Adsorption-entanglement layers in flowing high molecular weight polymer solutions III: Solution concentration and solvent power

Adsorption entanglement layers are thick, i. e. multimolecular layers formed at solid surfaces during the flow of high molecular weight polymer solutions. The present publication is concerned with the influence of solution concentration and solvent power on the formation of these layers. Three polymers have been used, polyethylene, polymethylmethacrylate and atactic polystyrene in various solvents. The formation and thickness of layers were monitored from their effect on flow behaviour. Two flow geometries were used: Couette flow — where the layers cause peaks in the shear stress during continuous shearing, and capillary flow where the layers reduce the flow rate. The results indicate that there is a critical concentration below which no layers form, and that this critical concentration is an order of magnitude less than the conventionally defined overlap concentration. It is also deduced that the layers form more quickly, and grow to be thicker when better solvents are used.     

A new look at the crystallization of polyethylene. II. Crystallization from the melt at low supercoolings

This article is part of the general project laid out in Part I (ref. 9) and is concerned with obtaining information on primary (unthickened) crystals of polyethylene formed at low supercoolings. For this, a technique had to be devised by which crystallization could be speeded up so as to eliminate or at least reduce lamellar thickening. Indeed we were able to increase the rate of crystallization by an order of magnitude using a technique which we have called enhanced self-nucleation. Using this technique we find that when viewed under an optical microscope, spherulites crystallize uniformly over the field of view, and not, as is usual, by a radial growth process. Isothermal crystallization in bulk linear polyethylene has been studied by means of the enhanced self-nucleation technique as a function of crystallization time by using Raman LAM and melting points to assess variations of fold length Data have been obtained at very much shorter times than before. At short times, we find a constant fold length; at longer times the crystals thicken linearly with the logarithm of time. Values of the initial fold length for crystallization temperatures between 118 and 130°C are presented. Associated with the thickening at short times we find an induction time which increases with temperature.     

Electrochemical materials science: tailoring intrinsically conducting polymers. The example: substituted thiophenes

A series of 3-(p-X-phenyl) thiophene monomers (X= –H, –CH₃, –OCH₃, –COCH₃, –COOC₂H₅, –NO₂) was electrochemically polymerized to furnish polymer films that could be reversibly reduced and oxidized (n- and p-doped). The oxidation potentials of the monomers and formal potentials of the n- and p-doping processes of polymers were correlated with resonance and inductive effects of the substituents on the phenyl ring as well as the semiempirically calculated heat of formation of the monomer radical cations. Presented at the 4th Baltic Conference on Electrochemistry, Greifswald, March 13-16, 2005     

Adsorption-entanglement layers in flowing high molecular weight polymer solutions IV. The rates of layer formation and decay

Adsorption-entanglement layers are thick (i. e. multimolecular) layers which form at solid surfaces during flow of high molecular weight polymer solutions. This publication, the final part of the present series, is concerned with the rates at which such layers are formed and at which once formed they decay. It is shown that the rate of layer formation depends linearly on the shear rate applied. It is also demonstrated that the rate at which the layers decay is much slower than the rate at which they form. From the results presented in this paper, together with those in previous parts of this series [1–3], a qualitative model which accounts for the observed behaviour is deduced.     

INAA for the characterization of airborne particulate matter from the industrial area of Islamabad city

Air particulate matter (PM) was collected in two size fractions using stacked filter units (SFUs) provided by the International Atomic Energy Agency (IAEA) from the industrial area of Islamabad. Nucleopore polycarbonate filters were used for collecting from Oct 98 to Jun 99 the particulate matter in coarse and fine size fractions. The samples were characterized by the instrumental neutron activation analysis (INAA). About 33 elements were quantified using different irradiation and counting protocols.     

Toward a Formyl-to-Phenyl Conversion: An Unexpected Photochemical Fulvene Rearrangement

Toward a conversion of aldehydes into arenes, we designed a sequence involving the initial reaction of an aldehyde to give a fulvene, followed by photochemical and platinum-catalyzed rearrangements into a Dewar benzene derivative, which finally isomerizes into the targeted arene. While computational studies support the plausibility of this route, we found that fulvene irradiation resulted in an unexpected isomerization into a spiro[2.4]heptadiene. This unusual photorearrangement has been investigated mechanistically and provides access to a variety of spiro[2.4]heptadienes with different substituents.     

Atomically Precise Defective Copper Nanocluster Catalysts for Highly Selective C−C Cross-Coupling Reactions

Point defects in nanoparticles have long been hypothesized to play an important role in governing the particle's electronic structure and physicochemical properties. However, single point defects in material systems usually exist with other heterogeneities, obscuring the chemical role of the effects. Herein, we report the synthesis of novel atomically precise, copper hydride nanoclusters (NCs), [Cu₂₈H₁₀(C₇H₇S)₁₈(TPP)₃] ( Cu₂₈ ; TPP: triphenylphosphine; C₇H₇S: o-thiocresol) with a defined defect in the gram scale via a one-pot reduction method. The Cu₂₈ acts as a highly selective catalyst for C−C cross-couplings. The work highlights the potential of defective NCs as model systems for investigating individual defects, correlating defects with physiochemical properties, and rationally designing new nanoparticle catalysts.     

Development and implementation of <Emphasis Type="Italic">k</Emphasis><Subscript>0</Subscript>-INAA standardization at PINSTECH

The k ₀ method has been introduced at the 30 kW miniaturized neutron source reactor (MNSR) at the Pakistan Institute of Nuclear Science & Technology (PINSTECH). It involved the full energy peak efficiency calibration of the HPGe detector for different counting geometries and the characterization of the neutron flux at four inner irradiation channels. The latter involved the determination of the thermal to the epithermal flux ratio, epithermal flux shape factor, the modified spectral index, Westcott’s g-factor, the Maxwellian neutron temperature and the fast flux. The method was validated by analyzing IAEA-SL1 (Lake Sediment) and NIST-SRM-1572 (Citrus Leaves) reference materials. All calculations were performed in Excel, including the optimization step. The results revealed that most of the elements were estimated with less than 10% relative deviation from the certified value.     

Density of states effective mass of n-type CuInSe2 from the temperature dependence of Hall coefficient in the activation regime

The Hall coefficient R_H of n-type CuInSe₂ single crystals is measured between 10 and 300 K in pulsed magnetic field up to 35 T. The threshold field B_th, above which the magnetic freezeout starts to occur, varies linearly with temperature. From the analysis of the temperature dependence of electron concentration in the activation regime above 100 K at different field values, it is established that the density of states effective mass is independent of the magnetic field B and the activation energy E_D, above around 6 T, varies as B^1/3. Similar B^1/3 dependence of the magnetoresistance in the high magnetic field regime, reported earlier in the same material, suggests that theoretical work that could explain this coincidence is needed.