Method of calibrating gel permeation chromatography with whole polymers

Column systems for gel permeation chromatography are usually calibrated by eluting a series of sharp polymer fractions of known molecular weights and by correlating their elution volumes with molecular size or weight. A different method for calibration is proposed in which only one polymer sample, with a broad, well-characterized molecular weight distribution, is used. The cumulative distribution and the integrated, normalized GPC chromatogram are successively superimposed, and molecular weights and corresponding elution volumes are correlated. It is found that calibration curves obtained in this manner show a definite curvature. A possible explanation and correction for this behavior is given, based on the concentration dependence of elution volumes.     

On the viscosity-temperature behavior of polymer melts

Based on the free volume concept and the equation by Doolittle, an empirical equation is offered for the flow activation energy, E ^*, for polymer melts for the range of over 150°C above glass transition temperature, T _g. This E ^* represents the temperature coefficient of viscosity for the Newtonian region which is also equal to the value measured at constant shear stress for non-Newtonian flow. Data show that the E ^* of linear polymers approaches a constant value for a temperature range above T _g+150°C. Data on 17 polymers are correlated. The proposed equation for this region predicts the E ^* of polymer melts from the volume expansion coefficient, _l, above T _g and also from the T _g.Correlations have also been developed between E ^* and _l and between E ^* and T _g by simplifying the equation by use of the Simha-Boyer expression. A polymer having a lower _l or higher T _g generally has a higher E ^*. However, more satisfactory results are obtained by calculating E ^* from both _l and T _g. The E ^* calculated is found to agree with measurements within the experimental precision of about ±1 Kcal/mole.The effects of polymer composition, molecular weight, branching and microstructure on E ^* are also discussed. These factors influence E ^* in the way in which they effect _l and T _g.     

Trapped modes due to narrow cracks in thin simply-supported elastic plates

We consider an elastic plate of infinite length and constant width supported simply along its two parallel edges and having a finite length crack along its centreline. In particular, we look for and find trapped modes (localised oscillations) in the presence of the crack. An explicit wide-spacing approximation based on the Wiener–Hopf technique applied to incident wave scattering by semi-infinite cracks is complemented by an exact formulation of the problem in the form of integro-differential equations. An application of a Galerkin method for the numerical calculation of results from the latter method leads to a novel explicit ‘small-spacing’ approximation. In combination with the wide-spacing results this is shown to provide accurate results for all lengths of crack.     

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