260 research outputs found
A detailed characterization and design of copolymerization
Many industrial polymerizations are copolymerizations in which two or more comonomers are copolymerized together to obtain a final product with a wide variety of properties originating from the related homopolymers. Crucial is the identification of the correct comonomer types and the reaction conditions so that the suited connectivity of monomer units is ensured in the copolymer chains. In view of this challenge a detailed characterization tool is indispensable. A sole focus on experimental tools is insufficient as they only allow the assessment of copolymer properties through relative properties and/or are limited to average properties [1-4]. The latter implies the lack of validation of intermolecular homogeneities, inhibiting process control on the polymer property level. To solve this issue and thanks to the advance in recent computer technologies, simulation tools have been developed which allow a characterization of copolymerization processes at the molecular level (see Figure 1; [3]). Monomer sequences of individual chains can be visualized allowing an unambiguous product qualification. In this contribution, the potential of these simulation tools is highlighted through several case studies. Focus in on both bulk/solution radical and cationic polymerizations and the interplay of chemistry and diffusional limitations [5-7].
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Designing controlled radical polymerization: A selection of a terminal or penultimate model for the intrinsic reactivities
In the past decades many efforts have been devoted to understand and design controlled radical polymerization (CRP) techniques such as atom transfer radical polymerization (ATRP) and nitroxide mediated polymerization (NMP). A crucial aspect is the use of detailed reaction schemes and the appropriate correction for diffusional limitations. Limited focus has however paid to the impact of penultimate monomer unit (PMU) effects, which can be explained by the complexity of the associated kinetic models with multiple reaction channels and the lack of data on reactivity ratios, in particular for NMP specific reactions. In the present contribution, it is demonstrated that depending on the comonomer pairs and the reaction conditions either a terminal [2] or penultimate model [3] is more suited. For copolymerizations with equimolar conditions for the comonomer amounts the PMU can be very pronounced even if based on the reactivity ratios as such this is not expected (Figure 1).
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Model-based design of MADIX under bulk and solution conditions
Macromolecular design by interchange of xanthates (MADIX) is a less studied controlled radical polymerization technique from a mechanistic and modeling point of view. In this contribution, MADIX of styrene and chain extension toward the synthesis of block copolymers is investigated, with azobisisobutyronitrile as conventional radical initiator and O-ethylxanthyl ethyl propionate as initial RAFT agent (R0X). Degenerative transfer coefficients for both the exchange with R0X and macro-RAFT agent are reported and their difference is highlighted to be relevant for the kinetic description. The model validity is supported by measurement of end-group functionality (EGF) data considering elemental analysis. Novel mechanistic insights are that in contrast to typical reversible addition fragmentation chain transfer (RAFT) polymerizations the macroradical CLD follows a Schulz-Flory distribution and that both during the homopolymerization and the chain extensions an exchange, so with monomer incorporation, only takes place once [1].
[1] D.J.G. Devlaminck, P.H.M. Van Steenberge, M.-F. Reyniers, D.R. D’hooge, Polym Chem. 2017, 8, 694
Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators
While the first binary post-AGB stars were serendipitously discovered, the
distinct characteristics of their Spectral Energy Distribution (SED) allowed us
to launch a more systematic search for binaries. We selected post-AGB objects
which show a broad dust excess often starting already at H or K, pointing to
the presence of a gravitationally bound dusty disc in the system. We started a
very extensive multi-wavelength study of those systems and here we report on
our radial velocity and photometric monitoring results for six stars of early F
type, which are pulsators of small amplitude. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. All of the six objects are binaries, with orbital periods ranging
from 120 to 1800 days. Five systems have non-circular orbits. The mass
functions range from 0.004 to 0.57 solar mass and the companions are likely
unevolved objects of (very) low initial mass. We argue that these binaries must
have been subject to severe binary interaction when the primary was a cool
supergiant. Although the origin of the circumstellar disc is not well
understood, the disc is generally believed to be formed during this strong
interaction phase. The eccentric orbits of these highly evolved objects remain
poorly understood. With the measured orbits and mass functions we conclude that
the circumbinary discs seem to have a major impact on the evolution of a
significant fraction of binary systems.Comment: 13 pages, 15 figures, accepted for Astronomy and Astrophysic
The rotation and coma profiles of comet C/2004 Q2 (Machholz)
Aims. Rotation periods of cometary nuclei are scarce, though important when
studying the nature and origin of these objects. Our aim is to derive a
rotation period for the nucleus of comet C/2004 Q2 (Machholz). Methods. C/2004
Q2 (Machholz) was monitored using the Merope CCD camera on the Mercator
telescope at La Palma, Spain, in January 2005, during its closest approach to
Earth, implying a high spatial resolution (50km per pixel). One hundred seventy
images were recorded in three different photometric broadband filters, two blue
ones (Geneva U and B) and one red (Cousins I). Magnitudes for the comet's
optocentre were derived with very small apertures to isolate the contribution
of the nucleus to the bright coma, including correction for the seeing. Our CCD
photometry also permitted us to study the coma profile of the inner coma in the
different bands. Results. A rotation period for the nucleus of P = 9.1 +/- 0.2
h was derived. The period is on the short side compared to published periods of
other comets, but still shorter periods are known. Nevertheless, comparing our
results with images obtained in the narrowband CN filter, the possibility that
our method sampled P/2 instead of P cannot be excluded. Coma profiles are also
presented, and a terminal ejection velocity of the grains v_gr = 1609 +/- 48
m/s is found from the continuum profile in the I band.Comment: 11 pages, 9 figures, accepted by A&
Comet Machholz (C/2004 Q2): morphological structures in the inner coma and rotation parameters
Extensive observations of comet C/2004 Q2 (Machholz) were carried out between
August 2004 and May 2005. The images obtained were used to investigate the
comet's inner coma features at resolutions between 350 and 1500 km/pixel. A
photometric analysis of the dust outflowing from the comet's nucleus and the
study of the motion of the morphological structures in the inner coma indicated
that the rotation period of the nucleus was most likely around 0.74 days. A
thorough investigation of the inner coma morphology allowed us to observe two
main active sources on the comet's nucleus, at a latitude of +85{\deg} \pm
5{\deg} and +45{\deg} \pm 5{\deg}, respectively. Further sources have been
observed, but their activity ran out quite rapidly over time; the most relevant
was at latcom. = 25{\deg} \pm 5{\deg}. Graphic simulations of the geometrical
conditions of observation of the inner coma were compared with the images and
used to determine a pole orientation at RA=95{\deg} \pm 5{\deg}, Dec=+35{\deg}
\pm 5{\deg}. The comet's spin axis was lying nearly on the plane of the sky
during the first decade of December 2004.Comment: 29 pages, 8 figures, 3 table
Folding ion rays in comet C/2004 Q2 (Machholz) and the connection with the solar wind
The appearance of folding ion rays in cometary comae is still not very well
understood, so our aim is to gain more insight into the role of the local solar
wind in the formation of these structures. Comet C/2004 Q2 (Machholz) was
intensively monitored during its closest approach to Earth (January 2005) with
the CCD camera Merope mounted on the Flemish 1.2m Mercator telescope, in three
different bands (Geneva U and B and Cousins I). Spectacular ion rays, thin
ionic structures rapidly folding tailward, were recorded in the U band during
one night, January 12th. Data from the SOHO satellite that was extrapolated
corotationally to the position of the comet showed that the ion rays were
formed during a sudden change in the in-situ solar wind state. We were able to
succesfully correlate a high-speed solar wind stream with the appearance of
folding ion rays. To our knowledge, this is the first clear observational
evidence that folding ion rays in cometary comae are produced by a sudden
change in the local solar wind state.Comment: 4 pages, 4 figures, accepted by A&
Detailed abundance study of four s-process enriched post-AGB stars in the Large Magellanic Cloud
Analysis of 26 Barium Stars II. Contributions of s-, r- and p-processes in the production of heavy elements
Barium stars show enhanced abundances of the slow neutron capture (s-process)
heavy elements, and for this reason they are suitable objects for the study of
s-process elements. The aim of this work is to quantify the contributions of
the s-, r- and p-processes for the total abundance of heavy elements from
abundances derived for a sample of 26 barium stars. The abundance ratios
between these processes and neutron exposures were studied. The abundances of
the sample stars were compared to those of normal stars thus identifying the
fraction relative to the s-process main component. The fittings of the sigmaN
curves (neutron capture cross section times abundance, plotted against atomic
mass number) for the sample stars suggest that the material from the companion
asymptotic giant branch star had approximately the solar isotopic composition
as concerns fractions of abundances relative to the s-process main component.
The abundance ratios of heavy elements, hs, ls and s and the computed neutron
exposure are similar to those of post-AGB stars. For some sample stars, an
exponential neutron exposure fits well the observed data, whereas for others, a
single neutron exposure provides a better fit. The comparison between barium
and AGB stars supports the hypothesis of binarity for the barium star
formation. Abundances of r-elements that are part of the s-process path in
barium stars are usually higher than those in normal stars,and for this reason,
barium stars seemed to be also enriched in r-elements, although in a lower
degree than s-elements. No dependence on luminosity classes was found in the
abundance ratios behaviour among the dwarfs and giants of the sample barium
stars.Comment: 30 pages including 24 figures, accepted to A&
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