Purification and characterization of the alpha-agarase from Alteromonas agarlyticus (Cataldi) comb. nov., strain GJ1B

The phenotypic features of strain GJ1B, an unidentified marine bacterium that degrades agar [Young, K. S. Bhattacharjee, S. S. & Yaphe, W. (1978) Carbohydr. Res. 66, 207-212], were investigated and its agarolytic system was characterized using 13C-NMR spectroscopy to analyse the agarose degradation products. The bacterium was assigned to the genus Alteromonas and the new combination A. agarlyticus (Cataldi) is proposed. An alpha-agarase, i.e. specific for the alpha(1-->3) linkages present in agarose, was purified to homogeneity from the culture supernatant by affinity chromatography on cross-linked agarose (Sepharose CL-6B) and by anion-exchange chromatography (Mono Q column). The major end product of agarose hydrolysis using the purified enzyme was agarotetraose. Using SDS/PAGE, the purified alpha-agarase was detected as a single band with a molecular mass of 180 kDa. After the affinity-chromatography step, however, the native molecular mass was approximately 360 kDa, suggesting that the native enzyme is a dimer which is dissociated to active subunits by anion-exchange chromatography. The isolectric point was estimated to be 5.3. Enzyme activity was observed using agar as the substrate over the pH range 6.0-9.0 with a maximum value at pH 7.2 in Mops or Tris buffer. The enzyme was inactivated by prolonged treatment at a pH below 6.5, or by temperatures over 45 degrees C or by removing calcium. In addition, a beta-galactosidase specific for the end products of the alpha-agarase was present in the alpha-agarase affinity-chromatography fraction, probably as part of a complex with this enzyme. The degradation of agarose by this agarase complex yielded a mixture of oligosaccharides in the agarotetraose series and the agarotriose series, the latter consisting of oligosaccharides with an odd number of galactose residues.     

Experimental investigation in the TOSQAN facility of heat and mass transfers in a spray for containment application

TOSQAN is an experimental program undertaken by the Institut de Radioprotection et de Sûreté Nucléaire (IRSN) in order to perform thermal hydraulic containment studies. The TOSQAN facility is a large enclosure devoted to simulate typical accidental thermal hydraulic flow conditions in nuclear-pressurized water reactor (PWR) containment. The TOSQAN facility which is highly instrumented with non-intrusive optical diagnostics is particularly adapted to nuclear safety CFD code validation. The present work is devoted to studying the interaction of a water spray injection used as a mitigation means in order to reduce the gas pressure and temperature in the containment, to produce gases mixing and washout of fission products. In order to have a better understanding of heat and mass transfers between spray droplets and the gas mixture, and to analyze mixing effects due to spray activation, we perform detailed characterization of the two-phase flow.     

In situ generation of nanoparticulate lanthanum(III) oxide-polyimide films: characterization of nanoparticle formation and resulting polymer properties

An in situ and single-step route to creating a uniform dispersion of lanthanum(III) oxide nanoparticles in a polyimide is described. The process of thermally evolving, from the diaquotris(2,4-pentanedionato)lanthanum(III) complex, to a homogeneous dispersion of lanthanum(III) oxide nanoparticles within a 6FDA/1,3(3)-APB polyimide matrix has been characterized. The report also describes the resulting changes in the final properties of the hybrid material relative to the neat polyimide. Characterization techniques include dielectric spectroscopy, small angle X-ray scattering, X-ray photoelectron spectroscopy, and gas permeability.     

Characteristic time of strain induced crystallization of crosslinked natural rubber

Real time Wide-Angle X-ray Scattering (WAXS) measurements during cyclic tensile tests at high strain rates (from 8 s^?1–280 s^?1) and at room temperature on crosslinked Natural Rubber (NR) are performed thanks to a specific homemade device. From the observed influence of the frequency on the crystallization index at the maximum sample elongation, a characteristic crystallization time is deduced. This is done taking into account the material self-heating during such unusually high strain rates. Two regimes for the dynamic process of strain induced crystallization are evidenced. For the NR tested, the obtained characteristic time is around 20 ms when the material average elongation during the cyclic test is above a critical elongation value λ_c. λ_c is the minimum elongation needed to induce crystallization during low strain rate tensile tests. Moreover, a rapid increase of this characteristic time is found when the average elongation decreases below this critical value.     

Polyelectrolyte behaviour of acrylonitrile methallylsulphonate copolymers in dimethylformamide

The properties in organic solvent (DMF) of two ionic copolymers are investigated and compared to those of polyelectrolytes in aqueous solutions. First, the viscometric behaviour is disccussed: it is demonstrated that [η] varies linearly with $\text{C}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}{}_{\mathrm{T}}$, where C_T is the ionic concentration of the solution. At infinite salt concentration we obtain data in agreement with unperturbed dimensions. Using isoionic dilution, we deduce ø_p, the osmotic coefficient. From osmometry, the dependence of ø_p on the concentration is obtained and compared to the theoretical value. In the presence of neutral salt, the osmotic pressure is determined as a function of salt and polymer concentrations. The results are interpreted in terms of a Donnan equilibrium.     

WAXS study of the structural reorganization of semi-crystalline polylactide under tensile drawing

This work deals with the in situ and ex situ WAXS study of the strain-induced structural changes in relation to the mechanical behavior of a semi-crystalline polylactide containing 4% of d-stereomer units. The crystals isothermally generated at 120 °C were in the α form. The draw temperature was in the range 65 °C < T_d < 90 °C, i.e. just above T_g. In this T_d range isothermal crystallization was so slow that only strain-induced crystals could be generated during the time scale of the experiments. This allowed studying samples with various initial levels of crystallinity, X_cr. For X_cr = 5%, the early occurrence of strain-hardening contrasted with the case of the amorphous material. This was due to the physical crosslinking of the macromolecular network by the crystallites that prevented chain relaxation since the onset of drawing. Strain-induced α′ crystals developed in parallel with the mesophase, the respective amount of the two phases being dependent on T_d. For X_cr = 24%, the drawing behavior was that of a crosslinked and filled elastomer. Again, both strain-induced α′ crystals and mesophase occurred in various proportions in relation to T_d. For X_cr = 40%, it was shown that the initial α crystals underwent partial destruction and subsequent reorganization into mesophase and/or α′ crystals. Whatever T_d and X_cr, the total amount of ordered phases, i.e. crystal + mesophase, did not exceed the value of 40% that was the maximum accessible crystal content for the present material. A mechanism of plastic deformation involving twinning is proposed from the WAXS analysis of the crystalline texturing in the case X_cr = 40%. AFM observations as a function of strain corroborate the proposed deformation scheme.     

Solution properties of dextran-polyacrylamide graft copolymers

Copolymers obtained by grafting polyacrylamide (PAA) onto polydextran (DEX) have been synthesised by ceric-ion-reduced redox initiation method. These copolymers have been characterized by light scattering, self-exclusion chromatography and viscometry in aqueous solutions. The short range molecular structure has been studied by small-angle X-ray scattering. From these results it is suggested that close to the anchoring point the grafted chains are extended while they take on a random conformation far from this point. A few investigations have been carried out in water/DMF (dimethyl formamide) mixtures. It is observed that the molecular weight depends strongly on the dimethyl formamide fraction. These results are discussed by contemplating the existence of PAA/DMF complexes.     

Binary blends of linear ethylene copolymers over a wide crystallinity range: Rheology, crystallization, melting and structure properties

This study deals with the physical properties of melt-compounded blends of three linear ethylene copolymers covering a large crystallinity range, namely 77% - 46% - 16% for the high density - linear low density - ultra low density copolymers, respectively. The melt behavior assessed from the zero-shear viscosity (η_o) reveals immiscibility of the three binary systems over the whole composition range. However, the change from positive to negative deviation of η_o with respect to the log-additivity mixing law as a function of composition suggests a structural transition from partial miscibility at the interface of the phase-separated domains to incompatibility. Crystallization and melting behaviors of the blends corroborate the occurrence of phase separation in the three systems. For most blends, the temperature shift of the crystallization (T_c) and melting (T_m) peaks as compared to the ones of the pure copolymers yet indicates partial miscibility in the crystalline and/or in the amorphous regions. It is pointed out that miscibility in the amorphous phase resulting from partial miscibility in the melt may, on its own, entail T_m depression of the crystals via surface free energy effect without necessarily implying cocrystallization and crystal thickness reduction. In several cases, the presence of intermediate endotherm and exotherm between the two main peaks of the melting and crystallization traces, respectively, discloses hybrid crystals assigned to a composition gradient at the interface of the phase-separated domains. A marked positive deviation of the upper T_c from the linear mixing rule is observed for the three systems. A nucleating effect from the interface of the phase-separated domains is suggested to promote early crystallization in the upper T_c phase. The SAXS data reveal electron density fluctuations at a much larger scale than that of the semi-crystalline structure demonstrating the occurrence of micro-phase separation in the melt prior to crystallization. Solubility of low T_m chain species in the amorphous layers of the high T_m phase is also evidenced. AFM and DMTA support micro-phase separation in the three systems and provide complementary information on the crystalline habits in the phase-separated domains of the blends.     

X-ray radiolysis induced formation of silver nano-particles: A SAXS and UV-visible absorption spectroscopy study

We show that X-ray irradiation of metal salt aqueous solutions in the absence of any stabilizer leads to the synthesis of metal nano-particles, similarly to γ-ray irradiation. The chemical route is the reduction of silver metal ions induced by radiolysis of water. Moreover, X-rays used for the synthesis allow following in situ the formation of silver nano-particles by Small Angle X-ray Scattering associated with UV-visible absorption spectroscopy. The synthesized nano-objects appear spherical with a radius of about 14 nm.     

Influence of Output Optical Losses on the Dynamic Characteristics of 1.55-μm Wafer-Fused Vertical-Cavity Surface-Emitting Lasers

Semiconductors - The results of studying the dynamic characteristics of 1.55-μm single-mode vertical-cavity surface-emitting lasers (VCSELs) formed by the fusion of wafers of high-quality...