1,860 research outputs found
Novel "green" catalysts for controlled ring-opening polymerization of lactide
Syntéza polylaktidu (PLA) polymerací za otevření kruhu cyklického monomeru (ROP) může být uskutečněna různými způsoby. Literatura uvádí více než 100 katalytických systémů, jejichž pomocí lze polylaktid a jiné biodegradabilní alifatické polyestery získat. Například organokovové katalyzátory na bázi Sn, Zn, Al atd. se po splnění své polymerační funkce stávají kontaminanty a pro humánní implantáty je použití takového materiálu diskutabilní. V současné době jsou v centru výzkumné pozornosti nové N-heterocyklické karbenové katalyzátory. Tyto „metal-free“ katalytické struktury jsou schopné reprodukovatelně řídit syntézu polymerů předem definované molekulové hmotnosti s definovanými koncovými skupinami a nízkou polydisperzitou, která je charakteristická pro živý průběh polymerace. Nabízí se možnost syntézy blokových kopolymerů a různorodých makromolekulárních architektur. Předložená diplomová práce se zabývá studiem polymerace cyklického monomeru D,L-laktidu katalyzované N-heterocyklickým karbenem. Polymerace byly vedeny v přítomnosti benzylalkoholu jako iniciátoru v roztoku THF. Byl sledován vliv složení reakčního systému monomer – iniciátor – katalyzátor. Dále byly připraveny polymery opticky čistého L-laktidu s makroiniciátory PEG s Mn = 1000 a 2000 g/mol. Střední číselná molekulová hmotnost (Mn) a polydisperzita (PDI) byly stanoveny pomocí GPC. Definovatelnost koncových skupin vybraných polymerů byla prokázána pomocí 1H NMR.The synthesis of polylactide (PLA) by ring-opening polymerization (ROP) of cyclic monomer can be realized by different routes. More than 100 catalysts for the synthesis of polylactide and other biodegradable aliphatic polyesters are published in the literature. For example organometallic catalysts based on Sn, Zn, Al etc. after finishing polymerization function became contaminants and using obtained polymer material in human body is controversial. At present, the research is focused on novel N-hererocyclic carbene catalysts. These metal-free catalysts are able to produce polymers with controlled molecular weight, narrow polydispersity, end-group fidelity with high reproducibility as well as to synthesize the block copolymers and complex macromolecular architectures, which is characteristic for living polymerization system. This diploma thesis is focused on study of polymerization of cyclic monomer D,L-lactide catalyzed by N-hererocyclic carbene. Polymerizations were carried out at the presence of benzylalcohol as initiator at THF. We were focused on the influence of composition of reaction system monomer – initiator – catalyst. Polymers of optically pure L-lactide with macroinitiators PEG with Mn of 1000 a 2000 g/mol were prepared as well. Number average molecular weight (Mn) and polydispersity index (PDI) was determined by GPC. 1H NMR was used to prove end-group fidelity.
Nucleosynthesis of Nickel-56 from Gamma-Ray Burst Accretion Disks
We examine the prospects for producing Nickel-56 from black hole accretion
disks, by examining a range of steady state disk models. We focus on relatively
slowly accreting disks in the range of 0.05 - 1 solar masses per second, as are
thought to be appropriate for the central engines of long-duration gamma-ray
bursts. We find that significant amounts of Nickel-56 are produced over a wide
range of parameter space. We discuss the influence of entropy, outflow
timescale and initial disk position on mass fraction of Nickel-56 which is
produced. We keep careful track of the weak interactions to ensure reliable
calculations of the electron fraction, and discuss the role of the neutrinos.Comment: 10 pages, 9 figure
Neutrino Interactions in the Outflow from Gamma-Ray Burst Accretion Disks
We examine the composition of matter as it flows away from gamma ray burst
accretion disks, in order to determine what sort of nucleosynthesis may occur.
Since there is a large flux of neutrinos leaving the surface of the disk, the
electron fraction of the outflowing material will change due to charged current
neutrino interactions. We calculate the electron fraction in the wind using
detailed neutrino fluxes from every point on the disk and study a range of
trajectories and outflow conditions for several different accretion disk
models. We find that low electron fractions, conducive to making r-process
elements, only appear in outflows from disks with high accretion rates that
have a significant region both of trapped neutrinos and antineutrinos. Disks
with lower accretion rates that have only a significant region of trapped
neutrinos can have outflows with very high electron fractions, whereas the
lowest accretion rate disks with little trapping have outflow electrons
fractions of closer to one half.Comment: 11 pages, 10 figure
Open Sourcing Social Change: Inside the Constellation Model
The constellation model was developed by and for the Canadian Partnership for Children's Health and the Environment. The model offers an innovative approach to organizing collaborative efforts in the social mission sector and shares various elements of the open source model. It emphasizes self-organizing and concrete action within a network of partner organizations working on a common issue. Constellations are self-organizing action teams that operate within the broader strategic vision of a partnership. These constellations are outwardly focused, placing their attention on creating value for those in the external environment rather than on the partnership itself. While serious effort is invested into core partnership governance and management, most of the energy is devoted to the decision making, resources and collaborative effort required to create social value. The constellations drive and define the partnership. The constellation model emerged from a deep understanding of the power of networks and peer production. Leadership rotates fluidly amongst partners, with each partner having the freedom to head up a constellation and to participate in constellations that carry out activities that are of more peripheral interest. The Internet provided the platform, the partner network enabled the expertise to align itself, and the goal of reducing chemical exposure in children kept the energy flowing. Building on seven years of experience, this article provides an overview of the constellation model, discusses the results from the CPCHE, and identifies similarities and differences between the constellation and open source models
The impact of global nuclear mass model uncertainties on -process abundance predictions
Rapid neutron capture or `-process' nucleosynthesis may be responsible for
half the production of heavy elements above iron on the periodic table. Masses
are one of the most important nuclear physics ingredients that go into
calculations of -process nucleosynthesis as they enter into the calculations
of reaction rates, decay rates, branching ratios and Q-values. We explore the
impact of uncertainties in three nuclear mass models on -process abundances
by performing global monte carlo simulations. We show that root-mean-square
(rms) errors of current mass models are large so that current -process
predictions are insufficient in predicting features found in solar residuals
and in -process enhanced metal poor stars. We conclude that the reduction of
global rms errors below keV will allow for more robust -process
predictions.Comment: 5 pages, 3 figures, invited talk at the 15th International Symposium
on Capture Gamma-Ray Spectroscopy and Related Topics (CGS15), to appear in
EPJ Web of Conference
Prospects for obtaining an r-process from Gamma Ray Burst Disk Winds
We discuss the possibility that r-process nucleosynthesis may occur in the
winds from gamma ray burst accretion disks. This can happen if the temperature
of the disk is sufficiently high that electron antineutrinos are trapped as
well as neutrinos. This implies accretion disks with greater than a solar mass
per second accretion rate, although lower accretion rates with higher black
hole spin parameters may provide viable environments as well. Additionally, the
outflow from the disk must either have relatively low entropy, e.g. around s =
10, or the initial acceleration of the wind must be slow enough that it is
neutrino and antineutrino capture as opposed to electron and positron capture
that sets the electron fraction.Comment: 8 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8
proceeding
On the Contribution of Gamma Ray Bursts to the Galactic Inventory of Some Intermediate Mass Nuclei
Light curves from a growing number of Gamma Ray Bursts (GRBs) indicate that
GRBs copiously produce radioactive Ni moving outward at fractions of the speed
of light. We calculate nuclear abundances of elements accompanying the
outflowing Ni under the assumption that this Ni originates from a wind blown
off of a viscous accretion disk. We also show that GRB's likely contribute
appreciably to the galactic inventory of 42Ca, 45Sc, 46Ti, 49Ti, 63Cu, and may
be a principal site for the production of 64Zn.Comment: 11 pages, 1 figur
- …