Thixotropy, yielding and ultrasonic Doppler velocimetry in pulp fibre suspensions

This paper reports thixotropy in concentrated pulp fibre suspensions and studies their transient flow behaviour using conventional rheometry coupled with a velocimetry technique. Specifically, an ultrasonic Doppler velocimeter is used in conjunction with a rate-controlled rheometer to deduce the local velocity profiles of pulp fibre suspensions. Pulp suspensions are found to exhibit a plateau in their flow curves where a slight increase in the shear stress generates a jump in the corresponding shear rate, implying the occurrence of shear banding. The velocity profiles were found to be discontinuous in the vicinity of the yielding radius where the Herschel–Bulkley model failed to predict the flow. Shear history and the time of rest prior to the measurement were found to play a significant role on the rheology and the local velocity profiles of pulp suspensions.     

Wall slip and melt fracture of poly(lactides)

The wall slip and melt fracture behaviour of several commercial polylactides (PLAs) as well as their rheological properties under shear and extensional have been investigated. The PLAs have had weight-average molecular weights in the range of 10⁴–10⁵ g/mol and studied in the temperature range of 160–200°C. The solution properties and linear viscoelastic behaviour of melts indicate linear microstructure behaviour. PLAs with molecular weights greater than a certain value were found to slip, with the slip velocity to increase with decrease of molecular weight. The capillary data were found to agree well with linear viscoelastic envelope once correction for slip effects was applied. The onset of melt fracture for the high molecular weight PLAs was found to occur at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt.% of a polycaprolactone (PCL) into the PLA that exhibits melt fracture was found to be effective in eliminating and delaying the onset of melt fracture to higher shear rates. This is due to significant interfacial slip that occurs in the presence of PCL.     

A 1,3,2‐ox­aza­borolidine dimer derived from (S)‐α,α‐di­phenyl­prolinol

The reaction of (S)‐α,α‐di­phenyl­prolinol with an excess of borane–tetra­hydro­furan complex yields a stable crystalline material with the composition C₃₄H₃₈B₂N₂O₂, which features a borane adduct of a spiro­cyclic structure with two ox­aza­borolidine rings joined by a central tetrahedral B atom. This dimeric ox­aza­borolidine complex, viz. 3,3,3′,3′‐tetra­phenyl‐1,1′‐spiro­bi(3a,4,5,6‐tetra­hydro‐3H‐pyrrolo­[1,2‐c][1,3,2]­ox­azaborole)–7‐borane, is the dominant product under various reaction conditions; its crystal structure is consistent with ¹¹B, ¹H and ¹³C NMR and IR analyses.     

Nitrogen modified nanostructured titania: electronic,structural and visible‐light photocatalytic properties

Nitrogen doped titanium dioxide (N‐TiO₂) powders were synthesized as the hydrolysis condensation product of tetrabutyl titanate reaction with urea. Following adequate thermal treatment, nitrogen was introduced at interstitial lattice sites, mostly near the surface of the nanocrystals. These defects, detectable by Raman spectroscopy, induce a shift of the energy band gap to the visible range (at about 2.3 eV) that justifies significant visible light photocatalytic activity towards degradation of methyl orange (MO) pollutant.

     

Relaxation effects of slip in shear flow of linear molten polymers

The transient shear response of a linear molten polymer (linear low-density polyethylene) in the nonlinear domain was studied using a true shear (sliding plate) rheometer with different gap spacings to detect slip effects. It was found that nonlinear viscoelasticity is further complicated by wall slip phenomena. Experimental evidence suggested that static slip models coupled with Wagner’s constitutive equation cannot adequately describe the experimental data at large and fast shear deformations. A new dynamic slip model involving multiple slip relaxation times is proposed in this paper, together with a method to assess the model parameters. Significant improvement in predicting the stress response is demonstrated by several examples of start-up of steady shear and large-amplitude oscillatory tests of a linear low-density polyethylene.     

Electrochemical promotion of conventional and bipolar reactor configurations for NO reduction

The reduction of NO by CO or C₃H₆ in the presence of O₂, a reaction of great technological importance, was investigated on porous polycrystalline Rh catalyst films using a single-chamber and a wireless bipolar cell configuration. In the latter case the Rh catalyst films were deposited on the inner side of a YSZ tube, while two Au films deposited on the outer side of the tubes were used to polarize the Rh catalyst surface. The experimental conditions used in this study were close to those in the exhaust of a lean burn or diesel engine, i.e., high flowrates and space velocities and in some cases, considerable excess of oxygen. It was found that both direct (conventional) and indirect (wireless) polarization of the catalyst causes significant enhancement in the reaction rates (up to a factor of 20) and in the reactant conversion. These rate increases are strongly non-Faradaic with apparent Faradaic efficiencies, A, in the order of 100, manifesting the effect of Electrochemical Promotion or Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA). The Rh catalyst films were subsequently promoted in a classical way, via dry impregnation with NaOH, followed by drying and calcination. The thus Na-promoted Rh films were found to exhibit higher catalytic activity than the unpromoted films, with a considerable decrease in their light-off temperature. The effect of Electrochemical Promotion was then studied on these, already Na-promoted Rh catalysts. The results showed that the effect of Chemical and Electrochemical Promotion on the catalyst performance can be synergetic and their combination may lead to interesting practical applications. This is further supported by the fact that such bipolar tube configurations: (a) do not need electrical connection to the catalyst and (b) can be adapted easier to commercial exhaust units. Paper presented at the 8th EuroConference on Ionics, Carvoeiro, Algarve, Portugal, Sept. 16 – 22, 2001.     

Systematische Fehler bei der Selenbestimmung im ng/g-Bereich in biologischen Matrices nach dem Hydrid-AAS-Verfahren

Zusammenfassung Vier biologische Standardreferenz-substanzen (NBS und IAEA) werden mit zwei unterschiedlichen Aufschlußmethoden, einem Naßaufschluß mit einem HNO₃/HClO₃/HClO₄-Gemisch nach optimiertem Temperatur-Zeit-Programm sowie einer Verbrennungsmethode im Sauerstoffstrom (Trace-O-Mat), mineralisiert und die Selengehalte der Aufschlußlösungen nach der Hydrid-AAS-Methode bestimmt. Dabei treten gegenüber den zertifizierten Selengehalten mit zunehmender Einwaage Minusbefunde auf, die — von den Aufschlußmethoden unabhängig — auf sich addierende Querstörungen durch Begleitelemente im g/g-Bereich bei der Selenhydridbildung zurückzuführen sind, wie bereits an anderer Stelle [3] beschrieben wurde.

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Systematic errors in the determination of selenium in the ng/g-range in biological matrices by the hydride-AAS method

Summary Four biological standard-reference materials (NBS and IAEA) were mineralized with two independent decomposition methods — a wet decomposition with a mixture of HNO₃/HClO₃/HClO₄ using an optimized temperature-time programme, and a combustion method in an oxygen stream (Trace-O-Mat). The selenium contents of the decomposition solutions were determined by the hydride-AAS method. The selenium concentrations were in agreement with the certified values at low sample weights, but decreased as the sample weights were increased. This effect is independent of the decomposition method, but is caused by interferences of concomitant elements in the g/g-range with the formation of selenium hydride, as has been previously discussed [3].

Diese Arbeit wurde in dankenswerter Weise von der Fa. Sandoz AG, Basel, unterstützt.     

Isolation and structure of providencin: a highly oxygenated diterpene possessing a unique bicyclo[12.2.0]hexadecane ring system from the sea plume Pseudopterogorgia kallos

[structure: see text] Providencin (1) is a naturally occurring cytotoxin isolated from the Caribbean gorgonian octocoral Pseudopterogorgia kallos. Its highly oxygenated hexacyclic structure is based on a previously undescribed bicyclo[12.2.0]hexadecane ring system and was established through spectroscopic analysis and X-ray crystallographic analysis. Providencin (1) was shown to exhibit modest anticancer activity against human breast (MCF7), lung (NCI-H460), and CNS (SF-268) cancer cell lines.     

Vibrational autoionization in rydberg states of ammonia

Rotationally resolved photoelectron spectroscopy is used to elucidate the mechanism for vibrational autoionization via the nontotally symmetric umbrella vibration of ammonia ( NH3). This mechanism depends strongly on the character of the autoionizing level, and the data demonstrate that both the electronic and rotational degrees of freedom can compensate for the change in vibrational symmetry that occurs for odd changes in the quantum number of the umbrella vibration. This result has implications for models of the mode dependence of vibrational autoionization in polyatomic molecules.