2,886 research outputs found
Continuum Theory of Polymer Crystallization
We present a kinetic model of crystal growth of polymers of finite molecular
weight. Experiments help to classify polymer crystallization broadly into two
kinetic regimes. One is observed in melts or in high molar mass polymer
solutions and is dominated by nucleation control with , where is the growth rate and is the super-cooling. The
other is observed in low molar mass solutions (as well as for small molecules)
and is diffusion controlled with , for small . Our
model unifies these two regimes in a single formalism. The model accounts for
the accumulation of polymer chains near the growth front and invokes an
entropic barrier theory to recover both limits of nucleation and diffusion
control. The basic theory applies to both melts and solutions, and we
numerically calculate the growth details of a single crystal in a dilute
solution. The effects of molecular weight and concentration are also determined
considering conventional polymer dynamics. Our theory shows that entropic
considerations, in addition to the traditional energetic arguments, can capture
general trends of a vast range of phenomenology. Unifying ideas on
crystallization from small molecules and from flexible polymer chains emerge
from our theory.Comment: 37 double-spaced pages including 8 figures, submitted to the Journal
of Chemical Physic
Sedimentological studies of the "Ilha de Marchantaria" in the Solimões/Amazon River near Manaus
Sedimentological studies on the Ilha de Marchantaria an island in the Amazon river near Manaus reveal the existence of four different structural main units. A: Sandy channel bars consisting of giant ripples constitute deposits for the formation of islands or newly formed areas of the Varzea. B: Gradual accumulation on the channel bars leads to the formation of point bar ridges which consist chiefly of small-scale ripples. C: Between point bar ridges of different ages there exist swales. During rising water level the river water flows from the downward side into the swales where finegrained sediments are deposited. D: Permanent lakes are formed mainly in the centre of the islands. During low water periods the lakes may be cut of from the river. During high water periods when the whole island is flooded by the river, additional sedimentation takes place especially in the upstream area of the island. There is however, extensive erosion of the banks on the upstream side of the island. By erosion of the upstream end and sedimentation processes on the downstream end the island moves slowly downstream
Systematics of the odd-even effect in the resonance ionization of Os and Ti
Measurements of the odd-even effect in the mass spectrometric analysis of Ti and
Os isotopes by resonance ionization mass spectrometry have been performed for ΔJ = + 1, 0 and -1 transitions. Under saturating conditions of the ionization and for ΔJ = + 1 transitions odd-even
effects are reduced below the 0.5% level. Depending on the polarization state of the laser large
odd isotope enrichments are observed for ΔJ = 0 and -1 transitions which can be reduced below
the 0.5% level by depolarization of the laser field
Laser-induced isotopic selectivity in the resonance ionization of Os
Isotope selective effects in resonance ionization mass spectrometry (RIMS) pose a potentially serious limitation to the application of this technique to the precise and reproducible measurement of isotope ratios. In order to identify some of the underlying causes of isotope selectivity in RIMS and to establish procedures for minimizing these effects, we investigated laser-induced isotope selectivity in the resonance ionization of Os. A single-color, one-photon resonant ionization scheme was used for several different transitions to produce Os photoions from a thermal atomization source. Variations in Os isotope ratios were studied as a function of laser parameters such as wavelength, bandwidth, power and polarization state. Isotope selectivity is strongly dependent on laser power and wavelength, even when the bandwidth of the laser radiation is much larger than the optical isotope shift. Variations in the ^(190)Os/^(188)Os ratio of ≈20% for a detuning of 0.8 cm^(−1) were observed on a transition with a small oscillator strength. Large even—odd isotope selectivity with a 13% depletion of ^(189)Os was observed on a ΔJ = +1 transition at low laser intensity; the odd mass Os isotopes are systematically depleted. For ΔJ = −1 and 0 transitions the isotope selectivity was reduced by polarization scrambling and for strongly saturating conditions. A technique employing the wavelength dependence of even—even isotope selectivity as an internal wavelength standard was developed to permit accurate and reproducible wavelength adjustment of the laser radiation. This technique provides control over laser-induced isotope selectivity for single-color ionization and enabled us to obtain reproducible measurements of ^(192)Os/^(188)Os and ^(189)Os/^(190)Os ratios in the saturation regime for a ΔJ = +1 transition with a precision of better than 0.5%. The application of this wavelength-tuning procedure should significantly improve the quality of RIMS isotope ratio data for many elements
Systematics of isotope ratio measurements with resonant laser photoionization sources
Sources of laser-induced even-even and odd-even isotopic selectivity in the resonance ionization mass
spectroscopy of Os and Ti have been investigated experimentally for various types of transitions. A set
of conditions with regard to laser bandwidth and frequency tuning, polarization state and intensity was
obtained for which isotopic selectivity is either absent or reduced below the 2 % level
Laser-induced isotopic effects in titanium resonance ionization
Titanium isotope ratios have been measured by resonance ionization mass spectrometry (RIMS) with special emphasis on the nature of laser-induced isotopic selectivity. A pronounced wavelength dependence of even mass isotope ratios is caused by large nuclear volume effects near the
magic neutron number 28 in ^(50)Ti . Optical isotope shifts, ranging from 0.07 to 0.21 cm^(-l), between ^(50)Ti and ^(46)Ti were measured for several transitions. The ^(50)Ti/^(46)Ti and ^(48)Ti/^(46)Ti ratios, nevertheless, exhibited only mass-dependent fractionation, in which the lighter Ti isotopes were enriched by ~2.5%/amu, when the laser operating parameters were properly controlled. Odd-even mass isotopic selectivity in the resonant ionization process was also examined for several transitions as a function of the laser polarization state and intensity. Under saturating conditions for a ΔJ= +1 transition and a high degree of laser depolarization for a ΔJ = 0 transition, the odd-even isotopic enhancement was reduced below the 2% level. The Ti isotope data agree with our previous results for Os and indicate that, by a careful choice of resonance transitions and laser operating parameters, isotope ratios can be measured accurately and reliably with RIMS
FlexRiLoG -- A SageMath Package for Motions of Graphs
In this paper we present the SageMath package FlexRiLoG (short for flexible
and rigid labelings of graphs). Based on recent results the software generates
motions of graphs using special edge colorings. The package computes and
illustrates the colorings and the motions. We present the structure and usage
of the package
Development of a Metabolomic Method to Define the Phenylalanome in Arabidopsis thaliana
In the study of metabolomics, one of the greatest challenges can be accurately identifying compounds detected in biological extracts, especially when standards are not readily available. Current metabolomic methods are also limited in that they provide little to no information about a compound’s metabolic origin. In this study, we sought to address these issues by developing a novel metabolomic method that employs stable isotope feeding, LC-MS, Xcms, and an analytical software algorithm to study the ‘phenylalanome’ of Arabidopsis thaliana. Using this approach we were able to develop a method that, based on current results, is capable of detecting over 30 distinct compounds derived from phenylalanine. These results are promising, and indicate that with further refinement the method should be capable of detecting a significantly greater number of compounds. If implemented, the greater understanding of metabolism provided by this new approach should prove invaluable in harnessing plant genetics to produce desired outcomes. Additionally, the techniques developed in this study have the potential, when applied more broadly, to map a multitude of secondary metabolic networks in a variety of biological systems
Spin Hall effect transistor
Spin transistors and spin Hall effects have been two separate leading
directions of research in semiconductor spintronics which seeks new paradigms
for information processing technologies. We have brought the two directions
together to realize an all-semiconductor spin Hall effect transistor. Our
scheme circumvents semiconductor-ferromagnet interface problems of the original
Datta-Das spin transistor concept and demonstrates the utility of the spin Hall
effects in microelectronics. The devices use diffusive transport and operate
without electrical current, i.e., without Joule heating in the active part of
the transistor. We demonstrate a spin AND logic function in a semiconductor
channel with two gates. Our experimental study is complemented by numerical
Monte Carlo simulations of spin-diffusion through the transistor channel.Comment: 11 pages, 3 figure
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