Surfactant-Assisted Synthesis of Semiconductor Nanotubes and Nanowires

Nanotubes and nanowires of CdSe and CdS have been obtained from solutions containing a surfactant such as Triton 100-X. They have been characterized by x-ray diffraction, electron microscopy, and optical spectroscopy

Conformational dynamics and internal friction in homopolymer globules: equilibrium vs. non-equilibrium simulations

We study the conformational dynamics within homopolymer globules by solvent-implicit Brownian dynamics simulations. A strong dependence of the internal chain dynamics on the Lennard-Jones cohesion strength ε and the globule size N [subscript G] is observed. We find two distinct dynamical regimes: a liquid-like regime (for ε ε[subscript s] with slow internal dynamics. The cohesion strength ε[subscript s] of this freezing transition depends on N G . Equilibrium simulations, where we investigate the diffusional chain dynamics within the globule, are compared with non-equilibrium simulations, where we unfold the globule by pulling the chain ends with prescribed velocity (encompassing low enough velocities so that the linear-response, viscous regime is reached). From both simulation protocols we derive the internal viscosity within the globule. In the liquid-like regime the internal friction increases continuously with ε and scales extensive in N [subscript G] . This suggests an internal friction scenario where the entire chain (or an extensive fraction thereof) takes part in conformational reorganization of the globular structure.American Society for Engineering Education. National Defense Science and Engineering Graduate Fellowshi

Interaction of cylindrical polymer brushes in dilute and semi-dilute solution

We present a systematic study of flexible cylindrical brush-shaped macromolecules in a good solvent by small-angle neutron scattering (SANS), static light scattering (SLS), and by dynamic light scattering (DLS) in dilute and semi-dilute solution. The SLS and SANS data extrapolated to infinite dilution lead to the shape of the polymer that can be modeled in terms of a worm-like chain with a contour length of 380 nm and a persistence length of 17.5 nm. SANS data taken at higher polymer concentration were evaluated by using the polymer reference interaction site model (PRISM). We find that the persistence length reduce from 17.5 nm at infinite dilution to 5.3 nm at the highest concentration (volume fraction 0.038). This is comparable with the decrease of the persistence length in semi-dilute concentration predicted theoretically for polyelectrolytes. This finding reveals a softening of stiffness of the polymer brushes caused by their mutual interaction

Semiconductor nanorods: Cu, Zn, and Cd chalcogenides

Semiconductor nanorods have been synthesized by the use of surfactants. The nanorods of CuS, ZnS, CdS, CuSe, ZnSe, and CdSe have diameters in the 5-150 nm range and exhibit high aspect ratios, with lengths of up to several microns. By using a high surfactant concentration, nanotube-like structures of CdS and CdSe with high aspect ratios are obtained. All the chalcogenide nanostructures possess hexagonal structures, independent of the structures of the stable phases of the materials

Polycarbonate synthesis by the melt phase carbonate-ester interchange reaction of bisphenol-a diacetate with dimethyl carbonate

The molecular weight of polycarbonate formed by the carbonate-ester interchange reaction of bisphenol-A diacetate (BPAC2) and dimethyl carbonate (DMC) was found to depend on the composition of the oligomer formed in its first stage. At a [DMC] : [BPAC2] ratio of 2, an oligomer with relatively higher acetate end group concentration was formed. This upon further polycondensation yielded a polycarbonate of inherent viscosity, 0.2-0.25 dL/g. Upon addition of DMC at a later stage in the oligomer-forming reaction, an oligomer containing relatively higher carbonate end groups could be obtained. These oligomers gave polycarbonates higher inherent viscosity, 0.4-0.5 dL/g. It is therefore proposed that oligomers having carbonate end groups are more reactive in self-polycondensation than those containing acetate end groups

Surfactant-assisted synthesis of semiconductor nanotubes and nanowires

Nanotubes and nanowires of CdSe and CdS have been obtained from solutions containing a surfactant such as Triton 100-X. They have been characterized by X-ray diffraction, electron microscopy, and optical spectroscopy