInP/GaAs is epitaxially grown using metal organic chemical vapor deposition technique. Using novel twice transferring process, the p-GaP layer is remained at the top side as both the current spreading and-window layer. Dot patterned GaAs contact dots are formed between main structure and rear mirror to improve light reflection and current spreading. Moreover, the surface of p-GaP window is further textured by nano-sphere lithography technique for improving the light extraction. Significant improvement in output power is found for AlGaInP LEDs with GaAs contact dots and roughened p-GaP window as compared with those of LEDs with traditional n-side up and p-side up structures without roughened surfaces.

Orderly Arranged NLO Materials Based on Chromophore-Containing Dendrons on Exfoliated Layered Templates

Three chromophore-containing dendrons were intercalated into montmorillonite layered silicates via an ion-exchange process. Enlarged d spacings ranging from 50 to 126 A were achieved for these novel organoclays. After the organoclays were blended with a polyimide, the steric bulkiness of the dendrons and the interaction between dendron and polyimide resulted in an ordered morphology. The orderly arranged nanocomposites were characterized by a UV-visible spectrophotometer, a variable-temperature infrared spectrometer, and electro-optical modulation. The dendrons in layered silicates were capable of undergoing a critical conformational change into an ordered structure, indicated by the drastic changes of interlayer distances at certain packing densities. Electro-optical coefficients increased sharply from 0 to 6 pm/V while the conformational change occurred. Furthermore, the addition of a polyimide capable of interaction-induced orientation was found to exert an enhancing effect on the degree of the noncentrosymmetric alignment.

Improved Light Extraction in AlGaInP-Based LEDs Using a Roughened Window Layer

Surface-textured p-GaP/AlGaInP/GaAs light-emitting diodes (LEDs) have been fabricated using a combination of natural lithography and dry etching techniques. Here, the randomly positioned polystyrene spheres are used as a mask for dry etching. The p-GaP window layer was successfully etched to achieve nanopillars on its surface. It was found that the surface-textured LEDs with the p-GaP nanopillars revealing a diameter of 300-320 nm, a density of 1.4 X 10 9 cm -2 , and a depth of 550-650 nm have maximum luminance intensity. Output power of the surface-textured LED increased about 70% compared with the planar-surface LED at a forward current of 20 mA. The LED window with nanopillar diameter (300-320 nm, half of wavelength) and depth (600 nm, ∼wavelength) resulting in much light extraction compared with flat-surface LEDs or deeper nanopillars has been demonstrated by experiment and simulation. After a 1000 h aging test, luminance intensity and voltage fluctuation do not exceed ± 10%. These results indicated that AlGaInP-based LEDs with a roughened surface did not exhibit deteriorated performance.

Self-assembled clay films with a platelet–void multilayered nanostructure and flame-blocking properties

Polymeric composite films with a high loading of nano-size silicates can hardly meet the increasingly stringent fireproof and smoke-free requirements during burning. Thus, it is desirable to prepare pure clay films that can block air, heat, and flame. Here we report an organic-free clay film capable of both flame- and heat-shielding. The film was prepared from the self-assembly of nanometer-thick silicate platelets derived from the exfoliation of natural clays. The self-assembled film has a highly regular multilayered nanostructure over a large area and an appreciable volume of air entrapped in between. The combination of regular structure and substantial air volume contributes to the low thermal conductivity and flame blocking property of the film. It was demonstrated that the film can shield flame over hour duration and prevent temperature rising on the backside of film. This remarkable clay film has a myriad of uses including gas barrier, heat insulator, and fireproof devices.