Synthesis of Polysaccharide Macrobolaforms

International audienceNeutral polysaccharides are very abundant renewable natural products that have been identified as alternative substrates for use in the field of polymer commmodities. Unfortunately beside other obstacles such as hydrophilicity, polysaccharides can display very low shape factors (molecular length/width) as compared to common synthetic polymers. This shape factor is often unsuited for creating the molecular entanglements, necessary for material physical cohesion for applications in the field of plastic material. In order to overcome this drawback various strategies have been applied to extend the length of these macromolecules, from controlled chemical cross-linking to macromolecular fractionation. Due to the very large number of chemically similar hydroxyl functions present on the macromolecular polymer backbone, selective chemical intermolecular-linking of hydroxyl groups is almost impossible to control, and very rapidly leads to unsuited tri-dimensional networks. Taking advantage of the unique chemical nature of one of the extremities (the reducing end) we developed several techniques of selective dimerisation of oligo-and polysaccharide, using diamine and dicarboxylic linkers. These dimerisations resulted in saccharidic bolaforms with improved shape factor by doubling of the molecular length. In the case of large polysaccharides the obtained macrobolaforms displayed interesting improved mechanical properties. Preliminary investigations of the structure/property relationships of these macrobolaforms show an improvement of mechanical behaviour, which was linked to the shape factor improvement and also to the ability to form oriented structures during material extrusion + dimerisation of polysaccharide

SPORT: A new sub-nanosecond time-resolved instrument to study swift heavy ion-beam induced luminescence - Application to luminescence degradation of a fast plastic scintillator

We developed a new sub-nanosecond time-resolved instrument to study the dynamics of UV-visible luminescence under high stopping power heavy ion irradiation. We applied our instrument, called SPORT, on a fast plastic scintillator (BC-400) irradiated with 27-MeV Ar ions having high mean electronic stopping power of 2.6 MeV/\mu m. As a consequence of increasing permanent radiation damages with increasing ion fluence, our investigations reveal a degradation of scintillation intensity together with, thanks to the time-resolved measurement, a decrease in the decay constant of the scintillator. This combination indicates that luminescence degradation processes by both dynamic and static quenching, the latter mechanism being predominant. Under such high density excitation, the scintillation deterioration of BC-400 is significantly enhanced compared to that observed in previous investigations, mainly performed using light ions. The observed non-linear behaviour implies that the dose at which luminescence starts deteriorating is not independent on particles' stopping power, thus illustrating that the radiation hardness of plastic scintillators can be strongly weakened under high excitation density in heavy ion environments.Comment: 5 figures, accepted in Nucl. Instrum. Methods

SuperMassive Black Holes in Bulges

We present spatially extended gas kinematics at parsec-scale resolution for the nuclear regions of four nearby disk galaxies, and model them as rotation of a gas disk in the joint potential of the stellar bulge and a putative central black hole. The targets were selected from a larger set of long-slit spectra obtained with the Hubble Space Telescope as part of the Survey of Nearby Nuclei with STIS (SUNNS). They represents the 4 galaxies (of 24) that display symmetric gas velocity curves consistent with a rotating disk. We derive the stellar mass distribution from the STIS acquisition images adopting the stellar mass-to-light ratio normalized so as to match ground-based velocity dispersion measurements over a large aperture. Subsequently, we constrain the mass of a putative black hole by matching the gas rotation curve, following two distinct approaches. In the most general case we explore all the possible disk orientations, alternatively we constrain the gas disk orientation from the dust-lane morphology at similar radii. In the latter case the kinematic data indicate the presence of a central black hole for three of the four objects, with masses of 10^7 - 10^8 solar masses, representing up to 0.025 % of the host bulge mass. For one object (NGC2787) the kinematic data alone provide clear evidence for the presence of a central black hole even without external constraints on the disk orientation. These results illustrate directly the need to determine black-hole masses by differing methods for a large number of objects, demonstrate that the variance in black hole/bulge mass is much larger than previously claimed, and reinforce the recent finding that the black-hole mass is tightly correlated with the bulge stellar velocity dispersion.Comment: 26 pages, 11 Postscript figures, accepted for publication on Ap

SAURON Observations of Disks in Early-Type Galaxies

We briefly describe the SAURON project, aimed at determining the intrinsic shape and internal dynamics of spheroids. We focus here on the ability of SAURON to identify gaseous and stellar disks and to measure their morphology and kinematics. We illustrate some of our results with complete maps of NGC3377, NGC3623, and NGC4365.Comment: 4 pages, 4 figures (newpasp.sty). To appear in ASP Conf. Series "Galaxy Disks and Disk Galaxies", eds. J.G. Funes S.J. & E.M. Corsini. Version with full resolution images available at http://www.strw.leidenuniv.nl/~bureau/pub_list.htm

Flux pinning in (1111) iron-pnictide superconducting crystals

Local magnetic measurements are used to quantitatively characterize heterogeneity and flux line pinning in PrFeAsO_1-y and NdFeAs(O,F) superconducting single crystals. In spite of spatial fluctuations of the critical current density on the macroscopic scale, it is shown that the major contribution comes from collective pinning of vortex lines by microscopic defects by the mean-free path fluctuation mechanism. The defect density extracted from experiment corresponds to the dopant atom density, which means that dopant atoms play an important role both in vortex pinning and in quasiparticle scattering. In the studied underdoped PrFeAsO_1-y and NdFeAs(O,F) crystals, there is a background of strong pinning, which we attribute to spatial variations of the dopant atom density on the scale of a few dozen to one hundred nm. These variations do not go beyond 5% - we therefore do not find any evidence for coexistence of the superconducting and the antiferromagnetic phase. The critical current density in sub-T fields is characterized by the presence of a peak effect, the location of which in the (B,T)-plane is consistent with an order-disorder transition of the vortex lattice.Comment: 12 pages, submitted to Phys Rev.

Discovery of a maximum damage structure for Xe-irradiated borosilicate glass ceramics containing powellite

In order to increase the waste loading efficiency in nuclear waste glasses, alternate glass ceramic (GC) materials are sought that trap problematic molybdenum in a water-durable CaMoO4 phase within a borosilicate glass matrix. In order to test the radiation resistance of these candidate wasteforms, accelerated external radiation can be employed to replicate long-term damage. In this study, several glasses and GCs were synthesized with up to 10 mol% MoO3 and subjected to 92 MeV Xe ions with fluences ranging between 5 × 10^12 to 1.8 × 10^14 ions/cm2. The main mechanisms of modification following irradiation involve: (i) thermal and defect-assisted diffusion, (ii) relaxation from the ion's added energy, (iii) localized damage recovery from overlapping ion tracks, and (iv) the accumulation of point defects or the formation of voids that created significant strain and led to longer-range modifications. Most significantly, a saturation in alteration could be detected for fluences greater than 4 × 10^13 ions/cm2, which represents an average structure that is representative of the maximum damage state from these competing mechanisms. The results from this study can therefore be used for long-term structural projections in the development of more complex GCs for nuclear waste applications.EPSRC (Grant No. EP/K007882/1

Energy deposition by heavy ions: Additivity of kinetic and potential energy contributions in hillock formation on CaF2

The formation of nano-hillocks on CaF2 crystal surfaces by individual ion impact has been studied using medium energy (3 and 5 MeV) highly charged ions (Xe19+ to Xe30+) as well as swift (kinetic energies between 12 and 58 MeV) heavy ions. For very slow highly charged ions the appearance of hillocks is known to be linked to a threshold in potential energy while for swift heavy ions a minimum electronic energy loss is necessary. With our results we bridge the gap between these two extreme cases and demonstrate, that with increasing energy deposition via electronic energy loss the potential energy threshold for hillock production can be substantially lowered. Surprisingly, both mechanisms of energy deposition in the target surface seem to contribute in an additive way, as demonstrated when plotting the results in a phase diagram. We show that the inelastic thermal spike model, originally developed to describe such material modifications for swift heavy ions, can be extended to case where kinetic and potential energies are deposited into the surface.Comment: 12 pages, 4 figure

Molecular Gas Dynamics in NGC 6946: a Bar-driven Nuclear Starburst "Caught in the Act"

We present high angular resolution ~1" and 0.6" mm-interferometric observations of the 12CO(1-0) and 12CO(2-1) line emission in the central 300pc of the late-type spiral galaxy NGC6946. The data, obtained with the IRAM Plateau de Bure Interferometer (PdBI), allow the first detection of a molecular gas spiral in the inner ~10" (270pc) with a large concentration of molecular gas (M(H_2) ~1.6x10^7M_sun) within the inner 60pc. This nuclear clump shows evidence for a ring-like geometry with a radius of ~10pc as inferred from the p-v diagrams. Both the distribution of the molecular gas as well as its kinematics can be well explained by the influence of an inner stellar bar of about 400pc length. A qualitative model of the expected gas flow shows that streaming motions along the leading sides of this bar are a plausible explanation for the high nuclear gas density. Thus, NGC6946 is a prime example of molecular gas kinematics being driven by a small-scale, secondary stellar bar.Comment: accepted for publication in the Astrophysical Journal; 47 pages, 17 figures, 1 tabl