461 research outputs found
Tailoring the flow of soft glasses by soft additives
We examine the vitrification and melting of asymmetric star polymers mixtures
by combining rheological measurements with mode coupling theory. We identify
two types of glassy states, a {\it single} glass, in which the small component
is fluid in the glassy matrix of the big one and a {\it double} glass, in which
both components are vitrified. Addition of small star polymers leads to melting
of {\it both} glasses and the melting curve has a non-monotonic dependence on
the star-star size ratio. The phenomenon opens new ways for externally steering
the rheological behavior of soft matter systems.Comment: 4 pages, 4 figures, accepted in Phys. Rev. Let
Field-theoretic simulation of block copolymers at experimentally-relevant molecular weights
Field-theoretic simulation (FTS) offers an efficient means of predicting the equilibrium behavior of high-molecular-weight structured polymers, provided one is able to deal with the strong ultraviolet (UV) divergence that occurs at realistic molecular weights. Here melts of lamellar-forming diblock copolymer are studied using a Monte Carlo version (MC-FTS), where the composition field fluctuates while the pressure field follows the mean-field approximation. We are able to control the UV divergence by introducing a new effective Flory-Huggins interaction parameter, , thereby permitting MC-FTS for molecular weights extending down to values characteristic of experiment. Results for the disordered-state structure function, the layer spacing and compressibility of the ordered lamellar phase, and the position of the order-disorder transition (ODT) show excellent agreement with recent particle-based simulation. Given the immense versatility of FTS, this opens up the opportunity for quantitative studies on a wide range of more complicated block copolymer systems
Phase separation in star polymer-colloid mixtures
We examine the demixing transition in star polymer-colloid mixtures for star
arm numbers f=2,6,16,32 and different star-colloid size ratios. Theoretically,
we solve the thermodynamically self-consistent Rogers-Young integral equations
for binary mixtures using three effective pair potentials obtained from direct
molecular computer simulations. The numerical results show a spinodal
instability. The demixing binodals are approximately calculated, and found to
be consistent with experimental observations.Comment: 4 pages, 4 figures, submitted to PR
Discovery of the Binary Pulsar PSR B1259-63 in Very-High-Energy Gamma Rays around Periastron with H.E.S.S
We report the discovery of very-high-energy (VHE) gamma-ray emission of the
binary system PSR B1259-63/SS 2883 of a radio pulsar orbiting a massive,
luminous Be star in a highly eccentric orbit. The observations around the 2004
periastron passage of the pulsar were performed with the four 13 m Cherenkov
telescopes of the H.E.S.S. experiment, recently installed in Namibia and in
full operation since December 2003. Between February and June 2004, a gamma-ray
signal from the binary system was detected with a total significance above 13
sigma. The flux was found to vary significantly on timescales of days which
makes PSR B1259-63 the first variable galactic source of VHE gamma-rays
observed so far. Strong emission signals were observed in pre- and
post-periastron phases with a flux minimum around periastron, followed by a
gradual flux decrease in the months after. The measured time-averaged energy
spectrum above a mean threshold energy of 380 GeV can be fitted by a simple
power law F_0(E/1 TeV)^-Gamma with a photon index Gamma =
2.7+-0.2_stat+-0.2_sys and flux normalisation F_0 = (1.3+-0.1_stat+-0.3_sys)
10^-12 TeV^-1 cm^-2 s^-1. This detection of VHE gamma-rays provides unambiguous
evidence for particle acceleration to multi-TeV energies in the binary system.
In combination with coeval observations of the X-ray synchrotron emission by
the RXTE and INTEGRAL instruments, and assuming the VHE gamma-ray emission to
be produced by the inverse Compton mechanism, the magnetic field strength can
be directly estimated to be of the order of 1 G.Comment: 10 pages, 8 figures, accepted in Astronomy and Astrophysics on 2 June
2005, replace: document unchanged, replaced author field in astro-ph entry -
authors are all members of the H.E.S.S. collaboration and three additional
authors (99+3, see document
Observations of Mkn 421 in 2004 with H.E.S.S. at large zenith angles
Mkn 421 was observed during a high flux state for nine nights in April and
May 2004 with the fully operational High Energy Stereoscopic System (H.E.S.S.)
in Namibia. The observations were carried out at zenith angles of
60--65, which result in an average energy threshold of 1.5 TeV
and a collection area reaching 2~km at 10~TeV. Roughly 7000 photons from
Mkn~421 were accumulated with an average gamma-ray rate of 8 photons/min. The
overall significance of the detection exceeds 100 standard deviations. The
light-curve of integrated fluxes above 2~TeV shows changes of the diurnal flux
up to a factor of 4.3. For nights of high flux, intra-night variability is
detected with a decay time of less than 1 hour. The time averaged energy
spectrum is curved and is well described by a power-law with a photon index
\egamm and an exponential cutoff at \ecut~TeV and an average integral flux
above 2~TeV of 3 Crab flux units. Significant variations of the spectral shape
are detected with a spectral hardening as the flux increases. Contemporaneous
multi-wavelength observations at lower energies (X-rays and gamma-rays above
~GeV) indicate smaller relative variability amplitudes than seen
above 2~TeV during high flux state observed in April 2004.Comment: 5 pages, 4 figures, published in A&
Very high energy gamma rays from the direction of Sagittarius A*.
We report the detection of a point-like source of very high energy (VHE) -rays coincident within 1' of Sgr A *, obtained with the HESS array of Cherenkov telescopes. The -rays exhibit a power-law energy spectrum with a spectral index of and a flux above the 165 GeV threshold of m -2 s -1. The measured flux and spectrum differ substantially from recent results reported in particular by the CANGAROO collaboration
A possible association of the new VHE gamma-ray source HESS J1825--137 with the pulsar wind nebula G18.0--0.7
We report on a possible association of the recently discovered very
high-energy -ray source HESS J1825--137 with the pulsar wind nebula
(commonly referred to as G 18.0--0.7) of the year old
Vela-like pulsar PSR B1823--13. HESS J1825--137 was detected with a
significance of 8.1 in the Galactic Plane survey conducted with the
H.E.S.S. instrument in 2004. The centroid position of HESS J1825--137 is offset
by 11\arcmin south of the pulsar position. \emph{XMM-Newton} observations have
revealed X-ray synchrotron emission of an asymmetric pulsar wind nebula
extending to the south of the pulsar. We argue that the observed morphology and
TeV spectral index suggest that HESS J1825--137 and G 18.0--0.7 may be
associated: the lifetime of TeV emitting electrons is expected to be longer
compared to the {\it XMM-Newton} X-ray emitting electrons, resulting in
electrons from earlier epochs (when the spin-down power was larger)
contributing to the present TeV flux. These electrons are expected to be
synchrotron cooled, which explains the observed photon index of , and
the longer lifetime of TeV emitting electrons naturally explains why the TeV
nebula is larger than the X-ray size. Finally, supernova remnant expansion into
an inhomogeneous medium is expected to create reverse shocks interacting at
different times with the pulsar wind nebula, resulting in the offset X-ray and
TeV -ray morphology.Comment: 5 pages, 3 figures, to appear in Astronomy and Astrophysics Letter
Polypeptide-grafted macroporous polyHIPE by surface-initiated N-Carboxyanhydride (NCA) polymerization as a platform for bioconjugation
A new class of functional macroporous monoliths from polymerized high internal phase emulsion (polyHIPE) with tunable surface functional groups was developed by direct polypeptide surface grafting. In the first step, amino-functional polyHIPEs were obtained by the addition of 4-vinylbenzyl or 4-vinylbenzylphthalimide to the styrenic emulsion and thermal radical polymerization. The obtained monoliths present the expected open-cell morphology and a high surface area. The incorporated amino group was successfully utilized to initiate the ring-opening polymer-
ization of benzyl-L-glutamate N-carboxyanhydride (BLG NCA) and benzyloxycarbonyl-L-lysine (Lys(Z)) NCA, which resulted in a dense homogeneous coating of polypeptides throughout the internal polyHIPE surfaces as confirmed by SEM and FTIR analysis. The amount of polypeptide grafted to the polyHIPE surfaces could be modulated by varying the initial ratio of amino acid NCA to amino-functional polyHIPE. Subsequent removal of the polypeptide protecting groups yielded highly functional polyHIPE-g-poly(glutamic acid) and polyHIPE-g- poly(lysine). Both types of polypeptide-grafted monoliths responded to pH by changes in their hydrohilicity. The possibility to use the high density of function (âCOOH or âNH2) for secondary reaction was demonstrated by the successful bioconjugation of enhanced green fluorescent protein (eGFP) and fluorescein isocyanate (FITC) on the polymer 3D-scaffold surface. The amount of eGFP and FITC conjugated to the polypeptide-grafted polyHIPE was significantly higher than to the amino- functional polyHIPE, signifying the advantage of polypeptide grafting to achieve highly functional polyHIPEs
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PEGâpeptide conjugates
The remarkable diversity of the self-assembly behavior
of PEGâpeptides is reviewed, including self-assemblies formed by PEGâpeptides with ÎČ-sheet and α-helical (coiled-coil) peptide sequences. The modes of self-assembly in solution and in the solid state are discussed. Additionally, applications in bionanotechnology and synthetic materials science are summarized
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