Absorbing phase transitions of branching-annihilating random walks

The phase transitions to absorbing states of the branching-annihilating reaction-diffusion processes mA-->(m+k)A, nA-->(n-l)A are studied systematically in one space dimension within a new family of models. Four universality classes of nontrivial critical behavior are found. This provides, in particular, the first evidence of universal scaling laws for pair and triplet processes.     

Quantum automorphism groups of finite graphs

A quantum analogue of the automorphism group of a finite graph is introduced. These are quantum subgroups of the quantum permutation groups defined by Wang. The quantum automorphism group is a stronger invariant for finite graphs than the usual automorphism group. We get a quantum dihedral group .

     

Nanoscopic scale studies of LiFePO4 as cathode material in lithium-ion batteries for HEV application

We present a review of the structural properties of LiFePO₄. Depending on the mode of preparation, different impurities can poison this material. These impurities are identified and a quantitative estimate of their concentrations is deduced from the combination of X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and magnetic measurements. An optimized preparation provides samples with carbon-coated particles free of any impurity phase, insuring structural stability and electrochemical performance that justify the use of this material as a cathode element a new generation of lithium secondary batteries.     

Organocatalytic depolymerization of poly(ethylene terephthalate)

We describe the organocatalytic depolymerization of poly(ethylene terephthalate) (PET), using a commercially available guanidine catalyst, 1,5,7‐triazabicyclo[4.4.0]dec‐5‐ene (TBD). Postconsumer PET beverage bottles were used and processed with 1.0 mol % (0.7 wt %) of TBD and excess amount of ethylene glycol (EG) at 190 °C for 3.5 hours under atmospheric pressure to give bis(2‐hydroxyethyl) terephthalate (BHET) in 78% isolated yield. The catalyst efficiency was comparable to other metal acetate/alkoxide catalysts that are commonly used for depolymerization of PET. The BHET content in the glycolysis product was subject to the reagent loading. This catalyst influenced the rate of the depolymerization as well as the effective process temperature. We also demonstrated the recycling of the catalyst and the excess EG for more than 5 cycles. Computational and experimental studies showed that both TBD and EG activate PET through hydrogen bond formation/activation to facilitate this reaction. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Using rheometry to determine nucleation density in a colored system containing a nucleating agent

A new suspension-based rheological method was applied to experimentally study the crystallization of a nucleating agent (NA) filled isotactic polypropylene. This method allows for determination of point nucleation densities where other methods fail. For example, optical microscopy can fail because nucleation densities become too high to be counted (materials with effective NA) or crystallites are not easily visible (colored materials), while differential scanning calorimetry does not allow the effect of flow to be studied. Both quiescent and mild-shear-induced crystallization were investigated. The results show that the addition of a nucleating agent increases the nucleation density by six decades for quiescent crystallization. The effect of shear on crystallization in the presence of a nucleating agent was assessed, and it is demonstrated that, at least for this system, the effect of shear is much smaller than the effect of the nucleating agent.     

Static SIMS study on surfaces of chalcogenide glasses modified by an organic layer

Chalcogenide glasses are useful optic materials that find applications in infrared spectroscopy, sensors and thermal imaging. A route for direct surface modification of such glasses with organic layers has been investigated by static Time of Flight Secondary Ion Mass Spectrometry (ToF-SIMS). The GAS (germanium–arsenic–selenium) glasses are modified by deposition followed by UV-irradiation of disulfide- or/and silane-functionalized organic molecules. SIMS analysis shows the characteristic fragments of the grafted molecules and organic–inorganic fragments which prove unambiguously the binding mode to the surface: disulfides, after S–S cleavage, are linked to arsenic and selenium; triethoxysilanes bind exclusively to oxidized germanium. The successive grafting of disulfide and silane compounds on the same substrate (IG2 glass with 33% of Ge) affords a “mixed” organic layer on the glass surface. From water contact angle measurements and X-ray Photo-electron Spectroscopy (XPS), the coverage density is not significantly improved comparatively to the “non-mixed” layers. However, the grafting of both types of molecules allows to reach a more homogeneous coverage.     

Probing the Hydrogen Bonding Structure in the Rieske Protein

The use of the far‐infrared spectral range presents a novel approach for analysis of the hydrogen bonding in proteins. Here it is presented for the analysis of Fe? S vibrations (500–200 cm^?1) and of the intra‐ and intermolecular hydrogen bonding signature (300–50 cm^?1) in the Rieske protein from Thermus thermophilus as a function of temperature and pH. Three pH values were adequately chosen in order to study all the possible protonation states of the coordinating histidines. The Fe? S vibrations showed pH‐dependent shifts in the FIR spectra in line with the change of protonation state of the histidines coordinating the [2Fe? 2S] cluster. Measurements of the low‐frequency signals between 300 and 30 K demonstrated the presence of a distinct overall hydrogen bonding network and a more rigid structure for a pH higher than 10. To further support the analysis, the redox‐dependent shifts of the secondary structure were investigated by means of an electrochemically induced FTIR difference spectroscopic approach in the mid infrared. The results confirmed a clear pH dependency and an influence of the immediate environment of the cluster on the secondary structure. The results support the hypothesis that structure‐mediated changes in the environment of iron? sulfur centers play a critical role in regulating enzymatic catalysis. The data point towards the role of the overall internal hydrogen bonding organization for the geometry and the electronic properties of the cluster.     

Mechanical properties of natural rubber nanocomposites reinforced with cellulosic nanoparticles obtained from combined mechanical shearing,and enzymatic and acid hydrolysis of sisal fibers

In a previous work (Siqueira et al. 2010b) the preparation of cellulosic nanoparticles from sisal fibers using different processing routes, viz. a combination of mechanical shearing, acid and enzymatic hydrolysis was reported. It was shown that the pre-enzymatic hydrolysis treatment of bleached sisal pulp helps the preparation of well individualized rod-like nanocrystals. An amorphous polymer (natural rubber—NR) was chosen as model matrix to investigate the effect of these nanoparticles on the thermo-mechanical properties of nanocomposites. Both tensile tests and dynamic mechanical analyses showed improved stiffness for all nanocomposites. The enzymatic treatment allowed production of a huge range of cellulosic nanoparticles which provided completely different mechanical properties to NR matrix.