A raman LAM study of lamellar thickness of virgin polyethylene powder

The lamellar thickness of polyethylene virgin reactor powder has been successfully measured using the Raman longitudinal acoustic mode (LAM). Relatively high lamellar thicknesses have been found. These values change with polymerization temperature in an unusual way, with lamellar thickness being larger for lower polymerization temperature, i.e., at higher undercooling for crystallization. Agreement within an experimental error of 10% has been obtained between values from LAM and those calculated by the Thompson—Gibbs equation from the melting point measured by differential scanning calorimetry.     

The influence of pressure on the capillary flow of poly(methyl methacrylate)

Summary Pressure effects in the capillary flow of a single sample of poly(methyl methacrylate),M _v = 1.33 · 10⁵, were evaluated. The length/diameter ratios of the different capillaries used varied from 4 to 100. The tests were made with an Instron rheometer in the range 160–250 °C. The pressure-viscosity model, derived from the free volume-WLF equation, was used to pressure correct the capillary data. The corrected data agreed well with data obtained at atmospheric pressure using aWeissenberg rheogoniometer. A derived expression to calculate an increase in the flow activation energy with increasing stress predicts the observed increase in activation energy.

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Zusammenfassung Druckeffekte bei der Kapillarströmung einer einzelnen Polymethylmethacrylat-ProbeM _v = 1,33 · 10⁵ werden abgeschätzt. Die Längen/Durchmesserverhältnisse der verschiedenen verwendeten Kapillaren variierten zwischen 4 und 100. Im Temperaturbereich zwischen 160 und 250 °C wurden die Versuche mit einem Instron-Rheometer durchgeführt. Das Druck-Viskositätsmodell, das aus der WLF-Gleichung abgeleitet war, wurde zur Druckkorrektur der Kapillardaten verwendet. Die korrigierten Werte zeigten eine gute Übereinstimmung mit den Werten, die bei Atmosphärendruck mit Hilfe einesWeissenberg-Rheogoniometers gewonnen worden waren. Ein abgeleiteter Ausdruck zur Berechnung des Anstiegs der Strömungs-Aktivierungsenergie mit steigender Spannung sagt den beobachteten Anstieg in der Aktivierungsenergie voraus.

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On leave from Montecatini Edison, Milan (Italy).     

Synthesis of PbSeTe single ternary alloy and core/shell heterostructured nanocubes

We report a robust method for synthesis of monodisperse PbSeTe single ternary alloy and core/shell heterostructured nanocubes, respectively. The key synthetic strategy to produce such different classes of nanocubes is to precisely control the time of reaction and successive growth. The crystallinity, shape/size distributions, structural characteristics, and compositions of as-prepared nanocubes, both ternary alloy and core/shell, were carefully studied. A plausible growth mechanism for developing each type of lead chalcogenide nanocubes is proposed. These delicately designed PbSeTe nanoscale architectures offer tunable compositions in PbSeTe ternary alloy and nano-interfaces in core/shell nanocubes, which are the critical factors in controlling thermal conductivity for applications in thermoelectrics.     

Eisenuntersuchungen

Ohne Zusammenfassung     

QUENCHING OF CHLOROPHYLL FLUORESCENCE BY NITROBENZENE

Abstract—Nitrobenzene quenching of chlorophyll fluorescence in ethanol has been investigated. Steady state relative quantum yields have been measured and fluorescence decay rates were determined using both nanosecond photon counting and picosecond pulses from a mode-locked Nd³⁺ glass laser.
The fluorescence decay is described by
1( t )= I ₀ exp (- t/τ−At^1/2 )
the form predicted for decay governed by the kinetics of the continuum model of diffusion controlled reactions. From the parameters of the fluorescence decay, the encounter distance is 5–7 A° the mutual diffusion coefficient is 0.62 × 10^--⁵ cm2s^-1± 12%.
Some of the fluorescence quenching is also attributed to static quenching by a nitrobenzene-chlorophyll, ground-state complex. The equilibrium constant for formation of this ground-state complex was determined to be 4.1 M ^-1. The combined dynamic and static quenching model allows calculation of quantum yields of fluorescence in good agreement with the experimentally determined quantum yields.     

N -Fold ?ech Derived Functors and Generalised Hopf Type Formulas

The notion of n-fold Čech derived functors is introduced and studied. This is illustrated using the n-fold Čech derived functors of the nilization functors Z_k. This gives a new purely algebraic method for the investigation of the Brown–Ellis generalised Hopf formula for the higher integral group homology and for its further generalisation. The paper ends with an application to algebraic K-theory. (Received: December 2004)     

Analysis of volumetric changes through melting using a dilatometer

Summary A conventional push-rod dilatometer is modified in order to accurately correlate the measured density to the predicted sample temperature of alloys in the phase-change regime. This new configuration makes use of a standard furnace assembly; however, the specimen is now symmetrically encased in a well-instrumented, graphite cylindrical shell. The combination of system geometry and high-conductivity sample holder material promotes the development of a simplified heat transfer model. The solution of this model properly correlates the measured density to that of the actual sample temperature based on using remote, sample-holder temperature measurements. Preliminary results using aluminum A356 provide insight into the proposed configuration.