412 research outputs found
Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor
Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales
Impact modification of poly(caprolactam) by copolymerization with a low molecular weight polybutadiene
Caprolactam and a reactive, low molecular weight polybutadiene were polymerized in an autoclave, followed by post-condensation in the solid state. The rubber concentration was varied (0–30 wt%). The morphology of the reaction products was studied by transmission electron microscopy. In the materials with 10 and 20 wt% polybutadiene, large spherical polybutadiene-rich domains with nylon sub-inclusions were observed. The 30 wt% polybutadiene product was shown to have a co-continuous structure. The moduli of the materials decrease rapidly with rubber content and only the material with the highest polybutadiene content had a high notched Izod impact strength
Copolymers of PBT and nylon 4T
Polybutylene terephthalate-nylon 4T copolymers (PBT-PA 4T) are synthesized from the diamide of diaminobutane and dimethyl terephthalate (DMT) with butane diol and more DMT in a concentration range of up to 50% PA 4T. The polymerization conditions were similar to those for PBT: first, a melt polymerization, followed by solid-state post-condensation. The materials were studied by differential scanning analysis (DSC) (melting and crystallization behavior) and dynamic mechanical thermal analysis (DMTA) (glass transitions and torsion moduli). The water absorptions were determined at 100% RH. By increasing the PA 4T content in the copolymers, melting temperatures increased strongly, heats of fusion decreased slightly, and glass transition temperatures increased linearly. The torsion moduli above the glass transition temperature were higher
Functionalization of arrays of silica nanochannels by post-condensation
Functionalized arrays of silica nanochannels (ASNCs) were prepared by post-condensation of amino-functional alkoxysilanes (NH-silanes). The method of post-condensation combines postsynthetic grafting and co-condensation – the classical pathways for the functionalization of mesoporous silica – to afford a convenient one-pot reaction in aqueous medium. The structure of the NH-silanes determines the distribution of the surface-bound groups in the ASNCs. Accumulation at the channel entrances was observed by confocal laser scanning microscopy in the case of sterically hindered NH-silanes, whereas smaller NH-silanes led to a more uniform distribution. In all cases, the channels remained accessible for additional postsynthetic treatment or introduction of guests after extraction of the structure-directing agent. The particle size and morphology, as well as the pore size and the degree of ordering of the amino-functionalized ASNCs were comparable to pristine ASNCs
Ugi post-condensation copper-triggered oxidative cascade towards pyrazoles
International audiencePyrazolidinones were prepared in a two-step sequence starting from a-hydrazonocarboxylic acids. After a four-component Ugi coupling, the resulting hydrazone was engaged in a copper triggered [3 + 2] cycloaddition/aerobic oxidation cascade. © 2011 Santos et al; licensee Beilstein-Institut
Nylon 46-polytetramethylene oxide segmented block copolymers
Block copolymers were synthesized from amine-terminated polytetramethylene oxide (PMTO) (Mw 800 and 1130) and polyamide 4,6 salt. First prepolymers were prepared at 200–210°C in the presence of a solvent (pyrrolidone). The prepolymers were postcondensed at 255°C (where possible in the solid state) to a high molecular weight. In the materials the endgroups were analysed, and the inherent viscosities and ethanol extraction values measured. The thermal properties were determined with d.s.c. and d.m.a. With ethanol, polyether could be extracted. Thus it seems that not all the polyether is present as part of the block copolymer. Melt phasing seems to have taken place and occurs even more with the higher molecular weight PTMO. With d.m.a. two glass transitions were observed; one of the polyether phase and one of the polyamide-rich phase. The melting temperatures of these polyamide 4,6 block copolymers are high (200–270°C), and the torsion modulus remained fairly constant up to these high melting temperatures
The abundance of SiS in circumstellar envelopes around AGB stars
New SiS multi-transition (sub-)millimetre line observations of a sample of
AGB stars with varying photospheric C/O-ratios and mass-loss rates are
presented. A combination of low- and high-energy lines are important in
constraining the circumstellar distribution of SiS molecules. A detailed
radiative transfer modelling of the observed SiS line emission is performed,
including the effect of thermal dust grains in the excitation analysis. We find
that the circumstellar fractional abundance of SiS in these environments has a
strong dependence on the photospheric C/O-ratio as expected from chemical
models. The carbon stars (C/O>1) have a mean fractional abundance of 3.1E-6,
about an order of magnitude higher than found for the M-type AGB stars (C/O<1)
where the mean value is 2.7E-7. These numbers are in reasonable agreement with
photospheric LTE chemical models. SiS appears to behave similar to SiO in terms
of photodissociation in the outer part of the circumstellar envelope. In
contrast to previous results for the related molecule SiO, there is no strong
correlation of the fractional abundance with density in the CSE, as would be
the case if freeze-out onto dust grains were important. However, possible
time-variability of the line emission in the lower J transitions and the
sensitivity of the line emission to abundance gradients in the inner part of
the CSE may mask a correlation with the density of the wind. There are
indications that the SiS fractional abundance could be significantly higher
closer to the star which, at least in the case of M-type AGB stars, would
require non-equilibrium chemical processes.Comment: Accepted for publication in A&A (14 pages, 7 figures
Prominence Mass Supply and the Cavity
A prevalent but untested paradigm is often used to describe the
prominence-cavity system: the cavity is under-dense because it is evacuated by
supplying mass to the condensed prominence. The thermal non-equilibrium (TNE)
model of prominence formation offers a theoretical framework to predict the
thermodynamic evolution of the prominence and the surrounding corona. We
examine the evidence for a prominence-cavity connection by comparing the TNE
model with diagnostics of dynamic extreme ultraviolet emission (EUV)
surrounding the prominence, specifically prominence horns. Horns are correlated
extensions of prominence plasma and coronal plasma which appear to connect the
prominence and cavity. The TNE model predicts that large-scale brightenings
will occur in the SDO/AIA 171\AA\ bandpass near the prominence that are
associated with the cooling phase of condensation formation. In our
simulations, variations in the magnitude of footpoint heating lead to
variations in the duration, spatial scale, and temporal offset between emission
enhancements in the other EUV bandpasses. While these predictions match well a
subset of the horn observations, the range of variations in the observed
structures is not captured by the model. We discuss the implications of our
one-dimensional loop simulations for the three-dimensional time-averaged
equilibrium in the prominence and the cavity. Evidence suggests that horns are
likely caused by condensing prominence plasma, but the larger question of
whether this process produces a density-depleted cavity requires a more tightly
constrained model of heating and better knowledge of the associated magnetic
structure
Circumstellar molecular line emission from S-type AGB stars: Mass-loss rates and SiO abundances
The main aim is to derive reliable mass-loss rates and circumstellar SiO
abundances for a sample of 40 S-type AGB stars based on new multi-transitional
CO and SiO radio line observations. In addition, the results are compared to
previous results for M-type AGB stars and carbon stars to look for trends with
chemical type. The circumstellar envelopes are assumed to be spherically
symmetric and formed by a constant mass-loss rate. The mass-loss rates are
estimated from fitting the CO observations using a non-local, non-LTE radiative
transfer code. Once the physical properties of the circumstellar envelopes are
determined, the same radiative transfer code is used to model the observed SiO
lines in order to derive circumstellar abundances and the sizes of the SiO
line-emitting regions. We have estimated mass-loss rates of 40 S-type AGB stars
and find that the derived mass-loss rates have a distribution that resembles
those previously derived for similar samples of M-type AGB stars and carbon
stars. The estimated mass-loss rates also correlate well with the corresponding
expansion velocity. In all, this indicates that the mass loss is driven by the
same mechanism in all three chemical types of AGB stars. In addition, we have
estimated the circumstellar fractional abundance of SiO relative to H2 in 26 of
the sample S-type AGB stars. The derived SiO abundances are, on average, about
an order of magnitude higher than predicted by stellar atmosphere thermal
equilibrium chemistry, indicating that non-equilibrium chemical processes
determines the abundance of SiO in the circumstellar envelope. Moreover, a
comparison with the results for M-type AGB stars and carbon stars show that for
a certain mass-loss rate, the circumstellar SiO abundance seems independent
(although with a large scatter) of the C/O-ratio.Comment: 24 pages, 11 figure
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