Skin formation and bubble growth during drying process of polymer solution

When a polymer solution with volatile solvent is dried, skins are often formed at the surface of the solution. It has been observed that after the skin is formed, bubbles often appear in the solution. We conducted experiments to clarify the relation between the skin formation and the bubble formation. We measured the time dependence of the thickness of the skin layer, the size of the bubbles, and the pressure in the solution. From our experiments, we concluded that i) the gas in the bubble is a mixture of solvent vapor and air dissolved in the solution, ii) the bubble nucleation is assisted by the pressure decrease in the solution covered by the skin layer, and iii) the growth of the bubbles is diffusion limited, mainly limited by the diffusion of air molecules dissolved in the solution.     

Construction, operation and output characteristics of a small, long-life, cataphoretic HeSe laser

The properties and main design features of a small, cataphoretic HeSe⁺ laser are described. By improving both the He buffer gas containment and the Se vapour deposition, laser life can be increased considerably to several thousand hours. The length of the active laser region can be reduced to 100 mm without appreciable loss in single pass gain by reduction of the capillary diameter to 1 mm. Up to 3 mW laser power in six blue-green and green Sell lines can be extracted from this small active volume. By synchronizing the spontaneous discharge striations in the He-Se vapour mixture, laser noise is reduced to less than 3% r m s. A few potential applications for this type of laser are listed to encourage commercial interest.     

Nafion/mordenite hybrid membrane for high-temperature operation of polymer electrolyte membrane fuel cell

Nafion/mordenite hybrid membranes for the operation of polymer electrolyte membrane fuel cells (PEMFCs) above 100 °C were prepared by mixing of H⁺-form mordenite powder and perfluorosulfonylfluoride copolymer resin. PEMFC operation above 100 °C reduces CO poisoning as well as passivation of the Pt anode electrocatalyst by other condensable species. The physico-chemical properties of hybrid membranes were investigated by tensile strength and proton conductivity measurements. As the mordenite content increases at the high temperature region, the proton conductivity of hybrid membranes increased due to the late dehydration rate of existent water in the mordenite. Also, from the results of current–voltage relationship for single cells under 130 °C of operation condition, the hybrid membrane cell with 10 wt.% mordenite showed better performance than that of the others over the entire current density range. This result indicated that the existent water in the hybrid membrane containing 10 wt.% mordenite was higher than that with the others, thereby maintaining its conductivity. The Nafion/mordenite hybrid membrane prepared by this present method is thought to be a satisfactory polymer electrolyte membrane for PEMFC operation above 100 °C.     

Construction and operation of high power gas lasers

In the present paper some typical gas laser construction and their performances will be described. Aspects of transition selective systems and high pressure operation will be treated.     

A review of polymer electrolytes: fundamental,approaches and applications

In this paper, we review different types of polymer electrolytes, recent approaches and technological applications of polymer electrolytes. The report first discusses the characteristics, advantages and applications for three types of polymer electrolytes: gel polymer electrolytes, solid polymer electrolytes and composite polymer electrolytes. Next, we discuss the features and performance of different polymer hosts based on some important and recently published literature. Recent progress of some approaches used in improving the performance of the polymer electrolytes is highlighted. The last part of the review includes the technological applications of some electrical energy storing/converting devices: electrochemical capacitors, batteries, fuel cells and dye-sensitized solar cells. It is also stressed that the technological advancement in the polymer electrolytes plays a pivotal role in the development of energy storing/converting systems.     

Ionization equilibrium and radiative energy loss rates for C,N, and O ions in low-density plasmas

The results of calculations of the ionization equilibrium and radiative energy loss rates for C, N and O ions in low-density plasmas are presented for electron temperatures in the range 10⁴–10⁷ °K (～1–10³ eV). The ionization structure is determined using the steady-state corona model, in which electron impact ionization from the ground states is balanced by direct radiative and dielectronic recombination. Using an improved theory, detailed calculations are carried out for the dielectronic recombination rates in which account is taken of all radiative and autoionization processes involving a single-electron electricdipole transition of the recombining ion. The radiative energy loss processes considered are electron-impact excitation of resonance line emission, direct radiative recombination, dielectronic recombination, and electron-ion bremsstrahlung. For all three elements, resonance line emission resulting from 2s?2p transitions produces a broad maximum in the energy loss rate near 10⁵°K(～ 10 eV).     

Wetting, drying, and layering of colloid-polymer mixtures at porous interfaces

The influence of interface porosity on the wetting properties of colloid-polymer mixtures is studied within density functional theory for the Asakura-Oosawa-Vrij model at the surface of a quenched hard-sphere matrix. While the porosity hardly changes the location of the transition from partial to complete wetting at colloidal bulk gas-liquid coexistence, the onset of wetting, as signaled by the first discontinuous layering transition, can be efficiently controlled by tailoring the porosity. We furthermore find that the penetrability of the porous interface induces complete drying into the matrix upon approaching capillary coexistence.     

Thermal stability of alkylbiphenyls in the subcritical,critical, and supercritical states

The kinetics of gas-phase thermal transformations of 4-methyl-, 4,4′-dimethyl-, 4-tert-butyl-, and 4,4′-di-tert-butylbiphenyls was studied over the temperature ranges 813–848, 733–793, 703–763, and 703–763 K, respectively. Competition between cracking and alkyl substituent isomerization at aromatic nuclei of the initial substance and products formed was observed for tert-butylbiphenyls. The kinetic parameters of the overall transformation of alkylbiphenyls (ABPs) were determined. Recommendations for the determination of the critical temperatures of ABPs, their synthesis, processing, and use of articles with structural fragments in molecules identical to those studied in this work were given. It was found experimentally that 2-, 3-, and 4-methylbiphenyls and their mixtures with toluene were thermally unstable over the whole composition range at critical temperatures (600–780 K).     

In situ production of CuS particles in polymer electrolyte matrix for mixed ion+electron conduction

Composites of polymer electrolyte polyethylene oxide (PEO) complexed with NH₄ClO_4, (PEO:NH₄ClO₄), having different weight ratios of dispersed semiconductor CuS (0–5 wt.%) have been prepared and characterized. The dispersal of CuS was achieved by its in situ formation in the viscous solution of polymer electrolyte (PEO:NH₄ClO₄) by sulfuration of CuSO₄ using H₂S. The band gap of CuS dispersed in the composites was found to be ~2.4 eV, which is higher than that of the bulk CuS for which it is 2.2 eV. Scanning electron microscopy studies show that the particle size varies from ~200 nm to several hundreds of nanometers. Polarization studies show that the semiconductor dispersed polymer composite so obtained has mixed ionic and electronic conduction. Detailed I–V studies show that the dispersoid is a p-type semiconductor.     

Rheology of polymer suspensions: II. Translation,rotation and viscosity

The theory of average local velocity fields developed in the preceding article is applied to a suspension of spherically symmetric polymers. Three transport properties of the suspension are analyzed, namely the concentration dependence of the translational and rotational friction coefficient of a polymer, and the concentration dependence of the viscosity of the solution.     

Carboxylated magnetic polymer nanolatexes: Preparation,characterization and biomedical applications

Carboxylated magnetic polymer nanolatexes were prepared by miniemulsion polymerization using 4,4′-azobis(4-cyanopentanoic acid) (ACPA) as initiator, which provided carboxyl end groups on the latex surface directly. The colloidal stability and the magnetic properties showed that these resulting carboxylated magnetic polymer nanolatexes were applicable in biomedical separation, which was performed by covalent coupling of activated antibody.     

Holographic polymer materials with diffusion development: Principles, arrangement, investigation, and applications

We present a review of information that has been published in the scientific literature on the mechanism of formation (development, enhancement) of holograms at the expense of the photochemical attachment of phenanthrenequinone molecules and other compounds to polymer chains and diffusion mixing of unreacted molecules, on different types of light-sensitive materials that realize this mechanism, on the application of these materials to create volume holographic elements, on recording of information, and for research purposes.     

Comparison of TSE, TGSE, and CPMG measurement techniques for MR polymer gel dosimetry

A radiation dose distribution that optimally conforms to the target volume is of major interest for stereotactic radiotherapy. For this purpose treatment plans have to be verified experimentally before transferring to the patient. The requirements regarding dose accuracy and spatial resolution can be fulfilled with tissue equivalent polymer gel dosimeters which offer the possibility to visualize 3D dose distributions. Herewith, dosimetry can be performed by the spin-spin relaxation rate R2 which varies with the absorbed dose. In this work, different MR measurement techniques were evaluated: The standard Carr-Purcell-Meiboom-Gill (CPMG) method, a modified Turbo-Spin-Echo (TSE) sequence, and a modified Turbo-Gradient-Spin-Echo (TGSE) sequence. Experiments were performed both with a homogeneous water phantom and an irradiated polymer gel. The results show that TGSE and especially TSE are suited well for MR polymer gel dosimetry: The acquisition time of both techniques can be reduced in comparison to CPMG by a factor of 5. The accuracy of dose determination for doses between 2 Gy and 13 Gy lies between 5.6% and 2.0% (TSE), 9.0% and 3.2% (TGSE), and 7.9% and 2.7% (CPMG). These investigations show that especially TSE can be handled as a substitute or at least an alternative to CPMG for the verification of treatment plans in stereotactic radiotherapy.     

Characterization of PEO-lithium triflate polymer electrolytes: Conductivity, DSC and Raman Investigations

Polymeric membranes to be used as electrolytic separators in lithium batteries were prepared from Poly Ethylene Oxide (PEO) and Lithium Triflate salt (LiTf) via a solventfree procedure. Several membranes, having different PEO/LiTf molar ratios, were characterized by Electrochemical Impedance Spectroscopy (EIS) to obtain their ionic conductivity. Most of the compositions exhibit an Arrhenius-like behaviour with two different activation energies above and below 60 °C. Only the sample having the highest LiTf concentration showed a non-Arrhenius trend. Differential Scanning Calorimetry (DSC) study was performed to determine the various phases present in the system. The degree of association of the mobile ions changes vs. composition and temperature was investigated by Raman spectroscopy, a powerful technique which allows to follow these changes in details by studying the spectral parameters of the SO₃ stretching vibration of triflate ions. These data were correlated with the conductivity and thermodynamic data. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16–22, 2001.     

Drops of biological fluids drying on a hard substrate: Variation of the morphology,weight, temperature,and mechanical properties

The time variation of the morphology, weight, temperature, and integral mechanical properties of drying drops of biological fluids are investigated with the aim of defining more exactly the mechanism of self-organization in the drops. Test fluids in experiments are distilled water, a physiological salt solution (0.9 wt. % NaCl), a solution of bovine serum albumin (BSA) in water, and a solution of BSA (7 wt. %) in the physiological solution. It is shown that the variation of the weight of the drying drops can be described by a sloping straight line with two slightly nonlinear portions at the beginning and at the end of the drying process. The earlier nonlinear portion correlates with a more rapid fall of the temperature, which slows down and stabilizes as a saturated vapor layer forms over the drop. The later nonlinear portion in the drop weight variation is associated with the retardation of water diffusion through the solidifying medium. The temperature variation of the drops is a superposition of endothermal (water evaporation) and exothermal (salt crystallization and gelation) phase transitions. Phase transitions may change the mechanical properties of the drying drops, which can be detected by the method of acoustic impedancemetry.     

Spin-cast, thin, glassy polymer films: Highly metastable forms of matter

Ultra thin films of glassy polymers such as polystyrene (PS) can show a) anomalously large thickness changes, b) unexpected dewetting properties, c) large shifts in the glass temperature T_g. The present discussion focusses mainly on point a). A certain cascade of metastable states is presented together with (tentative) explanations. Received 1 March 2001 and Received in final form 10 May 2001     

Dynamics of concentrated colloidal suspensions during drying --aging,rejuvenation and overaging

We report on the slow dynamics of concentrated colloidal suspensions during drying and rewetting under conditions of reversible concentration changes without coalescence or aggregation. We used multispeckle diffusing-wave spectroscopy to monitor the slow dynamics of particles. We found that the relaxation of the suspensions exhibits successively slowing-down, acceleration and a stationary regime during drying at constant rates. Under rewetting conditions, we observed slowing-down and a stationary regime. The characteristic time of the stationary regime is inversely proportional to the rate of concentration change and identical for both drying and rewetting. We explain these regimes as aging (overaging), rejuvenation and plastic flow of the suspensions induced by a deviatoric stress (a combination of compressive and elongational stresses) which is induced by the uniaxial compressive strain generated by evaporation.Received: 3 June 2004, Published online: 3 August 2004PACS: 82.70.Dd Colloids - 47.55.Mh Flows through porous media - 42.25.Dd Wave propagation in random media