Transient rheological behaviour of poly-para-henylenetherephthalamide (PpPTA) in sulphuric acid

Nearly all the available information on the transient flow behaviour of liquid crystalline polymers has been obtained on model systems, especially on solutions of polybenzylglutamate (PBG) and hydroxypropylcellulose (HPC). The assessment of rheological models has been based almost entirely on these model systems. It is not clear how much of the available theoretical and experimental knowledge can be applied to systems of industrial relevance, which have quite different molecular structures. Here, an industrial lyotropic system, poly(p-phenylenetherephthalamide) (PpPTA) in sulphuric acid (TWARON from AKZO), is investigated. Various techniques to study transient behaviour are used, these include measurements of transient shear and normal stresses after sudden changes in shear rate, dynamic moduli and stress relaxation after cessation of flow and elastic recoil. At all shear rates studied the PpPTA solution is shear thinning, and the first normal stress difference remains positive. For the stress transients a strain scaling applies reasonably well as it did in model systems. The moduli increase with time upon cessation of flow, indicating that the molecules become less oriented in the previous flow direction. This particular behaviour is similar to that of HPC. Transients also resemble more closely those of HPC rather than those of PBG. This latter difference might be attributed to the higher flexibility of HPC and PpPTA chains as compared with PBG molecules.     

Investigation of a catalyst ink dispersion using both ultra-small-angle X-ray scattering and cryogenic TEM

The dispersion of Nafion ionomer particles and Pt/C catalyst aggregates in liquid media was studied using both ultra-small-angle X-ray scattering (USAXS) and cryogenic TEM. A systematic approach was taken to study first the dispersion of each component (i.e., ionomer particles and Pt/C aggregates), then the combination of the components, and last the catalyst ink. Multiple-level curve fitting was used to extract the particle size, size distribution, and geometry of the Pt/C aggregates and the Nafion particles in liquid media from the scattering data. The results suggest that the particle size, size distribution, and geometry are not uniform throughout the systems but rather vary significantly. It was found that the interaction of each component (i.e., the Nafion ionomer particles and the Pt/C aggregates) occurs in the dispersion. Cryogenic TEM was used to observe the size and geometry of the particles in liquid directly and to validate the scattering results. The TEM results showed excellent agreement.     

Separation,Identification, and Quantitative Analysis of the Epimers of Cefaclor by TLC-Densitometry

JPC – Journal of Planar Chromatography – Modern TLC - The epimers of cefaclor have been separated, identified, and analyzed quantitatively by TLC. The stationary phase was silica gel 60...     

Photochemical Reduction of Carbon Dioxide Catalyzed by a Ruthenium‐Substituted Polyoxometalate

A polyoxometalate of the Keggin structure substituted with Ru^III, ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] in which ⁶Q=(C₆H₁₃)₄N⁺, catalyzed the photoreduction of CO₂ to CO with tertiary amines, preferentially Et₃N, as reducing agents. A study of the coordination of CO₂ to ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉] showed that 1) upon addition of CO₂ the UV/Vis spectrum changed, 2) a rhombic signal was obtained in the EPR spectrum (g_x=2.146, g_y=2.100, and g_z=1.935), and 3) the ¹³C NMR spectrum had a broadened peak of bound CO₂ at 105.78 ppm (Δ_1/2=122 Hz). It was concluded that CO₂ coordinates to the Ru^III active site in both the presence and absence of Et₃N to yield ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉]. Electrochemical measurements showed the reduction of Ru^III to Ru^II in ⁶Q₅[Ru^III(CO₂)SiW₁₁O₃₉] at ?0.31 V versus SCE, but no such reduction was observed for ⁶Q₅[Ru^III(H₂O)SiW₁₁O₃₉]. DFT‐calculated geometries optimized at the M06/PC1//PBE/AUG‐PC1//PBE/PC1‐DF level of theory showed that CO₂ is preferably coordinated in a side‐on manner to Ru^III in the polyoxometalate through formation of a Ru? O bond, further stabilized by the interaction of the electrophilic carbon atom of CO₂ to an oxygen atom of the polyoxometalate. The end‐on CO₂ bonding to Ru^III is energetically less favorable but CO₂ is considerably bent, thus favoring nucleophilic attack at the carbon atom and thereby stabilizing the carbon sp² hybridization state. Formation of a O₂C–NMe₃ zwitterion, in turn, causes bending of CO₂ and enhances the carbon sp² hybridization. The synergetic effect of these two interactions stabilizes both Ru–O and C–N interactions and probably determines the promotional effect of an amine on the activation of CO₂ by [Ru^III(H₂O)SiW₁₁O₃₉]^5?. Electronic structure analysis showed that the polyoxometalate takes part in the activation of both CO₂ and Et₃N. A mechanistic pathway for photoreduction of CO₂ is suggested based on the experimental and computed results.     

Investigation of the Mechanism of Colloidal Silicalite‐1 Crystallization by Using DLS,SAXS, and 29Si NMR Spectroscopy

Colloidal silicalite‐1 zeolite was crystallized from a concentrated clear sol prepared from tetraethylorthosilicate (TEOS) and aqueous tetrapropylammonium hydroxide (TPAOH) solution at 95 °C. The silicate speciation was monitored by using dynamic light scattering (DLS), synchrotron small‐angle X‐ray scattering (SAXS), and quantitative liquid‐state ²⁹Si NMR spectroscopy. The silicon atoms were present in dissolved oligomers, two discrete nanoparticle populations approximately 2 and 6 nm in size, and crystals. On the basis of new insight into the evolution of the different nanoparticle populations and of the silicate connectivity in the nanoparticles, a refined crystallization mechanism was derived. Upon combining the reagents, different types of nanoparticles (ca. 2 nm) are formed. A fraction of these nanoparticles with the least condensed silicate structure does not participate in the crystallization process. After completion of the crystallization, they represent the residual silicon atoms. Nanoparticles with a more condensed silicate network grow until approximately 6 nm and evolve into building blocks for nucleation and growth of the silicalite‐1 crystals. The silicate network connectivity of nanoparticles suitable for nucleation and growth increasingly resembles that of the final zeolite. This new insight into the two classes of nanoparticles will be useful to tune the syntheses of silicalite‐1 for maximum yield.     

Ring Opening of Pymisyl‐Protected Aziridines with Organocuprates

The pyrimidine‐2‐sulfonyl (pymisyl) group is introduced as a new protecting group that can be used to activate aziridines towards ring opening. It is readily introduced and removed under mild conditions. Regioselective ring opening of pymisyl‐protected 2‐methyl‐aziridine with organocuprates gives the corresponding sulfonamides in high yields, and the pymisyl group can subsequently be removed upon treatment with a thiolate. The versatility of this new nitrogen protecting group is illustrated with a new synthesis of Selegiline, a monoamine oxidase‐B inhibitor marketed for the treatment of Parkinson’s disease.     

Ordered mesoporous MFe(2)O(4) (M = Co, Cu, Mg, Ni, Zn) thin films with nanocrystalline walls, uniform 16 nm diameter pores and high thermal stability: template-directed synthesis and characterization of redox active trevorite

In this paper, we report on ordered mesoporous NiFe(2)O(4) thin films synthesized via co-assembly of hydrated ferric nitrate and nickel chloride with an amphiphilic diblock copolymer, referred to as KLE. We establish that the NiFe(2)O(4) samples are highly crystalline after calcination at 600 °C, and that the conversion of the amorphous inorganic framework comes at little cost to the ordering of the high quality cubic network of pores averaging 16 nm in diameter. We further show that the synthesis method employed in this work can be readily extended to other ferrites, such as CoFe(2)O(4), CuFe(2)O(4), MgFe(2)O(4), and ZnFe(2)O(4), which could pave the way for innovative device design. While this article focuses on the self-assembly and characterization of these materials using various state-of-the-art techniques, including electron microscopy, grazing incidence small-angle X-ray scattering (GISAXS), time-of-flight secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS), as well as UV-vis and Raman spectroscopy, we also examine the electrochemical properties and show the benefits of combining a continuous mesoporosity with nanocrystalline films. KLE-templated NiFe(2)O(4) electrodes exhibit reasonable levels of lithium ion storage at short charging times which stem from facile pseudocapacitance.     

Ab initio characterization of the Mg-HF van der Waals complex

The equilibrium structure and the three-dimensional potential energy surface of the Mg-HF van der Waals complex in its ground electronic state have been determined from accurate ab initio calculations using the coupled-cluster method, CCSD(T), in conjunction with the basis sets of triple- through quintuple-zeta quality. The core-electron correlation, high-order valence-electron correlation, and scalar relativistic effects were investigated. The Mg-HF complex was confirmed to be linear at equilibrium, with a vibrationless dissociation energy (into Mg and HF) D(e) of 280 cm(-1). The vibration-rotation energy levels of two isotopologues, (24)Mg-HF and (24)Mg-DF, were predicted using the variational method. The predicted spectroscopic constants can be useful in a further analysis of high-resolution vibration-rotation spectra of the Mg-HF complex.