Percolation transition and conductivity in macroscopically anisotropic two-component polymer membranes

Percolation conductance has been studied for polypropylene/poly(acrylic acid) membranes (PP-g-PAA) prepared by volume grafting of an ionic component onto polypropylene foil. In these membranes the volume fraction of the conducting phase decreases from the surface to the center, brining some anisotropy into the composition. For this material, the “insulator- to-conductor” transition proceeds at a critical volume fraction V_c equal to 0.395. It has been proved that this critical volume fraction and the critical exponent t, found to be below that predicted by theory, i.e., 1.2 ± 0.03 instead of 1.6 ± 0.2, result from the gradient in the distribution of the conducting component in the direction of the current flow. Computations of V_c and t for the same PP-g-PAA copolymer with a randomly distributed conducting component have led to V_c = 0.08 and t^* = 1.53. A critical exponent t in that range is characteristic of three-dimensional systems; however, V_c is rather low, suggesting a nonspherical shape for the conducting domains.     

Percolation conductivity in Nafion membranes

This article deals with the method of determination of a threshold volume fraction of the conductive phase within perfluorosulfonic acid ionomer Nafion. Experiments have been performed with the commercial Nafion-120 and Nafion-427 membranes equilibrated with concentrated sodium chloride and sodium hydroxide solutions at 353 K. It has been stated that the insulator-to-conductor transition in membranes occurs at the critical volume fraction of the conductive phase (V_c) equal to 0.1. The same V_c has been estimated for a geometrical cluster-network model. Lower than the theoretical V_c for a classical dense-packed-hard-sphere model (V_c = 0.15), the volume fraction for the membranes is caused mainly by channels connecting the ionic clusters. The critical exponent t has been calculated for both membranes and found to be equal to 1.6 for Nafion-120 and 1.5 for Nafion-427. Both these constants correspond to those theoretically predicted for 3D systems. The ratios of sodium ion mobility in the internal membrane solution to its mobility in the equilibrating NaCl or NaOH solutions (u ₊/u₊) are below unity, and they are dependent on the nature and concentration of the electrolyte.     

N-bromo-poly(styrene-co-divinylbenzene) sulphonamide metal salts. Synthesis and basic properties

Macromolecular pendant group N-monobromosulphonamide metal salts ? SO₂NBr^?M⁺ having a macroporous styrene-divinylbenzene matrix structure have been obtained by two methods: (a) by the action of aqueous metal hypobromites on the appropriate macromolecular pendant group sulphonamide, and (b) by the action of an aqueous metal bromide on the appropriate macromolecular N-chlorosulphonamide metal salt. In each case the sodium salt product was obtained with a 100% functional yield and it contained 2.1 mmol.g^?1 ? SO₂NBr^?Na⁺ groups, i. e., 4.2 meq · g^?1 of active bromine. The stability of the resin was investigated in the range 20–100°C in the dry state as well as in aqueous media. A thermoanalysis (TG, DTG, and DTA curves) was carried out up to 1000°C and its course was interpreted. The resin has brominating, oxidative, microbiocidic, and ionexchanging properties. It reacts in a reversible way: after the loss of the active bromine, it can be reactivated again.     

Low temperature techniques for X-ray microanalysis in pathology: Alternatives to cryoultramicrotomy

Many diseases are associated with a change in the distribution of diffusible ions at the cell or tissue level. These diseases can profitably be studied by X-ray microanalysis. This technique for the study of ion distribution requires the use of cryoprepared specimens. Analysis at low or medium resolution can be carried out on thick or semi-thick cryosections, or on frozenhydrated or freeze-dried embedded bulk samples. Such analyses are particularly useful in the initial stages of an investigation or when data from a large number of samples have to be acquired. Also X-ray microanalysis of cultured or single cells prepared by freeze-drying can be used to rapidly collect information on a large number of cells. Analysis at high resolution has to be carried out on thin sections: Cryosections or sections of freeze-substituted or freeze-dried embedded tissue. For the latter type of specimens, the use of low-temperature embedding methods may have important advantages.     

Why low temperature embedding for X-ray microanalytical investigations?

Freeze-drying followed by infiltration with resin and polymerization by UV light at low temperatures and under constant vacuum conditions is an alternative tissue preparation technique for microprobe analysis. Embedding is carried out with the nonpolar low-temperature embedding resin (Lowicryl HM20) which allows infiltration and polymerization at temperatures down to ?50°C. Sections of low temperature embedded material can be cut dry at ?60°C or at room temperature. Sectioning at low temperatures is an alternative for preparations that are difficult to cut at room temperature. The morphological preservation is adequate for the identification of structures such as mitochondria, lysosomes and different types of endoplasmic reticulum in liver cells. Some physical properties of Lowicryl resins, such as mass loss under the electron beam and high contrast, are positive characteristics for the analysis of semi-thick sections. No significant differences in the elemental composition could be detected between tissue which was freeze-dried or freeze-substituted prior to embedding. Freeze-drying is less time consuming. By avoiding contact with organic solvents the risks of ion loss and redistribution are diminished. In contrast to freeze-dried thin cryosections, low temperature embedded material can be sectioned for light microscopy and areas of interest chosen for further thin sectioning. This is of great importance in work with tissues with complicated morphology and heterogeneous cell populations. The initial preparative step—the cryofixation— determines to a high degree the morphological preservation of freeze-dried and embedded tissue.     

Experimental and numerical study of the load distribution in a ball-screw system

In this work, load distribution on ball-screw systems (BSS) is determined by experimental techniques. Two optical techniques are used: photoelasticity for stress-field measurement and the mark-tracking method for displacement-field determination. In parallel to the experimental study, finite element method (FEM) and analytical solutions are used to calculate the loads applied on each ball of the BSS. Experimental results are used to validate the choice of boundary conditions and contact conditions between ball-screw and ball-nut in the FEM solution. The validation criterion is the correspondence between numerical and experimental fringes representing the differences of principal stresses. In addition to the study of load distribution, this paper presents the influence of the angle of contact direction on the stress distribution in BSS.     

Organocatalytic Activation of Pyruvates Toward Michael Addition to Nitroalkenes: Synthesis of γ-Aminobutyric Acid (GABA) Derivatives

Despite the significant role of pyruvic acid in biotransformations, the utilization of pyruvates in the field of catalytic asymmetric synthesis has been unexpectedly limited. This statement also applies to enamine catalysis, in which pyruvates seem to be challenging substrates. Recently, the first illustration of the asymmetric addition of pyruvic acid to β-nitrostyrenes, catalyzed by a NahE enzyme using the enamine mechanism, has appeared in the literature. Now, we demonstrate pyrrolidine-based catalyst can mimick enamine mechanism used by this enzyme to activate pyruvates in the asymmetric addition to structurally diverse nitroalkenes to produce highly enantioenriched Michael adducts without the limitations in substrate structures associated with biocatalysts. In addition to confirming that optically pure pyrrolidines can serve as ultimately small organic catalysts in pyruvate activation, we present an exceedingly simple and versatile method for the synthesis of optically pure γ-nitro acids, which are precursors for well-known active pharmaceutical ingredients namely GABA derivatives     

Inverse Flood Routing Using Simplified Flow Equations

Water Resources Management - The paper considers the problem of inverse flood routing in reservoir operation strategy. The aim of the work is to investigate the possibility of determining the...     

Molecular mobility and physical ageing of plasticized poly(lactide)

Molecular mobility and physical ageing of amorphous plasticized polylactide (PLA) with two different contents of mesolactide are studied with the help of thermal analysis. Used plasticizers are acetyl tributyl citrate (ATBC) and triacetin (TA), two molecules with established miscibility and plasticizing efficiency. Lower plasticizer permanence of TA compared with ATBC is found. The plasticizer molecules decreased the size of the cooperativity domains at the glass transition temperature T_g and likely in the glassy state by decreasing intermacromolecular interactions and notwithstanding the mesolactide content of PLA and the chemical identity of the plasticizer. The recovery function is given and shows a significant effect of the plasticizer on the physical ageing. The supplementary free volume brought by the plasticizer enhances molecular mobility in the glassy state and increases structural relaxation at one order of magnitude. The comparison between different plasticizers reveals that the structural relaxation is however independent from the type of plasticizer and the percentage of mesolactide in PLA. POLYM. ENG. SCI., 55:858–865, 2015. © 2014 Society of Plastics Engineers