Strong visible up-conversion emission in Tb3+, Tm3+ and Tb3+-Tm3+ co-doped tellurite glasses sensitized by Yb3+

Up-conversion luminescence characteristics under 975 nm excitation have been investigated with Tb3+/Tm3+/Yb3+ triply doped tellurite glasses. Here, green (547 nm: D-5(4) --> F-7(4)) and red (660 nm: D-5(4) --> F-7(2)) up-conversion (UC) luminescence originating from Tb3+ is observed strongly, because of the quadratic dependences of emission intensities on the excitation power. Especially, the UC luminescence was intensified violently with the energy transfer from the Tm3+ ions involves in the Tb3+ excitation. To the Tb3+/Tm3+/Yb3+ triply doped glass system, a novel up-conversion mechanism is proposed as follows: the energy of (3)G(4) level (Tm3+) was transferred to D-5(4) (Tb3+) and the 477-nm UC luminescence of Tm3+ was nearly quenched. (C) 2006 Elsevier B.V. All rights reserved.

Effects of melting temperature on the broadband infrared luminescence of bi-doped and Bi/Dy co-doped chalcohalide glasses

Bismuth (Bi)-doped and Bi/Dy co-doped chalcohalide glasses are investigated as promising materials for amplifiers in optical communication. The samples synthesized at lower melting temperatures (MTs) are characterized by more intensified infrared emissions. With respect to the redox process of a liquid mixture at different MTs, we attribute an emission at 1230 nm to low-valent Bi ions. The lower MT favors the formation of LVB ions, i.e. Bi+ or Bi2+, while the higher MT promotes the production of higher-valent Bi ions Bi3+. An enhanced broadband infrared luminescence with the full-width at half-maximum over 200 nm is achieved from the present Bi/Dy co-doped chalcohalide glasses.

Complete genomic sequences for hepatitis C virus subtypes 6e and 6g isolated from Chinese patients with injection drug use and HIV-1 co-infection

In one of our recent studies, two HCV genotype 6 variants were identified in patients from Hong Kong and Guangxi in southern China, with injection drug use and HIV-1 co-infection. We report the complete genomic sequences for these two variants: GX004 and

The morphology and thermal properties of multi-walled carbon nanotube and poly(hydroxybutyrate-co-hydroxyvalerate) composite

Nanocomposites based on poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) and multi-walled carbon nanotubes (MWNTs) were prepared by solution processing. Ultrasonic energy was used to uniformly disperse MWNTs in solutions and to incorporate them into composites. Microscopic observation reveals that polymer-coated MWNTs dispersed homogenously in the PHBV matrix. The thermal properties and the crystallization behavior of the composites were characterized by thermogravimetric analysis, differential scanning calorimetry and wide-angle X-ray diffraction, the nucleant effect of MWNTs on the crystallization of PHBV was confirmed, and carbon nanotubes were found to enhanced the thermal stability of PHBV in nitrogen.

Multiple melting behavior of isotactic polypropylene and poly(propylene-co-ethylene) after stepwise isothermal crystallization

The multiple melting behavior of several commercial resins of isotactic polypropylene (iPP) and random copolymer, poly(propylene-co-ethylene) (PPE), after stepwise isothermal crystallization (SIC) were studied by differential scanning calorimeter and wide-angle X-ray diffraction (WAXD). For iPP samples, three typical melting endotherms appeared after SIC process when heating rate was lower than 10 degreesC/min. The WAXD experiments proved that only alpha-form crystal was formed during SIC process and no transition from alpha1- to alpha2-form occurred during heating process. Heating rate dependence for each endotherm was discussed and it was concluded that there were only,two major crystals with different thermal stability. For the PPE sample, more melting endotherms appeared after stepwise isothermal crystallization. The introduction of ethylene comonomer in isotactic propylene backbone further decreased the regularity of molecular chain, and the short isotactic propylene sequences could crystallize into gamma-form crystal having a low melting temperature whereas the long sequences crystallized into alpha-form crystal having high melting temperature.

Spectrometric investigation on the thermooxidative degradation of ethylene oxide and tetra-hydrofuran co-polyether

The thermooxidative degradtion of ethylene oxide and tetra-hydrofuran (EO-THF) co-polyether has been studied by electron spin resonance (ESR), Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. The initial degradation site was found to be at the a-carbon of the ether bond. Two free radicals which derived from dehydrogenation and oxygen addition were successfully detected by spin-trapping technique which used alpha -phenyl-N-tert-butyl nitrone(PBN) as spin trap. Both FT-IR and NMR have been used to follow structural changes of the copolyether during degradation. Nearly 20 product fragments including formate, carbonate, methyl, alcohol, methylene-dioxy, hydroperoxide and semiformal have been characterized by D-1 and D-2 NMR. The thermooxidtion of co-polyether preferred to occur on the THF units especially at the alternating linkage of EO and THF. Antioxidant (BHT) not only retarded the thermooxidation but also modified the degradation products with less ester and methylene-dioxy groups hut more hydroxyl and methyl groups.

Crystallization kinetics in mixtures of poly(epsilon-caprolactone) and poly(styrene-co-acrylonitrile)

Isothermal crystallization kinetics in the miscible mixtures of poly(epsilon-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) have been investigated as a function of the composition and the crystallization temperature by optical microscopy. The radial growth rates of the spherulites have been described by a kinetic equation including the interaction parameter and the free energy for the formation of secondary crystal nuclei. Fold surface free energies decrease slightly with the increase of SAN content. The experimental findings show that the influence of the glass transition temperature of the mixture, which is related to the chain mobility, on the rate of crystallization predominates over the influence of the surface free energies. This indicates that the glass transition temperature of the mixture should be of more importance, so that the growth rates decrease when the content of the noncrystallizable component increases. In addition, the Flory-Huggins interaction parameter obtained by fitting the kinetic equation with experimental data is questionable.

The miscibility of homopolymer random copolymer blends .1. Poly(styrene-co-acrylonitrile) poly(ethyl methacrylate) and poly(styrene-co-acrylonitrile) poly(methyl methacrylate) blends

The miscibility of blends of poly(styrene-co-acrylonitrile) (SAN) with poly(methyl methacrylate) (PMMA) or poly(ethyl methacrylate) (PEMA) has been investigated by means of NMR and DSC techniques. It is found that there are intermolecular interactions between the phenyl groups in SAN and carbonyl groups in PMMA or PEMA, and the strength of this intermolecular interaction strongly depends on the properties of ester side groups in PEMA or PMMA, composition of the blends and a certain composition of the copolymer. It is this specific interaction instead of the intramolecular repulsion force within the copolymer that plays a key role for the miscibility of SAN/PMMA and SAN/PEMA blends.

CRYSTALLIZATION KINETICS OF MIXTURES OF POLY(EPSILON-CAPROLACTONE) AND POLY(STYRENE-CO-ACRYLONITRILE)

The crystallization kinetics in mixtures of poly(epsilon-caprolactone) (PCL) and poly(styrene-co-acrylonitrile) (SAN) has been investigated as the function of composition and crystallization temperature. The isothermal growth rates of PCL spherulites decrease with increasing concentration of SAN. Because of the miscibility of PCL/SAN mixtures, the radial growth rates of the spherulites are described by a kinetic equation including the interaction parameter and the free energy for the formation of crystal nuclei. The interaction parameter obtained from the fitting of the kinetic equation with experimental data is in good agreement with that obtained from melting point depression. Folding surface free energies decrease with the increase of SAN concentration. In light of these results, it is suggested that, for the PCL/SAN mixtures, the noncrystallizable SAN polymer reduces the mobility of crystallizable PCL polymer so that the growth rates decrease with the increase of noncrystallizable component fraction.

MISCIBILITY OF POLY(EPICHLOROHYDRIN) WITH POLY(VINYL ACETATE) AND POLY(STYRENE-CO-ACRYLONITRILE)

The phase behaviours of poly(vinyl acetate) (PVAc) and poly(styrene-co-acrylonitrile)s (SAN) with poly(epichlorohydrin) (PECH) were examined using differential scanning calorimetry and an optical method using a hot plate. The PECH/PVAc blends showed LCST behaviour. The observed miscibility is thought to be a result of hydrogen-bonding interactions between the alpha-hydrogen atoms of PECH and the carbonyl groups of PVAc. Two SAN copolymers with an acrylonitrile (AN) content of 18 wt% (SAN18) and 25 wt% (SAN25), respectively, were also found to exhibit miscibility with PECH. No phase separation occurred by heating up to about 280-degrees-C, and the individual blend has a single, composition-dependent glass transition temperature. The formation of miscible PECH/SAN blends can be considered as a result of the intramolecular repulsion between styrene and AN units in SAN.