Room‐Temperature Phosphorescence From Films of Isolated Water‐Soluble Conjugated Polymers in Hydrogen‐Bonded Matrices

It is well known that luminescent conjugated polymers suffer serious loss of photoluminescence quantum yield (PLQY) in the solid state compared to dilute solution. This is due to efficient exciton migration in the solid, which enables the excitons to readily find low energy quenching sites. Here a new method to fabricate solid films with densely packed non‐interacting luminescent polymer chains, which yield very high PLQY and more astonishingly room temperature phosphorescence, is reported. Using water‐soluble conjugated polymers (WSCP) and polymeric surfactants such as poly(vinyl alcohol) (PVA) and poly(vinyl‐pyrrolidone) (PVP), films at 1:1 wt% or higher WSCP are produced and show room temperature phosphorescence; such behavior has never been observed before and clearly shows the very high degree of chain isolation that can be achieved in these hosts. The PVA or PVP not only breaks up WSCP aggregates in solution as an effective surfactant, PVA‐PVA or PVP‐PVP hydrogen bond formation upon drying locks in the isolation of the WSCP, avoiding segregation and yielding long time stability to these polymer/polymer nanomixtures. The method is found to work with a wide variety of WSCPs.     

Speech compression and encryption based on discrete wavelet transform and chaotic signals

Multimedia Tools and Applications - To increase transfer and storage efficiencies of the information, data compression has emerged as a significant issue in the communication environments. This...     

Novel poly(thiourea‐ether‐imide)s derived from 4,4′‐oxydiphenyl‐bis(thiourea): probing the possibility for high‐temperature applications

Our interest in the fabrication of high‐performance polyimides has led to thiourea‐substituted poly(thiourea‐ether‐imide)s (PTEIs) with good retention of thermal properties along with flame retardancy. A new aromatic monomer, 4,4′‐oxydiphenyl‐bis(thiourea) (ODPBT), was efficiently synthesized and polymerized with various dianhydrides (pyromellitic dianhydride, 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride and 4,4′‐(hexafluoroisopropylidene)diphthalic dianhydride) via two‐stage chemical imidization to fabricate a series of PTEIs. The structural characterization of ODPBT and the polymers was carried out using Fourier transform infrared, ¹H NMR and ¹³C NMR spectral techniques along with crystallinity, organosolubility, inherent viscosity and gel permeation chromatographic measurements. PTEIs bearing C?S and ? O? moieties in the backbone demonstrated an amorphous nature and were readily soluble in various amide solvents. The novel polymers had inherent viscosities of 1.16–1.23 dL g^?1 and molecular weights of ca 90 783–96 927 g mol^?1. Their thermal stability was substantiated via 10% weight loss in the temperature range 516–530 °C under inert atmosphere. The polyimides had glass transition temperatures of 260–265 °C. Incorporation of thiourea functionalities into polymer backbones is demonstrated to be an effective way to enhance their thermal properties and flame retardancy. Thus, ODPBT can be considered as an excellent candidate for use in the synthesis of high‐performance polymeric materials. Copyright © 2010 Society of Chemical Industry     

Novel Hybrids of Polystyrene Nanoparticles and Silica Nanoparticles-Grafted-Graphite Via Modified Technique

We have designed polystyrene nanoparticles through modified nanoprecipitation cosolvent evaporation technique and conventional nanoprecipitation and solvent evaporation. Polystyrene nanoparticles were embedded with silica nanoparticles and graphite to analyze scope of nanoprecipitation cosolvent evaporation. Ultraviolet–visible spectroscopic revealed decreased band gap of polystyrene nanoparticles obtained via nanoprecipitation cosolvent evaporation. Scanning electron microscopic showed uniform morphology of polystyrene nanoparticles and polystyrene nanoparticles-based nanocomposites engendered by nanoprecipitation cosolvent evaporation. X-ray diffraction disclosed presence of crystalline domains due to silica nanoparticles content in amorphous structure. Glass transition temperature was increased from 94 (polystyrene) to 124°C (PSNPs/SiNPs 0.6) and 137°C (PSNPs/SiNPs/G 0.6) with filler loading. Electrical conductivity of PSNPs/SiNPs/G 0.6 was also found to be higher (1.53 S/cm).     

Narrow emission linewidths of positioned InAs quantum dots grown on pre-patterned GaAs(100) substrates

We report photoluminescence measurements on a single layer of site-controlled InAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) on pre-patterned GaAs(100) substrates with a 15 nm re-growth buffer separating the dots from the re-growth interface. A process for cleaning the re-growth interface allows us to measure single dot emission linewidths of 80 μeV under non-resonant optical excitation, similar to that observed for self-assembled QDs. The dots reveal excitonic transitions confirmed by power dependence and fine structure splitting measurements. The emission wavelengths are stable, which indicates the absence of a fluctuating charge background in the sample and confirms the cleanliness of the re-growth interface.     

Unconditional nonlinear stability for double‐diffusive convection in a porous medium with temperature‐dependent viscosity and density

In this study, fluid flow in a porous medium is analyzed using a Forchheimer model. The problem of double‐diffusive convection is addressed in such a porous medium. We utilize a higher‐order approximation for viscosity‐temperature and density‐temperature, such that the perturbation equations contain more nonlinear terms. For unconditional stability, nonlinear stability has been achieved for all initial data by utilizing the or norms. It also shows that the theory of is not sufficient for such unconditional stability. Both linear instability and nonlinear energy stability thresholds are tested using three‐dimensional (3D) simlations. If the layer is salted above and salted below then stationary convection is dominant. Thus the critical value of the linear instability thresholds occurs at a real eigenvalue , and our results show that the linear theory produces the actual threshold. Moreover, it is known that with the increase of the salt Rayleigh number, R_c, the onset of convection is more likely to be via oscillatory convection as opposed to steady convection. The 3D simulation results show that as the value of R_c increases, the actual threshold moves towards the nonlinear stability threshold, and the behavior of the perturbation of the solutions becomes more oscillatory.     

Thermal and mechanical properties of carbon nanotube/epoxy nanocomposites reinforced with pristine and functionalized multiwalled carbon nanotubes

In this study, the effects of functionalization and weight fraction of mutliwalled carbon nanotubes (CNTs) were investigated on mechanical and thermomechanical properties of CNT/Epoxy composite. Epoxy resin was used as matrix material with pristine‐, COOH‐, and NH₂‐functionalized CNTs as reinforcements in weight fractions of 0.1, 0.5, and 1.0%. Varying (increasing) the weight fraction and changing type (pristine or functionalized) of CNTs caused increment in Young's modulus and tensile strength as observed during mechanical tests. CNT reinforcement improved thermal stability of the nanocomposites as observed by thermogravimetric analysis. Thermomechanical analysis showed a slight reduction in free volume of the polymer, that is a drop in coefficient of thermal expansion, prior to glass transition temperature (T_g) beside a slight increase in T_g value. Dynamic mechanical analysis indicated an increase in storage modulus and T_g owing to the strength addition of CNT to the matrix alongside the hardener. Scanning electron microscopy analysis of the fractured surface(s) revealed that CNTs were well dispersed with no agglomeration and resulted in reinforcing the matrix. POLYM. COMPOS., 36:1891–1898, 2015. © 2014 Society of Plastics Engineers     

Material Characterization of Ge1−x Sn x Alloys Grown by a Commercial CVD System for Optoelectronic Device Applications

High-quality compressive-strained Ge_1?x Sn _x /Ge films have been deposited on Si(001) substrate using a mainstream commercial chemical vapor deposition reactor. The growth temperature was kept below 450°C to be compatible with Si complementary metal–oxide–semiconductor processes. Germanium tin (Ge_1?x Sn _x ) layers were grown with different Sn composition ranging from 0.9% to 7%. Material characterizations, such as secondary-ion mass spectrometry, Rutherford backscattering spectrometry, and x-ray diffraction analysis, show stable Sn incorporation in the Ge lattice. Comparison of the Sn mole fractions obtained using these methods shows that the bowing factor of 0.166 nm (in Vegard’s law) is in close agreement with other experimental data. High-resolution transmission electron microscopy and atomic force microscopy results show that the films have started to relax through the formation of misfit and threading dislocations. Raman spectroscopy, ellipsometry, and photoluminescence (PL) techniques are used to study the structural and optical properties of the films. Room-temperature PL of the films shows that 7% Sn incorporation in the Ge lattice results in a decrease in the direct bandgap of Ge from 0.8 eV to 0.56 eV.     

Retained stones in the common bile duct: results of management

Seventeen patients underwent treatment for retained common bile duct stones. In 7 patients the stones were removed via a T-tube tract using steerable catheters while 5 patients underwent ERCP and sphincterotomy and 5 underwent surgical re-exploration. Considering morbidity, mortality, success rate and patients' stay in the hospital, non-operative modalities should be the treatment of choice for retained common bile duct stones.